FABRICATION METHODS FOR ... - Stanford University

FABRICATION METHODS FOR ENVIRONMENTALLY HARDENED SENSORS

A DISSERTATIONSUBMITTED TO THE DEPARTMENT OF ELECTRICAL ENGINEERING

AND THE COMMITTEE ON GRADUATE STUDIESOF STANFORD UNIVERSITY

IN PARTIAL FULFILLMENT OF THE REQUIREMENTSFOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

Anthony F. Flannery Jr.

May 2011

http://creativecommons.org/licenses/by-nc/3.0/us/

This dissertation is online at: http://purl.stanford.edu/jc894rw7986

© 2011 by Anthony Francis Flannery, Jr. All Rights Reserved.

Re-distributed by Stanford University under license with the author.

This work is licensed under a Creative Commons Attribution-Noncommercial 3.0 United States License.

ii



http://purl.stanford.edu/jc894rw7986

I certify that I have read this dissertation and that, in my opinion, it is fully adequatein scope and quality as a dissertation for the degree of Doctor of Philosophy.

Roger Howe, Primary Adviser


Thomas Kenny


Beth Pruitt

Approved for the Stanford University Committee on Graduate Studies.

Patricia J. Gumport, Vice Provost Graduate Education

This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file inUniversity Archives.

iii

iv

Abstract

Micromachined sensors have continued to open exciting new doors in metrology. Appli-

cations in biology, pharmacology, genetics, chemistry, and other fields are driving cross-

disciplinary research and development of sensors and sensing systems. The need for sensors

that can function successfully in a broad range of environments is clear. Whether the ob-

jective is to produce devices for harsh chemical environments or those that will not interfere

with delicate biochemical processes, developments in both materials and fabrication tech-

niques will continue to be necessary.

In the first stage, PECVD alumina, diamond-like carbon (DLC), flame-front diamond,

and PECVD silicon carbide were evaluated for chemical resistance in 49% hydrofluoric acid,

4:1 sulfuric peroxide, concentrated HCL, and 25% tetramethylammonium hydroxide. Only

PECVD silicon carbide demonstrated an etch rate less than 0.05 nm/min. While developing

an optimized recipe for the STS 310PC PECVD reactor, bimodal behavior was discovered

among the films tested; some withstood etching in 22% potassium hydroxide (KOH) at

80 °C, some etched relatively quickly. Rutherford backscattering with hydrogen forward

scattering was performed to analyze the stoichiometry with a sample set of representative

films. Stoichiometry did not explain the behavior. Further analysis with FTIR showed a

correlation with the amount of terminal -CH3 present in the film. These results support a

stoichiometric bond model which states that the etch behavior is due to a critically high level

of silicon-silicon bonds within the film, which are susceptible to a attack by KOH. With this

information, an optimized, etch-resistant, low-stress (<50 MPa), CMOS compatible recipe

for PECVD silicon carbide could be selected. Deposition conditions were 1600mTorr, a

methane flow rate of 1440 sccm, a 2% silane in argon flow rate of 2840 sccm, high frequency

power of 100W for 4.5 s, and low frequency power of 100W for 4.5 s. The use of PECVD

silicon carbide was demonstrated in several applications.

The film was deposited on an off-the-shelf pressure sensor and package to improve its

v

media compatibility. As part of this experiment, its mechanical properties were evaluated in-

cluding Young’s modulus (52GPa), hardness (7.3GPa), and coefficient of thermal expansion

(CTE) (2.5 ppm/°C). The net effect on the parametric behavior of the pressure sensor was

measured. For a 0.4 mm thick film, the offset shifted 8.6% FSO, and sensitivity was reduced

approximately 25%. Both of these shifts could be accounted for by design. The addition of

PECVD silicon carbide had the beneficial effect of reducing the temperature coefficient of

sensitivity (TCS) by 100 ppm/°C, roughly 4%. Tests of the carbide coated pressure sensor

by linear polarization and electrochemical impedance spectroscopy in sea water and nitric

acid (HNO3) showed that the silicon diaphragm was protected, but the sensor failed because

the PECVD failed to stick to the bond wires and package.

PECVD silicon carbide was deposited on an iridium microelectrode array. The micro-

electrode array was designed for the measurement of heavy metal ion concentration by square

wave anodic stripping voltammetry (SWASV). With the carbide coating, successful analysis

of 50 ppb of Cu+2 could be performed in HF over periods of several days.

A microfluidic gasket structure was designed that enabled the complete coating of a

channel structure with PECVD silicon carbide. It was incorporated into a novel microfluidic

total organic carbon (TOC) sensor. The principals of the TOC sensor were first tested with

a benchtop version that demonstrated the capability of measuring its theoretical limit of

~10 mM. Fabricating the gasket required adapting Riston®, a dry-film printed circuit board

photoresist, to wafer-level processing. When completed, extensive cracking developed in

the microfluidic version. Finite element analysis supports the conclusion this was due to

excessive stress from bonding around the gaskets.

vi

Acknowledgments

The research presented in this dissertation has been supported by many different funding

sources over the years: STTR Grant DMI-9461869, DARPA Contract N66001-96-C- 8631,

and NSF/SRC Grant P423740. Additional support in the form of materials or services

has been provided by John Meador and Beth Hassler from Medtronic, Inc., Minneapolis,

MN; Andy McQuarry from STS Ltd., Newport, U.K.; Dave Monk from Motorolla, Tempe,

AZ; Dr. Dave Burns from Maxim Integrated Products, Inc., Sunnyvale, CA; and Dianne

Dougherty, Dr. Samantha Tan, and Dr. Stan Tsai from ChemTrace Corp, Hayward, CA.

One of the joys in a graduate career is the development of relationships within a special

community bound together by its passion for knowledge and understanding. So many within

that community have been supportive of this work. For those who are deserved of thanks and

whose name I have not included, such is by error of ommission and not a lack of gratitude.

Thank you to Marnel King, Margaret Prisbe, Nancy Latta, Pat Burke, Mahnaz Mansour-

pour, Jeannie Perez, Uli Thumser, Dick Crane, Keith Gaul, Bob Wheeler, and Tony Souza

and other staff of the Stanford Nanofabrication Facility, both past and present. Without

your efforts, the facility and all the work it enables would not be possible.

During the late nights and long hours that work in the clean room typically requires,

collegues trade the most valuable knowledge and secrets that comprise the artistry of process

development. Thanks are due to Eric Perozziello, Aaron Partridge, and Sam Calmes for the

wealth of insight they have so generously shared.

I would like to thank Prof. Bruce Darling from the University of Washington in Seattle,

WA for guidance during his time at Stanford. His willingness to work directly with students

was a gift of time and talent and of particular importance to the efforts with heavy metal

ion sensing. A special thanks goes to Chris Storment, a walking encyclopedia of knowledge,

who shared willingly and was often there to the wee hours of the morning in support of

others.

vii

I would like to thank my advisor, Prof. Greg Kovacs. His tremendous effort to secure

funding and raise support for the lab is what enabled access to the critical resources for this

work. Despite the grind of the realities in running a research group, his appreciation for the

marvels of science and technology and wonder at invention were inspirational. Thanks to

his example, I recognize that we must not lose site of just how “cool” technology can be. It

was and is a daily privilege to work in this field.

My deep gratitude goes to my advisor, Prof. Roger Howe for your encouragement and

support to finalize this work. They were indispensable. I would also like to thank Prof.

Beth Pruitt and Prof. Thomas Kenny for their willingness to review this dissertation and

serve on my orals defense committee and to Prof. Andrea Goldsmith for serving as chair of

that committee.

Through the trials of graduate work, the members of one’s research group become akin

to commarades-in-arms, having weathered the (sometimes painful) battle of scholarship to-

gether. The Kovacs Research Group provided a rich environment of support and inspiration:

Bart Kane, Todd Whitehurst, Rich Reay, Erno Klaussen, Dave Borkholder, Ken Honer, John

Suh, Glen McLaughlin, Derek Debusscherre, Matt Hills, Gaylin Yee, Nick Mourlas, Dominik

Jaeggi, Dirk Lange, Laurent Giovangrandi, and Sandy Plewa.

If the path God laid out for my graduate career was not what I would have wanted,

in many ways it was what I needed for growth emotionally, intellectually and spiritually.

Thank you to the friends of Christ the King Anglican Church for their prayers of affirmation

that helped to set my feet back on the path.

Nothing would be worthwhile without the support and encouragement of my family. I

would like to thank my father and mother, Anthony and Diana Flannery for a lifetime of

self-sacrifice and effort that gave me every opportunity I could dream of. Particularly now

that I have children of my own, I am humbled by their example. I would like to thank

Ross and Jeanne Sackman, not only for allowing me to marry their wonderful daughter, but

for all their prayers and encouragement for the completion of this work. To my daughters,

Rachel and Devon, thank you for making the completion of my doctorate meaningful. To

my wife Jackie, thank you for making the completion of this work possible. Nothing could

have been done without your love, support.

viii

Contents

Abstract v

Acknowledgments vii

1 Development of a Chemically Robust Dielectric 3

1.1 Initial Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2 Background of PECVD Silicon Carbide . . . . . . . . . . . . . . . . . . . . . . 5

1.3 Development of a PECVD Silicon Carbide Process . . . . . . . . . . . . . . . 13

1.4 Thin Film Comparisons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

1.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2 Passivation for Pressure Sensors 28

2.1 Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

2.2 Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

2.3 Effect on Pressure Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.4 Corrosion Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3 An Iridium Microelectrode Array 47

3.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.2 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.3 Electrode Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

3.4 Experimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

ix

4 Microfluidic Gaskets 67

4.1 Self-sealing Microfluidic Channels . . . . . . . . . . . . . . . . . . . . . . . . . 67

4.2 Gasket Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

4.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

5 Total Organic Carbon Sensor 81

5.1 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

5.2 Benchtop Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

5.3 Microfluidic Version . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

6 Conclusions and Future Work 98

Bibliography 100

A Silicon Carbide DOE 113

B Etch Mask For Glass 120

C Laser Ablation Trials 122

C.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

C.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

D Mechanical Drawing TOC Die 131

x

List of Tables

1.1 Measured etch rates of candidate dielectrics (nm/min) . . . . . . . . . . . . . 4

1.2 Properties of α-SixC1-x:H . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.3 Process parameter space for PECVD silicon carbide process development . . . 15

1.4 Resistivity Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1.5 FTIR peak locations for PECVD silicon carbide . . . . . . . . . . . . . . . . . 22

1.6 Comparison of SiC and a-H:Si1-xCx . . . . . . . . . . . . . . . . . . . . . . . 25

1.7 Comparison of LPCVD SiN and PECVD α-SiCx:Hy . . . . . . . . . . . . . . 26

2.1 Deposition conditions for PECVD silicon carbide DOE . . . . . . . . . . . . . 30

2.2 DOE for evaluation of PECVD silicon carbide . . . . . . . . . . . . . . . . . . 31

2.3 Mechanical properties of trials in DOE . . . . . . . . . . . . . . . . . . . . . . 36

3.1 Coefficients of thermal expansion . . . . . . . . . . . . . . . . . . . . . . . . . 54

3.2 Progressive reduction of stress in evaporated iridium . . . . . . . . . . . . . . 55

3.3 Element screening by SWASV . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

3.4 Four-day exposure to 50:1 HF . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

4.2 Etchant testing on Riston . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

4.3 Optimized process flow for Riston liftoff . . . . . . . . . . . . . . . . . . . . . 75

xi

List of Figures

1 Stainless steel pressure package . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 MEMS applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1 Thin-film microelectrode array . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 α-SiC photodetector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.3 Histogram of refractive indices from process trials . . . . . . . . . . . . . . . . 17

1.4 Histogram of stress from process trials . . . . . . . . . . . . . . . . . . . . . . 18

1.5 Resistivity measurement configuration . . . . . . . . . . . . . . . . . . . . . . 18

1.6 Atomic percentages from RBS and HFS . . . . . . . . . . . . . . . . . . . . . 20

1.7 Carbon:silicon ratio by sample number . . . . . . . . . . . . . . . . . . . . . . 21

1.8 FTIR spectrogram of sample 488 . . . . . . . . . . . . . . . . . . . . . . . . . 22

1.9 Peak absorptions of Si-CH3 bond relative to etch behavior . . . . . . . . . . . 23

1.10 Isomers of silicon carbide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.1 Uncoated and coated pressure sensors after 45 minutes exposure to KOH. . . 29

2.2 Young’s modulus vs. normalized displacement . . . . . . . . . . . . . . . . . 34

2.3 Hardness vs. normalized displacement . . . . . . . . . . . . . . . . . . . . . . 34

2.4 Room temperature stress vs. thickness of deposited films . . . . . . . . . . . 35

2.5 Stress vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

2.6 Initial offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.7 Sensitivity vs. thickness of deposited film . . . . . . . . . . . . . . . . . . . . 39

2.8 TCS vs. film thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

2.9 The correlation of TCS with sensitivity for two thicknesses of PECVD SiC . 40

2.10 Etch rates of PECVD silicon carbide and PECVD silicon nitride . . . . . . . 42

2.11 Cross-section of die for electrochemical studies . . . . . . . . . . . . . . . . . . 42

2.12 Fixture for electrochemical corrosion studies . . . . . . . . . . . . . . . . . . 43

xii

2.13 Linear polarization plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

2.14 EIS of two PECVD silicon carbide samples . . . . . . . . . . . . . . . . . . . 45

3.1 Steps in square wave anodic stripping voltammetry . . . . . . . . . . . . . . . 49

3.2 Ratio of oxidized to reduced species vs. electrode potential . . . . . . . . . . . 51

3.3 Stripping phase of SWASV . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.4 Diagrammatic cross-section of the microelectrode array . . . . . . . . . . . . . 52

3.5 SEM of a failed iridium microelectrode pad . . . . . . . . . . . . . . . . . . . 53

3.6 SEM cross-section of Al-Ge liftoff . . . . . . . . . . . . . . . . . . . . . . . . . 57

3.7 Saturated calomel electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

3.8 Microelectrode array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

3.9 Optical interferometry scans of plated mercury hemispheres . . . . . . . . . . 62

3.10 Repeatability of ASV analysis over 70 hours . . . . . . . . . . . . . . . . . . . 65

4.1 Self-sealing microfluidic channel process . . . . . . . . . . . . . . . . . . . . . 68

4.2 Self-sealing microfluidic channels . . . . . . . . . . . . . . . . . . . . . . . . . 69

4.3 Concept of a microfluidic gasket . . . . . . . . . . . . . . . . . . . . . . . . . . 69

4.4 Trilayer Riston film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4.5 Serpentine with no PVA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

4.6 Serpentine with PVA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

4.7 Edge swelling after rinse in DI water . . . . . . . . . . . . . . . . . . . . . . . 73

4.8 Customized pressure spray developer . . . . . . . . . . . . . . . . . . . . . . . 74

4.9 Effect of descum on feature cleanup. . . . . . . . . . . . . . . . . . . . . . . . 75

4.10 Patterned Riston® in proximity to channel . . . . . . . . . . . . . . . . . . . 76

4.11 Gasket process flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

4.12 SEMS of aluminum mask structure . . . . . . . . . . . . . . . . . . . . . . . . 78

4.13 SEMs of carbide gasket . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

4.14 Process flow for glass wafer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

4.15 Cross-section of gasket structure . . . . . . . . . . . . . . . . . . . . . . . . . 80

5.1 Current TOC sensing methodology . . . . . . . . . . . . . . . . . . . . . . . . 82

5.2 Sievers 900 Benchtop TOC sensor . . . . . . . . . . . . . . . . . . . . . . . . . 82

5.3 Conceptual drawing of TOC sensor . . . . . . . . . . . . . . . . . . . . . . . . 84

5.4 Theoretical pH shift due to TOC concentration . . . . . . . . . . . . . . . . . 87

xiii

5.5 Differential voltage in benchtop TOC sensor . . . . . . . . . . . . . . . . . . . 89

5.6 Response of benchtop model compared with model . . . . . . . . . . . . . . . 89

5.7 Microfluidic TOC sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

5.8 Process flow for TOC silicon wafer . . . . . . . . . . . . . . . . . . . . . . . . 91

5.9 Process flow for TOC glass wafer . . . . . . . . . . . . . . . . . . . . . . . . . 92

5.10 Bonding flow for TOC sensor. . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

5.11 Gasketed microchannel images . . . . . . . . . . . . . . . . . . . . . . . . . . 93

5.12 Image of finished microfluidic TOC sensor . . . . . . . . . . . . . . . . . . . . 94

5.13 Cracking around gasket . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

5.14 FEA of anodic bond around gasket . . . . . . . . . . . . . . . . . . . . . . . . 95

5.15 Stress vs. the bonding void around the gasket . . . . . . . . . . . . . . . . . . 96

B.1 Isotropic wet etching of glass . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

C.1 Power: 50%, PPI: 1000, Res: 1000 . . . . . . . . . . . . . . . . . . . . . . . . 125

C.2 Power: 100%, PPI: 1000, Res: 1000 . . . . . . . . . . . . . . . . . . . . . . . . 126

C.3 Power: 100%, PPI: 500, Res: 500 . . . . . . . . . . . . . . . . . . . . . . . . . 127

C.4 Power: 100%, PPI: 333, Res: 1000 . . . . . . . . . . . . . . . . . . . . . . . . 128

C.5 Power: 100%, PPI: 333, Res: 500 . . . . . . . . . . . . . . . . . . . . . . . . . 129

C.6 Power: 100%, PPI: 333, Res: 333 . . . . . . . . . . . . . . . . . . . . . . . . . 130

D.1 Mechanical Drawing of Laser Ablation Drill Pattern . . . . . . . . . . . . . . 131

D.2 Drill pattern for TOC die and microfluidic fuel cell. . . . . . . . . . . . . . . . 132

xiv

Introduction

In the early days of microelectromechanical structures (MEMS), the majority of techniques

and materials were leveraged from the integrated circuit (IC) industry. With a few notable

exceptions (namely anisotropic etching), the processing capabilities in this toolbox were

sufficient to support the commercialization of MEMS’ first cornerstone products: pressure

sensors and accelerometers. But because MEMS sensors interact with their environment

in more diverse ways than traditional integrated circuits, commercialization also created

new challenges. As MEMS sensors left the relatively benign context of the laboratory for

real-world applications, they required more advanced solutions to protect them from any

undesirable effects caused by their environment and achieve acceptable lifetimes.

Early solutions relied primarily on packaging to accomplish this. A particular example

are the stainless steel diaphragm, oil-filled packages for pressure transducers that are still in

wide usage today. In figure 1, a typical package for a stainless steel pressure transducer is

shown together with a diagrammatic cross-section.

Figure 1: On the left, the MPM283 Pressure Sensor from Servoflo Corp., Lexington, MA. Onthe right, a diagramatic cross-section of a stainless steel, oil-filled pressure sensor package.

Drawbacks to this approach are readily evident. The materials and assembly cost of this

kind of package can exceed the cost of the silicon transducer by orders of magnitude, its

1

2

only function to isolate the sensor. It adds significant factors that must be addressed in the

design including the coefficient of thermal expansion (CTE) of the oil fill and the stresses in

the stamped, stainless steel diaphragm. The hermetic feedthroughs are also high risk points

of failure, particularly at higher pressures.

A significant motivation to develop more robust sensors is to lower the requirements and

cost of packaging for MEMS sensors to bring the cost of the package more in line with the

cost of the transducer. A second incentive is the evolution of transducer technology into

applications that interact with many different kinds of stimuli: light, chemicals (fluid and

gas), biological cells, extreme temperature, direct contact, radiation, etc. This has pushed

the need for materials and techniques that stand up to different conditions or have differ-

ent properties than can be accomplished with what was available for standard integrated

circuits. Examples are shown in Figure 2 illustrate high temperature, chemical and biologi-

cal applications. Techniques and materials to extend the operating environment of MEMS

sensors can also improve sensor lifetimes in existing applications as well as enable new ones

that would not otherwise be possible.

(a) (b) (c)

Figure 2: (a) A Fabry-Pérot pressure sensor [1](©IOP Publishing 2009), (b) a high tem-perature humidity sensor [2](©Elsevier 2009) , and (c) a biocompatible capacitive micro-machined ultrasonic transducer (CMUT) array [3] (©IOP Publishing 2007), reprinted withpermission.

The work presented here was done as two parts: the first focused on evaluation of

materials and development of processing techniques for PECVD silicon carbide and iridium,

the second on applications demonstration.

Chapter 1

Development of a Chemically Robust

Dielectric

Introduction

In a toolbox for designing environmentally hardened sensors, it is desireable to have a

dielectric that is relatively inert in a large range of chemistries. Popular dielectrics have

individual strengths, but none stand out as having remarkable robustness in an extraordinary

range of conditions. All variations of silicon dioxide (thermal, LPCVD, PECVD, spin-on-

glass (SOG)) etch quite readily in hydrofluoric acid. Silicon nitride in all of its forms, while

more resistant than oxides, also has finite, measureable etch rates in hydrofluoric acid [4].

Organics such as polyimide and bisbenzocyclobutene (BCB) have temperature limitations,

particularly in the presence of oxygen. They are also usually susceptible to degradation in

oxidizing chemistries such as sulfuric peroxide, solvents, or in some cases even water.

Polytetrafluoroethylene (PTFE) deposited by PECVD has shown some promise in this

area [5]. While this film does exhibit excellent chemical resistance, it has two drawbacks:

films of this material tend to be porous and they decompose at relatively low temperatures

(200 °C [6]). Many applications do not require operation near the decomposition tempera-

ture, but the limitation on further processing after deposition of the PTFE is a drawback.

Even photolithography singe temperatures of 150 °C can have an adverse affect on the film,

causing it to flow and deform. Furthermore, while the porosity can be a beneficial attribute,

as was demonstrated in a solid-state oxygen sensor [7], it is not desirable for an encapsulating

material that must protect the underlying substrate.

