MEMS: Characterization
Characterization of
MEMS Devices
Prasanna S. Gandhi
Assistant Professor,
Department of Mechanical Engineering,
Indian Institute of Technology, Bombay,
1
Recap
ƒ Fabrication of MEMS
ƒ Conventional VLSI fabrication
ƒ Nonconventional methods
ƒ Design and analysis of MEMS
ƒ Characterization of MEMS
2
Today’s Class
ƒ Why characterization?? Why optics??
ƒ Principles of optics useful in characterization
ƒ Tools for optical characterization
ƒ Profilometer
ƒ Microscope
ƒ Methods for characterization of mechanical
properties
ƒ SPM based tools: STM and AFM
3
Why Characterization?
ƒ Material properties change at micro-scale, different
from bulk properties due to grain boundary effect
ƒ Successful design/manufacturing of MEMS devices
need reliable knowledge of MEMS material properties
ƒ Verification of design and validation of models
proposed
ƒ Calibration of devices and signals
ƒ Electronic analysis: noise vs signal
„ Research various new effects: example Biosensor
devices
4
Why Optics for
Characterization?
ƒ
ƒ
ƒ
ƒ
ƒ
Noninvasive technique
Does not disturb sensitive MEMS device
Very high resolutions possible
Higher measurement range possible
Several optical phenomenon can be
made use of
5
Principles of Optics
Wave nature of light
ƒ Interference
ƒ Wave division
ƒ Amplitude division
ƒ Diffraction + Diffr. grating
ƒ Moire interference
ƒ Holography
6
Principles of Optics
ƒ Interference
ƒ Wave division
ƒ Amplitude division
Beam splitter
Young’s double slit
Reference
mirror
Michaelsons Interferometer
Analysis??
7
Principles of Optics
Interference
Test
device
Mach-Zehnder Interferometer
ƒ Used for laser-doppler vibrometer
8
Polarization
„
Concept of polarization of light
9
Principles of Optics
Interference
Source
Partially Reflecting
Mirrors
Lens
Febry-Parot Interferometer
Screen
ƒ Another method for interference
10
Principles of Optics
Diffraction grating
Source
Diffraction
Fringes
Diffraction Grating
Diffraction
Grating Fringes
11
Principles of Optics
Moire Fringes
Specimen Grating
Fringes
Master Grating
Rotational Mismatch
Translational Mismatch
12
Profilometer
„
„
A B
Profilometer principle
D C
„
Laser-photodetector
combination
As the scanning of sample
is done the laser spot
moves on the
photodetector (PSD)
because of bending of
cantilever over asperities
The movement results in
differential voltage output
from the PSD
13
Profilometer
Another technology
Sensor
copyright © Solarius Development Inc. 2003-04
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
14
Profilometer
Another technology
Sensor
copyright © Solarius Development Inc. 2003-04
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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
15
Microscope for
Measurement of Dimensions
„
Grating used in CD ROM
„
Taking image on CCD
camera and
processing with
precalibration for
measurement of
MEMS device
dimensions
Various types of
microscopes
16
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
17
SPM: STM and AFM
„
„
STM invented in early 80s by Binnig
and Rohrer.
Real limitations: only used to image
conducting materials. Cannot
distinguish between atoms of different
elements within a compound material.
18
Atomic Force
Microscope
19
AFM Image
ƒ Kriptan- polymer surface characteristics using AFM
20
Conclusions
ƒ Various optical principles
ƒ Characterization tools
ƒ Microscope
ƒ Ellipsometer
ƒ Profilometer
ƒ Various methods of characterization of
mechanical properties
21
Fundamentals of
Ellipsometry
Grating used in CD ROM
22
Fundamentals of
Ellipsometry
„
Change in
polarization
properties after
reflection
23
Ellipsometer
r
se
La
Detector
A
P
θ
Q
Surface
An 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.
24
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.
25
Next class
ƒ AFM technique and details of measurment
26
Next class
Polytec Laser Doppler Vibrometer [2]
27
Application of techniques
Characterization of
Mechanical Properties
Properties: E, ν, internal stress etc.
Various Techniques
„ Bending test
„
„
„
„
„
„
Cantilever
Beam
Bulge test
Resonance method
M-Test
Nanoindentation
28
Bending Test
„
Cantilever
Ebt 3
k=
4 1 −ν 2 l 3
(
)
k is the stiffness,
E is the elastic modulus,
b is the cantilever width,
v is Poisson’s ratio,
t is thickness, and
l is the length of cantilever
at the point of contact,
29
Bending Test
„
Fixed-fixed Beam
F = kbending z + kstress z + kstretching z3
wσ 0 π 2 t
Ewπ 4 t 3
Ewπ 4 t 3
=
⋅z+
⋅z+
⋅z
3
3
2L
6L
8L
bending, stress, and stretching components:
Small loads: - bending and stress
Large loads: - Stretching
E is the elastic modulus,
b is the cantilever width,
v is Poisson’s ratio,
t is thickness, and
l is the length of cantilever
at the point of contact,
30
Bulge Test
„
Pressure on circular membrane
4tσ 0
8t E 3
p = 2 h+ 4
h
r
3r 1 − ν
31
Resonance method
„
Vibrating cantilever
λt
f 0i =
4πl 2
2
i
 E 


 3ρ 
1
2
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
32
Profilometer
„
„
A B
Profilometer principle
D C
„
Laser-photodetector
combination
As the scanning of
sample is done the
laser spot moves on
the photodetector
(PSD)
The movement results
in differential voltage
output from the PSD
33