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Characterization of MEMS Devices

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

MEMS: Characterization

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Recap

Characterization of MEMSMotivationPrinciples of optics

Tools for optical characterizationMicroscopeEllipsometerProfilometer

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Today’s Class

Scanning Probe Microscopy based tools: STM and AFMMethods for characterization of mechanical properties

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Limitations of Microscope

Q: is it possible to increase the magnification of microscope indefinitely and expect improved resolution??

Minimum resolution possible is comparable with wavelength of light

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SPM: STM and AFM

STM invented in early 80s by Binnigand Rohrer.Real limitations: only used to image conducting materials. Cannot distinguish between atoms of different elements within a compound material.

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STM:Fundamentals

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STM: Fundamentals

Tunneling current at distance about 10ATwo methods

Constant current modeConstant height mode: faster

Remarkable sensitivity: current being exponential function of distance (1A change order of magnitude change in current)Measures surface of constant tunneling probabilitySurface has small area oxidized??Valid for conductors only

Tip

Cantilever

Surface

Electron Tunneling

University of SouthamptonSurface Science Group

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STM Image

http://spm.phy.bris.ac.uk/techniques/AFM/

STM image of copper and nickel atoms

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AFM: Fundamentals

Force of interaction between moleculesTip <100A in diaScanning of sample or tip to generate imageVan der Waals forces between tip and sampleContact: few A, noncontact: 10-100AForce balance in contact regime?Additional force: Capillary force + cantilever force = repulsive VW force (10-6-10-8N)Detection using photodiodes

Sample

Force

Tip-sample separation

Repulsive

Attractive

Contact

Non-Contact

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AFM: Operation

Contact ModeConstant-height

Fast speedsAtomic scale images

Constant-force: cantilever deflection used as feedback to adjust z to maintain deflection constant

Speed of scanning is limited

Non-Contact mode / tapping mode

Sample

Scan path

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AFM: Operation

Non-Contact mode / tapping modeVibration of AFM cantilever near surface of a sampleTotal force: 10-12N very smallStiffer cantilevers necessary Operation near resonance frequency (typically 100-400KHz), amplitude 10-100AChange in the resonance frequency during scanning of sampleControl can be used to keep resonance amplitude or freq constantSoft samples can be probed in this mode

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AFM: Operation

Other modesMFM: magnetic force microscopyLFM: lateral force microscopyEFM: Electrostatic force microscopyTSM: Thermal scanning microscopyNSOM: Near field scanning optical microscopyNanolithography

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Atomic Force Microscope Multi-mode nanoscope from

Digital Instruments: Physics Dept., IIT BombayActual system details

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Atomic Force Microscope

The SPM head

All figures of actual system are taken fromMultimode SPM installation manual, RevB,Digital Instruments, 2004.

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Atomic Force Microscope

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Atomic Force Microscope

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Atomic Force Microscope

Application to MEMS

Measurement of MEMS cantilever stiffness using AFMBioMEMS sensor characterization (ongoing activity)Nanoindentation using diamond tipThin film surface characterization

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AFM Image

Kriptan- polymer surface characteristics using AFM

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Characterization of Mechanical Properties

Properties: E, ν, internal stress etc. Various Techniques

Bending testCantileverBeam

Bulge testResonance methodM-TestNanoindentation

Application of techniques

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Bending Test

Cantilever

( ) 32

3

14 lEbtkν−

=

k is the stiffness, E is the elastic modulus, b is the cantilever width,v is Poisson’s ratio,t is thickness, andl is the length of cantilever at the point of contact,

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Bending Test

Fixed-fixed Beam

E is the elastic modulus, b is the cantilever width,v is Poisson’s ratio,t is thickness, andl is the length of cantilever at the point of contact,

F = kbending z + kstress z + kstretching z3

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420

3

34

826z

LtEwz

Ltw

zL

tEw⋅+⋅+⋅=

ππσπ

bending, stress, and stretching components:Small loads: - bending and stressLarge loads: - Stretching

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Bulge Test

Pressure on circular membrane

342

0

1384

hErth

rt

pν

σ−

+=

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Resonance method

Vibrating cantilever2

1

2

2

0 34

=

ρπλ E

ltf i

i

Where E, ρ, l and t are the Young’s modulus, density, length and thickness of the cantilever. λi is the eigen value, where i is an integer that describes the resonance mode number;for the first mode λ =1.875

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M-Test

Set of cantilever, fixed-fixed beam, circular diaphragm, fabricated on substrate: actuated by electrostatic pullCharacterization is based on pull in voltage No necessity of displacement measurement

+

=

ωγε 0

30

30

127

8g

gKV

n

effpi

{ }

−+

=

22

22

sinhcosh121

kLkL

kLeff

L

SK

30

330

~,~,12 gtEBtgSBSk === σ

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Nanoindentation: AFM

Additional attachment to AFM

AEdHdPS rπ

2==

( ) ( )i

i

r EEE

22 111 νν −+

−=

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Conclusions

Various optical principles Characterization tools

MicroscopeEllipsometerProfilometer

Various methods of characterization of mechanical properties

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Next class

Polytec Laser Doppler Vibrometer [2]

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Atomic Force Microscope

Laser Alignment

Crucial issues-Alignment-Calibration