3

4 CHAPTER 1. DEVELOPMENT OF A CHEMICALLY ROBUST DIELECTRIC

1.1 Initial Selection

After reviewing potential candidates, four films were selected for initial evaluation. Each

of these films had been reported either in literature or in interviews to be relatively in-

ert in a broad range of media and suitable for processing. The films tested were PECVD

diamond-like carbon (DLC)1,thin-film diamond (TFD)2, PECVD alumina3, and PECVD

silicon carbide4. Each was evaluated based on process compatibility and resistance to chem-

ical etching. As a first test, blank silicon wafers (100 p-type 10W-cm) were prepared and

sent out for sample deposition.

The TFD was deposited by a flame-front deposition process [8]. Several undesirable char-

acteristics of this film became immediately evident. Deposition temperatures were higher

than initially expected, upwards of 1000 °C, making it wholly incompatible with many mate-

rials (gold, aluminum, polyimide, etc.). The inherent stresses were so high that wafers were

visibly bowed. There was also a severe radial non-uniformity and in regions throughout the

wafer the film appeared like “soot” and was full of particles. For these reasons, TFD was

dropped from further testing.

The three remaining wafers were diced into squares and placed into the test etchants

listed in table 1.1. At intervals, each was withdrawn and rinsed. Any change in the thick-

ness of the films was measured by ellipsometry. Etch rates greater than 0.05 nm/min were

detectable by this method.

Table 1.1: Measured etch rates of candidate dielectrics (nm/min)

Film HF(49%)

HF(50:1)

BOE(6:1)

SulfuricPeroxide(4:1)

Conc.HCl

TMAH(25%)

DLC <0.05 <0.05 <0.05 <0.05 <0.05 <0.05Alumina 320 54 ~190 0.4 1.4 40SiliconCarbide

<0.05 <0.05 <0.05 <0.05 <0.05 <0.05

The chemical resistance of PECVD alumina as deposited was less than what had been

expected based on the chemical resistance of the bulk material. Significant etching occurred

1Stanford University, M. Cappelli2Lockheed Martin Corporation, Advanced Technology Center, Palo Alto, CA, Dr. K. Ravi3Alcatel Comptech, Inc., Fremont, CA, U.S.A.4Surface Technology Systems, Abercarn, Newport, Gwent, U.K.

1.2. BACKGROUND OF PECVD SILICON CARBIDE 5

in all of the media tested, making it unsuitable for use as an encapsulation material for

environmentally hardened sensors. In discussions with Alcatel, the probable cause of this

unexpected reactivity was the deposition temperature. Typically the film is deposited at

600 °C. In this trial, the temperature had been lowered to 350 °C in an attempt to reduce

the stress below 500MPa and achieve CMOS compatibility. While exploration of process

conditions could potentially have led to acceptable results, further work was directed towards

the remaining two films.

In the next phase, DLC and PECVD carbide were deposited on a thin-film microelectrode

array discussed in Chapter 3 and shown in Figure 1.1.

Figure 1.1: Thin-film microelectrode array

It was at this point that a problem

with the DLC deposition process became

apparent. The process tested required a

relatively high-energy ion bombardment to

achieve the desired film properties, specifi-

cally the stabilization of the necessary sp3

carbon hybridization [9]. This had the un-

fortunate side effect of sputtering the sur-

face of the wafer. Sputtering was so severe

that the metal electrodes and traces were

almost completely removed. This had not

been evident from examining the surface of

bare wafers. As with the PECVD alumina,

a set of process parameters that would yield

a suitable DLC thin film could potentially

be found, but because ion bombardment

was not necessary for PECVD silicon car-

bide, further effort was directed exclusively

at the carbide.

1.2 Background of PECVD Silicon Carbide

As a point of nomenclature, crystalline, stoichiometric SiC will be referred to by its chemical

formula. PECVD silicon carbide, amorphous silicon carbide, carbide, or α-SiC:H all refer to


α-SixC1−x:H. Before pressing forward into the study of PECVD silicon carbide, it is worth

reviewing the history of the material. Before methods of controlled plasma deposition were

developed, amorphous silicon carbide was deposited by pyrolitic methods which relied on the

combustion of reactants to generate the desired product [10]. The earliest reference found

on the deposition of silicon carbide by radio frequency plasma was by Sterling and Swann in

1965 [11]. At this time, PECVD deposition was referred to as the “glow discharge” method

of deposition. Silane and methane in helium were reacted to deposit a film that, like the

crystalline SiC films, showed a strong absorption peak at 12.6mm [12]. A more detailed

examination was done in 1977 by Anderson and Spear [13]. They made the first attempt to

analytically characterize the nature of PECVD silicon carbide by looking at stoichiometry,

optical band gap, and conductivity. In doing so, they were the first to reveal the complex

chemical and electronic character of the film.

Several aspects of PECVD silicon carbide are responsible for the complexity of its prop-

erties:

• Variable stoichiometry of deposited films: Stoichiometry can be tuned by process

conditions from amorphous silicon to amorphous carbon [14,15].

• Bond stoichiometry: For a given stoichiometry of α-SixC1−x:H there exist different

permutations of Si-C, Si-Si, and C-C bonds which satisfy that formula. Consequently

two films of identical stoichiometry can have different properties (conductivity, chem-

ical reactivity, etc.).

• Electronic properties of carbon: Carbon can bond either graphitically (sp2) or

tetrahedrally (sp3). As part of a Si-C matrix, carbon is almost always bound tetra-

hedrally. Process conditions and carbon concentrations, however, can create localized

regions of C-C graphitic bonds. The mechanical and electrical properties of these

bonds are very different. Consequently a change in the hybridization of carbon can

greatly affect film properties [13].

• Hydrogen content: The amount of terminating hydrogen bound into the silicon

carbide matrix can vary greatly up to an atomic percentage of more than 50% [14].

The range of conditions possible for a PECVD deposition influences each of these factors.

Consequently, the range of films from a typical PECVD silicon carbide process spans a

continuum of possibilities: hard to soft, insulator to semiconductor, chemically resistant to


moderately reactive. For this reason, the development of a PECVD carbide process requires

careful film analysis and testing to ensure that the film has the desired properties. It is also

why the development process is unique to every deposition system and configuration.

1.2.1 Precursors

While this work focused on the plasma reaction of silane and methane in argon as an inert

carrier, other precursors have been used. Silane has been used almost exclusively as a source

of silicon for a two-gas precursor system. A more diverse list of gases such as ethane (C2H6),

ethylene (C2H4), acetylene (C2H2), and butane (C4H10) have all been used as sources of

carbon [13,15–17]. The amount of carbon incorporated into the film and the deposition rate

is higher per molar volume for the less saturated precursors. This would be expected for

the energy of activation (radical formation) of acetylene (1.8 eV), ethylene (2.7 eV), ethane

(3.6 eV), and methane (4.3 eV).

PECVD silicon carbide has also been deposited using a single source precursor. Chiu and

Huang demonstrated the use of dodecamethylcyclohexasilane ((CH3)2Si)6, though at a

higher temperature than is typical for a PECVD process (1000 °C) [18]. Hexamethydisilane

(CH3)6Si2 [19], trimethylsilane (CH3)3SiH [20, 21], and 1,3-disilabutane [22] have also

been used. Processes with single precursors typically achieved deposition rates several times

higher than what was achieved with silane-organic mixtures, on the order of several tens

of nm/min. The stoichiometry of the films from trimethylsilane also tended to be carbon-

rich. This is different from films made using a silane-methane recipe. These tend toward

a silicon-rich stoichiometry, owing to the difference in bond energies between methane and

silane (98.8 kcal/mole vs. 70.9 kcal/mole) [23].

Another method of depositing α-SixC1−x:H which warrants mention is that of reactive

sputter deposition [24]. In this method, a silicon target is sputtered in the presence of

methane and argon. Results from Fourier transform infra-red (FTIR), ultraviolet (UV),

visible, near-infrared (NIR), and photoacoustical spectroscopy (PAS) showed that films pro-

duced in this manner were more disordered and inhomogeneous than the PECVD counter-

part. Among other differences in properties, this lead to a higher conductivity, which would

make it less desirable as a dielectric encapsulating layer.


1.2.2 Electrical and Optical Properties

The resistivity of PECVD silicon carbide has been measured under a variety of conditions.

Tyczkowskiet et al. [25] measured the temperature dependence of D.C. conductivity of films

produced from tetramethylsilane. Room temperature resistivity varied between 2x1014 and

1.5 x 1016 W-cm. Resistivity variations due to process conditions were measured across nearly

four orders of magnitude by Chattopadhyay et al. [26]: from 2x109 to 6x1012 W-cm. The

conductivity of carbide can also be altered by doping. When phosphine (PH3) was added

to a silane/ethylene deposition to produce n+ a-SiC:H, the conductivity increased from

2.2x10-12 S/cm for the undoped films to 2.8x10-3 S/cm [27].

Because of its potential for use in optical applications, the photoconductivity of PECVD

silicon carbide has been characterized [28]. The conductivity of carbide films illuminated

with a 50mw/cm2 white light source increased roughly four orders of magnitude. Dark con-

ductivity and photoconductivity both decrease with increasing carbon content. This trend

has been shown to continue for higher percentage carbon films, suggesting that increased

carbon content reduces generated carrier lifetimes and/or mobility.

The optical band gap (Eopt) of carbide has been investigated by a number of sources,

owing to the interest for its use in photovoltaic cells and other optical applications. The

optical band gap itself is not a well defined value, although it is referred to quite extensively

because of it usefulness in evaluating a materials optical performance. It has no physical

basis. Two definitions for Eopt have been found. The first is that point where the absorption

coefficient reaches 106m-1. The second, and that quoted more often is that determined from

the Tauc relationship

αhv = B (hv − Eopt)n (1.1)

where α is the absorption coefficient, hv is the photon energy, B is a proportionality constant,

and n a parameter corresponding to the density of states. Eopt is most often determined by

extrapolating a linearization of ((αhv)1n vs. hv and taking Eopt as the x-axis intercept [25].

The value obtained is dependent on the choice of n as the density of states. The majority

of sources use n = 2 for a-SixC1-x:H, though Tyczkowski et al. claim a better fit for higher

temperature films using n = 3. Values of the optical band gap for a-Si:H is 1.75 eV and for a-

C:H is 4 eV. As might be expected, the values for a-SixC1-x:H have been reported from 1.5 to

3.0 eV [13,15,26]. Several investigators have confirmed the linear dependence of the optical


band gap on x between 0.8 and 0.38 with a maximum near 3 eV at x = 0.38 [14, 16, 28].

Transmission as high as 60% down to 500 nm has been reported [29].

Values reported throughout the literature for the refractive index vary greatly, from 3.3

(silicon rich films) to 1.4. Nearly that same broad range of refractive indices were observed

in this work. An inverse relationship was found between the carbon atomic percentage and

the refractive index. This relationship is roughly linear at lower carbon concentrations,

up to roughly 30%, where it levels off and nf approaches 1.75 [14, 15, 26, 28]. It should be

noted, however, that the refractive index is not solely dependent on the percentage of carbon

alone. For a-Si0.75C0.25:H, Sussman and Ogden reported nf of roughly 2.4. That reported

by Kuhman et al. was roughly 1.9. Catherine et al. reported a variation from 2.2 to 2.5 with

deposition temperature for x=0.6. For films produced from hexamethyldisilane (HMDS),

Klumpp et al. reported an increase in refractive index with deposition temperature, from

2.48 to 2.65 at 200 °C and 300 °C respectively [19]. This trend was confirmed by He et

al. [29].

1.2.3 Mechanical Properties

In 1994, Khakani et al. compared the mechanical properties of PECVD silicon carbide with

that of SiC deposited by both laser ablation (LAD) and triode sputtering (TSD) [30]. They

found that the hardness of the PECVD films decreases in films where the C:Si ratio is less

than 1:1. Using FTIR, they were able to establish a direct correlation between the Si-C

bond density and both hardness and Young’s modulus. As is discussed in Section 1.3.3.2,

FTIR examination of the Si-C bond density can also provide a key to understanding the

chemical reactivity of PECVD silicon carbide in alkaline solutions. Additionally, it was

found that for films with an identical 1:1 Si:C stoichiometry, hydrogenation decreased the

hardness of the PECVD films (19.2GPa) relative to the non-hydrogenated LAD (30.6GPa)

and TSD (28.1GPa). Other work has shown that hardness increases with the radio frequency

power of deposition [29]. This tends to correlate with increasing carbon percentage, which

corresponds with the previous observation.

Because of interest for its use as a wear resistant coating, abrasion tests have been done

on deposited films. In an abrasion test, a calibrated abrasive is applied to the surface under

test and either weight loss or thickness loss is measured over time. While it is difficult to

correlate data from different tests and conditions, carbide has demonstrated a wear resistance

roughly 22 times greater than that of Corning 7059 B glass [29].


The density of deposited films can vary significantly. Sussman and Ogden reported higher

densities of 2.5 g/cm3 for silicon-rich films (x=0.8 ) compared with 2.0 g/cm3 for carbon rich

films (x=0.2) [28]. Their analysis, however, did not take into account the percentage of

hydrogen in the film, which can influence the resulting density greatly.

The stresses reported in as-deposited films are predominantly compressive. The magni-

tude ranged across the board from the low stress reported by Tong et al. of -34MPa [31] to

the high stress reported by Jeanet al. of -1,400MPa [23]. As discussed in more detail below,

stress can be altered greatly by annealing. Tensile stress, however, has been reported, both

in this work (430MPa) and in that by Loboda et al. (200MPa) [20].

1.2.4 Chemical Properties

As demonstrated in the screening process, PECVD silicon carbide is relatively unreactive in

many environments. It is not, however, impervious. Several papers characterize the etch be-

havior of carbide, though typically it is under extreme conditions. Tong et al. characterized

the etch rate in 49% HF of carbide produced under various conditions. They demonstrated

that the etch rate correlated inversely with the refractive index of various films. Low refrac-

tive index films (2.0) etched at 48 nm/min. Higher index films (2.3) etched at one fourth

that rate. Films produced by another source using HMDS, however, had undetectable etch

rates in both room temperature 48% HF (hydrofluoric acid) and in 33% KOH (potassium

hydroxide) at 80 °C [19]. Resistance to KOH was demonstrated by Jean et al. when they

fabricated PECVD silicon carbide membranes by etching through the wafer in a 30% KOH

solution at 80 °C. By depositing carbide on the both sides of the wafer, they were able to

use it both as the membrane material and as the etch mask.

An examination of failure mechanisms in thermal ink jet heads showed that carbide

is etched in alkaline solutions under repeated thermal cycling. The temperature of the

thermal cycle was 300 °C. The etch rate increased dramatically above pH11. Data from the

trial supports the conclusion that the mechanism of etching was one of SiO2 formation and

dissolution, facilitated by the high temperature [32].

Nodules have been observed by tunneling electron microscopy (TEM) in films deposited

at lower powers. Analysis by selected area diffraction showed these nodules to be carbon,

and attributed to plasma-phase polymerization of methane. Nodules were not observed in

higher power depositions. The theory put forth was that higher powers favored the ion-based

reactions near the deposition surface rather than the radical-based polymerization, which


occurs in plasma. Deposition temperature had an effect only on the size of the nodules; it

did not eradicate them. This was consistent with their hypothesis, as substrate temperature

would not have a significant impact on the plasma reaction [29].

Flourine-based plasmas can be used to pattern PECVD silicon carbide. Carbide can be

patterned using CF4, SF6, and CHF3/O2 using conventional photoresist as a mask [19].

1.2.5 Annealing

In numerous works, annealing PECVD silicon carbide can cause significant changes in the

character of the film. As a carbide film is annealed beyond its deposition temperature,

hydrogen is released, densifying the film and increasing the refractive index. As the annealing

temperature increases, hydrogen atoms bound to alkyl groups are released first, followed by

those bound to silicon. Evolution and densification continues until about 650 °C, after which

no hydrogen can be detected in the film by IR. Densification can cause as much as a 70%

decrease in film thickness under certain conditions [19,25,33]. The density of Si-C bonding

also increases, in one case by an order of magnitude when annealed at 1200 °C [34].

At 800 °C, the film begins to become to develop a microcrystalline structure. The size of

the crystallites increases with temperature until at 1200 °C, the film becomes polycrystalline

[34]. This is consistent with results reported elsewhere that plasma depositions at 1000 °C

are crystalline, showing patterns by X-ray diffraction like that of β-SiC [18].

Annealing has consistently been shown to shift mechanical stress towards the tensile

regime. Most carbide films are compressive (sometimes very compressive) as deposited, and

proper annealing can lead to a zero-stress film. In one experiment, rapid thermal annealing

(RTA) for 400 sec at 600 °C was shown to reduce the stress from -1,100MPa (compressive)

in the film as deposited to roughly zero. Further annealing for 800 sec resulted in a slightly

tensile film [23]. Klumpp et al. were also able to achieve a near-zero stress film after annealing

in a furnace for 12 h at 450 °C. Anneals at 600 °C and at 1000 °C produced successively more

tensile films, in one extreme case as much as 1,750MPa tensile [19]. Other examples that

demonstrate the shift towards tensile stress have been reported [33].

1.2.6 Applications

One of the earliest and most pervasive applications of PECVD silicon carbide has been in ink

jet devices. It is typically used as a coating in the fluidic channel of thermal ink jet heads to


protect the resistive heating elements from the corrosive effects of the ink [32,35,36]. Early

ink jet heads were often made from silicon and glass [36]. Research to address the corrosion

resistance in alkaline ink chemistries dates back to the early days of this application [37,38].

Because of its large optical gap, PECVD silicon carbide has been used to fabricate thin-

film, light-emitting diodes (TFLED). Electroluminscence in a p-i-n a-SiC TFLED is in the

visible wavelength region. Because Eopt can be adjusted by process conditions, TFLED’s

can be fabricated with colors ranging from red to yellow [39]. The wide optical band gap also

makes PECVD silicon carbide suitable for use as a window layer in a photovoltaic cell [15].

Low-leakage, high-gain a-SiC:H/a-Si:H p-i-n heterostructure photodetectors have been

demonstrated [27, 40]. A representation of the one by Dutta et al. is shown in Figure 1.2.

The intrinsic carbide layer serves as an insulator to lower leakage current and provides a

high-field region for avalanche multiplication of photogenerated carriers. Taking advantage

of the larger optical gap of carbide (2.35 eV in this case) enabled a quantum efficiency greater

than unity with a leakage current less than 10 nA/cm2 and a photocurrent multiplication of

9.

Figure 1.2: α-SiC:H photodetector, afterDutta et al. [27]

Because PECVD silicon carbide has a higher

coordination number than PECVD silicon ni-

tride, thin-film transistors (TFT’s) made with a

carbide gate have superior transport properties.

Gates made with PECVD nitride, however, are a

superior insulator (lower leakage), have a greater

εr, and break down at a higher voltage than

those of carbide. TFTs made with a laminated

a-SixC1-x:H/a-SiyN1-y gate show a two-fold in-

crease in transconductance due to the decrease

in interfacial defects, while preserving the supe-

rior qualities of a single-layer nitride gate [41].

Jean et al. successfully demonstrated the use

of a carbide membrane as a mask for X-ray

lithography [23]. As described previously, the low stresses necessary for membrane fab-

rication were achieved by RTA.

Because SiC is one of the harder, more wear resistant compounds available, work has

been done exploring the use of PECVD silicon carbide as a lower temperature coating for

1.3. DEVELOPMENT OF A PECVD SILICON CARBIDE PROCESS 13

mechanical protection. While some processes do produce relatively hard carbide layers, none

approach the mechanical resistance of crystalline or polycrystalline SiC.

In fabrication, the chemical resistance of carbide makes it an ideal masking layer. It

has been used during backside etches in the fabrication of microfluidic pumps and pressure

sensors [19]. It was also used in the formation thermally-isolated bridges for the fabrication

of a thin-film bolometer [42]. Because of its resistance to HF, free-standing carbide structures

can be fabricated in a surface micromachining process using silicon dioxide as a sacrificial

layer [31].

A selection of film properties pulled from the literature are summarized in table 1.2.

Table 1.2: Properties of α-SixC1-x:H

Property Value CommentDensity (g/cm3) 2.4 [23]

1.75 [34]2.1 [26]

x≈0.5

Index of Refraction 2.0 - 2.4 [31]1.79 - 2.37 [21]

Trimethylsilane

Dielectric Constant 4.13 - 6.28 [21] TrimethylsilaneOptical band Gap (eV) 2.0 - 2.6 [39]Resistivity (Ω- cm) 106- 1012 [13] Temperature and

stoichiometric dependentStress as Deposited (MPa) -1,400 to -300 [23]

-30 to -64 [31]-200 to +100 [21]

0.25 < x < 0.8

Coefficient of ThermalExpansion (ppm)

3.3 [33]

Hardness (GPa) 10.8-19.5 [25],17 [43]

x=0.5 - 0.67

Young’s Modulus 88 10, 154 16 [25], 100 17 [44] x=0.67x=0.5

Poisson’s Ratio 0.29 0.16, 0.23 0.11 [25] x=0.67x=0.5

1.3 Development of a PECVD Silicon Carbide Process

Development of an in-house carbide process was done in two phases. In the first phase,

process parameters were sought such that the resulting film satisfied the criteria for use in


a range of micromachined structures. The most useful film would be

• low stress as deposited,

• resistant to wet etchants,

• highly resistive,

• patternable, and

• conformal.

In the second phase, further investigation was carried out on the chemical, physical, and

electrical properties of a subset of these films.

1.3.1 Parameter Determination

The initial samples of silicon carbide that were evaluated in the dielectric screening were

prepared by Surface Technology Systems, (STS Ltd., Wales, UK) using their Multiplex CVD

system with a 380 kHz power supply. Deposition parameters used were 600mTorr, 10 sccm

SiH4, 250 sccm CH4, 300 sccm Ar, 300 °C, and a power level of 60W. The remainder of

the work was carried out using a modified STS 310PC PECVD deposition system at Stan-

ford University. Two RF power supplies were used in a switched mode, a high frequency

(13.5MHz) unit and low frequency (187 kHz) unit. The system was plumbed with 2% SiH4

in Ar, CH4and purged with Ar. Switching the carrier case for the silane (2% SiH4) from

nitrogen to argon was done to avoid the possibility of forming a carbide-nitride hybride

(which might bear investigation in its own right). The electrode area was 730 cm2. With

two exceptions, all films were deposited using a dual frequency, switched plasma technique

based on prior success using this method to control the stress in PECVD silicon nitride.

