Download AN INHERENTLY-ROBUST 300 DEGREES C MEMS

An Inherently-Robust 300◦C MEMS Temperature
Sensor for Wireless Health Monitoring of Ball and
Rolling Element Bearings
Sean Scott∗ , Farshid Sadeghi† , and Dimitrios Peroulis∗
∗ School
of Electrical and Computer Engineering
of Mechanical Engineering
Purdue University, West Lafayette, Indiana 47907
Email: {scottsm, sadeghi, dperouli}@purdue.edu
† School
Abstract— Presented is an inherently-robust wireless capacitive
MEMS temperature sensor capable of operating to 300◦ C. The
heart of the sensor is an array of bimorph (metal-dielectric) cantilevers whose deflections are sensed by an array of appropriatelyplaced electrodes. The key advantages of this configuration are
the following. First, its dielectric layer is SiO2 thermally-grown
at 1,100◦ C as opposed to conventional low-temperature PECVD
or sputtered films. Second, the lack of a sensing surface directly
beneath the movable structures renders stiction nearly impossible. Third, the fringing-field sensing results in constant sensitivity
throughout the entire temperature range. Fourth, the employed
passive approach is immune to high-temperature reliability issues
faced by active devices. Furthermore, the fabrication yield is
over 99%, even in an academic cleanroom (Birck Nanotechnology
Center at Purdue University). When configured with an inductor
and wirelessly interrogated, the measured resonant frequency has
a linear shift from 206 MHz at room temperature to 199 MHz
at 300◦ C.
as silicon carbide may seem to be a desirable option. However,
these devices have reliability issues at high temperatures [2],
[3], and require expensive materials and fabrication processes.
A potential solution which has been explored involves two
inductively-coupled coils. One of the coils is located away
from the bearing, and is used for interrogation. The other
is located directly on the bearing, and is connected to a
temperature-sensitive capacitor which works to the necessary
operating temperatures (Figure 1).
I. I NTRODUCTION
Knowledge of a component’s temperature or strain can be
an excellent indication of the component’s overall health and
remaining lifetime [1]. For example, in bearings, spalling or
development of small cracks on the rolling elements will result
in an increase in friction. This friction generates an increase in
temperature, which will further increase as a crack propagates
over time. As a result, early knowledge of this temperature
increase can lead to prediction of time until failure. Currently,
sensors on military aircraft monitor either the oil temperature
outside of the bearing, or the housing temperature containing
the entire assembly. However, because it is much closer to
the point of failure, the bearing cage temperature has much
faster response time and higher resolution than either of these
options. As such, miniature, harsh environment sensors which
can be integrated directly on the cage and read wirelessly
are in high demand for real-time health monitoring. This
becomes challenging, however, as this is an oily, and potentially contaminated environment, and the cage rotates at
thousands of revolutions per minute. Average temperatures
may vary based on the speed of rotation, but a sensor must
be able to operate to at least 300◦ C. Because typical CMOS
electronics are not functional at this temperature, a remotelypowered active circuit utilizing a high-bandgap material such
978-1-4244-5335-1/09/$26.00 ©2009 IEEE
Fig. 1. A telemetry concept is employed to allow the temperature to be read
wirelessly. A temperature sensitive capacitor is used to change the resonant
frequency. MEMS bimorphs deflect downward as temperature increases, thus
increasing the capacitance. This MEMS-based approach allows for much
higher temperatures than commercial devices.
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IEEE SENSORS 2009 Conference
II. D ESIGN AND FABRICATION C ONSIDERATIONS
A common approach employed for wireless temperature
sensing involves using temperature-dependant commercial capacitors in an LC resonator configuration, which is interrogated remotely [4]. However, no commercial capacitors
today simultaneously achieve small size, monotonic temperature response, and high-temperature operation (over 300◦ C).
More appropriate thermal capacitors have been made with
MEMS bimorph devices [5], [6]. These devices feature metaldielectric cantilevers which deflect as temperature increases.
Unfortunately, these typically suffer from large creep and
hysteresis at high temperatures due to a poor quality dielectric
layer (typically PECVD, sputtered or other low-temperature
films). These sensors usually involve a bimorph and electrode
beneath to sense the capacitance in a parallel-plate topology.
This technique allows for large changes in capacitance, but
this increase is most significant when the beam is nearly
deflected. As a result, the sensitivity at lower temperatures,
and linear frequency response of the resonator are lost. By
employing a bimorph made with a high-temperature dielectric
in a fringing-field fashion, both of these problems are solved
simultaneously, and additional benefits are gained as well.
A bimorph is created by stacking two materials with differing coefficients of thermal expansion (CTE). As temperature
increases, the material with the higher CTE will expand to a
greater extent, and cause the structure to deflect. To generate
large deflection, a metal is commonly used for the top layer,
and a dielectric for the bottom. These are placed in a fringingfield configuration in which stationary electrodes are placed
adjacent to the released bimorph (Figure 2). This technique
helps to maintain a linear frequency response in the resonator
(Figure 3).
In our case, gold is used on top, and we make use of
the mechanical properties of the thermally-grown oxide as
our dielectric layer. Because of thermal mismatch in the
processing, the beams are naturally deflected upwards at room
temperature. The thicknesses of these layers and their respective CTEs will determine the amount of deflection at a specific
temperature. As such, the thicknesses and materials should be
selected so that the beams will be flat at the maximum desired
operating temperature. In our case, about 0.5 µm of both oxide
and gold yields a cantilever that lays flat at about 300◦ C.
The beams used are typically several tens of µm wide and
several hundreds of µm long. Because the mass of each beam
is so small, the inertial effects of the bearing have virtually
no impact on the sensor’s capacitance. For instance, with an
acceleration of 100 Gs, the maximum tip displacement is about
90 nm, or less than 0.1% of its range.
