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Lecture 22Chapter 29

Microelectromechanical Systems (MEMS)

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IntroductionThe University of New Mexico

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• MEMS– Micro – scale on which the devices exist

– Electro – devices have electrical input/output

– Mechanical – devices have mechanically moving parts

– System – integration of components

• Why develop MEMS?– Sensors (Analog Devices – Accelerometer (Airbag Sensor)

• Small package– Lightweight

– Higher sensitivity

» Small forces – sub-mN

» Small displacements – sub-μm

– Actuators (Texas Instr. Micromirror Device)

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Types of MEMSThe University of New Mexico

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• Bulk Micromachining

– Removal of bulk Si from wafer to define

a device

– Examples

• SCREAM

• SCALPEL

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Conceptual Design for Thin Film Tester

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Load Cell Fabrication

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Load Cell Force Calibration

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Gold Nanofilm Characterization• Analysis of Au Film

– Au film sputtered on Si (100) substrate

– Stoney’s Formula• ~10 MPa residual

compressive stress

– XRD• Main orientation (111)• ~2.5 MPa residual

compressive stress

– TEM• Average grain size

~57nm

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Gold Nanofilm Characterization• Average grain size ~57nm

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The University of New Mexico

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Experimental Setup

Leseman, Z. C. and Mackin, T. J. Indentation testing of axisymmetric freestanding nanofilms using a MEMS load cell, Sensors and Actuators A, in press. (Invited paper)

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Gold Nanofilm Data

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Fracture

500 μm

•Au Film–100 nm thick

–(111) Texture

–10 MPacompressive residual stress

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Creep of Au Films

0

5

10

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20

25

30

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45

50

0 2 4 6 8 10 12 14

Time (minutes)

Dia

met

er o

f Are

a C

reep

ing

( μm

)

Creep Behavior of 100 nm Thick Au Film at Room Temperature

Thickness = 100 nmDiameter = 500 μmAverage Grain Size = 20 nmIndenter Diameter = 300 μmMax Applied Load = 761 μNTemperature = 20 oC

t = 0 minφ = 48.6 μm

t = 1.5 minφ = 32.2 μm

t = 12 minφ = 0 μm

57 nm

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Types of MEMSThe University of New Mexico

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• Surface Micromachining

– Selective deposition and removal of thin films from a substrate (Si)

to create a multi-level device

– SUMMiTTM Process (Sandia)

– DMD Process (Texas Instruments)

Stiction

• Stiction- Unintended structural collapse due to secondary forces

• Causes– During etching of

sacrificial layers– Application of surface

treatments

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Experimental Concept

Fixed PositionMoves Incrementally in y-direction (50 nm)

• Stiction failed cantilever (s-shaped failure)– Fixed end of beam raised incrementally– Crack length (s) is measured before and after increment

• Arrest value for the critical strain energy release rate is found

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Experimental SetupThe University of New Mexico

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Microcantilevers Used in this Work

•SUMMiT IVTM from Sandia•Beam dimensions: 30 μm wide and 1500 μm long

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0

2

4

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Substrate = poly-SiLiquid = IPA

Substrate = poly-SiLiquid = DI

Substrate = hydrophobicLiquid = IPA

Substrate = hydrophobicLiquid = DI

Substrate = hydrophilicLiquid = IPA

Substrate = hydrophilicLiquid = DI

G (mJ/m2) 47% RH

at 20oC

47% RHat 20oC

58% RHat 20oC

58% RHat 20oC

58% RHat 20oC

58% RHat 20oC

‘Wet’ Experimental Results

• Wet experiments in IPA vs. DI– GIPA < GDI

• poly-Si substrate• Hydrophilic gold• Hydrophobic gold

} Surface Tension EffectσDI = 73 mJ/m2 ; σIPA = 21.7 mJ/m2

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Types of MEMSThe University of New Mexico

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• LIGA – Lithographie Galvanoformung und abformung1. Deposit PMMA onto a substrate

2. Develop PMMA

3. Deposition of metal onto primary substrate

4. PMMA removed

5. Injection molding of another plastic

– Forms• Freestanding metal structure

• Plastic injection-molded structure

• Investment-cast metal structure from injection-molded structure

• Slip-cast ceramic part from injection-molded structure

LIGA DynamometerThe University of New Mexico

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