Two additional trials were done with constant low frequency and constant high frequency

excitation. All films were deposited at 350 °C. Before discussing the optimization process,

however, it is essential to establish that results apply strictly to the STS310 PC PECVD

deposition system. While the work here describes what is believed to be an optimal PECVD

silicon carbide film for use in environmentally hardened transducers for that system, many

other systems have greater degrees of freedom in their process variables (electrode spac-

ing, DC bias, electrode symmetry, frequency adjustment, temperature >350 °C, methods of

excitation, precursor selection, etc.). This greater parameter space effects both the opti-

mization process and the resulting film properties. It is also not implied that the carbide


films produced in this work are optimal for the material; merely that they are optimal given

the technical resources of the available deposition system. Together with the greater pro-

cess control in different deposition systems; broader range of precursor choices; and newer

plasma technologies, such as electron cyclotron resonance (ECR), inductively coupled plas-

mas (ICP), and triode configured systems superior quality films are undoubtedly possible.

The process development of PECVD silicon carbide began by establishing the range

of deposition parameters. Those used are listed in Table 1.3. The pressure range was

chosen because preliminary trials showed that higher pressures tended towards lower stress

films. The upper pressure was a limitation of the pumping capabilities of the machine.

Temperature was fixed at the system maximum and the upper bound of CMOS compatibility.

As described earlier, higher temperature films tended to have higher level of Si-C bonding.

The flow rate of 2% SiH4 in argon was fixed at the system maximum as this had been shown

to yield maximum uniformity across the platen. Power parameters were bound on the low

end by the minimum required for detectable deposition (50W) and on the upper end by

the system power supplies (150W). The period of excitation was bound by that required

to achieve a steady state plasma (2 s) and that required to deposit a monolayer (7 s). The

latter boundary ensured the deposition of a homogeneous film rather than a laminate. A

switched frequency method of deposition was chosen because of previous success depositing

a low-stress PECVD silicon nitride. A complete list of process conditions and results of film

properties can be found in Appendix A. The parameters for the “best” film produced are

also shown in Table 1.3.

Table 1.3: Process parameter space for PECVD silicon carbide process development

Parameter Range SelectedPressure (mTorr) 1000 - 1600 1600Temperature (°C) 350 (Fixed) 350

2% SiH4Flow Rate (sccm) 2840 (Fixed) 2840CH4Flow Rate (sccm) 360, 900, 1440 1440Hi Freq. Power (W) 50, 100, 150 100Hi Freq. Period (s) 2 -7 4.5

Low Freq. Power (W) 50, 100, 150 100Low Freq. Period (s) 2 -7 4.5

During deposition, a substantial layer of particles accumulated on the sidewalls of the

chamber and, in particular, under the platen. This is not the case with either the PECVD


silicon nitride or the PECVD silicon dioxide processes run in the same chamber. However,

no particles were visible on the platen itself or on the wafers. Analysis of the powder-like

substance by FTIR revealed strong Si-H stretching with little evidence of carbon bonding,

It was also observed that this “silanacious dust” was pyrophoric, and on occasion would

spontaneously react with the air upon chamber venting. This level of particle generation in

a carbide process is consistent with results reported elsewhere [21]. In addition to concern

about pinhole problems in the film, it raises a potential concern for vacuum pump operation.

In-line filters were already being used in the system and examination of the pump did

not reveal any accumulation of deposits. By lowering the pressure to 1,200mTorr, a set

of process parameters were found that could also produce a good quality, low-stress film

while decreasing the amount of particulate accumulation in the chamber. This adjustment

reduced, but did not eliminate, the amount of particle deposition. As a result, total chamber

accumulation was limited to 2 mm before an etchback cleaning cycle.

1.3.2 Thin-film Characterization

1.3.2.1 Optical Properties

As mentioned previously, the refractive index of carbide has a broad range. Values in this

work measured by ellipsometry ranged from 1.8 to 3.2. An average value of those films

produced during the process development phase was 2.53 ± 0.28. The distribution of all

films from the trial can be seen in Figure 1.3. Even after fixing the deposition conditions

on a set of parameters that produces a film of desirable characteristics, the refractive index

tended to increase under those same process conditions as the thickness of the deposited

film increased.

1.3.2.2 Mechanical Properties

Stress was measured with an SMSI 3800 stress gauge (Scientific Measurement Systems, Inc.,

San Jose, CA) using wafers for which reference measurements had been made. Extremes

of stress measured between -401MPa (compressive) and 431MPa (tensile). Films tended

to be slightly compressive, averaging around -96MPa. It was possible to determine a set

of parameters that could consistently produce films with stresses lower in magnitude than

-30MPa. As with the refractive index, the stress distribution had a broad range of values

as shown in Figure 1.4.


Figure 1.3: Histogram of refractive indices from process trials

Further stress and coefficient of thermal expansion analysis was done as part of an

investigation of carbide for use in media compatible pressure sensors. This is discussed in

detail in Chapter 2.

1.3.2.3 Electrical Properties

Surface and volume resistivity were measured with a Keithley 6517 electrometer and an 8009

test fixture (Keithley Instruments, Inc., Cleveland, OH). This fixture is especially designed

to accurately measure the resistivity of dielectrics.

If done incorrectly, accurate measurement of the resistivity properties in a dielectric

can be confounded by the current from capacitive charging. To avoid this and improve re-

peatability, the resistivity of samples was measured with a square wave differential sampling

technique [?]. The square wave was ±50V with a 15 s half cycle time. Current was sampled

just prior to the end of each half cycle. The last four measurements were summed as a

weighted average.


Figure 1.4: Histogram of stress from process trials

(a) Volume Resistivity (b) Surface Resistivity

Figure 1.5: Configuration of Keithley 8009 resistivity test fixture for (a) volume and (b)surface resistivity measurements

Iavg =I1 − 3I2 + 3I3 − I4

8(1.2)

A minimum of seven measurements were made for each sample. Wafers were baked for

1 hour at 140 °C prior to measurement to remove surface moisture. Volume resistivity was


calculated based on the dimensions of the test chamber using

ρv =22.9V

tIavgΩ − cm (1.3)

where ρv is volume resistivity, V is the test voltage, t is the average thickness of the sample

in centimeters. Surface resistivity is calculated from

ρs =53.4V

IavgΩ/ (1.4)

where ρs is volume resistivity, V is the test voltage, and Iavg is the measured current. Results

are shown in Table 1.4.

Table 1.4: Resistivity Results

Sample ρv (Ω − cm) ρs (Ω/)499 2.8e7 1.8e11500 9.9e8 1.5e13496 2.7e8 5.0e11507 2.2e8 3.8e9532 6.9e8 7.8e11522 6.0e10 5.1e14512 1.0e8 5.2e12525 9.2e7 2.1e12

Average 7.8e9 6.7e13

1.3.3 Etch Resistance

As stated in the introduction, the media trials of the test samples showed no detectable etch

rate to the time limits of the tests. This was not the case with all the carbide films deposited

during the process development phase. While all the films showed a consistent resistance

to etching by any of the acidic media tested, roughly 40% of the films tested exhibited a

very high etch rate in 22% potassium hydroxide at 80 °C. Behavior of the films tested was

bi-modal; either the films tested as etch-resistant, or they etched relatively quickly. One of

the objectives in the subsequent chemical analysis was to discover the reason behind this

difference.

It was observed that the surfaces of the carbide before exposure to any test media were

typically hydrophobic. After exposure to a potassium hydroxide test solution, the surfaces


were consistently hydrophylic. This is most likely due to the formation of a surface layer of

silicon dioxide, as has been previously reported [31]

1.3.3.1 Stoichiometry

The etch behavior indicates that there is some critical aspect in the chemistry of PECVD

silicon carbide that leads to its susceptibility to etching in solutions of KOH. As a first step,

the stoichiometries of representative samples were analyzed by Rutherford backscattering

(RBS) and hydrogen forward scattering (HFS) spectroscopy at Charles Evans and Associates

(Redwood City, CA). Results of the analyses are summarized in Figure 1.6.

Figure 1.6: Atomic percentages from RBS and HFS

Several important insights are revealed by the stoichiometry data. First, there is almost

no apparent correlation between the films stoichiometry and etch behavior. It would be

reasonable to assert that a minimum amount of carbon is necessary in order to establish

etch resistance. In the limit, the film would be a-Si:H and would etch rapidly in KOH. This

might explain the behavior of sample 488 which had a particularly low atomic percentage of

carbon. A minimum level of carbon notwithstanding, films of nearly identical stoichiometry

show significantly different etch behavior. Second, as is typical of PECVD films produced at

350 °C, the films in this study have a relatively high atomic percentage of hydrogen. Despite


the fact that some films are nearly 50% hydrogen, this also does not correlate with etch

behavior. Lastly, the films were silicon-rich. As shown in Figure 1.7, the C:Si ratio ranged

from 0.11:1 to 0.88:1 , never achieving the stoichiometric parity that would be expected in

crystalline silicon carbide.

Figure 1.7: Carbon:silicon ratio by sample number

1.3.3.2 Bond Stoichiometry

To gain further insight into why films of nearly identical stoichiometry would have radically

different etch behaviors in KOH, spectral analysis by Fourier transform infrared (FTIR)

spectroscopy was done. An example spectrogram is shown in Figure 1.8. Several key peaks

in the film were identified and are listed in Table 1.5.

As samples were ranked in the order of peak area for Si-CH3 stretching and deformation

(normalized for film thickness), the order correlated with susceptibility to etching in KOH,

as shown in Figure 1.9. This would suggest that for two films with identical stoichiometries,

the one with the greater amount of terminal -CH3 groupings is more likely to etch in KOH.

This can be explained by a stoichiometric model of bond types. For a fixed stoichiometry in

PECVD silicon carbide, the number of bonds can be expressed by the following relationship:


Figure 1.8: FTIR spectrogram of sample 488

Table 1.5: FTIR peak locations for PECVD silicon carbide

Peak Location (cm-1) Description700 - 800 Si-C stretching, SiCH3rocking or wagging970 - 1020 Si-CH2stretching

& 1250 -CH3symmetric deformation

& 2100 Si-H stretching2887 -CH3asymmetric stretch2949 -CH3symmetric stretch

4C = (C −H) + 2(C − C) + (Si− C)

4Si = (S −H) + 2(Si− Si) + (Si− C)

4C + 4Si = (C −H) + (S −H) + 2(Si− C) + 2(C − C) + 2(Si− Si) (1.5)

Where

C = Number of Carbon Atoms

Si = Number of Silicon Atoms

(Si-C) = Number of Si-C bonds


Figure 1.9: Peak absorptions of Si-CH3 bond relative to etch behavior

(C-C)= Number of C-C bonds, etc...

By way of acknowledgment, this is a simplified model that does not take into account

C=C double bonds, pentavalent Si or any other less energetically favorable but possible

distributions of bonding types. This assumption is partially supported by the lack of C=C

stretching in the infrared region near 1650 cm-1.

The argument that a greater number of terminal -CH3 groups leads to a greater number

of Si-Si bonds can be proven logically. Given a fixed stoichiometry, the total number of –H

bonds must remain constant, and therefore (C-H) + (S-H) is a constant. Also, C and Si are

constants. Increasing the number of (C-H) bonds (correlating with a higher percentage of

terminal –CH3 groups), the number of (C-Si) or (C-C) bonds must decrease. Regardless of

the greater contributor to the overall decrease of non-terminating carbon bonds, because the

number of Si and the number of C atoms are constant and the right side of the relationship

is constant, (Si-Si) must increase. Data from FTIR analysis suggests that at some critical

point, the number of Si-Si bonds makes the film susceptible to etching in KOH because this

bond is susceptible to attack.


Isomers of silicon carbide are shown in Figure 1.10. It is easy to see how an increase in

terminal -CH3 groups can lead to an increase in Si-Si bonds which are susceptible to attack

by KOH.

(a) Isomer with terminal -CH3. 1 Si-C bond, 1Si-Si bond

(b) Isomer with no terminal -CH3. 0 Si-Sibonds, 1 Si-C bond

Figure 1.10: Isomers of silicon carbide

Interesting to note is that films prepared by Tawada et al. with an ethylene precursor

showed none of the peaks for a terminal -CH3 group. This would seem to indicate that

ethylene might be a superior precursor where the chemical resistance of carbide were of

tantamount concern [15].

1.4 Thin Film Comparisons

1.4.1 Comparison with Silicon Carbide

The majority of literature on deposition processes for silicon carbide focuses on CVD pro-

cesses for the crystalline or polycrystalline form, and its use as a semiconductor for high

temperature electronics, light emitting diodes, mechanical barriers, and other applications.

That research tends to focus on the electrical properties of SiC, particularly at higher tem-

peratures. The operating temperature limit of conventional silicon electronics is limited in

range from 200 to 250 °C depending on the particular technology. This limitation is a result

of silicon’s intrinsic carrier concentration and band gap (1010 cm-3 and 1.11 eV [45]). The

particular crystal forms 6H-SiC and 4H-SiC have much lower intrinsic carrier concentra-

tions (10-6 and 10-10 cm-3 [46]) and higher band gaps (3.02 and 3.26 eV [47]). This has the

1.4. THIN FILM COMPARISONS 25

effect of extending the temperature range where these materials continue to function as a

semiconductor to beyond 700 °C where other considerations such as dielectric or contact per-

formance become the temperature limiting factor. A 6H-SiC differential amplifier has been

demonstrated to 450 °C [48] and a SiC junction field effect transistor (JFET) to 500 °C [49].

Polycrystalline 3C-SiC has been proposed as a mechanical layer for surface microma-

chining. Stress is controlled through control of deposition parameters [50], annealing, and

doping [51]. This same material has been demonstrated in the process flow for a resonator.

As the poly-SiC is deposited, it first coats the device, and then seals the cavity as part of a

wafer-level package. In addition to the packaging, the device benefits from the properties of

the carbide, low stiction being the most relevant. [52]

The material explored in this work bares little resemblance to these other forms. Its

electronic structure is one of localized rather than extended states, leading to behavior as a

dielectric rather than a semiconductor [25]. Because the temperatures of plasma depositions

are typically too low to produce any crystallization [34], it is amorphous rather than crys-

talline or polycrystalline. PECVD carbide films also tend to be hydrogenated. In some cases

the atomic percentage of hydrogen approaches 50%. This results in very different mechan-

ical and optical properties. SiC for instance has a hardness approaching that of diamond

(9.3 on the Mohs scale), whereas some formulations a-H:Si1-xCx are quite soft, scratching

easily with a fingernail. Several critical differences between the materials are summarized

in Table 1.6. Values reported for PECVD silicon carbide are ranges found in the literature,

produced under a variety of conditions.

Table 1.6: Comparison of SiC and a-H:Si1-xCx

SiC a-H:Si1-xCx

Electrical Character Semiconductor InsulatorResistivity (W-cm) 1011 [53, 54] 1010 - 1016

Dielectric Constant 9.7 - 10.03 [55] 4.13 - 6.25Refractive Index 2.65-2.69 [55] 1.4 - 3.3

Structure Crystalline (Manytypes - 2H, 3C, 6H,15R, 21R, etc.)

Amorphous

Hardness (GPa) 24.5 [55] 8.8 - 19.2Young’s Modulus 447 [56] 56 - 153

Stoichiometry (Si1-x Cx) x=0.5 0.33 < x < 0.90Hydrogen Atomic % 0% 0-55%


1.4.2 LPCVD Silicon Nitride with PECVD Silicon Nitride

As a point of interest, Table 1.7 compares some literature values for SiN and its PECVD

counterpart, α-SiNx:Hy. Like the carbides, the change from crystalline to amorphous with

the addition of hydrogen affects several properties in a similar way: reduced dielectric con-

stant, lower hardness, and a reduced Young’s modulus.

Table 1.7: Comparison of LPCVD SiN and PECVD α-SiCx:Hy

SiN PECVD SiNElectrical Character Insulator InsulatorResistivity (W-cm) >1015 [57] 1.21x107-

2.06x1012 [58]2x1014 [59]

2x1014-≥ 1016 [60]Dielectric Constant 6.7-7 [61] 4.64 [61]

5.4-6.6 [58]5.4-6.2 [60]

Refractive Index 1.95-2.02 [61]1.98-2.02 [62]

1.98 [63]

2.14 [61]1.85-3.2 [58]

Structure Crystalline (3 types -a,b,g)

Amorphous

Hardness (GPa) 21.0 ± 0.9 [64] 10-12 [65]Young’s Modulus (GPa) 193-310 [66]

222 ± 3 [64]90-130 [65] [67]

Poisson’s Ratio 0.23 ± 0.02 [68]0.28 ± 0.05 [64]

0.25optimization [69]0.23 ± 0.02,

0.23 ± 0.01 [70]Stoichiometry (SiNx) x=0.75 0.74 <x<0.98 [71]Hydrogen Atomic % 0 8-25 [72]

1.5 Summary

A review of available thin films was conducted focusing on their potential to function as

environmentally resistant dielectrics. Four candidates were selected and testing was done

to eliminate all but PECVD silicon carbide. A process was developed on the STS 310

PECVD deposition system for silicon carbide and the films from that development were

characterized. Of particular importance was the analysis done to understand the root cause

1.5. SUMMARY 27

behind the differences in etch behavior among the films. It was discovered through FTIR

that differences in the amount of terminal -CH3 and by inference the amount of Si-Si bonding

correlated to the etch test results. The data on film properties and an understanding of the

root cause behind etch resistance facilitated the selection of an optimal film for use in the

fabrication of environmentally hardened transducers described throughout this work.

Chapter 2

Passivation for Pressure Sensors

As discussed in the introduction, media compatibility in pressure sensors is typically achieved

by packaging the transducer behind a stainless steel diaphragm and using a relatively in-

compressible fluid (oil) to couple pressure from the package diaphragm to the transducer

diaphragm. This approach typically involves welding a stamped, corrugated diaphragm to

a stainless housing with hermetic glass feedthroughs. There are many drawbacks to this

approach:

• The stainless steel diaphragm must be carefully designed or it can introduce significant

non-linearities in the response of the sensor.

• The CTE of a typical oil for this application such as Dow Corning’s DC-550 is 750

ppm. This high thermal expansion creates challenges to minimize the total oil volume

and accommodate this expansion without adversely affecting the sensor performance.

• A mechanism must be incorporated to accommodate filling with an oil under vacuum

and then hermetically sealing. This is typically done with glassed feedthroughs which

can be costly. These are also mechanically delicate and can lead to failures in the field

from even mildly stressful handling.

A much simpler (and cheaper) approach could be used if the transducer itself was capable

of withstanding long term exposure to the media of interest and the diaphragm could be

exposed directly.

PECVD silicon carbide shows promise as a material capable of extending the environ-

mental compatibility of silicon pressure transducers. A preliminary test was done on an

28

29

NPC-103 pressure sensor from Lucas NovaSensor (Fremont, CA). The entire sensor, ce-

ramic substrate and bond wires were coated with approximately 300 nm of PECVD silicon

carbide carbide deposited using 1000mTorr, 1200 sccm 2% Si in Ar, 300 sccm CH4, a high

frequency power of 75W for 5 s, and a low frequency power of 150W for 10 s.

A discoloration around the periphery of the ceramic observed after the deposition indi-

cated the die attach material had experienced some level of oxidation or decomposition from

exposure to the high deposition temperature. A process for deposition on a fully packaged

part would require using a high temperature packaging material such as benzocyclobutene

(BCB).

Figure 2.1: Uncoated and coated pressure sensors after 45 minutes exposure to KOH.

The coated, packaged pressure sensor was then placed in 22% KOH at 80 °C along with

an uncoated sensor. After 45 min, the diaphragm of the uncoated sensor was completely

etched away. The coated sensor was still intact. Both are shown in Figure 2.1. Tests on

the coated pressure sensor showed that although stress in PECVD silicon carbide was low

enough so as not to induce offsets, the signal was significantly reduced, indicating that the

mechanical properties of the carbide films must be accounted for in sensor design.

Based on these promising results, a more detailed study was done on the characteristics

of PECVD silicon carbide that would specifically affect pressure sensor design. Mechanical

characterization, measurement of sensor performance, and analysis of corrosion resistance

was performed in collaboration with Thomas Kim, Bishnu Gogoi, Slobodan Petrovic, and

David Monk of the Motorola Sensor Products Division, and John Heck of the Berkeley

Sensors and Actuator Center.

30 CHAPTER 2. PASSIVATION FOR PRESSURE SENSORS

2.1 Optimization

In order to obtain an optimal set of deposition conditions specifically for a pressure sensor, a

two-level, four variable DOE was performed set up and samples were generated. Deposition

conditions for this DOE are listed in Table 2.1

Table 2.1: Deposition conditions for PECVD silicon carbide DOE

Deposition Parameters Set PointSilane Flowrate (sccm) 2840

Methane Flowrate (sccm) 1000, 1440, 1880Pressure (mtorr) 1600

High Frequency Power (W) 100High Frequency Cycle Time (s) 2.0, 4.5, 7.0

Low Frequency Power (W) 50, 100, 150Low Frequency Cycle Time (s) 2.0, 4.5, 7.0

Temperature (°C) 350

Due to equipment failure and availability, four points in the DOE were unable to be

completed. The list of samples and deposition conditions can be found in Table 2.2.

2.2 Mechanical Properties

2.2.1 Experimental Setup

The first step in assessing the effect of adding PECVD silicon carbide to a piezoresistive

diaphragm pressure sensor is to measure its mechanical properties. Establishing these pa-

rameters is important for use in analytical and finite element models used to predict sensor

performance. Young’s modulus and hardness were measured by nanoindentation using a

Nano Indenter II® mechanical properties microprobe (MPM) (MTS Nano Instruments,

Inc., Oak Ridge, TN). A Berkovich indenter was used to generate load-displacement curves

for two test samples. Samples for nanonindentation were prepared with thicknesses of 1 mm

and 2 mm using the centerpoint deposition condition. Wafers were sawn into one-inch squares

for testing. Poisson’s ration was assumed to be 0.25 following literature values recorded for

PECVD silicon carbide [30]. Khakani et al. have correlated hardness, Young’s modulus,

and Poisson’s ratio to the stoichiometric ratio of Si to C for α-SixC1-x:H with 0.4 < x <0.67.

