Masters Defense presentation

MicroSystems Engineering Team

Louisiana State University

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Thesis Defense – April 3, 2001

Design and Fabrication of a

Thermomechanical Microactuator

Proyag Datta Department of Mechanical Engineering


April 3, 2001

Thesis Defense



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Acknowlegdements

Project was funded by AFOSR



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PRESENTATION OUTLINE

•  Introduction •  Design and Modeling •  Fabrication Process Developement •  Conclusion



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INTRODUCTION Trapped Vortex(TV) Combustors

•  Continuous interest towards improving the performance of aircraft propulsion systems

•  Improved fuel efficiency, better specific energy release, extended life, extended lean flammability limit and reduced emission of environmental pollutants

•  A Trapped Vortex combustor is a means to implement a stabilized combustion process in an engine



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INTRODUCTION Concept of ‘Breathing Wall’

•  TV-combustors experience thermo-acoustic instabilities and ‘hot spots’, which lead to lowered efficiency in the combustor

•  Hot spots can be controlled by injecting cooler air through dilution holes on the combustor walls

•  Distributed air injection would –  control local stoichiometry –  lead to uniform temperature distribution –  minimize wall temperature –  minimize NOx formation



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INTRODUCTION Schematic of TV Combustor



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DESIGN and MODELING Microvalves

•  Properties of an ideal valve –  Low leakage –  Low power consumption –  Low dead volume –  Large differential pressure capability –  Insensitivity to particulate contamination –  Low response time –  Potential for linear operation –  Ability to handle fluids of any density/viscosity/chemistry

•  Valves are designed for specific conditions of operation



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DESIGN and MODELING Microvalves

•  Valves are classified as ‘passive’ or ‘active’ •  Passive Valves

–  No external power or control –  Usually one-way or check-valves

•  Active Valves –  Powered actuation mechanism –  Driving Mechanisms

•  Electrostatic •  Piezoelectric •  Magnetic •  Shape Memory •  Pneumatic



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DESIGN and MODELING Overview

•  Design Criteria •  Recurve Architecture •  Quasistatic Modeling •  Finite Element Analysis •  Dynamic Modeling



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DESIGN and MODELING Design Criteria

•  Survival at elevated temperatures •  Actuation distance (~500 µm) •  Force Required

•  Compactness of design •  Integrable into combustor walls •  Frequency response (>100Hz) •  Rugged design for operation in harsh

environment



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DESIGN and MODELING Design Considerations

•  Most methods of active actuation fail due to high temperature (e.g. piezoelectric, magnetic)

•  Passive actuation chosen •  Temperature gradient as energy source to drive

the actuator •  Thermal expansion as method of actuation •  Array structure chosen

–  Resistant to particulates –  Tailored to meet force and deflection requirements



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DESIGN and MODELING Recurve Architecture

•  Direct thermal expansion produces insufficient deflection

•  Deflection of a single bimetallic element is insufficient for the amount of deflection reqd.

•  Bimetallic elements cannot be stacked as tip rotation nullifies deflection

•  Recurve architecture suggested by Ervin and Brei (1998) chosen



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DESIGN and MODELING ‘Recurve’ Schematic



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DESIGN and MODELING Recurve Architecture

•  Basic building block - composite beam made of two materials with different coefficients of thermal expansion

•  Produces a parallel displacement of the endpoint relative to the base

•  Can be combined into arrays to obtain greater net deflections or forces

•  By reversing positions of high and low CTE materials, pull type actuators can be fabricated.



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DESIGN and MODELING Behavior of a Recurve Element

3-D solid model of a recurve element shown in undeformed(Left) and deformed(Right) state



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DESIGN and MODELING Recurve Array



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DESIGN and MODELING Quasi-static Modeling

•  Strain energy based analytical derivation using Castigliano’s second theorem

•  Equations derived for –  Displacement –  Force



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•  Equation for Recurve

•  Moment in bimetallic strip

DLM

DL

mF

nez

412.

