IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 12, DECEMBER 2002
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Fabrication and Characterization of
Polycrystalline SiC Resonators
Shuvo Roy, Member, IEEE, Russell G. DeAnna, Christian A. Zorman, Member, IEEE, and
Mehran Mehregany, Senior Member, IEEE
Abstract—This paper presents the development of polycrystalline 3C silicon carbide (polySiC) lateral resonant devices, which
are fabricated by a three-mask surface micromachining process
using silicon dioxide (SiO2 ), polysilicon, and nickel (Ni) as the
isolation, sacrificial, and contact metallization layers, respectively.
The polySiC resonators are packaged for operation in high
temperature environments using ceramic-based materials and
nickel wirebonding procedures. Device operation is successfully
demonstrated over 10 5 –760 torr and 22–950 C pressure
and temperature ranges, respectively. Quality factors ( s) of
100 000 at 10 5 torr and resonant frequency drifts of 18
ppm/h under continuous operation are achieved using an scanning
electron microscope (SEM) setup. Device resonant frequency
varies nonlinearly with increasing operating temperature. Finite
element modeling reveals that this variation resulted from the
interplay between the Young’s modulus of polySiC and induced
stresses, which occur due to mismatch in thermal expansion
coefficients of the polySiC film and the underlying silicon (Si)
substrate.
Index Terms—High temperature transducers, lateral resonant
devices, microelectromechanical systems (MEMS), microsystems,
polySiC, resonators, silicon carbide (SiC), surface micromachining.
I. INTRODUCTION
M
ICROMACHINED polysilicon resonators are emerging
as potential on-chip replacements for conventional
discrete oscillators and filters in high performance communication transceivers [1]. The integrated microelectromechanical
polysilicon devices exhibit frequency selectivity characteristics
active filtering techniques
that are superior to integrated
based upon traditional electron devices. Micromachined resonator s of 80 000 under vacuum conditions and resonant
frequency coefficients of 10 ppm/ C have been reported
transceiver components, the utility
[2]. In addition to high
of polysilicon resonators has also been demonstrated in a
number of other applications including mechanical properties
Manuscript received May 30, 2002; revised October 14, 2002. This work was
supported by DARPA MTO under Grant DABT63-98-1-0010 and ARO/MURI
under Grant DAAH04-95-10097. The review of this paper was arranged by
Editor K. Najafi.
S. Roy is with the Department of Biomedical Engineering, The Cleveland
Clinic Foundation, Cleveland, OH 44195 USA (e-mail: roys@bme.ri.ccf.org).
R. DeAnna was with the U.S. Army Research Laboratory, Vehicle Technology Center, NASA Glenn Research Center at Lewis Field, Cleveland, OH
44135 USA. He is now with Advanced Engineering Technologies, Norcross,
GA 30071 USA.
C. Zorman and M. Mehregany are with the Department of Electrical Engineering and Computer Science, Case Western Reserve University, Cleveland,
OH 44106 USA.
Digital Object Identifier 10.1109/TED.2002.807445
testing, pressure sensing, and inertial navigation systems
[3]–[5]. However, the electrical and mechanical properties of
polysilicon begin to rapidly degrade at elevated temperatures
( 350 C), making it increasingly unsuitable for high temperature applications [6]–[8]. In contrast, SiC is well known for its
mechanical characteristics, such as high Young’s modulus and
yield strength, chemical inertness, high thermal conductivity,
and electrical stability at temperatures well above 600 C [8],
[9]. Although these material properties and microfabrication
compatibility of SiC make it an attractive structural material
and/or high temperature microelecfor fabrication of high
tromechanical devices, the development of SiC as a structural
material has been limited by a combination of fabrication,
packaging, and testing challenges.
The lack of a durable wire bonding technology is a major impediment to the implementation of SiC-based devices for high
temperature applications. We have developed a nickel (Ni) wire
bonding process for Ni contact pads on 3C-SiC films using conventional tools and wire diameters and demonstrated reliable
operation up to 550 C [10]. Ni is attractive as a contact metal
for high temperature SiC devices in that Ni has a melting temperature of 1453 C, forms an ohmic contact to both n-type and
p-type 3C-SiC substrates, and is also available in wire diameters
similar to that of the Al and Au bonding wires (e.g., 25 m).
For high temperature applications, it is desired that the contact
pad and the wire be of the same material because intermetallic
growth and interface corrosion, which can occur between dissimilar metals at high temperatures, results in a rapid degradation of bond strength and a shift in the principal failure mode
from wire breaks to bond lifts. The stability of the wire bonds
is threatened by such a problem, especially at elevated temperatures. Therefore, reliability should be improved when pad metallization and bonding wires are of the same material.
