CHAPTER 2
EXPERIMENTAL STUDY
2.1
T-MEMS Fabrication
The T-MEMS beams are fabricated based on standard surface micromachining
techniques, as outlined in Figure 2.1. The substrate wafer is an n-type, <100> Si wafer
with 3” diameter. Three thin films are deposited on the wafer to begin the fabrication.
First layer (bottom) consists of ~1 m thermal oxide, deposited at 1100 °C. Second layer
is polycrystalline silicon, deposited by low pressure chemical vapor deposition (LPCVD)
at 610 °C. The thickness of this layer is approximately 0.6 m. Finally, a top layer of
SiO2 is deposited by LPCVD at 420 °C. The top layer is approximately 0.2 m thick.
Photoresist is deposited on the film structure to outline the top beam layer. The two top
layers (SiO2 and poly-Si) are etched to form the top layer. After rinsing the photoresist, a
thin layer (~0.2 m) of thermal SiO2 is grown on the structure at a process temperature of
1100 °C. This layer is a protective coating to prevent the poly-Si layer from being etched
during the final release process. Since thermal oxide grows primarily on silicon (in this
case, on the sides of the poly-Si layer), the increase in thickness of top SiO2 film is
believed to be negligible. Next, mask 2 is used to define the bottom layer of the beam.
The thermal SiO2 layer is etched to form the bottom layer. Final release process involves
etching the Si substrate to form a well under the beam. The process is an anisotropic etch
at 80 °C, which results in a 60 ° slope in the walls.
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1.
Deposit three layers of thin films
on n-type, single sided Si wafer
mask 1
2.
Deposit photoresist
(PR) to pattern top layer
using mask 1
LPCVD low thermal SiO2
LPCVD poly-Si
thermal SiO2
Si substrate
photoresist
3.
Pattern top oxide layer
4.
Pattern poly-Si layer
5.
Remove PR, and grow ~ 0.2 m thermal
SiO2 at 1100 °C (protective layer)
6.
Deposit PR to pattern
bottom layer using mask 2
mask 2
7.
Pattern bottom oxide layer
8.
Remove PR
9.
Release the beams
from substrate
Figure 2.1 T-MEMS fabrication process
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top view of
completed beams
The completed beams consist of 0.19 m SiO2, 0.54 m poly-Si, and 1.03 m SiO2
layers (from top), with ~23 m gap between the beam and Si substrate. The completed
beams have film thicknesses that are near, but not identical to, the originally deposited
thickness due to the various etching and rinsing solutions used; therefore, the layer
thicknesses were found by applying thin film interference model to the experimentally
measured spectral reflectivity. The model and results are discussed in following chapters.
The 23 m gap used in the samples are much larger than the original design dimension,
which was 6 m. The large depth allows the beams to be tested repeatedly for materials
property determination since the beams will not bend enough to touch the substrate. The
beam lengths range from 100 m to 50 m, at 1 m increment. Beams were fabricated
with 14 types of top-to-bottom width ratios, ranging from ~0.4 to ~0.8. The beams were
arranged on a die in 14 columns (for varying width ratios) and 51 rows (in order of
descending lengths). The layout of the beams on the die is shown in Figure 2.2.
changing widths
1
2
3
.
.
.
14
.
.
.
.
.
.
.
.
.
.
.
.
.
decreasing lengths
100 m
99 m
.
.
50 m
~ 4 mm × 4 mm
Figure 2.2 Layout of beams on T-MEMS die
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Figure 2.3 SEM micrograph of bending T-MEMS showing the upward initial
curvature. Beams shown are approximately 50 m in length.
SEM micrograph of the beams taken at room temperature reveals that they are bending
up due to residual stresses in the beams (Figure 2.3). This upward deflection typically
measures ~7 m for a 100 m beam. Residual stress in the beams is likely a combination
of thermal stress (from the mismatch in thermal expansion coefficient as beams cool from
deposition to room temperature) and deposition stress (such as interfacial mismatch).
2.2
Curvature Measurements
The curvature of T-MEMS at high temperature was measured in situ by an innovative
optical method. The setup, shown in Figure 2.4, consists of a heat source, sample holder,
sample illuminator, and a CCD camera. The heat source is a single tungsten-halogen
lamp with a parabolic-mirror housing that reflects light into a line. The sample, which
typically measures ~5 mm × 5 mm, is supported above the heater by two quartz rods and
a Si wafer. Next to the sample, a 5 mm square of a wafer with embedded thermocouple
(Sensarray) is placed to monitor temperature. An aluminum reflector plate, as well as a
quartz window with aluminum reflector foil, is placed above the wafer to reduce natural
convective currents and to reflect radiation back onto the sample for maximum heating.
A fan is positioned to force airflow over the quartz to further keep down natural
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CCD camera
Beam splitter
Light source
with collimator
Quartz plate
Al reflector
Thermocouple
Si wafer
Quartz rods
Sample
W-halogen lamp
Figure 2.4 Schematic of experimental setup for measurement
of thermally induced curvatures of T-MEMS
convection currents, which lead to image distortion. Under these conditions, the sample
can reach a temperature of up to 850 ºC.
For visualization, the sample is illuminated by a nearly-collimated beam from fiberoptics
bundle and a collimator, which reflects onto the sample through a 45 º cube beamsplitter.
The cube beamsplitter is superior to ordinary beamsplitter in minimizing “ghost” images
from secondary reflections within the beamsplitter. The light that is reflected from the
sample passes through the cube beamsplitter and is captured by the CCD camera. The
camera is equipped with telescopic lens with a minimum field-of-view of approximately
400 m.
The determination of beam curvature from the images is based on the narrow numerical
aperture of the camera lens, which limits the angle of light entering the camera. When
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collimated light reflects off the curved beam, light is scattered into some angle, the beam
tip having the largest scatter. Consequently, only the portion of the beam closest to its
base appear illuminated on the image captured by the CCD. The resulting “apparent
length” of the beam, lbeam, is the shorter for beams with larger curvature. This relation is
shown in Figure 2.5. There are three geometrical relations to be noted from the figure:
h  rc 1 cos  