23

+=Δ

( )IE

hTMeΔ−

+=

).(.1224.2 21 αα

ϕ



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Force vs Height

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0 100 200 300 400 500 600

Height of Recurve(Micrometers)

Blo

ckin

g F

orc

e(N

)

Deflection vs Height

0

10

20

30

40

50

60

0 100 200 300 400 500 600

Height of Recurve(Micrometers)

Def

lect

ion

(Mic

rom

eter

s)



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Deflection vs Thickness

050100150200250

0 100 200 300Thickness (Micrometers)

Def

lect

ion

(M

icro

met

ers)

Force vs Thickness

0.00

0.20

0.400.60

0.80

1.00

0 100 200 300Thickness (Micrometers)

Blo

ckin

g F

orc

e(N

)



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Deflection vs Length

0

20

40

60

80

100

0 5000 10000 15000Length (Micrometers)

Def

lect

ion

(M

icro

met

ers)

Force vs Length

0.000.501.00

1.502.002.50

0 5000 10000 15000Length (Micrometers)

Blo

ckin

g F

orc

e(N

)



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DESIGN and MODELING ANSYS Modeling

•  3-D ANSYS model created •  Steady state analysis carried out •  Alternate configurations simulated •  Coupled field analysis carried out •  Sequential Method of analysis used



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DESIGN and MODELING ANSYS Modeling

•  Meshed with Solid87 3-D, 10-Node Tetrahedral elements for thermal analysis

•  Uniform steady state temperature attained •  Nodal results read in for structural

analysis •  Elements changed to Solid92, a 10-node

tetrahedral structural solid



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DESIGN and MODELING ANSYS Modeling – Model I

Meshed ANSYS Model - I



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DESIGN and MODELING ANSYS Modeling – Model I

Deflection of Recurve Model- I



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DESIGN and MODELING Comparison of Results

Comparison of Deflections Predicted by Analytical Model and ANSYS (Model I)

0

50

100

150

200

250

0 100 200 300 400 500Temperature (C)

Def

lect

ion

(mic

rom

eter

s)

AnalyticANSYSError



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DESIGN and MODELING ANSYS Modeling – Model II

Meshed ANSYS Model - II



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DESIGN and MODELING ANSYS Modeling – Model II

Deflection of Recurve Model- II



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DESIGN and MODELING Comparison of Results

Comparison of Deflections Predicted by Analytical Model and ANSYS(Model II)

0

100

200

300

400

500

0 100 200 300 400 500Temperature (C)

Def

lect

ion

(mic

rom

eter

s)

AnalyticANSYSError



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DESIGN and MODELING ANSYS Modeling-Max Stress

Max Stress predicted by analytical model =1.482E-5 kgf/sq µm



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DESIGN and MODELING Dynamic Modeling

•  Assess the order of dynamic response of the passive actuator

•  Graphical system-modeling tool •  Uniform treatment of various energy domains •  Lumped parameter pseudo bondgraph model of

heat transfer in the recurve elements developed •  Coupled with the mechanical system bond graph

using signal bonds



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DESIGN and MODELING Bond Graph

R

..0

Thermal Part

RD

1..

CM2

0..

SET

RD

1..

RC RC

C RS

0.. 1..

C

..0

I

1..

SE

..

R

0..0

SE

1..

CM1 I

RC RC

C

0..

RS C

1.. 0..

I

..1

SESE

1....

R

0..0

I CM1

..

R

0..0

CM2

Mechanical Part



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DESIGN and MODELING Results



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DESIGN and MODELING Results



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DESIGN and MODELING Valve Design

‘Push-pull’ valve arrangement



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DESIGN and MODELING Valve Design

Recurve Actuator

Buckling Valve Cover

Motion of Valve Cover

Motion of Actuator

Recurve driven buckling valve cover



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FABRICATION PROCESS Overview

•  Ni-Fe Plating •  Mask Fabrication •  Prototype Fabrication

–  Multi-layer fabrication process –  Photolithography –  LIGA –  Conventional Machining Processes



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FABRICATION PROCESS Materials

•  Nickel chosen as high CTE material –  High melting point –  High CTE –  Ease of electroplating

•  Invar-like Ni-Fe alloy chosen as low CTE material –  High melting point –  Low CTE



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FABRICATION PROCESS Ni-Fe Electroplating

•  Electrolyte formulated to electrodeposit an Invar-like Ni-Fe alloy (64% Fe, 36% Ni)

•  Hull cell experiments were carried out to determine a suitable current density for plating

•  500 µm high, 120 µm X 120 µm cross section posts were plated as test structures