This paper presents the development of polycrystalline
SiC (polySiC) folded beam lateral resonant devices (subsequently called resonators) that are fabricated, packaged, and
successfully operated over a wide range of pressures and
temperatures. First, the fabrication of polySiC resonators by a
surface micromachining process using silicon dioxide (SiO ),
polysilicon, and Ni as the isolation, sacrificial, and contact
metallization layers, respectively, is presented. Details of the
resonator packaging scheme using ceramic-based materials and
Ni wirebonding procedures are then outlined. Next, testing procedures and optical and scanning electron microscope-based
experimental setups to investigate changes in resonator
with pressure and resonant frequency with temperature are
described. Finally, resonator testing results at both room and
0018-9383/02$17.00 © 2002 IEEE
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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 12, DECEMBER 2002
Fig. 1. Schematic description of polySiC resonator fabrication showing cross-sections after (a) sputter deposition of Al; (b) definition of resonator pattern by
plasma etching; (c) sputter deposition of Ni; (d) definition of Ni contact regions by photolithography and wet etching; and (e) release of polySiC resonators in
KOH.
high temperature are presented and the behavior of resonant
frequency with temperature is examined using finite element
analysis.
II. FABRICATION POLY SiC RESONATORS
BY SURFACE MICROMACHINING
A. PolySiC Film Growth
Substrates are prepared by depositing polysilicon films on oxidized Si wafers. A 1.5 m-thick SiO film is grown thermally
on 100-mm diameter, (100) Si wafers. A 3.5 m-thick polysilicon film is then deposited on the SiO film in a low-pressure
chemical vapor deposition (LPCVD) furnace, using silane as the
precursor gas. The deposition pressure, temperature, and growth
rate are 270 mtorr, 610 C, and 0.42 m/h, respectively. Afterwards, the polysilicon substrates are rinsed in deionized (DI)
water and spin-dried in nitrogen to remove any particulate contaminants.
PolySiC films are grown in a cold-wall, rf-induction-heated,
vertical APCVD reactor using a growth procedure similar to a
three-step process to grow epitaxial 3C-SiC films on (100) silicon wafers [11], [12]. The three-step deposition process begins
with an in situ hydrogen etch, followed by the formation of a
carbonized layer on the polysilicon surface, and continued by
bulk polySiC film growth. The hydrogen etch is performed at
1000 C and is used to remove the native oxide, as well as any
metallic and organic contaminants from the polysilicon surface
[11]. After the hydrogen etch, the susceptor is cooled to below
500 C. Carbonization is then initiated by heating the susceptor
to the growth temperature of 1280 C under a stable flow of 15%
propane in hydrogen (84 sccm) and hydrogen (25 slm). Once the
growth temperature is attained, temperature and flow rates are
held constant for 90s. During carbonization, propane flowing
over the heated susceptor decomposes into reactive hydrocarbon
radicals, which adsorb on the substrate surface and react with
free Si atoms to form SiC. The hydrogen carrier gas reacts with
and removes nonstoichiometric deposits in the carbonized silicon, leaving a thin polySiC film on the substrate surface. This
reaction will continue until the thickness of the polySiC film
is 100 Å. At this thickness, Si no longer diffuses to the surface in amounts sufficient to sustain film growth at reasonable
rates [13]. Growth is continued by simultaneously reducing the
propane flow to 26 sccm and introducing 5% silane in hydrogen
at 102 sccm. Temperature and flow rates are held constant for
the duration of the deposition. Using this procedure, 2 m-thick
polySiC films were grown on single wafers at a growth rate of
1 m/h. Secondary ion mass spectroscopy (SIMS) measurements of SiC films grown on silicon under identical conditions
revealed that the films were unintentionally doped with nitrogen
to a concentration of 4E18 cm [11].
B. Surface Micromachining Process
The polySiC films deposited on the polysilicon layers
are mechanically polished using a SiC slurry to reduce the
ROY et al.: FABRICATION AND CHARACTERIZATION OF POLYCRYSTALLINE SiC RESONATORS
Fig. 2. SEM micrograph of a released polySiC lateral resonant device.
The suspension beam lengths and widths are nominally 100 m and 2.5
m, respectively. Exposed polySiC shows up as dark gray, while the Ni
metallization appears light gray.
surface roughness (Ra) from 400 Å on the as-grown films to
40 Å [14]. The polishing process results in a final polySiC
film thickness of 1.75 m. Fig. 1 outlines the fabrication of
polySiC resonators by surface micromachining. A 5000 Å-thick
aluminum (Al) film is deposited on the polished polySiC films
by sputtering and subsequently patterned using photolithography and aluminum (Al) etchant to delineate the resonator
geometry. Next, the resonator pattern is defined in the polySiC
by dry etching in a CHF /O /He plasma with the patterned Al
acting as etch mask [15]. Afterwards, the Al mask is stripped
and a 7500 Å-thick Ni film is sputter deposited, patterned by
photolithography, and wet etched using commercial Ni etchant
to define nickel contacts to the polySiC. Finally, the resonator
is released by a timed etch of the sacrificial polysilicon in
40 wt.% KOH at 40 C and dried using a supercritical CO
drying process. Fig. 2 presents a SEM micrograph of a released
polySiC resonator.
