MEMS: Characterization

Characterization of MEMS Devices

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

Recap

Fabrication of MEMSConventional VLSI fabricationNonconventional methods

Design and analysis of MEMS

Characterization of MEMS

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

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

Why Optics for Characterization?

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

Principles of Optics

Wave nature of light

InterferenceWave divisionAmplitude division

Diffraction + Diffr. gratingMoire interferenceHolography

Principles of Optics

InterferenceWave divisionAmplitude division

Beam splitter

Michaelsons InterferometerAnalysis??

Young’s double slit

Referencemirror

Principles of OpticsInterference

Testdevice

Mach-Zehnder Interferometer

Used for laser-doppler vibrometer

Polarization

Concept of polarization of light

Principles of OpticsInterference

Febry-Parot Interferometer

Partially Reflecting Mirrors

Lens

Screen

Source

Another method for interference

Principles of OpticsDiffraction grating

Diffraction Fringes

Source

Diffraction Grating

Diffraction Grating Fringes

Principles of OpticsMoire Fringes

Specimen Grating

FringesMaster Grating

Rotational Mismatch Translational Mismatch

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

ProfilometerAnother technology

Sensor Camera

Spot size [µm] 1,5 Integrated in-axis camera

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

0,6x0,8

Measurement frequency [Hz] 10,000

Stand off [mm] 2 or 5 Laser diode Class I

Linearity [%] <0,08 Wavelength [nm] 630 copyright © Solarius Development Inc. 2003-04

ProfilometerAnother technology

copyright © Solarius Development Inc. 2003-04

Sensor Camera

Spot size [µm] 2 Integrated off-axis camera

Vertical resolution [µm] 0,1 Magnification 200x

Measurement frequency [Hz] 1400

Stand off [mm] 5 Laser Class II

Linearity [%] ±0.5 Wavelength [nm] 670

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

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

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.

Atomic Force Microscope

AFM Image

Kriptan- polymer surface characteristics using AFM

Conclusions

Various optical principles Characterization tools

MicroscopeEllipsometerProfilometer

Various methods of characterization of mechanical properties