•  EDXRF and WDS on an electron microprobe were used for analysis of composition



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Composition of Deposit as a Function of FeCl Concentration

0

10

20

30

40

50

60

70

80

0.1 0.12 0.14 0.16 0.18

Moles of Ferrous Chloride

Perc

enta

ge C

ompo

sitio

n

NiFe



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Composition along post varies - Microprobe analysis

Ni-Fe Post 490 micron High (Data points from bottom to top)

0

20

40

60

80

100

120

0 10 20 30 40 50 60

Length along post(in micrometers)

Perc

enta

ge C

ompo

sitio

n

Fe

Ni

Total



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FABRICATION PROCESS Image – Ni-Fe posts

Top view of posts Side view of single post



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FABRICATION PROCESS Image – Stress in Ni-Fe posts (20 mA/sqcm)



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FABRICATION PROCESS Image – Stress in Ni-Fe posts (10 mA/sqcm)



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Polarization Curve

0.000

5.000

10.000

15.000

20.000

25.000

30.000

35.000

40.000

45.000

50.000

55.000

60.000

65.000

70.000

0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800 0.900

Negative Potential (V) vs SCE

Neg

ativ

e C

urre

nt D

ensi

ty(m

A/s

qcm

)

E0Ni/Ni

2+

E0Fe/Fe

2+

Ohmic corrected polarization curve for nickel-iron bath



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FABRICATION PROCESS Ni-Fe Electroplating - Issues

•  Stress generation – cracks, brittleness •  Passivation – required hard is hard to

obtain, plating stops/slows down for no apparent reason

•  Composition varies from top to bottom •  Rusting



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FABRICATION PROCESS Mask Fabrication – Optical Mask

•  Autocad drawings –  Multilayered

•  Optical Mask –  Autocad file conversion –  5x5 inch commercial wafer with Chrome &

Positive resist –  Exposure on MANN 3600 pattern generator –  Development –  Chrome etch –  Resist removal



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FABRICATION PROCESS Mask Fabrication – Autocad Drawings



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FABRICATION PROCESS Mask Fabrication – X-Ray Mask

•  X-Ray Mask Fabrication –  Glass ring cut by waterjet –  DFP3 graphite cleaned and stuck to glass ring using UV-

cured glue –  50 A of Chrome and 300 A of gold E-beam deposited –  SU-8 spun on wafer and baked –  Wafer exposed using optical mask

Glass Ring

Evaporated Chrome & Gold

SU-8

DFP-3 Graphite

Glass UV Chromium Mask

UV Exposure



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FABRICATION PROCESS Mask Fabrication

–  Post-bake and developed –  Gold and chrome etched from around alignment marks –  Plasma ashing to clean wafer –  20 µm of gold electrodeposited in SU-8 mold –  Mask mounted on standard NIST ring –  Process was used to manufacture two X-Ray masks

Gold and Chrome etched from around alignment mark

Gold Plated into pattern

Alignment Mark

Developed Pattern



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FABRICATION PROCESS Image – Mask on Glass Ring

Gold on Graphite X-Ray mask mounted on glass ring



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FABRICATION PROCESS Image – Close up on mask

SU-8 structures with gold plated around them



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FABRICATION PROCESS Image – Alignment Marks on Mask

Complementary alignment marks on mask



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FABRICATION PROCESS Mask Fabrication - Issues

•  Glass surface should be clean & blemish-free •  Alignment marks need not be complementary –

two crosshairs work better •  Distance of alignment marks from structures is

critical •  SU-8 layer sinks into graphite, depending on

graphite density •  SU-8 removal still a problem



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FABRICATION PROCESS Substrate Preparation

•  4 inch Titanium plate –  Clean with HF for 1 min –  Rinse in DI

•  Oxidation –  Sodium Hydroxide and Hydrogen Peroxide –  65°C for 20 min

•  Copper Plating –  Copper Sulphate based bath –  20mA/sqcm for 30 min

•  Hand polished to improve surface

Titanium

Titanium

Titanium Oxide

Copper



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FABRICATION PROCESS Photolithography

•  Spin coat photoresist –  SJR-5740 positive photoresist –  2000 rpm for 30 sec to give 10 µm thick coat