PolySiC resonators are fabricated on three different substrates (i.e., three separate polySiC growth runs), termed A, B,
and C. After release and supercritical drying, but prior to packaging, all devices are examined under an optical microscope for
possible fabrication flaws and/or damage from handling. Resonators are also manually probed using a micromanipulator to
check for stiction. Flaw-, damage-, and stiction-free resonators
are then examined using SEM to confirm structural integrity
and measure resonator geometry. Using this procedure, 5
resonators from each substrate are selected for packaging and
testing. These 15 resonators are subsequently termed A1–5,
B1–5, and C1–5, where the letter identifies the substrate and
the numeral indicates the particular device.
III. PACKAGING OF POLY SiC RESONATORS
The released resonator chip is packaged for high temperature
operation onto a ceramic plate with patterned gold pads and
steel posts as shown in Fig. 3. The chip is attached to the
ceramic plate using commercially available zirconia cement
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(AREMCO Ultra-Temp 516), which is thermally conductive,
stable to 1760 C, and resistant to chemical attack. The gold
pads on the ceramic plate enable electrical connections from
the resonator package to external circuitry through the stainless
steel screws, nuts, and washers. Electrical connections from the
Ni contact pads on the resonator chip to the gold pads on the
ceramic plate are realized by 25 m-diameter Ni wires, which
are attached by a thermosonic wirebonding process optimized
for maximum pull strength [10]. In a typical thermosonic wirebonding procedure, the resonator package is first mounted on a
stage and heated to 250 C. The Ni bonding wire is then guided
to the first bonding site on a Ni contact pad of the polySiC
resonator and pressed onto the surface with a force of 340
mN by a titanium carbide (TiC) wedge mounted on a Kulicke
& Soffa 4123 ultrasonic wire bonder. While the wire is firmly
clamped between the contact pad and TiC wedge, a 12 ms
pulse of 520 mW ultrasonic vibration is applied to the wedge.
The ultrasonic energy causes localized wire deformation that
breaks up the surface oxides at the bonding site, resulting
in cold weld between the wire and contact metallization as
shown in Fig. 4. Afterwards, the wedge, along with the wire,
is lifted and positioned at a second bonding site on the gold
pad on the ceramic plate and bonded similarly, forming a wire
loop anchored at the nickel and gold pads. The wire clamps
then retract, pulling the wire and severing it near the end of
the second bond. The wirebonding procedure is repeated to
connect other contact pads of the resonator to corresponding
gold pads on the ceramic plate.
IV. TESTING PROCEDURES
The packaged resonators are tested under different pressure
and temperature conditions. Fig. 5 presents the electrical circuit scheme for resonator testing. A signal generator consisting
of a HP 33 210A variable function generator connected to a
Krohn-Hite 7602M wide-band power amplifier is used to apply
a 0–200 Vpp sinusoidal excitation voltage to the comb drive of
the resonator. The polySiC shuttle and silicon substrate are both
electrically grounded during testing and a high value series resistance (10 M ) is incorporated to provide short circuit protection. Resonance is determined visually by adjusting the excitation voltage frequency applied from the signal generator to
the drive pad of the resonator until maximum resonator amplitude is observed. The application of a pure sinusoidal signal to
the comb drive leads to a frequency doubling effect, and consequently, the resonant frequency is twice the frequency of the
excitation voltage at which maximum resonator amplitude is observed [16]. The resonant frequency is related to device geometry and material properties according to [17]:
(1)
is the resonant frequency, is the Young’s modulus
where
is the suspension beam
of polySiC, is the film thickness,
is the shuttle mass,
width, is the suspension beam length,
is the mass of the folding trusses, and
is the total mass
of the suspension beams. The 3 dB bandwidth of the resonator
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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 12, DECEMBER 2002
Fig. 3. Packaged device chip suitable for high temperature operation: (a) photograph showing device mounted on ceramic plate with gold pads and stainless steel
screws, washers, and hex nuts; and (b) cross-sectional schematic view across line AA′ in (a).
Fig. 4. SEM micrograph of nickel wirebonds on a nickel contact pad of a
polySiC resonator at 22 C.