The value of 0.25 for Poisson’s ratio was chosen as it represents the mean value at the upper

2.2. MECHANICAL PROPERTIES 31

Table 2.2: Conditions are listed below for a two-level, four-variable DOE of PECVD siliconcarbide deposition parameters. Missing cells are designated with a dash (-).

Wafer CH4Flowrate

(sccm)

hf Time(s)

lf Power(W)

lf Time(s)

- 1000 2 50 2B-08 1000 2 50 7B-01 1000 2 150 2A-17 1000 2 150 7A-23 1000 7 50 2B-06 1000 7 50 7B-04 1000 7 150 2- 1000 7 150 7

A-15 1880 2 50 2- 1880 2 50 7

A-05 1880 2 150 2- 1880 2 150 7- 1880 7 50 2- 1880 7 50 7- 1880 7 150 2

A-22 1880 7 150 7D-01 1440 4.5 100 4.5D-02 1440 4.5 100 4.5

end of x. To calculate Young’s modulus, the reduced modulus (Er) was employed, defined

as

S =2√πEr√A (2.1)

and

Er =

[1− ν2fEf

+1− ν2iEi

]−1

where Ei and νi are Young’s modulus and Poisson’s ration for the indenter, respectively,

and Ef and νf are Young’s modulus and Poisson’s ration for the thin film.

Because the stress in deposited thin films affect the electrical performance of piezore-

sistive transducers [73], this is an important property to measure over temperature. Films


were deposited with different thicknesses on 4 inch diameter, n-type (100) blank wafers. The

resulting radius of curvature change was measured using a Tencor FLX-2320 (Tencor Instru-

ments, Milpitas, CA). As-deposited stress was calculated from the radius of curvature (R)

using Stoney’s equation:

σ =Est

2s

6 (1− νs)Rtf(2.2)

where Es/ (1− νs) is the biaxial modulus of the silicon substrate (1.805 x 1011 Pa for <100>

silicon [74], ts is the thickness of the substrate, tf is the thickness of the deposited film, and

σ is the calculated film stress. Differences in the coefficients of thermal expansion (CTE)

between the film and substrate were calculated using the measurements from the Tencor

FLX-2320 system programmed with a selected temperature ramp. The temperature was

ramped from 20 ºC to 400 ºC over 60 minutes. Samples were cooled back down to 40 ºC over

150min using a nitrogen gas flow. Stress vs. temperature measurements were performed on

all available samples from the DOE table. The slope between 300 ºC and 40 ºC during the

ramp down period was used to calculate the CTE for carbide using:

Slope =dσ

dT=

Ef1− νf

∆α (2.3)

where Ef/ (1− νf ) is the biaxial modulus of the thin film and ∆α is the difference in CTE

between the silicon and the thin film. Multiple runs were performed on a centerpoint wafer

to determine both hysteresis and changes in dσdT .

Stiffness is derived from the initial slope of a load vs. displacement curve (assumed to

behave as a power law function) at the beginning of the unloading cycle. The equations of

relevance are:

P = αhm (2.4)

and

S =dP

dh

∣∣∣∣∣Pmax

dσ

dT(2.5)

where P is the load, h is the displacement, α is a constant, m is a constant assigned

according to the punch geometry [75], and S is stiffness. Hardness (H) is calculated by


dividing the load by the area of contact (A) of the indenter on the film.

H =P

A(2.6)

2.2.2 Results

Nanoindentation measurements revealed that PECVD a-SiC:H undergoes both plastic and

elastic deformation as the indenter is loaded. Unexpected profiles with non-zero slopes were

observed in plots of both hardness and modulus of elasticity as a function of normalized

displacement. The abscissa value is the displacement of the indenter divided by the total

film thickness for the given sample; the x-value is the fraction of penetration by the indenter

on the total available film thickness. Each point on the graph represents the mean value of

several measurements. Error bars are plotted as ±1 standard deviation.

2.2.2.1 Young’s Modulus

Figure 2.2 shows a plot of Young’s modulus versus normalized displacement for two samples

deposited with 1 mm and 2 mm. The Young’s modulus profiles for the two samples are similar

with a slight increase in the y-direction for the sample with 2 mm of silicon carbide. The

surprising feature of the profiles is that there is a slope to the calculated Young’s modulus,

indicating that the film responds differently (albeit in a linear fashion) at different levels of

load indentations. Typically, the profiles would plateau after about 10% of the test film has

been penetrated. The plateau observed for the 1 mm sample near the end of the normalized

displacement from 0.9 to 1 is more a feature of the silicon substrate than the silicon carbide

film. It appears that is that the silicon substrate, which has a higher value of Young’s

modulus (~160GPa) is confounding the expected non-zero slope of the silicon carbide film

profile. To obtain a good value of the Young’s modulus for the carbide itself, the results are

extrapolated to the zero displacement point at the y-intercept.

2.2.2.2 Hardness

Figure 2.3 shows a plot of hardness versus normalized displacement for two samples deposited

with 1 mm and 2 mm. As with the Young’s modulus, the hardness (H) also increases as the

film is being penetrated and the hardness must be determined by extrapolating to the zero

displacement point at the y-intercept.


Figure 2.2: Young’s modulus vs. normalized displacement

Figure 2.3: Hardness vs. normalized displacement

The PECVD a-SiC:H deposited at optimal centerpoint conditions has a hardness of

7.3± 0.5GPa and a Young’s modulus of 52± 8GPa. This is less than the range of 10.8 to

19.7GPa for H and the range of 88 to 154GPa for Young’s Modulus reported by Khakani,


et. [30], indicating that the film is silicon-rich PECVD a-Six C1-x:H film with x > 0.67. This

is consistent with the FTIR results discussed in chapter 1.

2.2.2.3 Stress

The stress of deposited films for centerpoint deposition conditions on blank wafers is shown

in Figure 2.4. For films deposited at 400 nm, the stress ranged from -47 to -9MPa. While

the term “low stress” is somewhat subjective, lower than 50MPa in absolute stress value

is generally considered acceptable, particularly in the context of this application. Room

temperature stress measurements were also made on samples of 0.8, 1.0, and 1.2 mm. Results

show that the stress of PECVD silicon carbide shifts more tensile as the film thickness

increases and approaches 10MPa almost asymptotically.

Figure 2.4: Room temperature stress vs. thickness of deposited films

2.2.2.4 Stress and CTE

Stress, CTE, and dσdT values are recorded in Table 2.3. Recall that dσ

dT is the slope from

the linear portions taken from 300 °C to 40 °C during the ramp down cycle of stress vs.

temperature plots. Since not all of the DOE cells were fulfilled, a complete screening analysis

could not be run. Analyzing the data that was available showed no first order interactions


of the four deposition variables (to 95% confidence levels) to either the stress or CTE values

for PECVD a-SiC:H. There was no unique set of coefficients to form a linear combination.

Due to the lack of degrees of freedom, second order parameters were also biased and no

higher order model could be generated. However, it seems apparent from comparing the

changes of the output levels for repeated runs with variations in only one of the four input

variables that each deposition condition was highly significant for room temperature stress

values (20°C) but not so significant for CTE values. Centerpoint PECVD a-SiC:H films

had a CTE value of 2.5 ppm/°C at 25 °C and 3.1 ppm/°C at 125 °C. CTE values of 2.4 and

3.0 ppm/°C at 25 and 125°C respectively were used for the silicon substrate. It is interesting

to note that PECVD a-SiC:H has a higher coefficient of thermal expansion than silicon,

since most passivation films have lower CTE values. This material property is significant

in analyzing devices with silicon carbide where the performance metric value is based on

temperature.

Table 2.3: Mechanical properties of trials in DOE

Wafer CH4Flowrate

(sccm)

hfTime(s)

lfPower(W)

lfTime(s)

Stress20 ºC(MPa)

dσdT

(MPa/ºCCTE25 ºC

(ppm/ºC

CTE125 ºC

(ppm/ºC)- 1000 2 50 2 - - - -

B-08 1000 2 50 7 -28.5 -0.116 2.5 3.1B-01 1000 2 150 2 83.2 -0.365 2.8 3.4A-17 1000 2 150 7 -8.1 -0.122 2.5 3.1A-23 1000 7 50 2 -102.5 -0.256 2.7 3.3B-06 1000 7 50 7 119.5 -0.442 2.8 3.4B-04 1000 7 150 2 -12.8 -0.111 2.5 3.1- 1000 7 150 7 - - - -

A-15 1880 2 50 2 -19.0 -0.114 2.5 3.1- 1880 2 50 7 - - - -

A-05 1880 2 150 2 -44.3 -0.149 2.5 3.1- 1880 2 150 7 - - - -- 1880 7 50 2 - - - -- 1880 7 50 7 - - - -- 1880 7 150 2 - - - -

A-22 1880 7 150 7 -32.7D-01 1440 4.5 100 4.5 -42.7D-02 1440 4.5 100 4.5 -46.2

Stress versus temperature profiles were strikingly similar for the DOE points investigated,


including the centerpoint. The hot temperature excursion consistently decreased the stress

value by 11± 5MPa at T=40 °C, making the films more tensile. Wafers from run A-23,

which had a room temperature stress of -103MPa, showed greater stress hysteresis with a

delta value of 60 MPa Wafer B-06, which had a room temperature stress value of 120MPa,

showed little hysteresis. Annealing occurs beyond the deposition temperature of 350 °C and

the stress versus temperature lines criss-cross randomly between 325 °C and 400 °C. Figure

2.5 illustrates multiple hot temperature excursions on a centerpoint test wafer deposited with

400 nm PECVD α-SiC:H. With each consecutive run, the stress value decreases. Annealing

still occurred after four runs but the stress hysteresis was lessened after each temperature

cycle. The initial change of stress was 11MPa. On run four, the delta was 4MPa at

T=40 °C. The slope of stress change, dσdT , remained consistent over multiple runs. This

demonstrates that the residual stress from a PECVD silicon carbide layer can be affected by

an anneal cycle. The sensitivity and offset of a pressure sensor would be affected by adding a

protective layer. Having the ability to minimize this impact by reducing the intrinsic stress

is an attractive quality of PECVD α-SiC:H. Because the anneal temperature is metal and

CMOS compatible, it can be easily incorporated into a production flow at minimal cost and

complication.

Figure 2.5: Stress vs. temperature


2.3 Effect on Pressure Sensors

The addition of a thin film to a piezoresistive pressure sensor affects the stiffness and stress

of the diaphragm, which in turn influences parametric behavior. A series of experiments

were conducted where various thicknesses of carbide were deposited on a low range (0 to 6

kPa) pressure sensor. Studying the variations of sensitivity and offset with thickness were

used to correlate the mechanical properties of the film with their effect on the behavior of

the sensor. The PECVD α-SiC:H was deposited onto the die as the last film in a series

of passivations. The passivation-film stack makes up about 5 % of the total diaphragm

thickness (18 mm).

2.3.1 Initial Offset

The offset induced by stresses in the encapsulating film are shown in Figure 2.6. As carbide

thickness increases, offset increases, suggesting that PECVD silicon carbide is tensile at

higher film thickness. This agrees with the material analysis of the stress levels as a function

of film thickness, as shown in Figure 2.4. A thickness of 400 nm of carbide shifted the initial

offset positive by approximately 1.5mV/V. The 800 nm showed the same rate of effect per

thickness, shifting the initial offset voltage positively by roughly 3.0mV/V.

Figure 2.6: Initial offset

2.3. EFFECT ON PRESSURE SENSORS 39

2.3.2 Sensitivity

As expected, the sensitivity decreased as thicker layers were deposited. The loss in sensitivity

indicates that the stiffness of the diaphragm increased due to the compressive stress applied

by the carbide film and the increase in passivation film thickness. During the material

characterization of carbide, compressive stress was observed for the 400 nm thickness, but

not the 800 nm thickness. Although carbide was seen to be tensile for films thicker than 800

nm, higher thickness of the film may decrease the sensitivity.

Figure 2.7: Sensitivity vs. thickness of deposited film

2.3.3 Temperature Coefficient of Sensitivity (TCS)

An important result is the decrease in magnitude of the temperature coefficient of sensitivity

(TCS) with the addition of PECVD silicon carbide. The TCS vs. film thickness is shown in

Figure 2.8. Typically the addition of an LPCVD silicon dioxide or PECVD silicon nitride

film on the pressure sensor diaphragm increases the magnitude of TCS, making the device’s

performance in sensitivity more temperature dependent. Differences in the CTE values for

these films may explain the variable effect on TCS; specifically, silicon oxide and PECVD

silicon nitride have CTE values less than silicon, whereas PECVD silicon carbide had a

CTE value greater than silicon. Additionally, correlations between TCS and sensitivity for


devices with variable thicknesses of silicon dioxide have shown that an increase in oxide film

thickness increases the magnitude of TCS. The opposite is observed with the addition of

PECVD silicon carbide. At a given sensitivity, the magnitude of TCS decreases, reducing

the level of electronic trimming required.

Figure 2.8: TCS vs. film thickness

Figure 2.9: The correlation of TCS with sensitivity for two thicknesses of PECVD SiC

2.4. CORROSION RESISTANCE 41

2.4 Corrosion Resistance

Several mechanisms can cause the breakdown of a protective coating. The etchant can attack

the film directly and remove it. A second mechanism is through defects. These defects can

be present in the film as deposited due to particles in the system. In some cases, nodules or

nucleation sites can form as a natural part of the deposition process and become points that

are not technically defects, but which are chemically different from the intended film and

susceptible to chemical attack. Defects can also be created by scratching during handling.

Defects allow the etching chemistry to reach the substrate and undercut the protective film.

The third mechanism is a function of the diffusive permeability of the film. If the film is

permeable to the etchant, or even to potentially corrosive gases in the environment such as

oxygen, long term exposure will lead to failure.

2.4.1 Etch Resistance

The etch resistance of the specific films in this study was measured using the same technique

as discussed in chapter 1. The change in thickness was measured over time using ellipsometry.

Two blank silicon wafers were coated with 400 nm of the carbide using the centerpoint set

of parameters in Table 2.1. For comparison, two separate PECVD silicon nitride films were

tested on two wafers each. This etch resistance test was done with 23wt.% KOH at 95 °C

using a fixture for single-sided wafer etching to prevent excessive overetching of the wafer

backside.

The etch depth results are shown in Figure 2.10. All the nitrides etched at a similar

etch rate, 5.29± 0.49 nm/min. The carbides etched at 0.79± 0.08 nm/min. This measurable

etch rate is different than what was in measured for 33wt.% KOH at 80 °C, which showed

no etch rate above 0.05 nm/min. The difference in etch rates between this experiment and

the previous one shows that bulk etching may be a mechanism of failure for long-term

exposure in extreme chemistries. Evaluation of the suitability of PECVD silicon carbide

for a particular application must be done on a case-by-case basis, tailoring the qualification

tests for specific conditions.

2.4.2 Linear Polarization

Linear polarization or linear sweep voltammetry can be used to investigate the diffusive per-

meability of the film [76]. Test substrates were prepared by patterning aluminum electrode


Figure 2.10: Etch rates of PECVD silicon carbide and PECVD silicon nitride

arrays on a thermally oxidized wafer. PECVD silicon carbide was deposited to a thickness

of 400 nm using the centerpoint recipe from table 2.1. A test substrate was also prepared

using 300 nm of PECVD silicon nitride for comparison.

Figure 2.11: Cross-section of die for electrochemical studies

The wafers were then patterned and etched to expose the bond pads. Wafers were diced

and packaged into a 6-pin unibody package. A chemically resistant epoxy was used to protect

2.4. CORROSION RESISTANCE 43

the bond pads, bond wires, and edges of the die from corrosion while exposing the coated

electrode. The experimental setup is shown in Figure 2.12. A platinum wire was used as

the counter electrode. A Ag/AgCl reference electrode was used. The coated die was the

working electrode as the device under test (DUT).

Figure 2.12: Fixture for electrochemical corrosion studies

The prepared devices were immersed in 23wt% KOH at room temperature for 48 h with

measurements taken at 0, 24, and 48 h. The corrosion of the underlying aluminum was

expected to proceed via an oxidation (Al→Al+3+ 3e-). The leakage current through the

film was measured by applying a positive potential, ramped at 100mV/s from 0 to 5V

As can be seen from the results at 48 h, the PECVD silicon carbide had negligible leakage

currents and no signs of permeability under the conditions tested. The PECVD silicon

nitride film tested had a linear, resistive current profile indicating diffusion through the

membrane. Some nitride films have been tested and shown results similar to the carbide [77].

The permeability of this particular PECVD silicon nitride is more a function of the deposition

conditions than the chemistry of the film. Exact determination of the resistance of the

carbide sample was not possible because the current at 5V was still below the noise floor

of the potentiostat. The resistance of the sample was greater than 200MW. The equivalent

resistance of the nitride sample was 220 kW.


Figure 2.13: Linear polarization plot of PECVD silicon carbide and a failing sample ofPECVD silicon nitride for comparison.

2.4.3 Electrochemical Impedance Spectroscopy

One drawback to linear polarization is that the absolute value of the current measurements

are dependent on surface area. Electrochemical Impedance Spectroscopy (EIS) has long

been established as a more sophisticated approach to analyzing corrosion behavior. Complex

equivalent circuit models are replete throughout the literature [78–81]. The DC potential

across the film was set at 0V and 5mV sine wave was swept between 0.1 Hz and 1 MHz.

The results are shown in Figure 2.14.

The resistance and capacitance vary inversely relative to each other with respect to both

area and thickness. The cutoff frequency of the dominant pole, f c, can be used as a figure

of merit to compare the quality of different films, taking care to account for any changes in

dielectric constant. The cutoff frequency of the failing sample, Sample 2, is approximately

600Hz. The apparent resistance between 1Hz and 1 kHz is 6MW with a capacitance of

47 pF. Sample 1 does not show a cutoff frequency in the range examined, indicating that

the resistance of this sample is greater than 5GW. The fact that one sample did not fail and

another did is a strong indication that the failure mechanism results from a defect, either

2.5. SUMMARY 45

Figure 2.14: EIS of two PECVD silicon carbide samples after 48 hours. Sample 2 showscorrosion behavior.

due to processing or due to handling. It is not an intrinsic property of the film.

2.5 Summary

Preliminary results using PECVD silicon carbide to protect silicon diaphragm pressure sen-

sors were promising. The mechanical properties that would be significant for the design of

a diaphragm pressure sensor were determined through direct characterization. The results

showed that carbide could be incorporated into a design with minimal effort. This was con-

firmed by depositing the carbide as a passivation layer to an existing pressure sensor design.

Measurements of the impact it had on the performance were consistent with the expected

effects from the mechanical properties measured. PECVD silicon carbide was compatible

with, and in some respects was beneficial to a diaphragm pressure sensor.

Accelerated corrosion studies showed that while the carbide protected the silicon di-

aphragm sufficiently, it was not successful when applied to existing packaging and bond

wires. Additionally, results showed that some failures are a result of fabrication defects and

not a failure of the inherent film. The mixed results gives an indication of promising avenues


for future work. One would be to improve the compatibility of bond wires and packaging

with the deposition process, particularly the deposition temperature of 350 °C. Both the

deterioration of the package and the adhesion to the bond wires are concerns. Second, the

deposition system requires optimization, specifically for PECVD silicon carbide, to reduce

the level of particles and point defects so that reliable thin films can be deposited at a

reasonable thickness.

Chapter 3

An Iridium Microelectrode Array

Introduction

Microelectrodes have been fabricated since the early days of micromachining. They have

been used in a variety of applications ranging from extracellular recording of electrical poten-

tials in live tissues to analytical chemistry. By using such inert materials as PECVD silicon

carbide and iridium in the fabrication of a microelectrode array, it is possible to fabricate

a chemical transducer that can withstand continual exposure to such media as hydrofluoric

acid (HF).

The work presented in this chapter includes improved methods for deposition and pat-

terning of iridium for micromachining applications, the application of PECVD silicon carbide

as a chemically resistant coating of a microelectrode array, and the demonstration of its use

in hydrofluoric acid.

The analysis of the microelectrode array in HF was done in collaboration with Dr. Stan

Tsie of ChemTrace Corporation, Hayward, CA. The original design of the array and was

done by Chris Storment, Stanford University, Stanford, CA.

3.1 Background

Microelectrodes have a number of characteristics that result in improved performance over

their macroscopic counterparts for many applications. A brief overview is presented here,

but a detailed discussion can be found in Analytical Electrochemistry by Joseph Wang [82].

Advantages offered by microelectrodes include:

47

48 CHAPTER 3. AN IRIDIUM MICROELECTRODE ARRAY

• localized measurements (an advantage in such endeavors as cellular recording),

• reduced resistive drop (allows measurements in low conductivity environments),

• reduced capacitance (faster time constants), and

• enhanced mass transport (enhanced signal-to-noise ratios).

The last property is of particular interest to this work as it leads to the improved performance

of microelectrodes for heavy metal analysis using square wave anodic stripping voltammetry

SWASV, discussed below.

3.1.1 Diffusion Behavior

One of the main advantages of a microelectrode is the nature of its diffusion profile. Standard

planar electrodes establish a planar diffusion profile. Hemispheric electrodes establish a

hemispheric diffusion profile, which has a planar and a radial component. It is the radial

component of the diffusion profile that leads to improved mass transport for a given electrode

area. Conditions for hemispheric diffusion are that

r0 √D0t (3.1)

where D0 is the diffusion layer thickness, t is the electrolysis time, and r0 is the smallest

dimension of the electrode. This effect has been shown to begin in electrodes with radii

as large as 300 mm [83]. The working electrode for the SWASV studies done here was

approximately 10 mm, well into the regime where this effect is established. Experimental

data have shown that this hemispheric profile is established on the order of 1 s [84].