–  Bake at 95 °C for 8 min •  Exposure

–  G-line UV-exposure station at CAMD

–  400 mJ/sqcm –  Only alignment marks

exposed •  Development

–  Microposit 354 developer for 8-12 min

Copper

Titanium Oxide

Photoresist

UV Exposed Photoresist



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FABRICATION PROCESS Photolithography

•  Nickel plating –  Activation using C-12 –  Sulphamate Bath –  20 mA/sqcm for 20 min

•  Strip photoresist –  Acetone

•  Oxidation –  Better visibility & adhesion

Copper

Nickel Alignment Marks

Copper

Copper Oxide



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FABRICATION PROCESS Image – Visible Alignment Marks

Wafer after oxidation – alignment marks visible



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FABRICATION PROCESS Photolithography - Issues

•  Contact printing process – optical mask has to be cleaned regularly

•  Perfect contact essential for good exposures •  Optical mask should have as much clear field as

possible •  Sacrificial electrode essential for controlling

plating into small areas



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FABRICATION PROCESS LIGA-Process Overview



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FABRICATION PROCESS LIGA

•  Bond PMMA –  500 µm thick stock PMMA sheet –  MMA based glue –  20 psi bonding pressure

•  Alignment –  X,Y displacement and rotation adjustments

•  Exposure –  X-ray exposure on CAMD beamline XRLM3

Copper

PMMA



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X, Y - Displacement Setscrews

Alignment Mark on Mask

Alignment Mark on Substrate

Glass

Rotational Displacement Setscrew

Optical Microscope

Schematic of Alignment Process Alignment Jig



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•  Development –  GG developer –  20 min in Developer, 40 min in Rinse –  1 cycle for every 100 µm of PMMA –  Rinse in DI

•  Etch Copper Oxide –  Vacuum wafer under etch solution

Exposed PMMA

Copper oxide etched to expose copper



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•  Nickel-Iron Electroplating •  Polishing •  Bond 500 µm thick PMMA sheet •  Flycut down to 100 µm above previous layer

Exposed PMMA

Electroplated nickel-iron, polished down to level

Second layer of PMMA flycut down to 100 µm



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FABRICATION PROCESS Image – Post 1st Electroplating

Wafer after electroplating for 1st layer



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FABRICATION PROCESS Image - Post 1st Electroplating

Part of structure after Nickel-Iron electroplating and polishing



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•  Alignment and 2nd exposure •  Development •  Copper oxide etch •  Nickel electroplating

–  Nickel Sulfamate bath –  Current density of 20 mA/sqcm Second Exposure of PMMA

Nickel plated into exposed PMMA mold

Copper oxide etched to expose copper



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•  Polish Nickel •  Strip PMMA

–  Acetone –  Heat & Stir

•  Etch Copper –  50% NH4OH and 50% H202

•  Etch Titanium –  HF

Nickel polished down to level with nickel-iron

PMMA removed using Acetone

Copper oxide and copper etched to release structures



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FABRICATION PROCESS Image – Post 2nd Exposure



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FABRICATION PROCESS Image – Alignment Error



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FABRICATION PROCESS Image – Post 2nd Electroplating



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FABRICATION PROCESS Image – Post final polish



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FABRICATION PROCESS Issues

•  Accurate alignment is difficult •  Unpredictable X-ray exposure results

–  Mask setting faulty •  Electroplated Ni-Fe has poor mechanical

properties •  Bond strength between Ni & Ni-Fe suspect •  Adhesion on titanium is poor



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FABRICATION PROCESS Future Work

•  Alignment Issues –  Reduce alignment steps by fabricating alignment marks

with first PMMA layer –  Use better alignment marks –  Use better alignment system

•  Ni-Fe plating –  Additives –  Varied pulse times at lower currents –  Better understanding of material properties of

electroplated Ni & Ni-Fe alloy



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ACKNOWLEDGEMENTS

Dr. Michael Murphy •  Committee Members:

–  Dr. Elizabeth J. Podlaha –  Dr. Sumanta Acharya –  Dr. Wajun Wang

•  CAMD Staff –  Yohannes Desta –  Zhong Geng Ling –  Kun Lian –  Jost Gottering –  Harish Manohara



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ACKNOWLEDGEMENTS

•  Kevin Zanca •  Abhinav Bhushan •  Kabseog Kim •  John Fuller •  Tracy Morris •  Summer Dann-Johnson •  Dawit Yemane •  Jason Sevin

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