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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

Fabrication of MEMSConventional VLSI fabricationNonconventional methods

Design and analysis of MEMS

Characterization of MEMS

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Today’s Class

Why characterization?? Why optics?? Principles of optics useful in characterizationTools for optical characterization

ProfilometerMicroscope

Methods for characterization of mechanical propertiesSPM based tools: STM and AFM

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Why Characterization?

Material properties change at micro-scale, different from bulk properties due to grain boundary effectSuccessful design/manufacturing of MEMS devices need reliable knowledge of MEMS material propertiesVerification of design and validation of models proposedCalibration of devices and signalsElectronic analysis: noise vs signalResearch various new effects: example Biosensor devices

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Why Optics for Characterization?

Noninvasive techniqueDoes not disturb sensitive MEMS deviceVery high resolutions possibleHigher measurement range possibleSeveral optical phenomenon can be made use of

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Principles of Optics

InterferenceWave divisionAmplitude division

Diffraction + Diffr. gratingMoire interferenceHolography

Wave nature of light

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Principles of Optics

InterferenceWave divisionAmplitude division

Beam splitter

Michaelsons InterferometerAnalysis??

Young’s double slit

Referencemirror

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Principles of Optics

Used for laser-doppler vibrometer

Mach-Zehnder Interferometer

Testdevice

Interference

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Polarization

Concept of polarization of light

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Principles of Optics

Another method for interference

Febry-Parot Interferometer

Partially Reflecting Mirrors

Lens

Screen

Source

Interference

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Principles of OpticsDiffraction grating

Diffraction Fringes

Source

Diffraction Grating

Diffraction Grating Fringes

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Principles of OpticsMoire Fringes

Rotational Mismatch

Specimen Grating

FringesMaster Grating

Translational Mismatch

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Profilometer

Laser-photodetectorcombinationAs the scanning of sample is done the laser spot moves on the photodetector (PSD) because of bending of cantilever over asperitiesThe movement results in differential voltage output from the PSD

Profilometer principle

BA

D C

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Profilometer

630 Wavelength [nm]<0,08Linearity [%]

Class ILaser diode2 or 5Stand off [mm]

10,000Measurement frequency [Hz]

0,6x0,8

Field of view [mm]0,020Vertical resolution [µm]

Integrated in-axis camera1,5 Spot size [µm]

CameraSensor

Another technology

copyright © Solarius Development Inc. 2003-04

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ProfilometerAnother technology

copyright © Solarius Development Inc. 2003-04

670Wavelength [nm]±0.5Linearity [%]

Class IILaser5Stand off [mm]

1400Measurement frequency [Hz]

200xMagnification 0,1 Vertical resolution [µm]

Integrated off-axis camera2Spot size [µm]

CameraSensor

All contents copyright © Solarius Development Inc. 2003-04

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Microscope for Measurement of Dimensions

Taking image on CCD camera and processing with precalibration for measurement of MEMS device dimensionsVarious types of microscopesGrating used in CD ROM

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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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Atomic Force Microscope

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AFM Image

Kriptan- polymer surface characteristics using AFM

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Conclusions

Various optical principles Characterization tools

MicroscopeEllipsometerProfilometer

Various methods of characterization of mechanical properties

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Fundamentals of Ellipsometry

Grating used in CD ROM

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Fundamentals of Ellipsometry

Change in polarization properties after reflection

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Ellipsometer

Surface

Q

P

Laser

A

Detector

θAn ellipsometer measures the changes in the polarization state of light when it is reflected from a sample. If the sample undergoes a change, for example a thin film on the surface changes its thickness, then its reflection properties will also change. Measuring these changes in the reflection properties can allow us to deduce the actual change in the film's thickness.

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Ellipsometer: Advantages

Non destructive character, High sensitivity due to the measurement of the phase of the reflected light, Large measurement range (from fractions of monolayers to micrometers ), The possibilities to control in real time complex processes.

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Next class

AFM technique and details of measurment

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Next class

Polytec Laser Doppler Vibrometer [2]

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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

Ltwz

LtEw

⋅+⋅+⋅=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

rtp

νσ

−+=

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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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Profilometer

Laser-photodetectorcombinationAs the scanning of sample is done the laser spot moves on the photodetector(PSD)The movement results in differential voltage output from the PSD

Profilometer principle

BA

D C