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Lecture 2: VLSI-based Fabrication for MEMS:

Fundamentals

Prasanna S. GandhiAssistant Professor,Department of Mechanical Engineering,Indian Institute of Technology, Bombay,

Recap: Last Class

What is MEMS?Why study of MEMS important?Practical examples and productsContents of the course

FabricationAnalysis Characterization

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Today’s Class

Materials for MEMSVLSI-based fabrication processes for MEMS: an overview

LithographyMaterial removalMaterial additionCombination of all these to make devices

Materials for MEMS

SiliconStronger than steelLight as AluminumCan be coated with varieties of materials

Available in form of wafers 2”, 4”, 8”, 12” diaOther materials

<100>

<010>

<001>

<111>

Silicon crystal orientation

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Materials for MEMS

Other materialsPolycrystalline silicon (polysilicon)Silicon dioxide (SiO2)Silicon nitride (Si3N4)Aluminum (thin film)Chromium, Gold (thin film)Several others now a days: various photoresists(polymers), tungston, copper, magnetic, etc.Doping of silicon

Fabrication processes

Lithography: patterningChemical etching

IsotropicAnisotropic

Plasma etching: RIEOxidationSputteringChemical vapor deposition (CVD)ElectroplatingSurface micromachiningLIGA

Materialremoval

Materialdeposition

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Lithography

Positive Photoresist (PPR)

MASK

U-V RAYS

Lithography

Negative Photoresist (NPR)

MASK

U-V RAYS

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Lithography

E-beam LithographyFeatures are written by scanning electron beamNo necessity of maskCan be used for preparation of maskVery fine size (sub-micron or <1 micron) features can be produced easilyNot suitable for higher length featuresTypical resist PMMA Polymethylmethacrylate

Chemical Etching

Isotropic etchingEtchant: HNA mixture.HNA can dissolve 550µm thick silicon wafer in about 20 min.HNA mixture removes silicon equally in all directions.SiO2 etch: 10-30nm/min

Without agitation (5)

With agitation (20)

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Chemical Etching

Anisotropic etchingEtchant: KOH, EDP<111> direction has lower etching rates than <100>Can produce grooves, slanted/vertical walls

<110> surface wafer

<100>

<010>

<001><111>

<100> surface wafer

Plasma Etching

Gas is exposed to electric and magnetic fieldsIonized gas hits the target surface to remove materialPlasma + chemical Reactive ion etching (RIE) – very efficient process

Vacuum chamber

Electrode

Electrode

Wafer V

PLASMA

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Oxidation

Oxidation of Si: keep in air at high temp (1000-1200oC)Well understood and controlled processParameters

TemperatureEnvironmentTime

Oxide: important patterning materialProblems: thermal stresses

BdtdTAT

BAtBATT

oxox

oxox

=+

+=+

)2(

Constants,)(2 τ

Sputtering

Target surface bombarded with a flux of inert ions (Ar, he)DC fields or RF for acceleratingDeposition rates 1µm/min for AlGranular deposited film under stressLow temperatures

Vacuum chamber

Electrode

Electrode

Wafer

VAr/He

PLASMA

Target

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Chemical Vapor Deposition (CVD)

Chemical reaction in vacuum chamberHigh temperatures (>300oC)Polysilicon, SiO2, Si3N4, tungston, titanium, copper etc. can be depositedLow pressure CVD (LPCVD)Plasma Enhanced CVD: low temperaturesPressure, temp, gas flow

Wafer

Gases

Temperature > 300oC

Surface Micromachining

Combination of lithography and etchingDefined with respect to deposited films instead of Si substrateWafer

U-V RAYS

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Surface Micromachining

At DARPA, USA

Surface Micromachining Material Systems

SiO2Oxygen plasma

PhotoresistAluminum

SiO2XeF2PolysiliconLPCVD Si3N4+ Al

SiO2PAN etch Aluminum Polyimide Si3N4 + SiO2Buffered HF SiO2Polysilicon

IsolationRelease Etch

SacrificialStructural

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More Complex Fabrication

Electroplating

SUBSTRATE

ADHESION LAYERCOPPER LAYER

U V LIGHT

OPTICAL MASK

CHROMIUM LAYER

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LIGA

Combined Lithography, electroplating and molding process for high aspect ratio (depth/width) structures

SUBSTRATE

PMMA RESIST

MASK

LIGA

Micromotor fabricated using LIGA

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Design Considerations

Design for fabrication: example, long shaft manufacture not possibleFabrication: series of selective deposition and etching processesArray of devices can be fabricatedCost: no. of lithography steps should be minimum for cost effective designOther consideration

Thermal expansion of devicesAir dampingIntegration of electronics along with mechanical devicePackaging and safety

Sensing Methods

Resistive sensingBased on strain: accelerometersBased on temperature:

Capacitive sensing: accelerometers, pressure sensorsBimetalic strips Thermocouple effectPiezoelectricUsing optics laser source and detectors

Diffraction effectsInterference effectsQuadrature photo diodes

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Conclusion

VLSI-based fabrication processes for MEMSLithographyMaterial removal: Etching: Isotropic, anisotropic, RIEMaterial Deposition: CVD, LPCVD, PECVD

Design: entirely new philosophyFabrication Analysis

Next class

Optical LithographyProcess detailsVarious typesImportant parameters