Introduction to Microeletromechanical Systems (MEMS)

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Texas Christian University Department of Engineering Ed Kolesar

Introduction toMicroeletromechanical Systems

(MEMS)

Lecture 2 Topics• Micromachining – Definitions and Terminology• Example Fabrication Process• Lithography

PhotolithographyElectron Beam LithographyOther Tools: Stereolithography, “soft” lithography

• Etching


Some Definitions• MEMS: Microelectromechanical System

Note: name is not really appropriate (anymore) since the field of MEMS includes also optical, magnetic, thermal, chemical, biological systems

Other definitions: MST: Micro Systems Technology Micro Sensors and ActuatorsMicromachined Transducers

• Transducer: converts one form of energy into another.This term encompasses “sensors” and “actuators.”For practical purposes, a transducer is a conduit for information.

• What is “micromachining” ?

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Micromachining• Broad term describing all precision

techniques to build small (mm to nm) structures

• Micromachining techniques were originally developed for the IC industry (1960’s on)

• Think of micromachining as the vocabulary with which we express microstructures


MicromachiningDistinguish:• Bulk micromachining:

use (silicon) substrate• Surface micromachining:

use deposited thin films

Aspect ratio:Relative height / width of features. Often, high aspect ratio is desired for MEMS

Figure: G. Kovacs, 1996.

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Micromachining

Three basic techniques:• Pattern Definition

(usually lithography)• Additive Processes

(deposition of material: thin film deposition, sputtering, evaporation, plating, spin-on, …)

• Subtractive Processes(removal of material: etching, …)


Sample Process• Walk through a common MEMS process• Ignore fabrication details for now, instead

focus on geometric operations:Patterning Deposition (addition of material)Etching (removal of material)Isotropic (same rate in all directions)Anisotropic (preferred directions)

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Sample Process

• Si wafer

• SiO2 depositionPECVD or furnace

• Photoresist spin-on

Wafer Cross Section (not to scale!)


Sample Process• Expose photoresist

lithography

• Develop photoresist

• Pattern transfer photoresist → SiO2anisotropic RIE etch

• Pattern transferSiO2 → Sianisotropic RIE etch (timed)

Trenches in substrate

Light SourceLithographic

Mask

10 µm …

100 µm

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Sample Process

• Remove photoresist

• Deposit SiO2PECVD (conformal - more or less)

Note: Thicker SiO2 on mesa

> 2 µm


Sample Process

• Etch trench SiO2RIE anisotropic (timed)

• Etch SiRIE isotropic (timed)

Released structures

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Sample Process

• Deposit insulating SiO2PECVD (conformal - more or less)

• Deposit Alsputter

Insulated mesa and trench metal


SCREAM Process• Single Crystal Reactive

Etching And Metallization(developed at Cornell CNF)

• Features:1 mask, “self-aligning”High aspect-ratio SCS structures

• Applications:Electrostatic actuatorsIntegrated STM, …

SCREAM cross section

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SCREAM Process

XYZ-stage for MEMS STM[Yang Xu et al. American Physical Society March Meeting, 1995]

Two-level SCREAM Actuators[K. Böhringer et al., IEEE MEMS, 1995]


MEMS Overview

Micromachining: lithography, deposition, etching, …

Processes & Foundries

Devices & Structures

Methodology

History & Market

Introduction&

Background

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Lithography• The basic technique used to define and transfer

patterns, in most micromachining and integrated circuit fabrication processes

• Optical lithography: light is directed through a mask to selectively expose a photosensitive organic material (photoresist)

• Patterned regions can then be manipulated (etching, deposition, etc.) or the resist can be used as a sacrificial (temporary spacer) layer.


LithographyDistinguish:

Contact lithography: mask in immediate proximity ofphotoresistStepper: geometric projection, e.g. 10:1 or 5:1

Positive resist: exposed resist is removed in developerNegative resist: non-exposed resist is removed in developer

Exposed regions can be then be manipulated (with etching, deposition, etc.)