The time necessary to reach steady state, spherical behavior is important, as it sets a

lower limit on deposition time or the preconcentration time for stripping techniques, and an

upper limit on the frequency of analysis. Theoretical calculations [85,86]

tε =d2

π3Dε2(3.2)

where tε is the time, d is the disk diameter in meters, D is the diffusion coefficient, and ε

is the percentage of final value obtained by time tε. For a typical diffusion constant of 1 x

10-9 m2/s, the current reaches 95% of the steady state value by 1.3 s.

3.1. BACKGROUND 49

3.1.2 Square Wave Anodic Stripping Voltammetry

SWASV analysis of heavy metal ion concentration was chosen to demonstrate the use of the

microelectrode array in caustic media. Previous work had demonstrated this application in

more conventional electrolytes such as perchloric acid and glacial acetic acid [87–89].

In the first step, a mercury hemisphere was plated onto the surface of the iridium elec-

trode. The choice of iridium as the electrode material was critical to the success of the

plating. Deposition on glassy carbon electrodes has poor reproducibility and adhesion is un-

reliable. With other metal electrodes such as platinum, mercury tends to form intermetallic

compounds (alloy) that leads to the dissolution and failure of the electrode. Iridium does

not suffer from either of these limitations [89].

Once the mercury had been plated, the electrode was then transferred to the solution

to be analyzed. SWASV is a two phase process. The first phase is called the preconcen-

tration phase. During this phase, the electrode is biased cathodically (negatively) relative

to solution. The mercury electrode can be considered equipotential. Cations of interest are

reduced at the surface of the mercury hemisphere. In their reduced or neutral state, the

metals of interest are much more soluble in the mercury than in the aqueous solution and

they concentrate within the electrode.

Figure 3.1: Steps in square wave anodic stripping voltammetry

The ratio of oxidized to reduced species is derived from the Nernst equation [90]:

E = Eφ +RT

nFln

(CνiOi

CνiRi

)(3.3)

where

E is the half-cell reduction potential,

Eφ is the standard half-cell potential,

R is the Universal Gas constant = 8.314 J/mol·K.


T is the absolute temperature,

F is the Faraday constant = 9.648 x 104 C/mol,

n is the number of electrons transferred in the reaction, and

CνiRi, CνiOi

are the concentrations of the reduced and oxidized species respectively.

The Nernst equation states that the electrochemical potential of an oxidized species

in equilibrium with its reduced species is proportional to the logarithm of their relative

concentrations. Inversely, the relative concentration of the species is dependent upon the

applied potential in a quasi-equilibrium (steady state) situation. For a given potential, at

the mercury interface, the relative concentrations of the different species can be determined

from

CνiRi

CνiOi

= enFRT (E

0−E). (3.4)

The concentration of the reduced metal species decreases exponentially as the electrode

potential moves beyond the standard half-cell potential. The floor on the preconcentration

voltage in an aqueous environment is approximately -1.2V relative to a Ag/AgCl reference

electrode. Below this potential the hydrolysis of water begins to compete and interfere with

the preconcentration reaction. As can be seen from the graph in Figure 3.2, the concentration

of the oxidized species at the interface can be considered to be zero even for species with

redox potentials as low as -1.1 V

As the concentration of reduced species at the interface decreases, a flux of metal atoms

from the center of the electrode to the surface is established. This progresses according to

Ficks first law, adjusted for spherical diffusion.

δC (r, t)

δt= D

δ2C (r, t)

δr2+

2

r

δC (r, t)

δr(3.5)

The longer the preconcentration time, the higher the concentration of the analyte in the

mercury hemisphere and the more sensitive the analysis. Preconcentration times from 100 s

to 15minutes are typical. Essentially all of the metal ion flux to the surface is a result of

steady state diffusion that is linearly dependent upon the concentration differential. Thus

the amount of analyte dissolved in the mercury hemisphere is linearly dependent on both

the concentration in solution and the preconcentration time.

The second phase is called the stripping phase. In the stripping phase, the voltage is

stepped anodically (positive relative to solution) using a staircase wave. Current is sampled

3.2. FABRICATION 51

Figure 3.2: Ratio of oxidized to reduced species vs. electrode potential

at the end of both the forward and reverse phase of the wave and subtracted (Figure 3.3).

By doing this, the Faradic current due to the metal oxidation can be separated from the

capacitive charging current of the electrode.

3.2 Fabrication

3.2.1 Overview

A diagrammatic cross section of the microelectrode array is shown in Figure 3.4. An overview

of the fabrication process is as follows. The starting material is a 4-inch (100) silicon wafer.

A layer of wet thermal oxide is first grown to a thickness of 500 nm and then patterned using

an oxide plasma etch (Applied Materials AMT 8100 Hexagonal Etching System, Applied

Materials, Inc., Santa Clara, CA). This provides a high quality underlying dielectric, reliable

alignment marks for later in the process, and a recess area surrounding the chip so that no

oxide is exposed along the edges after dicing.

The electrode layer is then deposited. A 25 nm tacking layer of chromium followed by a

350 nm layer of iridium are evaporated using an Innotec ES26C e-beam evaporator (Innotec


Figure 3.3: Stripping phase of SWASV

Figure 3.4: Diagrammatic cross-section of the microelectrode array

Group, Inc., Simi Valley, CA) and patterned into the electrode arrays using a germanium-

aluminum liftoff technique discussed in detail in section 3.2.2.3. Liftoff is accomplished with

4:1 sulfuric peroxide mixture.

The interconnect and bond pad layer is then deposited and patterned using the Shipley

liftoff layer (LOL) technique (Shipley Company, Marlborough, MA). This is a two-layer

technique whereby Shipley LOL is spun on, followed by Shipley 3612 photoresist. Both

layers are patterned in the same photolithography step. A 25 nm tacking layer of chromium

followed by a 500 nm layer of gold is evaporated and lifted off using acetone, followed by

isopropyl alcohol, and finally a 4:1 sulfuric peroxide mixture to clean the surface.

A 1 mm layer of PECVD silicon carbide is then deposited as the encapsulating layer.

3.2. FABRICATION 53

Openings to the electrodes and the bond pads are patterned using an SF6 plasma etch.

Details of carbide deposition and etching are discussed in Chapter 1.

3.2.2 Stress Control of Iridium

The initial electrodes were fabricated by sputter depositing 300 nm of iridium on top of a

25 nm chromium tacking layer. Because iridium is relatively inert, it is difficult to pattern

directly. A liftoff technique was required.

Figure 3.5: SEM of a failed iridium microelec-trode pad

One hindrance to repeatable production

of thin-film iridium electrodes is the stress

that is typical in both sputtered and evapo-

rated films of iridium. Stresses on the order

of a several GPa have been observed. In

early runs of the arrays, stress often caused

tearing of the electrodes as shown in Fig-

ure 3.5. Three methods were utilized to

overcome this limitation. The first was the

use of ion bombardment during evapora-

tion to reduce the tensile stress of the film.

The second was the use of a staggered de-

position process, in which cooling periods

were inserted between short deposition cy-

cles to reduce the stress from CTE mis-

match. The third was the development of

a stress-tolerant set of liftoff layers. Stress

was measured by wafer curvature using an SMSI 3800 stress gauge (Scientific Measurement

Systems, Inc., San Jose, CA).

3.2.2.1 Ion-assisted Deposition

Ion-assisted physical vapor deposition (IAPVD) has been used to help control stress, stoi-

chiometry, morphology, and tribological behavior of many different films. Some examples

from the literature include diamond-like carbon (DLC) [91], cubic boron nitride (c-BN) [92],

and silver [93]. The earliest reference of its specific use in this way is Mattox [94]. In

1994, Greer reported its use to control the stress of molybdenum thin films for use in flat


panel displays [95]. In IAPVD, a stream of inert ions is directed at the surface of the wafer

during thin-film deposition. The ions have enough kinetic energy to promote surface re-

arrangement, though not enough to sputter-etch the surface. It should be noted that this

technique is distinct from one in which the incident ions react chemically with the thin-film

material.

For the iridium deposition, a Mark I gridless ion source (Commonwealth Scientific Cor-

poration, Alexandria, VA) was used to generate a stream of argon ions. The stream of ions

was directed to cover a section of the planetary that was rotated during deposition. The

energy of the ion atoms was approximately 100 eV, which was too low to cause sputtering.

The ion current density based on the specified performance of the ion gun and distance from

the wafers was approximately 0.05mA/cm2. The deposition rate was kept low at 0.1 nm/s

to maximize the effect of the ion bombardment.

3.2.2.2 Staggered Deposition

The coefficient of thermal expansion (CTE) for the materials used in the electrode process

are listed in Table 3.1. Because the CTE for the metals are all larger than that of silicon, it

is clear that if the silicon wafer is heated during deposition, the thin film will become more

tensile as the wafer cools. Unfortunately, evaporation is problematic on two fronts. Thin

films are almost always tensile, and the energy directed at the crucible during evaporation

radiates outward and heats the wafers. Consequently the CTE mismatch acerbates, rather

than ameliorates the stress problem.

Non-reversible temperature labels from Omega (Omega Engineering, Stamford, CT)

were used on the backs of wafers during trial runs. Temperatures in excess of 180 °C were

observed during typical continuous depositions of a full thickness of iridium.

Table 3.1: Coefficients of thermal expansion

Element CTEIridium 6.4Gold 14.2

Chromium 4.9Silicon 2.6

To help decrease the temperature at which the metal is deposited, iridium was deposited

in short bursts interspaced with long cooling periods. For each deposition, the temperature

3.2. FABRICATION 55

was monitored and deposition stopped as soon as the temperature in the chamber reached

80 °C. The wafers were allowed to cool approximately 1 h before resuming depositions, to

roughly 35 °C. At 0.1 nm/sec., it was usually possible to deposit 15-20 nm per period.

Table 3.2 shows the stress reduction achieved from three separate trial runs. For the trial

runs, 25 nm of chromium were deposited as a tacking layer, followed by 120 nm of iridium.

Table 3.2: Progressive reduction of stress in evaporated iridium

Type of Trial Stress (MPa)Standard Evaporation 2500Evaporation with Ion Gun Assist 1630Evaporation with Ion Gun Assist and StaggeredDeposition

360

It is acknowledged that a staggered deposition is not a practical solution to high-stress

films. Achieving even a reasonable thickness film could take days. What the experiment

did illustrate successfully though was the significant improvement that controlling wafer

temperature can have on the stress of deposited films. It supports the case for the addition

of active wafer cooling, or, in some cases maybe heating, to a deposition system where stress

control is important.

3.2.2.3 High-Stress Liftoff Layers

Given the limitations of the accessible deposition systems, the steps taken above were suc-

cessful in reducing, but could not eliminate the measured stress. It was observed that

although the iridium itself could withstand stresses on the order of several hundreds of

MPa, the underlying photoresist-aluminum bilayer could not. Often during deposition, the

underlying photoresist layer would buckle and crinkle. This had the effect of generating par-

ticles during the deposition that would incorporate into the film and were suspected sources

of point defects.

Several material pairs were evaluated as alternate choices for a more stress-tolerant set

of liftoff layers. The materials had to satisfy several criteria.

• Suitable etch methods had to exist such that each layer could be patterned indepen-

dently in order to create the required liftoff undercut.

• These etch methods must be compatible with standard photoresist.


• A liftoff chemistry had to exist that did not attack silicon dioxide, chromium, or

iridium. Preferably the liftoff would proceed with both layers to reduce the time

required.

• The layers had to accommodate a high stress iridium deposition without severe defor-

mation (buckling, cracking, or peeling).

After several experiments with a series of possible layers, including aluminum-low-temperature

oxide (LTO), LTO-aluminum, and aluminum-silicon (sputtered), the best performance was

obtained with an aluminum-germanium bilayer.

• Aluminum can be patterned in aluminum etch. Germanium also etches in aluminum

etch, though at a much slower rate. The liftoff undercut can then be created with a

very low power SF6 plasma etch, which does not attack aluminum.

• Germanium etches very rapidly in an SF6. It etches so quickly, in fact, that it was

necessary to drop the power below 1.6W/cm2 to achieve a controllable, repeatable

undercut etch. Undercut etches were completed in 80 s. At this power and for this

duration, no significant attack of the underlying silicon dioxide was observed.

• Aluminum and germanium both etch rapidly in sulfuric peroxide. This is an ideal

liftoff etch as the cavitation from oxygen evolution aids in the process, and sulfuric

peroxide does not attack chromium or iridium.

• The Al-Ge bilayer was able to withstand 400MPa of stress from a 350 nm deposition

of iridium without failure.

Results of the Al-Ge bilayer are shown in Figure 3.6. The SEM shows ideal dimensional

control of the undercut. The ability to deposit thicker layers of either material ensures

that no “dog-ears” or “stringers” from inadequate separation of the deposited layer does not

occur.

3.3 Electrode Design

While microelectrodes typically have superior performance relative to their macroscopic

counterparts, the signals themselves are small enough that they can be lost in the parasitics

3.3. ELECTRODE DESIGN 57

Figure 3.6: SEM cross-section of an aluminum-germanium liftoff bilayer after deposition ofiridium

of the electrochemical system. By making a microelectrode array, the signals at each elec-

trode are added together, achieving both the superior performance of a microlectrode with

an adequate signal-to-noise ratio for the electronic system. The electrode design used in the

evaluation of PECVD silicon carbide was an adaptation of a second generation microelec-

trode array design done by Chris Storment of Stanford University.

3.3.1 Working Electrode

The balancing considerations in sizing the working electrode were that it be small enough

to exhibit hemispheric diffusion profile behavior, large enough to fabricate repeatably and

robustly, and spaced far enough apart that diffusion spheres from each electrode did not

overlap.

Once reaching steady state, the diffusion thickness can be calculated from

δss =2πr2nFD0C

∗0

iL(3.6)

where

r is the radius of the electrode,

D0 is the diffusion constant of the species,

C∗0 is the bulk concentration of the oxidized species in solution, and


iL is the limiting current for the electrode.

For a microelectrode, the limiting current of the electrode is

iL = 2πrnFD0C∗0 . (3.7)

Substituting and simplifying,

δss = r. (3.8)

.

The diffusion thickness is equivalent to the radius of the hemispheric electrode under

steady state [82] [96]. In a previous design, it was demonstrated by chronocoulometry

that for all time scales of interest, there was no significant overlap of diffusion spheres for

electrodes spaced 300 mm apart. In the design explored here, working electrodes were spaced

400 mm apart [84].

3.3.2 Reference Electrode

When it becomes necessary to establish a precise, absolute voltage within an electrochemical

cell, a reference electrode is used. With a reference electrode, the junction potential between

the electrode and the solution is stabilized to a known value by keeping the reference elec-

trode immersed in a solution of known concentration (typically saturated). This provides

an electrochemical “ground” into the solution. A diagram of a saturated calomel reference

electrode is shown in Figure 3.7.

Electrical contact is made to the test solution by bringing the reference and test solu-

tions into contact with each other across a glass frit. This is typical of reference electrode

configurations. A more thorough discussion of reference electrodes can be found in Brett

and Oliveira Brett [97].

The true reference electrode in microelectrochemistry is still a technical challenge. It

is often impractical to maintain a separate chemical environment within the microfluidic

system. Additionally, because electrical contact to the analyte is often made by bringing

the two solutions (test and reference) together across a glass frit, contamination of the test

solution is a problem. This is more of a problem on a microscale where small amounts of

contaminants can have a large impact on concentrations.

3.3. ELECTRODE DESIGN 59

Figure 3.7: Saturated calomelelectrode (after Brett andOliveira Brett) [97].

One method which can be used to overcome this lim-

itation is to use a pseudo-reference. A pseudo-reference

is an electrode which has demonstrated a relatively sta-

ble interface potential within a specific test solution, but

it is not kept within a separate chemical environment in

equilibrium. Consequently a pseudo-reference electrode

is more likely to exhibit small amounts of drift over the

course of an experiment.

It has been shown that iridium can function as a

pseudo-reference for certain electrochemical applications

[98]. For ease of fabrication, the reference electrode was

patterned from iridium at the same time as the working

and counter electrodes. It would be possible, however, to

use a separate mask to pattern the reference electrode,

enabling the use of other materials. Another material that has been used in both reference

and pseudo-reference electrodes is a matrix of silver and silver chloride. For some of the

experiments in this work, a separate, solid silver-silver chloride electrode from Invivometrics

(Invivometrics, P.O. Box 397, Healdsburg, CA 95448) was used.

3.3.3 Counter Electrode

The purpose of the counter electrode is to source or sink the current required to produce the

desired voltage or current at the working electrode. The material of the counter electrode

should be relatively inert and it should be sufficiently large in surface area that the expected

current density induce a negligible voltage drop across the interface. Iridium makes a superb

material for this application. The counter electrode was sized to be 104 times the surface

area of the combined working electrodes to minimize the interfacial impedance.

An individual electrode set is shown in Figure 3.8.


Figure 3.8: Microelectrode array

3.4 Experimentation

3.4.1 Optimization of Plating Procedures

Before beginning further experimentation, a procedure was developed to improve the robust-

ness and repeatability of the mercury plating process. The reliability of SWASV depends

foremost on the quality of the Hg hemispheres plated on the probe. With the criteria of

this procedure, a poorly plated microelectrode array will be detected and re-plated before

it leaves the plating solution.

Unless otherwise noted, all analyses were carried out in a 0.1 M solution of perchloric

acid. Dilutions of standards for inductively coupled mass spectroscopy (ICPMS) were added

to the perchloric acid to achieve the desired concentrations of the analytes in question (SPEX

Industries, Edison, NJ). SWASV scans were run at 60Hz, from -0.1V to -1.2V relative to a

solid Ag/AgCl pseudo-reference electrode from Invivometrics. The step height was 25mV,

the step increment was 5mV.

In the optimized Hg plating procedure, the microelectrode array is cleaned by dipping

the tip in concentrated nitric acid for 5min. The tip is then rinsed thoroughly in deionized

(DI) water. To achieve reproducibility, the array is pre-conditioned electrochemically in a

Hg plating solution of 8mM mercurous nitrate HgNO3 in 0.1M perchloric acid.

• The electrode is scanned between -0.5 V and 0.5 V vs. Ag/AgCl at least 10 times at

a scan rate of 1V/s.

• A sharp stripping wave is observed with a half-wave width of about 25mV. Repeat

3.4. EXPERIMENTATION 61

scanning or discard the array if the stripping wave remains broad (e.g., a half-wave

width larger than 50mV). A continous broad stripping wave indicates a failed array.

• The probe is then plated with Hg at -0.45V until 100 mC of charge is accumulated.

• The plated Hg is then stripped by applying a linear potential scan from -0.5V to 0.5V.

Charge under the wave is integrated and should be greater than 40 mC. It is necessary

to repeat electrochemical pre-conditioning from the beginning if the resulting stripping

charge is less than 40 mC.

The effect of potential cycling in an acid has been well established [99] to grow a stable,

hydrated iridium oxide layer on the surface. The stability of this layer has led to its use as

a pH sensor [100] [101].

Hg plating efficiency was determined by the ratio of integrated charge under the stripping

wave and the deposition wave. This ratio varies with the conditions of the electrolysis. A

maximum of about 40% was achieved in a degassed electrolyte with a clean electrode. It re-

mained constant regardless of the deposition potential and the amount of mercury deposited.

A stripping wave of 98-110 mC was observed if a microelectrode array was plated with 260 mC

of mercury. Well-formed mercury hemispheres were observed microscopically. By contrast,

if the electrode was not clean or was defective, the stripping charge was significantly less

than 100 mC.

The plating efficiency was confirmed using optical interferometry. It has been docu-

mented that gravity does not play a significant role in the shape of microlectrodes until

the diameter of the droplet approaches 2.4±0.2mm [102]. Consequently, the mercury hemi-

sphere can be assumed to be without any asymmetric distortions. Profiles of three separate

hemispheres plated with 260 mC are shown in Figure 3.9. Integrating the area under each,

the volume of the hemisphere is 381 mm3 or 0.38 pl ± 0.14%. The charge required to plate

out this volume from Hg+2 is 99 mC, or 38.1%.

ICPMS was also used as a third method to analyze the amount of mercury plated. 200 ml

of concentrated HNO3 was used to dissolve the mercury on a good electrode as observed

under the microscope. The solution was diluted to 10ml and then sampled by ICP-MS.

The average concentration of 10 ppb corresponds to an equivalent total of 100 ng of mercury

which is equivalent to 0.37 pl per working electrode which agrees with the previous data to

within 2.7%.


Figure 3.9: Optical interferometry scans of plated mercury hemispheres

From these results it can be concluded that using 100 mC as the required criteria for the

charge under the stripping wave is a strong indication that the plating successfully achieved

20 semi-spherical drops of mercury on the microelectrode array. Because plating efficiency

remains a constant, this number can be scaled accordingly if the desired size of the mercury

hemisphere is changed.

The variation of microelectrode array performance was well controlled by this optimized

mercury-plating procedure. With this procedure, the size of the mercury hemispheres on

the array was reproducible. Eight separate tests have been performed to strip the mercury

hemispheres from a probe deposited using the optimized procedure. The stripping charge

was used to calculate the size of the mercury hemispheres. The variation found was within

2%.

3.4.2 Nafion®

Nafion® is a sulfonated tetrafluoroethylene fluoropolymer from DuPont (DuPont, Wilm-

ington, DE). Membranes can be made from dispersions with a fluoropolymer (Teflon® )

backbone, which are permselective for cations. These membranes are not only chemically


resistant but can afford some degree of protection to the underlying materials by preventing

damaging anions from reaching the mercury surface without interfering with the transport of

the metal cations. One source of degradation is the fouling of the plated mercury electrode.

While the underlying substrate and iridium electrode are very robust, the mercury hemi-

sphere is more susceptible to attach. Chloride ions, for example, will react with mercury

and foul the electrode. These are common in many applications of interest. This property

of selective transport has been used previously to improve the performance of iridium and

silver/silver chloride electrodes for pH sensing [103].