Fig. 5. Electrical circuit scheme for testing polySiC resonators. The
suspension beam lengths and widths are 250 m and 2.5 m, respectively.
vibration amplitude spectrum is estimated by adjusting the frequency of the excitation voltage until the resonator amplitude is
71% of the maximum value [16]. Knowledge of the resonant
frequency and 3 dB bandwidth is used to determine the resonator using
(2)
is the 3 dB bandwidth.
where
Fig. 6 presents the experimental setup used to test devices
at atmospheric pressure (760 torr). The resonator package is
placed on a heater capable of attaining temperatures as high as
1000 C. The heater is mounted on a thermally-insulating ceramic tile inside an aluminum chamber with openings for argon,
electrical connections, and optical access. A variac is used to
control the power to the heater, and hence, the temperature of
the resonator, which is determined by the thermocouple. Probe
tips are pushed against the screws to electrically connect the resonator to the test circuitry. A long working distance microscope
is used to monitor device motion through the Pyrex watch glass
that protects the microscope optics from thermal damage during
high temperature operation. During testing, argon that is regulated to 2 psi is introduced into the chamber during testing to
minimize thermal oxidation effects on the probe tips and heater.
Fig. 6. Test setup for atmospheric pressure (760 torr) experiments. Heater
temperature is controlled using the variac. Device motion is observed using the
optical microscope. Measurement uncertainty in resonant frequency is 50 Hz.
6
Fig. 7 presents the experimental setup used to test devices
under vacuum conditions. The resonator package is placed on
a movable hot stage mounted inside an environmental SEM
(Philips XL30 ESEM), which uses a lanthanum hexaboride
(LaB ) filament in the electron gun. Electrical feedthroughs
are used to connect the resonator package to external test
circuitry. The temperature of the hot stage is measured using
ROY et al.: FABRICATION AND CHARACTERIZATION OF POLYCRYSTALLINE SiC RESONATORS
Fig. 7. Schematic description of the environmental SEM setup used for
experiments under vacuum conditions. Device motion is observed on an
attached monitor. Electrical feedthroughs are used to connect components to
1 Hz.
external circuitry. Measurement uncertainty in resonant frequency is
<6
a thermocouple and resonator motion is observed using a
video monitor attached to the electron microscope optics. In
addition to the room-temperature imaging capabilities of a
standard SEM under high vacuum (chamber base pressure of
2.6 10 torr) conditions, the environmental SEM can also
perform imaging under low vacuum (0.1–20 torr) and/or high
temperature ( 1000 C) conditions.
V. RESULTS
A. Room Temperature Testing
PolySiC resonators are tested using both the atmospheric and
environmental SEM setups at room temperature. Young’s modulus values are determined using (1) with experimentally-determined resonant frequencies, measured geometries of the resonators, and assumed polySiC density of 3230 kg/m to calculate the mass of the shuttle, trusses, and beams [18]. Resonator s are determined using (2) with the experimentally-determined 3 dB bandwidths. The wire bonds on seven of the
15 packaged polySiC resonators broke due to handling mishaps
during testing, and consequently, only three devices (A1, B2,
and C1) are characterized under all pressure conditions.
Table I presents a summary of resonator s and extracted
Young’s modulus values of polySiC films using the atmospheric
pressure setup at room temperature (22 C). Resonant frequencies and 3 dB bandwidths of the resonators are determined
by applying excitation voltages of 40–170 Vpp, depending on
specific device geometry, which provided a vibration amplitude
of 5 m at resonance. The extracted Young’s modulus values
range from a low of 250 GPa for Device #B3 to a high of 427
GPa for Device #B4. The variation in the extracted Young’s
modulus values can be attributed to errors in measurement of
resonator geometry and unintended fluctuations in growth conditions in the APCVD furnace during polySiC film growth. Nevertheless, the Young’s modulus are generally consistent with
the results of a preliminary study reported previously [19]. Resonator s range from a low of 25 for Device #A1 to a high of
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152 for Device #B3. Examination of the data does not reveal any
readily observable relationship between the extracted Young’s
modulus values and resonator s. However, there is a general
positive correlation between resonator and the resonant frequency: devices with higher resonant frequencies exhibit higher
s than lower resonant frequency devices. In the atmospheric
pressure test setup, damping resulting from Couette flow underneath the shuttle is the dominant energy dissipation mechanism,
and consequently, the resonator is proportional to stiffness of
the resonator suspension [16]. Therefore, resonators with higher
suspension stiffness, and hence, higher resonant frequencies,
exhibit higher s than lower resonant frequency devices.