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Lithography Process


Problems with Lithography“Features” of (photo-)lithography:• 2-dimensional• Requires flat surfaces• Resolution limited

by wavelength of light• Projection errors

Shapes that are difficult to fabricate with lithographic techniques[Figure: G. Kovacs, 1996]

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More Lithography• Electron beam lithography: scanning electron beam

for small feature sizes (smaller wavelength), Note: there are fundamental differences to optical lithography…

• X-ray and synchrotron lithographycan expose very thick layers of resist(mm and more)

Separation nozzle, Institute for Microstructure Technology Karlsruhe, Germany


Stereo Lithography

• Stereolithography:build up 3D structures from CAD model layer by layer withphotocurable resin

• Resolution < 5 µm is possible

Bertsch 1998 – 2001, EPFL Switzerland http://dmtsun.epfl.ch/~abertsch/album.html

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• Printing with elastomeric stampspatterning of a wide variety of materials;non-flat substrate surfaces are possible

Whitesides (Harvard) and Qin, Xia (UW)

• The key element is the elastomeric stamp or mold, usually made from polydimethylsiloxane (PDMS), having patterned relief structures on its surface.

• The stamp pattern is transferred in a printing process.

“Soft Lithography”


Etching• Subtractive Process - removing materials

• Wet etching - liquid etchants:AcidsHydroxides

• Dry etching - etching gases / plasma:Physical etching (impact of atoms/ions)Reactive chemicals / ionsEnhanced by RF energy (instead of temperature)

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Etchant PropertiesHow do we choose an etching method?

• Selectivity to masking layer(s) and their availability• Selectivity to metals (e.g., Al)• Etch rate• Anisotropy (crystal plane selectivity)• Surface roughness• Control of etch parameters• Safety of reactant(s) and product(s)• Cost (including disposal and fixed costs)• Capacity for etch-stops• Mode and ease of use (including throughput)• Other parameters?


Wet Etching• Most micromachining is presently done with silicon,

and a large amount of that is etching with wet chemicals.

• Isotropic etchants (e.g. HNA) give rounded profiles.• Anisotropic etchants (e.g. KOH, TMAH) slow down

markedly on (111) crystal planes of silicon, yielding flat surfaces.

• Dopants such as high concentrations of boron can be used to stop the progress of etchants such as KOH.

• Electrochemical etch-stop techniques can also be used since at certain potentials, silicon forms an anodic oxide that stops etching.

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Etching



Convex and Concave CornersAnisotropic Wet Etching:• Convex corners are

undercut• Concave corners stop at

[111] intersections

Figures: G. Kovacs, 1996.

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Anisotropic Si Etchants• Alkali Hydroxides (KOH, NaOH, etc.): very

smooth walls, can use isopropyl alcohol to increase selectivity (111) vs. (100), attacks aluminum, oxide etches somewhat, nitride good mask.

• Ethylene Diamine Pyrochatechol (EDP): similar to KOH but much more toxic, does not attack metals (even Al in some cases) nor oxide.

• Tetramethyl Ammonium Hydroxide (TMAH):similar to EDP but safer, in some cases will not attack Al, can be masked with oxide.

• Amine Gallates: similar to EDP but somewhat safer, not commonly used.


Anisotropic Etch Examples

Typical etch pit

Tuckerman and Pease (110) cooling channels


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Dopant Etch Stops• Many anisotropic etchants slow down markedly at

high boron concentrations (» 1020 cm-3).• Can diffuse or grow boron-containing epitaxial silicon.

Cantilever (Petersen, 1982) Membrane formation



Boron p++ Etch Stops


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Electrochemical Etching


• Use standard CMOS to form isolated single-crystal islands.

• Use “open” mask to leave bare silicon regions.

• With appropriate TMAH formulation, exposed Al is not attacked.

• n-wells are biased more anodic than passivation potential and are not etched.


Photon-pumped Etching• Use photogenerated

electron-hole pairs to supply holes for etching reaction.

• Holes are concentrated at high-field points (deliberately started) and can yield holes with aspect ratios > 70:1.

• (see Lebmann and Föll, 1990)


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Comparison of Si Etchants1 sublimation from solid

source2 With added Si,

polysilicic acid or pH control

3 Some formulations do not attach Al, but are not common

4 Includes cost of equipment

5 Varies with wt% TMAH, can be controlled to yield very low roughness

6 Defined as 1) allowing wafer to be immersed directly with no special measures and 2) no alkali ions

Source: G. Kovacs, 1996.

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Introduction to Microeletromechanical Systems (MEMS)