(2.1)
Lbeam
rc
lbeam  rc sin  
(2.2)
sin 
K
(2.3)
Combining Eqns. 2.1 and 2.2 results in the following:


L
h  rc 1  cos beam 
rc 

(2.4)

In the above equations and in Fig. 2.5, rc is the
rc
beam

radius curvature of the beam, K is the curvature,
 is the arc angle formed by the beam from the
rc
h

base to lbeam,  is the arc angle formed by the
lbeam
entire beam length, Lbeam, and h is the tip
(a)
deflection of the beam. The arc angle formed by
the base at any point is equal to the angle
(b)
between the beam and the horizontal at that
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Figure 2.5 Geometry of beam.
a) side-view, b) top-view
point, that is, at lbeam, the beam is at an angle  from the horizontal.
The curvature at high temperature is found by first calibrating the system to a room
temperature curvature.
At room temperature, h can be measured on an optical
microscope. From h and Lbeam, using Eqn. 2.4, the curvature at room temperature is
found through Eqn. 2.3. Next, a grayscale value is selected as the “threshold value,”
which is used to find the apparent beam length, lbeam, for the room temperature image.
The angle , which remains constant through the entire run, is found by applying Eqn. 2.3
to lbeam and K at room temperature. For all subsequent images taken at high temperatures,
the point on the beam that has the same grayscale value as the threshold is assigned the
angle . From Eqn.2.3, the curvature can be found for all beams. The specifications for
the system, including resolution and limitations, are discussed in Appendix A.
The above relations hold for beams that are curved up and down if the light source,
camera, and sample were aligned perfectly. When there is a misalignment of angle 
between the sample and the light source, Eqns. 2.1 – 2.4 must be adjusted accordingly.
In general, misalignment affects the measurements only when it passes through zerocurvature. It has the effect of off-setting the apparent beam length, lbeam, by some amount
when the beam is deflecting down. The correction for misalignment angle  is discussed
in more detail in Appendix B.
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2.3
Radiative Property Measurements
An integrating sphere was used to measure the hemispherical, spectral reflectivity of
surfaces at 8 º angle of incidence. The schematic of this setup is shown in Figure 2.6.
The light, emitted by a 100 W tungsten-halogen lamp, is divided into its spectrum by a
monochromator. Order-sorting filters are used to eliminate higher order diffractions.
The monochromatic light is guided though a fiber optics bundle to a collimator, and
enters the sphere through the light port. The sphere has two additional ports, 1” each, for
the sample and reference, and a ½” port for the detector located at the top of the sphere.
The light is directed to illuminate the sample port at 8 º angle of incidence, and is
reflected by the sample in all directions, and is eventually captured by the detector. Two
types of thermoelectrically cooled detectors are used: Si photodiode for 400 – 1100 nm
range, and PbS photodiode for 1000 – 2000 nm range. The voltage signal from the
detector is sent into the lock-in amplifier which, in conjunction with an optical chopper is
inserted between the tungsten-halogen lamp and the monochromator, eliminates the
effects of background noise such as ambient light. This system has a spectral range of
400 – 2000 nm.
To measure reflectivity, the voltage is recorded with sample placed on the sample port,
and the reference placed on the reference port (Vsample). A reference is a reflective
material for which the spectral reflectivity has been calibrated. Here, a first-surface