To test Nafion®’s ability to protect against mercury degradation, probes were dip-coated

in a 5 wt.% solution of Nafion® in ethanol. Probes were then air dried, followed by a 12 h

anneal at 80 °C. Trials were run in test solutions that were then spiked with 100 ml aliquots

of 0.1M HCl to a concentration of 1mM. Comparisons of scans before and after spiking for

bare mercury and Nafion®-coated probes were examined.

Results with Nafion® were inconclusive. Mercury plating was successful on coated

probes (i.e. through the coating). Results comparable to uncoated probes were obtained for

scans of 50 ppb Cu+2 in both HF and perchloric acid. However, when an electrochemical

cell with 50:1 HF was poisoned with 1mM HCl, the Nafion® coating peeled and flaked off

the surface after roughly 18 hours of exposure. These flakes did not dissolve in solution,

indicating that the HCl attacked the interface between the Nafion® and the probe. More

work would be necessary to improve the quality of the coating for it to be of use with the

chemistries that were tried.

3.4.3 Element Screening

An investigation of the metalic elements suitable for analysis by SWASV was carried out.

Eighteen common elements were tested: Al, As, Bi, Cd, Cr, Cu, Ge, In, Mg, Mn, Ni, Pb, Pt,

Sb, Sn, Ti, Tl and Zn. Tests were done under a standard set of conditions: 15 minute pre-

concentration at -1.2V vs. Ag/AgCl in 50 ppb single element solution. The solutions tested

were 0.1M of perchloric (HClO4), nitric (HNO3), sulfuric (H2SO4), phosphoric (H3PO4),

hyrdochloric (HCl) acids and a 50:1 solution of HF.

Results are summarized in Table 3.3. Among the eighteen elements, only Bi, Cd, Cu,

Pb, Sb, Tl and Zn showed positive responses in the 6 types of diluted process chemicals and

are therefore suitable for the ASV technique.


Table 3.3: Element screening by SWASV

Element 0.1MHClO4

0.1MHNO3

0.1MH2SO4

0.1MH3PO4

0.1MHCl

50:1HF

Al - - - - -As - - - - -Bi - Cd Cr - - - - -Cu Ga -Ge - -In - G# - -Mg - - - -Mn - - -Ni - - - -Pb Pt - -Sb - G# G#Sn - - - - G#Ti - - - -Tl -Zn

- DetectableG# - Detectable but low sensitivity- - Not detectable

3.4.4 Operation in HF

The ultimate goal of this project was to demonstrate the surviveability in a media that would

not have been possible without the addition of PECVD silicon carbide. 50:1 hydrofluoric

acid (HF) was chosen because of previous failures with PECVD silicon dioxide and silicon

nitride and the interest in monitoring semiconductor processing fluids. Normal probes that

have been insulated with some form of silicon dioxide have a very limited lifetime in HF.

The etch rate of a densified low-temperature oxide (LTO) in 50:1 HF has been reported as

8.7 nm/min [4]. An electrode array with 1 µm of oxide would last just under 2 h before the

metal traces would be exposed.

As a first demonstration, the measurement-to-measurement variation was characterized

by performing repeat analysis using the same Hg-microelectrode array in every ten minutes


in a 50:1 HF solution. The solution was continuously stirred to ensure homogeneity. At the

preconcentration step, a -1.2V vs. Ag/AgCl was applied for 4minutes. It was followed by a

square wave anodic potential sweep from -1.2 V to -0.1 V. The frequency of the square wave

signal was 60Hz. The probe was allowed to rest for 5min before the start of another cycle

of analysis. After one hour, the solution was spiked with 50 ppb Cu+2. After approximately

11 h of continuous operation, the solution was spiked again with an additional 10 ppb Cu+2.

The initial background charging current observed was 17 nA. After the spiking to 50 ppb,

the current showed an immediate increase to approximately 260 nA. This corresponds to

about 4.84 nA/ppb. After the additional spike the current jumped to about 313 nA, corre-

sponding to 4.93 nA/ppb. The stripping current was stable and repeatable with a variation

of ±1.8% over the time period examined. The data from the trial is plotted in Figure 3.10.

Figure 3.10: Repeatability of ASV analysis over 70 hours

A longer reproducibility study was also carried out. Two probes were soaked in 50:1

HF solution for four days. In the beginning of this period, stripping currents in 50 ppb

Cu+2 solution were collected. During the four day period, one probe was allowed to soak

undisturbed in the solution while the other was repeatedly cycled through a SWASV analysis.

Stripping currents after this period were collected under the same conditions. Results are

shown in Table 3.4.


Probe Soaking Condition Stripping Current Initial (nA) Stripping Current Final (nA)A Rest 220 190B Repeatedly Scanned 193 240

Table 3.4: Four-day exposure to 50:1 HF

The probe at rest showed a change of 13.6% while the probe run repeatedly showed a

change of 24%. While it might be desirable to improve the repeatability through development

of more advanced electrochemical techniques, the performance is sufficient for contamination

detection of a process bath. No degradation of the probe itself could be detected, either

from behavior during the analysis or under microscopic examination.

3.5 Summary

The previously published fabrication process for the microelectrode array was successfully

modified to improve the reliability of the thin-film iridium electrodes. The addition of

PECVD carbide as the outer dielectric enabled the successful demonstration for extended

periods of time in 50:1 HF. The plating process was optimized to improve repeatability

and develop criteria for determining the quality of both the microelectrode array and the

plated mercury hemisphere. Basic analysis parameters were investigated and then applied

to a broad range of elements and solutions to show which could be detected by the SWASV

method. The repeatability and viability for several days of exposure in 50:1 HF were demon-

strated.

While the use of the sensor in HF was demonstrated, future work should focus on the

development of an electrochemical system to practically deploy the sensor into an applica-

tion. Such a system would include the electronics (potentiostat, control systems) and some

form of sampling such as a flow cell. Additionally, modifications to the classical technique

of SWASV may be necessary to improve the repeatability and robustness of the system.

Chapter 4

Microfluidic Gaskets

Introduction

In developing a microfluidic system for a particular application, a designer might wish to

choose and control the materials with which the fluids of interest come into contact. It

may be that a reaction can be altered by certain surfaces. It could also be that chemicals

present can attack or corrode components of the fluidic system if they are not suitably

protected. To broaden the usefulness of microfluidic systems in general, a technique of

coating channels during fabrication was conceived and developed. This work focused on

making them chemically inert, but the approach has potential in other applications (e.g.

chromatography, electrophoresis, etc.) where the role of the channel surface is more involved

with the function of the device.

4.1 Self-sealing Microfluidic Channels

An initial approach to achieve a chemically resistant microfluidic channel relied on a fab-

rication process that was both coating and self-sealing in the same process step. The idea

was originally developed by Nick Mourlas at Stanford University. An oxide mask was used

during an isotropic SF6 plasma etch. The etch undercut the mask and defined the geometry

of the channel’s cross section. A thin film was then deposited (PECVD silicon carbide in this

case). The deposition process produced a film conformal enough that the the interior walls

of the channel were coated before the channel was sealed. A cross-section of the process flow

is shown in Figure 4.1.

67

68 CHAPTER 4. MICROFLUIDIC GASKETS

(a) Silicon channel is etched isotropically un-der an oxide mask

(b) PECVD silicon carbide is deposited untilwalls are coated and channel is sealed.

Figure 4.1: Self-sealing microfluidic channel process

The size of the mask opening is critical for proper coating and sealing. If the channel

is wider than 4 mm it is difficult to seal with a PECVD film; however, too narrow a mask

opening results in a channel that is sealed before a sufficiently thick conformal coating is

deposited on its internal surfaces. Figure 4.2 shows a channel that was etched with SF6

for 100minutes, with a flow rate of 150 sccm of SF6, at a pressure of 100mTorr, and power

of 550W in a Drytek D-100 plasma etcher with six electrodes of 232 cm2 each. A 3.8 mm

PECVD silicon carbide film was then used to coat and seal the channel. After dicing, a

1minute etch in 49% HF at room temperature and 1minute in 45% KOH at 80 °C were used

to highlight the SiO2/SiC/Si interfaces. The coating was nominally conformal; its minimum

thickness was approximately 200 nm. The fact that the channel wall deposition rate was

significantly smaller than the surface deposition rate dictates that a thorough understanding

of the deposition characteristics will be necessary to identify the dimension limits to produce

conformal coatings with sealed channels.

Self-sealed channels proved to be an impractical exercise. The geometry of closed chan-

nels is severely limited to sizes that can be anisotropically etched under a gap between 3

and 4 mm wide. The limitation on size and delicate structure of the top membrane makes

a practical solution for coupling to the channel difficult. Also, the region where the seal

comes together is very narrow and likely to create a dead volume fluids flowing through

the channel. For these reasons, another solution to making chemically inert channels was

developed.

4.2. GASKET PROCESS 69

Figure 4.2: The image on the left shows a cross-section of a self-sealing microfluidic channel.The close up on the right shows a conformal coating in the corner of the channel.

4.2 Gasket Process

Existing approaches to fabricating fluidic channels typically involve patterning into a sub-

strate (silicon or PDMS is commonly used) and then bonding with a cap wafer (e.g. Pyrex®).

Attempts were made to bond PECVD silicon carbide by both anodic and fusion bonding.

None of these attempts were successful. Consequently, a more versatile method was devel-

oped to fabricate a fluidic channel where all the materials present would be inert relative to

a broad range of chemistries.

Figure 4.3: Concept of a microfluidic gasket

The process first involved depositing PECVD silicon carbide on the etched channels and

surface of the wafer. Using dry film lithography (discussed next), a process flow was devised

that enabled the patterning of the carbide near the edge of the channel, leaving the coating


intact. Bonding around this patterned feature with a mating surface formed a microfluidic

gasket, sealing the channel by compression. A conceptual diagram is shown in Figure 4.3.

The gasket process was adapted for use in a flow-through total organic carbon sensor that

is discussed in more detail in Chapter 5.

One of the challenges in the proposed process flow is the required step of patterning back

the coating within a specified distance of the channel. Several alternatives were investigated

including spray film resists, CVD photoresist, and alternative coating techniques. A process

flow that took advantage of a dry film resist appeared most promising and was eventually

chosen as the best approach.

4.2.1 Dry Film Lithography

Previous work had been published that successfully demonstrated the use of dry film pho-

toresist in MEMS [104]. After review of the available materials and discussions with Martin

Hill of DuPont’s technical support group (E.I. DuPont de Nemours & Co., Research Trian-

gle Park, NC), two different formulations of Riston® were evaluated: SF125 and CM206.

CM206 was chosen as the best candidate because of its lower thickness. At 15 mm, it had

the potential to achieve the highest resolution of the dry film resists available. Riston® is

designed for automated printed circuit board manufacturing. Adapting it to MEMS fab-

rication required a number of modifications and optimizations from the standard process

flow.

Figure 4.4: Trilayer Riston® film

4.2.1.1 Lamination

Riston® is a trilayer laminated photosensitive film. The photosensitive layer is sandwiched

between two layers of transparent mylar as shown in Figure 4.4. In typical PC board


fabrication, the bottom layer of mylar is removed and the Riston® is exposed through the

top layer of mylar. The lamination, exposure, and development are done in an automated

conveyor system that is unsuitable for wafers. For this work, a standard identification

card laminator was adapted for use in the clean room. To support the wafer as it passed

through the laminator, a custom aluminum wafer carrier was designed with a 250 mm recess.

During the lamination process, the wafer was placed into the carrier. The bottom mylar was

removed from a sheet of Riston® and manually tensioned over the carrier and wafer. The

three layers, aluminum carrier, wafer, and Riston® sheet were then fed into the laminator

together.

Initial results showed that adhesion of Riston® to bare silicon and wafers coated with

silicon carbide was marginal. Films delaminated very easily, often when pulling off the top

layer of mylar. To achieve adequate adhesion, Riston® required heating the substrate either

during or after lamination, and deposition of a thin layer of metal. Because Riston® can

polymerize over 110 °C and the laminator did not have precise temperature control, wafers

were heated on a hot plate at 90 °C for 1min after lamination with a weight applied to the

mylar surface. Choosing a metal layer required screening potential candidates against the

planned processing later in the flow. The etchant used to pattern the adhesion layer must

not damage the Riston®, nor should the developer for Riston® undercut and damage the

adhesion layer. To avoid this potential incompatibility, combinations of Riston®, aluminum,

and chromium, and chromium/gold were tested against different etchants. Based on the

results from these tests shown in Table 4.2, a thin layer of chromium was selected as the

best choice for an adhesion layer.

4.2.1.2 Exposure

The top mylar sheet diffuses light and sets a limit in proximity of the mask to the photo-

sensitive layer of about 25 mm. In an attempt to improve resolution, the mylar was removed

and the mask was exposed in contact with the photosensitive layer. The Riston® stuck to

the mask so tenaciously that even after a long attempt to remove it in various strippers

the wafer could not be removed without damaging the mask. Polyvinyl alcohol (PVA) was

suggested by Martin Hill from DuPont as a spin on barrier layer that would allow intimate

contact without sticking and not interfere with the exposure or development of the film.

Because PVA does not dissolve readily in deionized (DI) water at room temperature,

preparation of a 10% solution required heating the solution to 50 °C for a several hours. Once


Etchant\Material Riston Al Au Cr3% KOH E ECr-141 -0- -0- E

Au Etch2 -0- A3 E -Al-114 E

PRX-1275 E -0-PRS-10005 E -0-

Glacial Acetic Acid F -0-3N HCl D -0-

Table 4.2: Etchant testing on Riston®

A - Attacked incompletely F - Etches fastE - Etches D - Delaminates but not etched

1Chromium Photomask Etchant, Cyantek Corp., Fremont, CA2Gold Etch, Olin Microelectronics Materials, Norwalk, CT3Aluminum is attacked slowly in gold etch by itself. The attach rate increases when incontact with Cr/Au, presumeably due to a galvanic reaction.4Aluminum Etchant, Cyantek Corp, Fremont, CA5Positive Resist Stripper, J.T. Baker, Philadelphia, PA

dissolved, the PVA remained in solution and was stable. No precipitation was observed. To

ensure that there were no particles or insoluble components the solution was filtered through

a 0.45 mm pore size syringe filter before storage.

To apply the PVA solution, the top layer of mylar was removed from the Riston® after

lamination on the wafer. The 10% PVA solution was spun on at 4000RPM for 20 s and

then baked for 1min at 90 °C. Once dried, the wafers coated with PVA could be exposed

in contact mode without sticking to the mask. Comparison of two test structures (figures

4.5 and 4.6) with and without PVA show an improved ability to define higher resolution

structures by exposing in soft contact with the PVA coating.

Based on requirements listed in the datasheet, the wafers were exposed for 1.5 s to

15mW/cm2 at 365 nm. Since radicals can continue to react in the film for 10-15 minutes after

exposure, they were baked post-exposure for 15min at 90 °C to allow the photochemistry

time to reach completion.


Figure 4.5: Serpentine with no PVA Figure 4.6: Serpentine with PVA

4.2.1.3 Development

In the first series of experiments, the wafers were rinsed in DI water to remove the PVA prior

to development, and to rinse away the developer after development. Exposure to DI water

caused Riston® to swell around the edges and fine features to delaminate. The datasheet

calls for rinsing in hard water, defined as containing an equivalent of 150-250 ppm CaCO3,

making it likely that the root cause behind this effect was the relatively high osmotic pressure

of DI water on the film. Adding 150 ppm of MgSO4 to DI water for rinsing was successful

at eliminating the swelling. After several more trials, no adverse affects were observed by

going straight into the developer after exposure and bake without rinsing the PVA, and the

first rinse was eliminated from the process flow.

Figure 4.7: Edge swelling after rinse in DI wa-ter

Initial trials using a tank develop proved

unsatisfactory. Development proceeded

slowly and unevenly. In most cases the pat-

tern failed to completely resolve. In typical

applications, the Riston® is passed through

an automated conveyor that sprays on de-

veloper under pressure. The datasheet calls

for a spray pressure of 140 kPa to 210 kPa.

Evidently the mechanical abrasion of the

pressured developer is a necessary compo-

nent in the removal of the exposed resist


material. To achieve satisfactory results, a custom pressure spray developer was designed

as shown diagrammatically in Figure 4.8.

Figure 4.8: Customized pressure spray developer

While the spray development was an improvement over the tank development, residues

and particles were still observed in some open areas and on the edges. The pressure in the

hand system was limited and possibly insufficient to properly clear exposed areas. Fortu-

nately an oxygen plasma descum proved very effective at improving the definition of features

and removing any lingering residue. Wafers were cleaned up using an MRCModel 55 reactive

ion etching system (Materials Research Corporation, Orangeburg, NY), 25 sccm, 25mTorr,

100W, for 15min. Images of a test feature before and after descum are shown in Figure 4.9.


(a) Before O2 descum. (b) After O2 descum.

Figure 4.9: Effect of descum on feature cleanup.

4.2.1.4 Results

An optimized process flow for Riston® is described in Table 4.3. Images of Riston liftoff

structures after an optimized lithography process are shown in Figure 4.10.

Table 4.3: Optimized process flow for Riston liftoff

Step CommentDeposit 25 nm Cr Adhesion layer for Riston®

Laminate Riston® Bottom layer mylar is removedBake hotplate at 90 °C for 1 min Promotes adhesionRemove top mylar Permits contact lithographySpin on 10% PVA Prevents sticking to maskBake hotplate at 90 °C for 1 min Dry PVAExpose 1.5 sec, 15 mW/cm2 at 365 nm Datasheet quotes 12-48 mJ/cm2 [105]Bake 90 °C for 15 minutes Allow photochemistry to work to completionSpray develop in 1 wt% Na2CO3 Spray is requiredRinse in hard water (150 ppm MgsSO4) Rinsing in DI water causes swellingO2 Descum Clear residue and clean up edges

4.2.2 Process Flow

Developing a dry-film resist process enabled the patterning of the gasket structure in the

proximity of the microfluidic channel. An overview that shows how the dry-film resist fits

into the process flow is shown in Figure 4.11.


Figure 4.10: Patterned Riston® in proximity to channel


Figure 4.11: Gasket process flow

Initially a thermal oxide is grown that serves to protect the wafer bonding surface during

processing. Because silicon carbide is etched in an SF6 plasma, it also functions as an etch

stop and prevents excessive roughening of the bond surface when the gasket is patterned.

This oxide will remain under the gasket on the lip of the channel, so a relatively thin oxide

of 75 nm was used to avoid making the gasket overly thick.

The microfluidic structure is then defined by DRIE etching in silicon. The target depth

of the channels on the front side was 50 mm. The minimum width of the channels was

limited to 50 mm. This set the aspect ratio of the channel to less than 1:1 ensuring that

there would be no difficulty getting good sidewall coverage of the PECVD silicon carbide or

the aluminum etch mask. The backside etch was used to establish input and output ports.


The etch depth was sufficient to connect to the microfluidic channel while leaving sufficient

material for the fluidic coupler.

The gasket material, 350 nm of PECVD silicon carbide in this case, was deposited on

both sides of the wafer using 2000 sccm of SiH4, 1440 sccm of CH4 at 1300mTorr with 3 s

of high-frequency power and 3 s of low-frequency power both at 100W. The trials with the

self-sealed channels (Section 4.1) demonstrated that good coverage on the side walls would

be achieved.

After the carbide, a 35 nm layer of chromium was deposited to improve the adhesion of

the dry-film resist as described in Section 4.2.1. After the process for the dry-film resist, the

chromium layer was removed in the exposed areas by etching for 5 min. At this point, the

wafers were ready for a deposition of the aluminum mask layer.

In order to ensure good sidewall coverage, the sputter deposition of the aluminum mask

was done with a heavy DC bias. A DC bias during deposition promotes substrate sputtering

and redeposition which promotes conformality [106]. To further ensure that the carbide

coating would be protected by the aluminum mask, an extra thick layer was deposited.

Wafers were deposited at Lance Goddard Associates (Foster City, CA); 2 mm was deposited

on the front side, 1.5 mm on the back side.

The mask was lifted off with PRX-127 which had been tested to be effective at removing

Riston® without damaging the aluminum. For fine features, a bi-layer liftoff structure is

preferred, but because the edge of the gasket is not a critical, any roughness or jagged edge

was acceptable. A SEM of the aluminum mask is shown in Figure 4.12.

Figure 4.12: Shown are SEM’s of aluminum mask structure: top view, upper corner, andlower corner.


Once the Riston® was removed, the underlying chromium adhesion layer was removed

using CR-14. The remaining aluminum mask protected the carbide gaskets and the coating

in the channel and ports. The carbide was etched in in Drytek Model D-100 reactive ion

etcher (Drytek, Inc., Wilmington, MA) using SF6 at 50 sccm, C2ClF5 at 70 sccm, 100mTorr,

55W. The etch time was 5:30min.The final step is a short etch in 6:1 buffered oxide etch

(BOE) to remove the underlying oxide etch. The wafer is now ready for anodic bonding

with the glass substrate. The patterned gasket is shown in Figure 4.13.

Figure 4.13: SEMs of carbide gasket

4.2.3 Glass wafer

The process for the glass wafer is relatively simple and shown in Figure 4.14. PECVD silicon

carbide was deposited on a double-side polished Pyrex® 7740 wafer in a mirror image to the

gasket pattern on the silicon with the microfluidic channels. The two wafers were bonded

at 350 °C with 600N of force, ramping to 1200V in 100V increments. A cross-section of the


resulting structure is shown in Figure 4.15.

Figure 4.14: Process flow for glass wafer

Figure 4.15: Cross-section of gasket structure

4.3 Summary

A gasket structure was proposed that enables the fabrication of microfluidic channels with a

uniform coating. Several processing challenges were overcome related to lithography in the

region of the channels. The gasket process was adapted for use in a total organic carbon

sensor and is discussed in Chapter 5.

Chapter 5

Total Organic Carbon Sensor

Introduction

In several diverse areas there is a need for a non-specific total organic carbon sensor (TOC).

In semiconductor and pharmaceutical manufacturing, the specific analytes are well-known

because the possible constituents are tightly controlled. In this instance a non-specific TOC

sensor can have benefits over a slower, more expensive chemical-specific measurement tech-

nique. Examples are waste water monitoring for photoresist or pharmacological precursors.

