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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