Table II presents a summary of resonator s and extracted
Young’s modulus values of polySiC determined using the
environmental SEM test setup at room temperature under low
vacuum conditions (0.1–0.2 torr). Exact pressure control in the
SEM is not possible under the low vacuum conditions and pressure determination is difficult since the pressure gauge readings
are not reliable near the base pressure of the mechanical pump,
which is approximately 0.1–0.2 torr. Resonant frequencies and
3 dB bandwidths of the resonators are determined by applying
excitation voltages of 2–10 Vpp, depending on specific device
geometry, which provided a vibration amplitude of 5 m
at resonance. Resonator s and Young’s modulus values are
determined in a manner similar to those for the atmospheric
pressure testing described earlier. Resonator s at low vacuum
are significantly larger than resonator s determined under
atmospheric pressure conditions due to decreased damping effects at 0.1–0.2 torr. However, Couette flow is still the dominant
energy dissipation mechanism under low vacuum conditions
since resonator s increase with resonant frequency from a
low 2875 for Device #A1 (10 055.3 Hz resonant frequency) to a
high of 3672 for Device #B5 (29 920.5 Hz resonant frequency).
The extracted Young’s modulus values ranging from a low of
310 GPa for Device #C1 to a high of 413 GPa for Device #B2
are consistently within the experimental error of those values
obtained using the atmospheric pressure test setup as well as
previous reports [19].
Table III presents a summary of resonator s and extracted
Young’s modulus values of polySiC determined using the environmental SEM test setup at room temperature under high
torr). Although chamber pressures
vacuum conditions (
as low as 2.6 10 torr are attainable, most determinations
of resonant frequencies and 3 dB bandwidths are conducted
at 5–9 10 torr. Fig. 8 presents a SEM micrograph of the
shuttle section of an actuating resonator at 9 10 torr. Resonators required excitation voltages of 0.3–0.8 Vpp, depending
on specific device geometry, which provided amplitudes of at
least 5 m at resonance. Resonator s and Young’s modulus
values are determined in a manner similar to those described
for the low vacuum testing earlier. Resonator s at high vacuum
are significantly larger than s determined under low vacuum
conditions, ranging from a low of 57 828 for Device #C1 to a
high of 107 926 for Device #B2. In contrast to resonator s
exhibited under atmospheric pressure and low vacuum conditions, s at high vacuum are not necessarily greater for higher
resonant frequency devices since thermoelastic friction and anchor losses, not Couette flow damping, are the primary energy
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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 12, DECEMBER 2002
TABLE I
RESONANT FREQUENCY,
Q, AND YOUNG’S MODULUS OF POLYSiC AT 760 torr
TABLE II
RESONANT FREQUENCY,
Q, AND YOUNG’S MODULUS OF POLYSiC AT 0.1–0.2 torr
RESONANT FREQUENCY,
Q, AND YOUNG’S MODULUS OF POLYSiC AT <10
TABLE III
dissipation mechanisms [17]. The extracted Young’s modulus
values, ranging from a low of 310 GPa for Device #C1 to a high
of 414 GPa for Device #B2, are consistently within the experimental error of those values obtained under low vacuum and atmospheric pressure conditions as well as previous reports [19].
An 800 million cycle, 16 h aging test performed at 22 C on
Device #A4 reveals a resonant frequency drift of 17.5 ppm/h.
torr
B. High Temperature Testing
Fig. 9 presents graphs showing the variation of resonant frequency with temperature for two polySiC resonators with comparable geometries. Device #B3 is actuated with an excitation
voltage of 150 Vpp inside the atmospheric pressure test setup.
The corresponding resonant frequency drops from 30 400 Hz at
ROY et al.: FABRICATION AND CHARACTERIZATION OF POLYCRYSTALLINE SiC RESONATORS
Fig. 8. SEM micrograph of shuttle section near a comb drive of a polySiC
10
torr and 22 C. Frequency of the
resonator under actuation at 9
excitation voltage is 10 059.94 Hz. The distance between blur edges is the
resonator amplitude.
2
22 C to 30 000 Hz at 410 C. Resonator actuation is not observed above 500 C in the atmospheric pressure setup. However, upon cooling to temperatures below 500 C, resonator
operation resumes, which suggests the possibility of dielectric
breakdown of the electrically isolating SiO layer above 500 C.
Device #C1 is actuated with an excitation voltage of 3.5 Vpp
under low vacuum (0.1–0.2 torr) conditions. The resonator operates over a 22–950 C temperature range. In contrast to device
operation in the atmospheric pressure setup, the lower excitation
voltage used to resonate the polySiC actuators under vacuum
conditions ensures that electrically isolating SiO layer does not
exhibit dielectric breakdown at elevated temperatures. The resonant frequency of the vacuum-tested device drops steadily from
28 930 Hz at 22 C to 28 402 Hz at 700 C, followed by an
increase to 28 544 Hz at 900 C, and then a slight decrease to
28 500 at 950 C. Resonator actuation at temperatures 950 C
is not attempted due to concerns about the lifetime of the LaB
filament in the electron gun of the environmental SEM.