The fabrication of the MEMS bimorph capacitors is completed with a relatively-simple process, done entirely in-house
at the Birck Nanotechnology Center cleanroom at Purdue
University. First, an oxide is thermally grown at 1,100◦ C on
a high-resistivity silicon wafer to a thickness of about 0.5 µm
(Figure 4(a)). Next, the oxide is patterned using traditional
lithography and a wet etch (Figure 4(b)). The metal layers
Fig. 2. Depiction of difference between parallel plate sensing and fringingfield sensing. Parallel-plate sensing generates larger magnitudes for a certain
area, but most of the increase is at high temperatures.
Fig. 3.
Simulated comparison between resonant frequency trends when
parallel-plate and fringing-field sensing capacitors are coupled with inductors.
Fringing-field sensing results in much more linear resonant frequency vs.
temperature trends.
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of titanium (about 50 nm for adhesion) and gold (about 0.5
µm) are then sputtered and patterned (Figure 4(c)). After this,
the devices are diced into individual samples and attached
and wirebonded to a commercially-available transistor outline
(TO) header for packaging. The cantilevers of the device
are next released with a dry-etch of the silicon using XeF2
(Figure 4(d)). The cantilevers naturally deflect upwards at
room temperature due to the thermal mismatches of the films
in the process. Finally, the TO can is resistively-welded to
the header creating a complete, hermetically-packaged sensor
(Figure 4(e)).
(a)
(b)
(c)
(d)
Fig. 5. The robust fabrication process results in extremely-high fabrication
yield across wafers. Around 500 individual cantilevers of 10 µm by 250 µm
are shown. An advantage of the small cantilevers is that the mass is so small
that inertial forces have virtually no impact on the cantilevers.
(e)
Fig. 4. Fabrication Process, after [7]: A thermal oxide is grown (a) and
patterned (b) on a bare silicon wafer. A Ti adhesion layer and thicker Au
layer are then sputtered and patterned (c). Dry etch of the silicon using XeF2
is used to release the cantilevers (d), and the devices are protected using a
commercial hermetic package (e).
This relatively-simple process, robust design, and dry-etch
for release result in an extremely high fabrication yield, nearlyperfect across a wafer (Figure 5). The sensor is then attached
to an inductor which is embedded in a plastic insert attached to
the bearing cage. This plastic insert is used to isolate the wire
from the metal, reducing eddy currents so significant coupling
between the interrogating coil and LC can be achieved. Finally,
a dummy sensor is placed opposite to dynamically balance the
cage (Figure 6).
Fig. 6. Image of the completed sensor integration on the bearing. A dielectric
insert isolates the coil from the metallic cage, reducing eddy currents. A
dummy sensor is placed opposite the original (but not connected electrically)
to balance the bearing.
to increase the temperature as capacitance is recorded (Figure
7). Once the packaged sensor is connected to the inductor, an
interrogating coil is connected to a network analyzer (Figure
8). The two coils are placed an appropriate distance apart, at
which the coupling is still large enough to see the resonance
(a few cm). After this, the sensor is heated with the hotplate,
while the interrogating coil remains at room temperature, and
the network analyzer records the data (Figure 9).
III. M EASURED R ESULTS
Both the capacitance versus temperature of the individual
sensor and the resonant frequency versus temperature of the
resonator are measured. To measure the capacitance change as
temperature increases, low capacitance probes are connected
to the Analog Devices AD7746 evaluation board which is
connected via USB to a PC. These probes touch measurement
pads connected to each side of the device, one side to the
anchor of the bimorph, the other touching the fixed electrodes.
The entire silicon piece is on a ceramic hotplate, which is used
IV. C ONCLUSION
A new, passive, wireless temperature sensor based on a
MEMS, thermally-tunable capacitance has been presented.
The thermal oxide employed allows the sensor to operate
to at least 300◦ C. The fringing-field method used results
in a linear change in resonant frequency from 206 MHz
at room temperature to 199 MHz at 300◦ C, with constant
sensitivity over the entire range. In addition, the dry-etching
of the substrate in the release step result in a high yield
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Fig. 7. Measured capacitance vs. temperature for a small array of bimorphs.
By scaling the number of bimorphs, the capacitance range can be scaled to
obtain the best resonant frequency for the application. Here, the capacitance
is only measured to 200◦ C to prevent damaging the measurement probes.
Fig. 9. Measured resonant frequency vs. temperature for the MEMS capacitor
attached to an inductor, and remotely read by an interrogating coil, as shown
in Figure 8.
ACKNOWLEDGMENT
The authors would like to thank the Patuxent River Naval
Air Station for financially supporting this project under grant
number 103625 entitled “Disruptive MEM Sensors for Monitoring Aircraft Drivetrains”.
R EFERENCES
Fig. 8.
Measurement setup to wirelessly record resonant frequency of
resonator. An interrogating coil magnetically couples with the LC formed by
the sensor and coil. The S11 of the coil determines the resonant frequency,
which is tuned by the temperature dependence of the sensor.
process in which stiction is virtually impossible in both the
release and in operation. All of these benefits, along with
the hermetic packaging of the MEMS component, give way
to an industry-ready device. It is the author’s belief this is
the first temperature sensor capable of surviving up to 300◦ C
integrated and tested on a real-world bearing cage. Moreover,
the flexibility of the process and high-quality dielectric show
no signs of failure at 300◦ C, so this device will likely be a
strong candidate for even higher temperatures. Although this
configuration requires close-proximity operation, the MEMS
can be reconfigured to operate in a far-field device as well
[8], resulting in robust, multi-situational solutions for harsh
environment temperature sensing.
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