aluminum mirror, calibrated to NIST-traceable standards, is used as the standard.
Typically, 100 voltage readings were averaged at each wavelength. Next, the voltage is
recorded after sample and reference is interchanged so that the light is directed onto the
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reference (Vreference). The two voltage readings are normalized with respect to the known
reference reflectivity:
 sam ple 
Vsam ple
Vreference
 reference
(2.5)
where sample and reference are the sample and reference spectral reflectivities,
respectively.
focusing mirror
sample port
monochromator
refe
integrating sphere
ren
8°
diffraction
gratings
ort
ce p
Si or PbS
detector
(on top)
Order-sorting
filters
Chopper
W-Hg lamp
fib
er
op
ti
cs
collimator
RS-232 interface
SR510 lock-in amplifier
chopper
controller
focusing mirror
PC
RS-232 interface
Figure 2.6 Schematic of experimental setup for spectral reflectivity measurements
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The integrating sphere was also used for high temperature measurements of spectral
reflectivity. Experimental methods and setup modifications were developed to enable the
high temperature measurements. Three key issues were addressed: 1) protecting the
sphere from thermal damage; 2) preventing detector heating due to radiation emitted from
heated sample port; and 3) minimizing the amount of transmitted radiation that reflects
on the heater and back into the sphere.
A combination of reflectors, convective fins, and passive cooling were developed to keep
the sphere at low temperatures. Aluminum reflectors placed around the sample port
prevented the radiation emitted by the heater from directly heating the sphere. The
reflectors also served as cooling fins by applying forced convection.
The performance of the two photodetectors proved to be highly sensitive to temperature,
and were cooled by passive coolants placed on the housing. The heating of the detector
element from radiation emitted by the hot sample also contributed to high noise level.
The sample port was reduced from the typical 1” used in room temperature measurement
to ¼” in order to reduce the amount of thermal radiation from the sample from entering
the sphere.
At room temperature, any incident radiation transmitted through the sample exits the
sphere into the environment. However, at high temperature, the transmitted radiation was
reflected from the heater, back into the sphere, which produced high reflectivity
measurements when Si became transparent. An aluminum ramp, sloped at 45 °, was
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developed to minimize this error
(Figure 2.7). The ramp is placed on
sphere wall
port reducer
the heater, which heats the sample.
45° reflector
The assembly is then pressed flush
heater
with the sample port, which is
sample
Figure 2.7 High temperature sample holder
for the integrating sphere
reduced to ¼” diameter by a port
reducer. The ramp was designed to reflect all light which is transmitted through the
sample away from the sphere to prevent re-entry. At room temperature, approximately
7% of the radiation hitting the ramp re-enters the sphere, which corresponds to an error of
up to 2% in the spectral reflectivity readings when the sample (silicon) is transparent.
This system has the same spectral range as the room-temperature system; however, noise
level in data is increased considerably, especially for the PbS detector, primarily because
less data points were used for averaging at each wavelength to reduce the total time
needed for the trials.
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