The resonator package survives repeated pressure
( 10 –760 torr) and thermal (22–950 C) cycling without
failure. Nickel wirebonds on resonator contact pads were examined in the SEM during high temperature operation at 500 C
and after cooling down to room temperature from 950 C. The
wirebonds do not exhibit any observable degradation due to the
thermal cycling or high temperature.
Finite element analysis is performed to investigate changes in
resonant frequency with increasing temperature. A preliminary
finite element model is analyzed using ANSYS5.3 software to
examine interactions between the polySiC resonator and underlying silicon substrate [20]. Fig. 10 presents the model of a 2
m thick resonator with 150 m long and 3 m wide suspension beams anchored on a 3.5 m thick polysilicon layer overlying a 1.5 m thick SiO film on the silicon substrate, which is
a typical geometric construction of devices tested. The polySiC
layer is meshed with 20-node brick elements, while underlying
layers are meshed with eight-node brick elements. The bottom
of the silicon substrate is fixed in the direction normal to the surface and allowed free thermal expansion in directions parallel
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to the surface. Fig. 11 presents a graph showing the variation of
thermal expansion coefficients with temperature for 3C-SiC (assumed equivalent to polySiC) and silicon, while Table IV lists
the other material properties used in the finite element analysis
[20]–[22].
Fig. 12 presents graphs of normalized resonant frequency
versus temperature based on the results of the finite element
analysis and the experimental data (vacuum conditions) that is
presented in Fig. 9. Although the geometry of the resonator in
finite element model is different from the resonator that is actually tested inside the SEM, there are certain similarities in the
behavior of resonant frequency with increasing temperature. In
both cases, there is an initial steady decrease in resonant frequency, followed by a slight increase, and another slight decrease. The differences between the finite element results and
experimental data can be attributed to differences in resonator
geometry as well as variations between the actual material property parameters and those used in the model as well as the influence of intrinsic stresses in the various material layers.
Examination of the material properties used in the finite element model provides qualitative insight into the resonant frequency versus temperature behavior for the polySiC resonators.
The thermal expansion coefficient of SiC is lower than that of
silicon from 22 C to 200 C, while at higher temperatures,
the thermal expansion coefficient of SiC is greater than that of
silicon. Consequently, at temperatures below 200 C, the silicon substrate expands faster than the polySiC layer, which induces a resultant compressive thermal stress in the suspension
beams of the resonator. This scenario is consistent with results
of the finite element analysis, which shows that the anchored
ends of the suspension beams of the modeled resonator experience 5 MPa compressive stress at 177 C. The net effect of
the induced compressive stress in the suspension beams and decrease in Young’s modulus of polySiC leads to a decrease in the
resonant frequency. As temperature increases above 200 C, the
polySiC film begins to expand faster than the silicon substrate,
which induces a resultant tensile stress in the resonator suspension beams. This shift in stress is also consistent with results
of the finite element analysis, which shows that the anchored
ends of the suspension beams of the modeled resonator experience 60 MPa tensile stress at 727 C. Consequently, the lowering effect on the resonant frequency due to the decrease in
Young’s modulus is counteracted by the induced tensile stress,
which tends to increase resonant frequency. The net effect of
increasing tensile stress and decreasing Young’s modulus with
increasing temperature leads to a resonant frequency minimum
( 475 C in Fig. 12), and then, an increase. However, as the
temperature increases even further ( 825 C in Fig. 12), the effect of the decrease in Young’s modulus dominates, leading to
the eventual decrease in resonant frequency.
VI. CONCLUSION
PolySiC resonators have been fabricated by a surface micromachining process using SiO , polysilicon, and Ni as the
isolation, sacrificial, and contact metallization layers, respectively. The resonators are packaged using ceramic-based materials, stainless steel fasteners, and nickel wirebonding proce-
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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 12, DECEMBER 2002
Fig. 9. Graphs showing variation of resonant frequency with temperature for two polySiC resonators—one tested at atmospheric pressure (760 torr) and the other
tested under vacuum conditions (0.1–0.2 torr). At atmospheric pressure, the resonator excitation voltage is 150 Vpp and the device does not operate above 500 C
due to breakdown of insulating SiO layer. In contrast, the resonator operates at temperatures as high as 950 C under vacuum conditions with an excitation voltage
of 3.5 Vpp.
Fig. 10.
Solid model of a 2 m-thick polySiC resonator investigated using finite element analysis. The suspension beams are 150 m long and 3 m wide.