In environmental monitoring where the specific nature of the contaminant is not known, a

non-specific TOC sensor can monitor for an entire class of contaminants without the need to

know their exact chemistry. A specific example is the use of TOC analysis to determine the

amount of bacteria in ground water. An overview of TOC sensing can be found in reports

that have been published by the U.S. Environmental Protection Agency [107,108].

Conventional TOC measurement techniques involve a two-step process. In the first

phase, the organic carbon is oxidized. The objective is to convert all organically bound

carbon into carbon dioxide (CO2 ). By converting all carbon into this form, detection

method can be used in the second phase to detect the CO2. Because all carbon is oxidized

to this form regardless of its precursor, the detection method can be optimized for a single

analyte.

Typical benchtop models are shown in Figure 5.2. One approach in this class of TOC

sensors is to oxidize the organic carbon via high temperature combustion (e.g. 1200 °C),

and then pass the resulting CO2 into a cell where a change in conductivity is measured.

The cost of these units is typically $18,000 to $24,000. In some applications it would be

81

82 CHAPTER 5. TOTAL ORGANIC CARBON SENSOR

Figure 5.1: Current TOC sensing methodology

desirable to monitor in-situ to prevent the accidental discharge of harmful chemicals, or avoid

contaminating a drinking water supply. These types of applications present a challenge for

the conventional units because of power, size, and cost requirements.

Figure 5.2: Sievers 900 benchtop TOC sensor (General Electric Company, Schenectady, NY)

Fortunately these are three areas in which a microfluidic approach to TOC sensing would

have several advantages. The size and cost advantages are inherent. The power requirement

can be addressed by resorting to an ultra-violet (UV) catalyzed oxidation along with a

differential pH sensing approach, described next. This greatly reduces the power required

to the point where a hand-held unit that would run for a reasonable time from a battery

source would be feasible. Sensing in remote locations would be possible without the need for

returning samples to a lab. This would eliminate the delay in results and the contamination

risks that bringing samples back for analysis entails.

5.1. THEORY 83

Thanks are due to Dr. Dolf Van der Heide for assistance with the FEA modeling.

5.1 Theory

To be rigorous, pH was originally defined in 1909 [109] as

pH = −log10([H+])

(5.1)

where [H+] is the molar concentration of the hydrogen ion. Often the hydronimum ion

[H3O+] is used instead if [H+], acknowledging that the hydrogen ion can protonate a water

molecule. pH was later defined to include the relative activity of hydrogen ions as

pH = −log10(mHγH

m0

)(5.2)

where

• mH is the concentration of the hydrogen ion as molality,

• gH is the molal activity coefficient, and

• m0 is the standard molality.

The activity coefficient takes into account the fact that in solution there is an interaction

between ions that can cause them to exhibit an effective concentration that deviates from

an ideal solution (where there is no interaction). Except for concentrated solutions, the

activity coefficient of hydrogen ions is essentially 1. To simplify calculations, equation 5.1

will be used.

5.1.1 Concept

The approach taken for a microfluidic, flow-through TOC sensor is to first mix the analyte

with a known volume and concentration of oxidizing species. The oxidizer should have a

reactivity that readily oxidizes the organic carbon in the presence of ultra-violet light, but

does not react without it. The analyte stream is split into two channels. One is exposed to

UV light, the other is shielded. The two streams pass over matched pH sensitive electrodes

and the pH change is sensed differentially. A conceptual drawing of the microfluidic structure

is shown in Figure 5.3.


Figure 5.3: Conceptual drawing of TOC sensor

5.1.2 Analytical Model

Organic molecules can be oxidized in the presence of oxygen to form carbon dioxide (CO2).

In an aqueous environment, CO2 reacts with water to form carbonic acid (H2CO3), which

causes a pH change in solution. The total organic carbon (TOC) sensor relies on sensing

this pH change to determine the amount of organic carbon originally present in solution.

Unlike commercially available TOC sensing systems that rely on measuring an absolute

change in pH, the differential TOC measures the difference in pH between two matched

channels. In one channel, ultraviolet light drives the oxidation of the organic carbon. The

other channel, the reference channel, is shielded from the UV light, and the oxidation of

the carbon does not proceed during the travel time of the analyte through the system. The

same advantages that apply to a differential amplifier are present here: high common mode

rejection and large possible gain.

Ideally, the intensity of the UV light should be high enough that all forms of organic

molecules are completely oxidized. The oxidizer should be sufficiently weak that without

the UV catalysis it does not react significantly with the hydrocarbon.

In its simplest representation, the complete oxidation of a simple alkane hydrocarbon

form can be shown as

CNH2N+1 + (3N + 1)O −−−−→ CO2 + (N + 1)H2O (5.3)

The above reaction is an oversimplification and can involve several different intermedi-

ates depending upon the oxidizing agents or catalysts present in solution. However, because

5.1. THEORY 85

intermediates are generally ephemeral in nature and the end result of the carbon oxida-

tion is the same, this oversimplification is useful for generating a predictive model for the

performance of an in-line TOC sensor.

Carbon dioxide is readily soluble in water, roughly 39mM/l at 20 ºC. A percentage of

the resulting CO2 reacts with water to form carbonic acid:

CO2 +H2Okh−−−−−−−−−− H2CO3 (5.4)

Carbonic acid is a diprotic acid and dissociates in two stages with two distinct dissocia-

tion constants.

H2CO3k1−−−−−−−−−− H+ +HCO−

3

k2−−−−−−−−−− 2H+ +CO−23 (5.5)

where k1 and k2 are the dissociation constants defined as

k1 =[H+]

[HCO−

3

][H2CO3]

(5.6)

and

k2 =[H+]

[CO−

3

][HCO−

3

] (5.7)

In order to derive a model for the pH response of the TOC system, it is also necessary

to take into account the following relationships:

koxCT =[CO2] +[CO−2

2

]+[HCO−

3

]+ [H2CO3] (5.8)

[H+]×[OH−] = kw (5.9)

[H+]=[OH−]+ [HCO−

3

]+ 2

[CO−2

3

](5.10)

where

CT is the initial total concentration of organic carbon,

kox is the percentage of carbon that oxidizes, and

kw is the dissociation constant of water.


Equation 5.10 is the charge balance equation and must be satisfied to ensure charge

neutrality. Equations 5.4 through 5.10 comprise a system of six equations with six unknowns

and all degrees of freedom are satisfied to derive a solution.

Combining equations 5.4, 5.6 and 5.7 with equation 5.8 yields

[CO−2

3

]= k1k2

[H2CO3]

[H+]2(5.11)

Substituting equations 5.6 and 5.11 into equation 5.8 yields

koxCT =[H2CO3]

kh+

k1k2 [H2CO3]

[H+]2+

k1 [H2CO3]

[H+]+ [H2CO3]

= [H2CO3]

([H+]

2+ k1k2kh + k1kh [H

+] + kh [H+]

2

kh [H+]2

)

Rearranging and solving for [H2CO3],

[H2CO3] = koxCTkh [H

+]2

[H+]2 + kh [H+]2 + k1kh [H+] + k1k2kh(5.12)

Substituting equations 5.12, 5.6, and 5.11 into the charge balance equation 5.10 yields

[H+]− kw

[H+]−(

k1[H+]

+2k1k2

[H+]2

)[koxCTkh [H

+]2

[H+]2 + kh [H+]2 + k1kh [H+] + k1k2kh

]= 0

[(1 + kh)

[H+]4

+ k1kh[H+]3

+ (k1k2kh − kw − kwkh − k1khkoxCT )[H+]2

−(kwk1kh + 2k1k2khkoxCT )[H+]−k1k2kwkh

]1

[H+]2 + kh [H+]2 + k1kh [H+] + k1k2kh= 0

Because we are only interested in non-zero, positive, real roots of the equation, the

numerator must be zero.

5.1. THEORY 87

(1 + kh)[H+]4

+ k1kh[H+]3

+ (k1k2kh − kw − kwkh − k1khkoxCT )[H+]2

− (kwk1kh + 2k1k2khkoxCT )[H+]− k1k2kwkh = 0

At 20 ºC, k1= 4.2 x 10−7, k2= 4.8 x 10−11, and kh=1.3 x 10-3. The second dissociation

lags four orders of magnitude behind the first and is generally disregarded for the pH effect

of dissolved CO2. There is now sufficient information to predict an expected pH shift due

to oxidation of organic carbon within a TOC sensor. The results are shown in Figure 5.4.

Figure 5.4: Theoretical pH shift due to TOC concentration

From the model, the method of pH sensing in an aqueous environment is limited by

the equilibrium between [CO2] and [H2CO3] and the dissociation of water. The amount of

[H2CO3] has to be above the ambient concentration of [H+].


5.2 Benchtop Model

Before fabricating a microfluidic version, a benchtop version of the differential, flow-through

TOC sensor was fabricated to validate the differential approach and perform preliminary

tests. Quartz tubes were fabricated with inlets for iridium wire electrodes. This provided

a mechanism to evaluate different choices as an oxidizer. An initial candidate, sodium per-

sulfate, acidified the solution and caused a parasitic signal when it dissociated. The second

choice, hydrogen peroxide, proved to be more suitable for UV-photocatalytic oxidation. This

effect was not observed. Experimental parameters like UV intensity and flow speed, initial

carbon content, as well as hydrogen peroxide concentration were investigated to provide a set

of experimental conditions that would behave as expected. In this way, first concentration

dependence measurements were carried out for ethanol dissolved in DI water.

5.2.1 Experiment

The solution was pumped through the tube using a peristaltic pump. The flow speed was

approximately 1.3ml/min as determined by the droplet formation rate and frequency at the

outlet. The passage through the tube was above 10 min. The sample solution was irradiated

using two Raypen UV mercury lamps.

Sample solutions were prepared by sparging DI water with nitrogen to remove oxygen

and carbon dioxide from the solution. Next, 3.5ml/l hydrogen peroxide was added as the

oxidant. Varying concentrations of ethanol were added to separate beakers and the tube

moved from beaker to beaker to inject the samples into the system.

5.2.2 Results

The results show a logarithmic response characteristic of the sensor typical for pH detection

schemes. The electrode response was greater than expected. Typical iridium response from

literature is -60mV/pH [110], though responses as high as -90mV/pH have been reported

[111]. The value of -90mV/pH was used in the model to compare with the data.

5.2. BENCHTOP MODEL 89

Figure 5.5: Differential voltage in benchtop TOC sensor

Figure 5.6: Response of benchtop model compared with model


5.3 Microfluidic Version

5.3.1 Design

A labeled image of the layout is shown in Figure 5.7. The fluidic channels were 30 mm

wide 50 mm deep. The serpentine loops were designed to provide additional travel time

in the channel to react with the oxidizer while being exposed to UV light. Three separate

electrodes were patterned in the channel to determine the effect of surface area on the sensor

response and provide opposing electrodes for additional experiments. The holes around the

input and output ports were designed to accommodate a microfluidic coupler (courtesy of

Nick Mourlas) [112].

Figure 5.7: Microfluidic TOC sensor

The flow in the system was expected to be laminar. This can be determined analytically

from a calculation of the Reynolds number. The Reynolds number is a useful index for any

flow system that can be used to determine whether or not a flow is laminar or turbulent. It

is defined as

Re =LV ρ

µ

where

5.3. MICROFLUIDIC VERSION 91

• L is a dimensional parameter equal to 4AP where A is the cross-sectional area and P is

the perimeter,

• V is the average velocity through the channel,

• m is the fluid viscosity,

• r is the fluid density, and

• r is the density of the liquid

A typical channel size used in this design would be 30 mm wide and 50 mm deep. Turbulent

flow requires a Reynolds number of approximately 2000. For aqueous solutions flowing at a

nominal rate of 1mm/s, the calculated Reynolds number was 0.05, well within the laminar

range.

5.3.2 Fabrication

The TOC process flow incorporates the microfluidic gasket process described in Chapter 4.

It is a two-wafer process. The first wafer is a silicon wafer. An overview of the process flow

is described in Figure 5.8.

Figure 5.8: Process flow for TOC silicon wafer

The second wafer is a glass wafer. The process flow is shown in Figure 5.9. Because

of the challenges in etching through glass, laser ablation was used to etch the majority of


the way through. This greatly reduced the amount of material required for removal at the

back end to clear the holes for the heat stake couplers. The details of the laser ablation are

covered in Appendix C.

Figure 5.9: Process flow for TOC glass wafer

The glass and silicon wafer were patterned with opposing gaskets. They were aligned

and anodically bonded as shown in Figure 5.10. Once the wafers were bonded, the remaining

membrane on the glass wafer was removed mechanically, and the fluidic coupler was heat

staked into the openings. The coupler provided a means to align and seal a glass capillary

into the microfluidic channel.

Micrographs of the final channel are shown in Figure 5.11. The bond around the gasket


Figure 5.10: Bonding flow for TOC sensor.

and the traces was not conformal. Large voided areas were created around the topology.

Some void area was to be expected since the compression seal required the forces created as

the bond is pulled around the protruding gasket. An attempt had been made to reduce this,

particularly around the traces by patterning recesses into the top of the glass, but either

due to misalignment or lack of depth control, the voids were larger than expected.

(a) Gasketed channel. (b) Channel with electrode

Figure 5.11: Gasketed microchannel images

After fabrication, the TOC sensors were mounted on specially designed PC boards and

interfaced to a fluidics test system. An image of a mounted part is shown in Figure 5.12.


Figure 5.12: Image of finished microfluidic TOC sensor

While some devices showed cracking during fabrication, some of them did not and ap-

peared to be in good condition to begin testing. As soon as initial fluidic tests began,

however, cracks in all the devices became evident. It appears from the location of cracks

around the gaskets that the cause of the cracking was the remaining stress in the glass wafer

bonded around the protrusion.

To look at the level of stress more analytically, a finite element model was built and

the stresses due to bonding around the height of the gasket were examined. A correlation

between the maximum stress in the glass and the size of the void surrounding the protrusion

was extracted. The symmetric half model used to simulate the affects of bonding around the

microfluidic gasket is shown in Figure 5.14. Contact surfaces were created on opposing sides

of the bond and pulled together with a progressively stronger force. Critical corners were

filleted to avoid any artifact of a singularity resulting from the finite element method. Several

assumptions were made in order to simplify the simulation of the bond. An initial estimate

of the interfacial force due to electrostatic attraction was taken from the literature [113].


(a) (b)

Figure 5.13: Cracking around gasket

At 300 °C, a voltage of 800V was found to produce an equivalent electrostatic pressure of

2.4 x 106 Pa. Simulations were only run to 300 kPa as this produced sufficient results to

determine that the magnitude of deformation around the gaskets was beyond the generally

accepted value of 100 kPa fracture strength of Pyrex®

Figure 5.14: FEA of anodic bond around gasket

A point was selected in the FEA model sufficiently close to the high stress region of the

simulation to represent the expected maximum stress experienced by the glass deforming


around the gasket. The maximum stress was plotted against the expected distance between

the edge of the gasket and the bond front. As can be seen from the graph of Von Mises

stress vs. void distance that with a combined height of 2 mm, the bond cannot propagate

closer than several hundred microns before exceeding the fracture strength of Pyrex®. This

model is only an approximation because the exact mechanical properties of the glass wafer

at the bonding temperature are unknown. While the bond temperature of 300 °C might

provide some measure of stress relief, the anneal point for Pyrex® 7740 is 560 °C, which was

beyond the temperature limits of the bonding chamber, and above what would be desirable

from a process compatibility point of view.

Figure 5.15: Stress vs. the bonding void around the gasket

5.4 Summary

An application for the microfluidic gasket was proposed that would take advantage of the

chemical resistant nature of the channel coating. A novel methodology was proposed for

sensing the total organic carbon concentration in a flow-though, microfluidic device. The

5.4. SUMMARY 97

theory behind the sensing methodology was explored and an analytical model was developed

for predicting the sensitivity of the technique. A bench-top unit was constructed and the

technique was tested on a macroscopic scale. The data from the bench trial showed reason-

able agreement with the analytical model. A process flow for the microfluidic device was

developed and devices were fabricated. The microfluidic devices showed a common failure

mechanism due to the stress inherent in the device. An FEA model was developed and sim-

ulated to confirm the behavior. Further iteration on a design for the gasketed microfluidic

structure would allow for conditions which do not exceed the yield strength of the materials

used.

The differential pH sensor has potential applicability in any number of analytic reactions

where a pH change is the resulting indicator. The TOC sensor used light shielding to

differentiate the two analyte streams, but microfluidic systems which differentiate based on

the introduction of different reactants can also be envisioned. Specifically for application in

TOC sensing, further work in choosing a candidate for oxidation and a detailed study of the

UV catalyzed oxidation of different analytes is warranted.

Chapter 6

Conclusions and Future Work

The techniques used to deposit and grow materials during microfabrication are many and

varied: sputter deposition, reactive sputtering, e-beam and thermal evaporation, plasma

enhanced and low pressure chemical vapor deposition, epitaxial growth, molecular beam

epitaxy, and atomic layer deposition to name just some. The materials may have been stud-

ied and characterized in their classical form but, as was shown in this work, the properties

of films deposited by different methods may differ significantly. Such differences can explain

the performance of materials such as alumina, which etched unexpectedly, and the PECVD

silicon carbide, which had very different mechanical properties from its stoichiometric cousin.

As was demonstrated, chemical analysis is a valuable tool to understanding the underlying

cause behind these behavioral differences and can be used to optimize materials for their ap-

plication. The amount of Si-Si bonding produced during the PECVD deposition of carbide

is a key factor in determining carbide’s etch resistance, particularly to alkaline etchants such

as KOH. The optimized PECVD silicon carbide produced was resistant to etching in a re-

markably broad range of chemistries and yet remained compatible with standard processing

techniques.

In the area of chemical analysis, the resistance of both PECVD silicon carbide and irid-

ium were successfully demonstrated as tools to extend the range of operation for existing

electrochemical probes. The use of these materials does not necessarily require signifi-

cant modification of electrode designs. The use of an electrode array, which was previ-

ously used for analysis of tap and ground water, was successful extended into more caustic

chemistries. Such capabilities create an exciting opportunity for the development of analyses

in chemistries that have not previously been possible. The results of these experiments show

98

99

that future work may be required to develop the system level support necessary for com-

mercial deployment. Some specific areas include modifications to existing electrochemical

techniques, sampling systems such as flow-through cells.

To incorporate an etch-resistant film into a mechanical transducer such as the pressure

sensor studied, the mechanical properties of the film must be known. The mechanical

characteristics of the PECVD silicon carbide in this work were measured and shown to have

a minimal impact on the fundamental operating parameters of an existing silicon diaphragm

pressure sensor. The observed effects could easily be accommodated in a new design. The

chemical resistance of the coated sensor and package were tested and while the protection

of the diaphragm was successful, the carbide failed to protect the package and bond wires.

Additionally, a point defect was identified as a cause of failure by electrochemical impedance

spectroscopy (EIS). Future work on this particular application should focus on two areas.

First, work on a more advanced deposition system would be required to achieve an optimized

film with fewer defects. Second, it would be desirable to design a packaging approach that

would be compatible with a deposition. The lower temperature limit of 350 °C is challenge

for most conventional packaging materials, and the observed adhesion problems to gold bond

wires also requires some effort to overcome.

The invention of a differential pH approach to chemical sensing shows promise for adap-

tation into microfluidic systems. Though the fabrication attempt of the TOC design was

unsuccessful, the successful demonstration of the technique with the benchtop version and

the elucidation of the failure mechanism by FEA justify future efforts to overcome the chal-

lenges encountered. Possible future approaches include the design of recesses around the

channel or relying on a more stress-tolerant bonding mechanism. Incorporating the chem-

ically resistant carbide and iridium into this structure, as with the microelectrode array,

would extend the range of compatible chemistries for this device.

Future work on the incorporation of materials for environmentally hardened sensors

would benefit from developments in fabrication systems. Examples demonstrated in this

work include the need for temperature control during e-beam evaporation of iridium. Also

the PECVD system used in this work was not optimal for the deposition of carbide. Some of

the problems observed including the deposition of silanacious “dust” and the defects observed

during the EIS studies of corrosion resistance. Future work on the tools themselves would

help transition this technology from the research lab into practical applications.

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App

endi

xA

Silic

onCarbide

DOE

Below

aretherecord

ofruncond

itions

forthedevelopm

entof

PECVD

silic

oncarbidean

dpreparationof

samples

foran

alysis.

Pre

ssure

(mTorr

)

Tem

p

(°C

)

Hi

Fre

q

Pow

er

(W)

Hig

h

Fre

q

Pe-

riod

(s)

Lo w

Fre

q

Pow

er

(W)

Lo w

Fre

q

Pe-

riod

(s)

Dep

Tim

e

(min

)

Dep

Rat

e

(nm

/m

in)

Avg.

Thic

k

(nm

)

St

Dev

.

(nm

)

Avg.

RI

St

Dev

.

Str

ess

(MPa)

C:S

i

Ra-

tio

H%

Etc

h

KO

H

Etc

h

HF

Surf

ace

Res

is-

tivity

Volu

me

Res

is-

tivity

1000

350

75

7.0

150

7.0

12.0

15.8

190.2

0.3

2.6

50.0

03

1200

350

150

--

-12.0

16.9

202.2

6.7

2.4

10.0

08

NN

1200

350

150

7.0

150

7.0

12.0

14.2

170.3

1.4

2.4

00.0

06

1600

350

150

7.0

150

7.0

12.0

19.7

236.1

2.7

2.4

00.0

24

-40.6

1600

350

50

7.0

150

2.0

12.0

7.5

90.0

1.3

2.7

80.0

12

430.5

0.1

149

YN

1000

350

150

2.0

150

2.0

12.0

14.7

176.4

3.7

2.4

60.0

11

-236.0

1.9

9E+

12

3.5

4E+

10

113

114 APPENDIX A. SILICON CARBIDE DOEPre

ssure

(mTorr

)

Tem

p

(°C

)

Hi

Fre

q

Pow

er

(W)

Hig

h

Fre

q

Pe-

riod

(s)

Low

Fre

q

Pow

er

(W)

Low

Fre

q

Pe-

riod

(s)

Dep

Tim

e

(min

)

Dep

Rat

e

(nm

/m

in)

Avg.