Fig. 11. Graphs of the thermal expansion coefficients of polySiC and silicon
as a function of temperature.
dures. Optical and SEM-based setups are built to test the devices
under atmospheric pressure (760 torr) and vacuum conditions
( 10 torr), as well as temperatures up to 950 C. Resonator
s increase with vacuum level to as high as 100 000 at 10
torr at 22 C due to decreased damping-related energy dissipation. Resonant frequency drifts of 18 ppm/h under continuous
operation have been observed. Resonator packaging survived
repeated pressure ( 10 –760 torr) and thermal (22–950 C)
cycling without failure. High temperature testing revealed that
device resonant frequency decreases by 2% between 22 C
and 700 C, followed by a slight increase at 900 C, and then,
eventual decrease at 950 C. Finite element analysis reveals that
changes in resonant frequency with increasing temperature depend on the interplay between the decrease in Young’s modulus
of polySiC and induced stress in the suspension beams of the
resonators, which occurs due to mismatch in thermal expansion
coefficients of the polySiC film and the underlying substrate.
ROY et al.: FABRICATION AND CHARACTERIZATION OF POLYCRYSTALLINE SiC RESONATORS
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TABLE IV
MATERIAL PROPERTIES FOR FINITE ELEMENT ANALYSIS
Fig. 12. Graphs showing the similarity in relative changes of device resonant frequency as a function of temperature for the finite element model in Fig. 10 and
experimental data in Fig. 9.
ACKNOWLEDGMENT
The authors would like to thank the following people for their
valuable assistance: A. K. McIlwain in the Department of Materials Science and Engineering, Case Western Reserve University (CWRU); A. J. Fleischman in the Department of Biomedical Engineering, The Cleveland Clinic Foundation; R. K. Burla,
currently at Microsoft Corporation; L. Dudik in the Electronics
Design Center, CWRU; and S. Yu in the Electronics Design
Center, CWRU.
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[10] R. K. Burla, S. Roy, V. M. Haria, C. A. Zorman, and M. Mehregany,
“High temperature testing of nickel wirebonds for SiC devices,” in
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[11] C. A. Zorman, A. J. Fleischman, A. S. Dewa, M. Mehregany, C. Jacob,
S. Nishino, and P. Pirouz, “Epitaxial growth of 3C-SiC films on 4-inch
diameter (100) silicon wafers by atmospheric-pressure chemical vapor
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1998.
[13] C. A. Zorman, S. Roy, C.-H. Wu, A. J. Fleischman, and M. Mehregany, “Characterization of polycrystalline silicon carbide films grown
by atmospheric pressure chemical vapor deposition on polycrystalline
silicon,” J. Mater. Res., vol. 13, pp. 406–412, 1998.
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using CHF /O and CHF /O /He plasmas at 1.75 Torr,” J. Vac. Sci.
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Shuvo Roy (M’95) received the B.S. degree (magna
cum laude) with General Honors for triple majors
in physics, mathematics (Honors), and computer
science from Mount Union College, Alliance, OH,
in 1992. He received the M.S. and Ph.D. degrees in
electrical engineering from Case Western Reserve
University, Cleveland, OH in 1995 and 2001, respectively. While pursuing his doctorate, he conducted
research in the areas of design, microfabrication,
packaging, and performance of MEMS for harsh
environments.
He is a Co-Director of the BioMEMS Laboratory in the Department of
Biomedical Engineering at The Cleveland Clinic Foundation (CCF). He also
investigated microstructural characteristics and mechanical properties of
MEMS materials, developed the requisite microfabrication technologies and
demonstrated operation of the first surface-micromachined silicon carbide
transducers at high temperatures (up to 950 C). He has also developed
miniaturized micro-relays for high performance electrical switching and
ice detection sensors for aerospace applications. He joined CCF in 1998 to
develop MEMS technology for various biomedical applications (BioMEMS)
including surgical instruments, noninvasive monitoring, bioartificial organs,
portable diagnostics, and drug delivery. He is also investigating new and novel
materials for BioMEMS including polymers and natural proteins. He has over
21 technical publications, co-authored one book chapter, and given over 20
invited presentations.
Dr. Roy is the recipient of a Top 40 under 40 award by Crain’s Cleveland Business in 1999 and the Clinical Translation Award at the 2nd Annual BioMEMS
and Biomedical Nanotechnology World 2001 meeting.
Russell G. DeAnna received the B.S. degree in mechanical engineering from The Ohio State University,
Columbus, in 1982, the M.S. degreefrom the University of California, Berkeley, in 1985 and the Ph.D. degree in mechanical and aerospace engineering from
Case Western Reserve University (CWRU), Cleveland, OH, in 1993.