Thic

k

(nm

)

St

Dev

.

(nm

)

Avg.

RI

St

Dev

.

Str

ess

(MPa)

C:S

i

Ra-

tio

H%

Etc

h

KO

H

Etc

h

HF

Surf

ace

Res

is-

tivity

Volu

me

Res

is-

tivity

1000

350

50

2.0

50

7.0

12.0

6.6

79.8

0.7

2.7

50.0

01

-376.0

5.5

4E+

12

4.6

6E+

10

1300

350

50

7.0

150

2.0

12.0

9.5

114.3

0.9

2.6

10.0

17

-45.4

NN

1600

350

150

4.5

100

7.0

12.0

13.0

155.5

1.6

2.4

40.0

30

3.4

8.5

9E+

11

2.2

9E+

10

1300

350

50

7.0

50

7.0

12.0

0.4

5.2

1.5

1.8

10.4

22

-337.0

Y1.2

2E+

13

1.3

7E+

12

1000

350

50

7.0

150

7.0

12.0

13.3

159.8

0.8

2.3

60.0

20

-188.7

N1.1

6E+

14

7.5

2E+

10

1600

350

150

7.0

50

2.0

12.0

16.5

198.1

1.4

2.2

30.0

05

-176.0

0.4

147.5

NN

2.3

1E+

12

1.3

0E+

10

1000

350

50

7.0

150

7.0

12.0

13.3

159.1

0.9

2.4

00.0

44

-209.6

0.7

540.5

NN

2.5

2E+

12

1.2

1E+

11

1600

350

150

2.0

150

2.0

12.0

16.7

200.2

1.0

2.7

10.0

26

100.0

N3.7

6E+

15

5.2

1E+

13

1000

350

50

7.0

100

4.5

12.0

13.5

162.2

0.6

2.8

50.0

11

-135.9

Y3.4

9E+

13

1.5

4E+

10

1600

350

150

2.0

50

7.0

12.0

17.9

215.4

1.3

2.7

10.0

28

22.7

0.8

840

YN

8.8

5E+

11

1.2

4E+

10

1600

350

50

7.0

150

2.0

12.0

16.9

203.4

0.7

2.8

50.0

07

236.7

0.4

145

YN

7.3

6E+

13

4.3

9E+

11

1300

350

150

2.0

100

7.0

12.0

9.4

113.1

1.3

2.8

00.0

42

-266.2

N1.4

7E+

13

1.2

7E+

11

1000

350

150

7.0

100

4.5

12.0

12.0

144.4

11.0

2.6

30.1

07

-334.0

N8.2

7E+

12

1.1

2E+

11

1000

350

150

7.0

150

2.0

12.0

21.3

255.5

8.5

2.7

30.0

79

-168.1

N1.2

2E+

13

1.9

4E+

10

1000

350

50

7.0

50

2.0

12.0

18.4

221.4

1.1

2.4

40.0

04

-42.3

NN

1000

350

150

7.0

50

7.0

12.0

14.5

173.8

8.0

2.6

20.0

24

-201.6

1.7

3E+

15

3.3

4E+

12

1000

350

150

7.0

50

7.0

12.0

18.9

227.4

6.9

3.0

40.1

20

-98.0

0.4

636.5

NN

1.9

0E+

10

9.8

4E+

10

1600

350

150

2.0

50

2.0

12.0

19.0

228.2

4.6

2.3

10.0

06

-107.5

Y1.5

5E+

13

2.3

2E+

13

115

Pre

ssure

(mTorr

)

Tem

p

(°C

)

Hi

Fre

q

Pow

er

(W)

Hig

h

Fre

q

Pe-

riod

(s)

Lo w

Fre

q

Pow

er

(W)

Lo w

Fre

q

Pe-

riod

(s)

Dep

Tim

e

(min

)

Dep

Rat

e

(nm

/m

in)

Avg.

Thic

k

(nm

)

St

Dev

.

(nm

)

Avg.

RI

St

Dev

.

Str

ess

(MPa)

C:S

i

Ra-

tio

H%

Etc

h

KO

H

Etc

h

HF

Surf

ace

Res

is-

tivity

Volu

me

Res

is-

tivity

1600

350

150

7.0

50

2.0

12.0

15.9

191.3

10.4

2.2

30.0

16

-230.0

5.1

7E+

14

1.3

7E+

14

1000

350

150

2.0

150

7.0

12.0

16.8

201.7

3.7

2.7

40.0

09

-328.6

0.6

734

NN

2.6

4E+

13

4.5

9E+

10

1600

350

100

2.0

50

2.0

12.0

17.0

204.4

7.9

3.0

20.1

17

-146.0

0.5

641

YN

5.2

4E+

15

3.4

1E+

13

1000

350

150

2.0

150

2.0

12.0

15.9

190.4

5.9

2.4

60.0

23

-258.8

1.4

9E+

13

9.3

7E+

10

1600

350

50

4.5

100

2.0

12.0

18.8

225.3

4.8

3.0

70.0

83

-131.1

1.0

2E+

15

8.0

5E+

11

1000

350

150

7.0

50

7.0

12.0

11.8

141.9

1.8

2.6

80.0

12

-242.3

3.7

1E+

10

1600

350

50

7.0

50

7.0

12.0

19.6

235.7

0.5

2.5

50.0

09

-95.0

2.6

1E+

14

5.5

5E+

11

1600

350

100

2.0

150

4.5

12.0

14.5

174.0

1.8

2.2

60.0

16

-80.4

N3.4

2E+

14

2.0

8E+

11

1600

350

50

2.0

150

7.0

12.0

10.9

131.2

1.1

2.8

20.0

19

-340.9

N8.1

1E+

12

2.4

2E+

10

1000

350

150

2.0

50

2.0

12.0

12.7

152.0

2.0

2.7

00.0

07

-235.6

N3.7

7E+

14

4.0

6E+

10

1600

350

50

4.5

50

2.0

12.0

14.1

169.0

8.1

2.1

70.0

82

165.5

0.3

643.5

YN

2.5

7E+

15

2.6

6E+

13

1000

350

50

2.0

150

2.0

12.0

15.4

185.2

0.8

3.1

10.0

09

261.8

2.4

6E+

13

3.6

9E+

10

1000

350

100

4.5

50

4.0

12.0

7.5

90.0

1.8

2.5

40.0

05

354.5

4.9

3E+

11

7.1

9E+

10

1300

350

150

4.5

150

4.5

12.0

15.1

181.1

3.2

2.6

30.0

16

-1.8

0.4

440

YN

1.0

6E+

13

4.0

8E+

10

1600

350

100

7.0

150

7.0

12.0

14.2

170.5

2.8

2.2

10.0

38

102.9

5.0

7E+

12

6.7

1E+

13

1000

350

100

2.0

100

7.0

12.0

14.2

170.5

2.8

2.2

10.0

38

-401.1

8.5

3E+

12

1.5

4E+

10

1000

350

50

2.0

50

2.0

12.0

6.5

78.4

0.8

2.5

00.0

02

-325.8

1.3

5E+

11

1600

350

150

7.0

50

2.0

12.0

18.7

223.8

0.8

2.4

70.0

19

18.3


ssure

(mTorr

)

Tem

p

(°C

)

Hi

Fre

q

Pow

er

(W)

Hig

h

Fre

q

Pe-

riod

(s)

Low

Fre

q

Pow

er

(W)

Low

Fre

q

Pe-

riod

(s)

Dep

Tim

e

(min

)

Dep

Rat

e

(nm

/m

in)

Avg.

Thic

k

(nm

)

St

Dev

.

(nm

)

Avg.

RI

St

Dev

.

Str

ess

(MPa)

C:S

i

Ra-

tio

H%

Etc

h

KO

H

Etc

h

HF

Surf

ace

Res

is-

tivity

Volu

me

Res

is-

tivity

1200

350

--

150

-12.0

14.3

171.0

1.8

2.4

90.0

02

218.7

0.4

939

N3.9

3E+

12

3.0

4E+

11

1600

350

100

2150

4.5

14

155

2168

2.3

2.4

60.0

07

-95

1000

350

150

4.5

100

4.5

15

139

2080

3.6

2.6

70.0

11

-506

1000

350

150

750

715

127

1903

2.2

3.2

10.0

49

-287

1600

350

100

2150

314

152

2128

8.2

2.9

90.1

12

-116

1600

350

100

4.5

150

4.5

14

170

2378

5.0

2.5

90.0

94

-61

1600

350

100

4.5

100

4.5

14

166

2322

1.9

2.4

40.0

06

-36

1200

350

100

4.5

100

4.5

14

115

1605

0.4

2.3

80.0

06

-212

1600

350

100

4.5

100

4.5

12

165

1975

4.0

2.3

80.0

06

-58

1600

350

100

4.5

100

4.5

25

150

3748

0.8

2.4

80.0

12

-28

1600

350

100

4.5

100

4.5

25

146

3643

17.7

2.5

10.0

56

-21

1600

350

100

4.5

100

4.5

54

154

8306

12.3

2.8

80.1

59

6

1600

350

100

4.5

100

4.5

54

154

8306

12.3

2.8

80.1

59

6

1600

350

100

4.5

100

4.5

12

161

1928

2.2

2.3

10.0

04

-38

1600

350

100

4.5

100

4.5

25.5

147

3741

0.9

2.5

10.0

26

4

1600

350

100

4.5

100

4.5

25.5

147

3741

0.9

2.5

10.0

26

4

117

Pre

ssure

(mTorr

)

Tem

p

(°C

)

Hi

Fre

q

Pow

er

(W)

Hig

h

Fre

q

Pe-

riod

(s)

Lo w

Fre

q

Pow

er

(W)

Lo w

Fre

q

Pe-

riod

(s)

Dep

Tim

e

(min

)

Dep

Rat

e

(nm

/m

in)

Avg.

Thic

k

(nm

)

St

Dev

.

(nm

)

Avg.

RI

St

Dev

.

Str

ess

(MPa)

C:S

i

Ra-

tio

H%

Etc

h

KO

H

Etc

h

HF

Surf

ace

Res

is-

tivity

Volu

me

Res

is-

tivity

1600

350

100

2150

210

119

1185

0.4

2.4

60.0

21

-72

1600

350

100

2150

235

102

3572

40.6

2.1

20.2

09

-86

1600

350

100

2150

235

95

3322

2.8

2.2

90.0

25

-99

1600

350

100

2150

235

100

3488

10.6

2.0

90.0

78

-62

1600

350

100

4.5

100

4.5

15

154

2315

3.4

2.4

10.0

10

-14

1600

350

100

4.5

100

4.5

54

148

8001

14.0

2.8

60.0

41

-3

1600

350

100

4.5

100

4.5

27

138

3738

37.2

2.7

10.1

40

-56

1600

350

100

7100

4.5

12

185

2222

3.8

2.3

50.0

03

-36

1600

350

100

7100

4.5

25

142

3544

2.5

2.1

40.0

44

-1

1600

350

100

7100

4.5

25

138

3461

0.9

2.2

10.0

26

-25

1600

350

100

4.5

100

4.5

27

165

4452

11.2

2.9

20.0

90

1600

350

100

4.5

100

4.5

27

171

4611

8.9

2.8

80.1

34

1600

350

100

4.5

100

4.5

27

1600

350

100

4.5

100

4.5

27

1600

350

100

250

710

102

1017

0.8

2.6

10.0

08

1600

350

100

250

740

117

4689

3.5

2.4

70.0

19

1

1600

350

100

250

727

176

4742

1.9

2.5

40.0

14

-31

1600

350

100

250

210


ssure

(mTorr

)

Tem

p

(°C

)

Hi

Fre

q

Pow

er

(W)

Hig

h

Fre

q

Pe-

riod

(s)

Low

Fre

q

Pow

er

(W)

Low

Fre

q

Pe-

riod

(s)

Dep

Tim

e

(min

)

Dep

Rat

e

(nm

/m

in)

Avg.

Thic

k

(nm

)

St

Dev

.

(nm

)

Avg.

RI

St

Dev

.

Str

ess

(MPa)

C:S

i

Ra-

tio

H%

Etc

h

KO

H

Etc

h

HF

Surf

ace

Res

is-

tivity

Volu

me

Res

is-

tivity

1600

350

100

250

238

101

3852

3.0

2.3

90.0

05

-40

1600

350

100

250

238

102

3882

1.3

2.4

00.0

06

1600

350

100

2150

710

1665

0.7

2.2

70.0

20

1600

350

100

2150

725

NA

NA

-17

1600

350

100

2150

725

NA

NA

-16

1600

350

100

7150

210

1861

5.0

2.2

00.0

04

1600

350

100

7150

223

150

3448

14.3

2.1

50.0

54

94

1600

350

100

7150

223

151

3469

12.8

2.1

60.0

45

53

1600

350

100

7150

710

118

1179

0.3

2.4

90.0

16

1600

350

100

7150

735

121

4241

5.6

2.4

40.0

03

-70

1600

350

100

7150

735

120

4190

4.5

2.4

4-7

2

1600

350

100

7150

710

164

1639

6.3

2.4

40.0

48

1600

350

100

750

226

141

3657

2.4

2.3

40.0

24

33

1600

350

100

750

226

141

3657

1.1

20.0

43

59

1600

350

100

2150

210

80

803

0.9

2.4

30.0

06

1600

350

100

2150

255

NA

NA

NA

251

1600

350

100

2150

255

NA

NA

NA

294

1600

350

100

7150

210

173

1731

9.8

2.1

50.1

05

119

Pre

ssure

(mTorr

)

Tem

p

(°C

)

Hi

Fre

q

Pow

er

(W)

Hig

h

Fre

q

Pe-

riod

(s)

Lo w

Fre

q

Pow

er

(W)

Lo w

Fre

q

Pe-

riod

(s)

Dep

Tim

e

(min

)

Dep

Rat

e

(nm

/m

in)

Avg.

Thic

k

(nm

)

St

Dev

.

(nm

)

Avg.

RI

St

Dev

.

Str

ess

(MPa)

C:S

i

Ra-

tio

H%

Etc

h

KO

H

Etc

h

HF

Surf

ace

Res

is-

tivity

Volu

me

Res

is-

tivity

1600

350

100

7150

225

153

3817

2.3

2.3

80.0

10

-65

1600

350

100

7150

225

153

3818

2.7

2.3

80.0

09

-59

1600

350

100

750

710

61

613

1.1

2.3

50.0

11

1600

350

100

750

710

63

630

0.6

2.3

60.0

07

1600

350

100

750

765

55

3578

1.0

2.4

90.0

46

393

1600

350

100

750

65

59

3821

34.8

2.4

30.0

84

421

1600

350

100

250

710

71

708

0.7

2.3

70.0

05

1600

350

100

250

760

74

4452

34.5

2.4

10.1

32

-114

1600

350

100

250

760

64

3827

2.7

2.4

80.0

51

-76

1600

350

100

4.5

100

4.5

135

1600

350

100

4.5

100

4.5

135

1600

350

100

4.5

100

4.5

135

1600

350

100

4.5

100

4.5

135

20700

37.6

3.2

025

1600

350

100

4.5

100

4.5

27

4304

15.0

0.1

20

-21

1600

350

100

4.5

100

4.5

27

3967

9.4

0.0

83

-46

1600

350

100

4.5

100

4.5

67

10108

2.3

3.2

022

1600

350

100

4.5

100

4.5

67

10766

2.8

3.2

019

Appendix B

Etch Mask For Glass

One challenging process step in micromachining is the deep or through wafer etching in glass.

A common approach is to use gold with a thin but necessary adhesion layer of chromium.

As can be seen from this image of a typical channel that was masked with 20 nm of Cr and

100 nm of Au, while the mask may not fail, etching proceeds along the interface in an erratic

pattern, often faster than the etch in the bulk.

Figure B.1: Isotropic wet etching of glass using Cr/Au mask on left (Reprinted with per-mission) [114] and PECVD SiC mask on right (Courtesy of Smart Sytems, Palo Alto, CA)

Of particular note on the channel masked with Cr/Au is the jagged edge of the etch.

This is a clear indication that the interface between the mask and the substrate is failing.

It has been demonstrated that this can be improved through annealing and is indicative of

stress at the interface [115].

120

121

As a demonstration of its use as an etch mask, 350 nm of PECVD silicon carbide was

deposited on a glass wafer and patterned using an MRC (Materials Research Corpora-

tion, Orangeburg, NY) model RIE- 51 reactive ion plasma etcher with an electrode area of

182 cm2. Parameters used were 50 sccm of SF6 at 100mTorr and 100 W of power. The

patterned wafer was etched for 3 hours in 49% HF as part of a process for preparing thin

film glass for anodic bonding. Post-process examination of the mask cross-section by SEM

did not reveal any degradation of the carbide. In comparing the performance of the two

masks, two things are noteworthy. The first is the depth of the etch. The channel masked

with Cr/Au is only 10 mm deep while that with the carbide mask is approximately 200 mm

deep. The second is that while some non-uniformity can be seen along the edge, it is much

less than that from the Cr/Au mask.

Appendix C

Laser Ablation Trials

The process sequence used for the TOC sensor relied on pre-drilling a percentage of the

way through the glass wafer prior to the start of fabrication. If the holes had been drilled

completely through the glass wafer, it would not have been possible to spin photoresist on the

surface without defects. The through-wafer features required were holes for the legs of the

fluidic couplers. The fluidic couplers were heat-staked to the chip. Because this procedure

is forgiving of minor imperfections and dimensional variations, trading off precision for ease

of fabrication was possible. Typical lithography and wet-etching is an order of magnitude

more precise than what is required. Laser ablation was investigated as an alternative.

Advantages

• No mask is required, negating the need for good mask adhesion during wet etching,

• Large amounts of material can be removed relatively quickly.

Disadvantages

• Ablation systems are typically not designed for precision placement and holding of

wafers. A custom fixture had to be developed.

• Slag is generated from most ablation processes. Its removal must be accounted for in

the process flow.

• Accuracy of position control in an ablation system is typically much less than that

in a mask writing system (even a laser writing mask system). Registration errors are

122

C.1. EXPERIMENTAL SETUP 123

common in matching lithography patterns to ablation patterns.

• Substrate damage (cracking) from the relatively violent process of ablation must be

investigated and considered in establishing limits for the speed and power.

C.1 Experimental Setup

The laser ablation system available was not specifically designed for MEMS or precision

fabrication. The M Class 30W CO2laser (Universal Laser Systems, Scottsdale, AZ) was

designed for precision engraving1. To establish the limits of the system and characterize

an optimal recipe, a matrix of conditions was run using a test pattern. The test pattern

consisted of a 300 mm diameter circle and 300 mm length/width cross. The parameters varied

were power, pulses per inch (PPI), resolution, and speed. The maximum resolution (mini-

mum size) of the spot size on the system was 75 mm using a fixed focal length of 3.8 cm. The

system was configured with a cone and air stream assist to prevent ablated material from

settling back on the optics. The substrates were 500 mm thick, double-side polished 100mm

wafers made of Pyrex® 7740 (Bullen Ultrasonics, Eaton, OH). Prior to ablation, 40 nm of

chromium was sputter deposited on the surface. This was to provide a “liftoff” layer that

would not interfere with ablation but would protect the surface from slag that would settle

back on the substrate during ablation. While chromium is typically reflective, the layer was

thin enough that it did not interfere with the initiation of ablation, yet it was thick enough

that it would protect the surrounding surface. Following ablation, the wafers were cleaned

in 10:1 sulfuric peroxide and the chromium layer removed by etching in chromium etchant

(CR-14, Cyantek Corporation, Fremont, CA)

Power was specified as a percentage of system maximum (30W) The differences between

50% and 100% power were investigated. The lower power tended towards smoother features

with less cracking, but at the expense of depth.

Speed and power together have the most influence on the depth of cut. Speed is specified

as the percentage of the system maximum of 25 cm/s.

Resolution does not refer to the spot size of the laser but to the minimum spot size

of control by the system driver, measured in dots per inch (DPI). Spot size was varied

between 75 mm (333 DPI) down to 25 mm (1000 DPI - system limitation). Because the lower

1Special thanks to Juan Santiago’s research group in the ME department at Stanford University for accessto their laser ablation system and to Gina Binnard for training.

124 APPENDIX C. LASER ABLATION TRIALS

resolution corresponds to the spot size, there is no overlapping of pulses. The effect of the

energy distribution in the beam can be seen in the corrugations that are visible in Figure C.6.

Etching to a more uniform depth requires overlapping pulses justifying the use of the highest

resolution.

Pulses per inch (PPI) refers to the number of times the laser fires per inch. Although

not directly tied to DPI, the two parameters work together to determine the quality of the

image.

C.2 Results

C.2. RESULTS 125

(a) Speed: 1%

(b) Speed: 5%

(c) Speed: 10%

(d) Speed: 25%

(e) Speed: 50%

(f) Speed: 75%

(g) Speed: 100%

Figure C.1: Power: 50%, PPI: 1000, Res: 1000


(a) Speed: 1%

(b) Speed: 5%

(c) Speed: 10%

(d) Speed: 25%

(e) Speed: 50%

(f) Speed: 75%

(g) Speed: 100%


C.2. RESULTS 127

(a) Speed: 1% (b) Speed: 5%

(c) Speed: 10% (d) Speed: 25%

(e) Speed: 50% (f) Speed: 75%

(g) Speed: 100%



(a) Speed: 1%

(b) Speed: 5%

(c) Speed: 10%

(d) Speed: 25%

(e) Speed: 50%

(f) Speed: 75%

(g) Speed: 100%


C.2. RESULTS 129




(g) Speed: 100%






(g) Speed: 100%


Appendix D

Mechanical Drawing TOC Die

Figure D.1: Mechanical Drawing of Laser Ablation Drill Pattern

131

132 APPENDIX D. MECHANICAL DRAWING TOC DIE

Figure D.2: Drill pattern for TOC die and microfluidic fuel cell.

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FABRICATION METHODS FOR ... - Stanford University