He worked at General Electric Nuclear Energy
Business, San Jose, CA in reactor safety analysis before joining NASA Glenn Research Center at Lewis
Field, Cleveland, in 1985. At NASA Glenn, he was
employed by the Vehicle Technology Center of the Army Research Laboratory
(ARL). This laboratory is the critical link between the scientific and military
communities. He has developed expertise in both empirical testing and analytical modeling of turbo machinery components. He later worked on MEMS
sensors and instrumentation for gas-turbine engines with ARL/NASA in collaboration with Prof. Mehran Mehregany of CWRU. In 2000, he joined Movaz
Networks, a telecommunications company developing an all-optical, cross connect. He is the principal designer of the MEMS micro-mirror switch. He joined
Advanced Engineering Technologies, Norcross, GA, in late 2002 to work on finite element modeling and analysis applications.
IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 49, NO. 12, DECEMBER 2002
Christian A. Zorman (M’96) received the B.S. degree (cum laude) in physics and the B.A. degree (cum
laude) in economics from the The Ohio State University, Columbus, in 1988, and the M.S. and Ph.D.
degrees in physics from Case Western Reserve University (CWRU), Cleveland, OH, in 1991 and 1994,
respectively. His doctoral research involved an investigation of the secondary electron emission properties
of CVD diamond films for vacuum electronics.
Dr. Zorman joined the MEMS program at CWRU
in 1994 as a Research Associate and immediately
began working in the SiC MEMS area. He was promoted to Senior Research
Associate in 1997 and Researcher in 2000. In addition to his research positions
within the University, he has held appointments as Adjunct Assistant Professor
in the Department of Electrical Engineering and Computer Science and Interim
Administrative Director of the Microfabrication Laboratory. He currently is
an Associate Professor in EECS at CWRU. He has been instrumental in the
construction of AP- and LPCVD reactors for SiC thin films, and has led the
development of recipes for the growth of single and polycrystalline 3C-SiC
films for micromachined sensors and actuators. In addition to the development
of novel bulk and surface micromachining techniques for SiC, he was a key
contributor in the development of novel polishing, wafer bonding, and low
defect density growth processes for SiC. He has published over 85 technical
papers, two book chapters, and has taught several short courses on SiC for
MEMS.
Dr. Zorman is a past chairman of the MEMS Technical Group in the American Vacuum Society and is currently serving as co-chairman.
Mehran Mehregany (SM’00) received the B.S.
in electrical engineering from the University of
Missouri, Columbia, in 1984, and the M.S. and
Ph.D. degrees in electrical engineering from the
Massachusetts Institute of Technology, Cambridge,
in 1986 and 1990, respectively.
From 1986 to 1990, he was a consultant to the
Robotic Systems Research Department at AT&T
Bell Laboratories, where he was a key contributor to
ground-breaking research in microelectromechanical
systems (MEMS). In 1990, he joined the Department
of Electrical Engineering and Applied Physics at Case Western Reserve University (CWRU), Cleveland, OH, as an Assistant Professor. He was awarded
the Nord Assistant Professorship in 1991, was promoted to Associate Professor
with tenure in July 1994, and was promoted to Full Professor in July 1997. He
held the George S. Dively Professor of Engineering Endowed Chair from January 1998 until July 2000, when he was appointed the BFGoodrich Professor
of Engineering Innovation. He served as the Director of the MEMS Research
Center at CWRU from July 1995 until July 2000. He is well known for his research in the area of MEMS, and his work has been widely covered by domestic
and foreign media. He has over 200 publications describing his work, holds 12
U.S. patents, and is the recipient of a number of awards/honors. He served as
the Editor-in-Chief of the Journal of Micromechanics and Microengineering
from January 1996 to December 1997, and is Assistant-to-the-President of the
Transducers Research Foundation. His research interests include silicon and
silicon carbide MEMS, micromachining and microfabrication technologies,
materials and modeling issues related to MEMS and IC technologies, and
MEMS packaging. He is the Founder and served as the President (July 1993 to
March 1999) of Advanced Micromachines Incorporated, Cleveland, a company
in the MEMS area. Advanced Micromachines Incorporated was acquired by
The BFGoodrich Corporation in March 1999. He founded NineSigma, Inc.,
an information technology company, in February 2000 and served as its CEO
(June 2000 to January 2001) and CTO (January 2001 to August 2001), during
which period he successfully completed initial rounds of private financing and
grew the company to 15 employees. He co-founded FiberLead, Inc., an optical
telecommunications company, in September 2000 and served as its CEO until
September 2001; during which period he successfully completed the early stage
round of venture capital financing and grew the company to five employees.