A Micromachined Polyurethane/Stainless-Steel Capacitive Pressure Sensor Without Cavity and Diaphragm,

A Micromachined Polyurethane/Stainless-Steel Capacitive Pressure Sensor Without Cavity and Diaphragm,
A MICROMACHINED POLYURETHANE/STAINLESS-STEEL CAPACITIVE PRESSURE SENSOR
WITHOUT CAVITY AND DIAPHRAGM
Kenichi Takahata* and Yogesh B. Gianchandani
Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, USA
ABSTRACT
This paper reports micromachined capacitive pressure
sensor intended for applications that require mechanical
robustness.
The device is constructed with two
micromachined metal electrode plates and an intermediate
polymer layer that is soft enough to deform in a target
pressure range. The plates are formed of micromachined
stainless steel with overall dimensions of 42.4 mm2
fabricated by batch-compatible micro-electro-discharge
machining. A polyurethane RTV liquid rubber is used as
the deformable material. This structure eliminates both the
vacuum cavity and the associated lead transfer challenges
common to micromachined capacitive pressure sensors.
For wireless interrogation of the capacitance, an L-C tank
is fabricated by combining the capacitive sensor and a 40turn coil that are formed by winding a copper wire on the
sensor. A preliminary measurement test performed by
monitoring frequency shift of a reactance peak of the tank
with varying pressure shows its response of 2.6-9.6 Hz/Pa
measured over a dynamic range of 350 KPa. Temperature
dependence of the tank is also experimentally evaluated.
in a target pressure range allows thickness of the polymer,
or capacitance of the parallel plate capacitor, to be
dependent on hydraulic pressure that surrounds the device.
This capacitive change can be interrogated by either a hardwired interface or a wireless setup in which the sensor
serves as a capacitor of an L-C tank (Fig. 1).
Proper choice of materials compatible with particular
environments will offer broader opportunities such as in
automobile and biomedical applications that include air
pressure monitoring in the tires [6] and bowel pressure
detection [7]. The inherent environmental compatibility is
a significant advantage because it allows us to circumvent
constrains and problems associated with the packaging that
in general degrades device performance and cost
effectiveness in the device manufacturing.
Keywords: capacitive pressure sensor, stainless steel,
polyurethane, micro-electro-discharge machining, resonant
tank
I. INTRODUCTION
Capacitive pressure sensors are favored for low-power
and telemetric applications since they draw no DC power,
and conveniently form passive L-C tank circuits.
Micromachined capacitive pressure sensors have typically
used an elastic diaphragm with fixed edges and a sealed
cavity in between the diaphragm and the substrate below [1,
2]. Since this configuration relies on deflection of a
relatively thin diaphragm against a sealed cavity, in some
applications there is a concern of robustness of the
diaphragm and leaks in the cavity seal. Lead transfer for
the sealed electrode has also been a persistent challenge.
This has motivated the development of innovative
fabrication methods that involve multilayer deposition,
planarization, and other remedies, but require relatively
high mask counts [3, 4]. Another approach to deal with
this has been to move the sense gap outside the cavity [5].
This research explores a capacitive pressure sensor
that consists of two micromachined metal plates with an
intermediate polymer layer. This configuration aims to
eliminate the need of diaphragms and cavities from the
sensors. Use of polymeric material soft enough to deform
*
Fig. 1: Concept of the capacitive pressure sensor with an
inductor wound on the sensor for constructing an L-C tank.
II. FABRICATION
In this effort, polyurethane rubber was selected to
form the polymeric layer. This material offers mechanical
robustness such as high tear and abrasive resistances,
chemical resistance, and controllability of its softness over
a wide range. It has been extensively used in medical
implant applications [8] and was also used to fabricate
micro/nanostructures for MEMS applications [9, 10, 11].
The fabrication process is illustrated in Fig. 2. Two
plates were patterned in stainless steel sheets with microelectro-discharge machining (EDM) technique [12] (Fig.
4). The base and top plates with the layout shown in Fig. 3
were cut from stainless steel sheets with thickness of 200
µm and 50 µm respectively. Since these plates potentially
have burrs at the edges, the top plate was designed to be
slightly smaller than the base plate (50 m off from all
sides of the base plate) as seen in Fig. 3 to minimize
Corresponding author: 1301 Beal Avenue, Ann Arbor, MI 48109-2122, Tel: +1-734-647-1782, Fax: 763-9324, Email: [email protected]
probability of physical/electrical contact between the two
plates at the edges. The base plate was still connected to
the original sheet through two tethers after the machining
as shown in Fig. 4. This effort used a two-part polyurethane
RTV liquid rubber (Poly 74-20, part A: polyurethane prepolymer, part B: polyol, Polytek Development Co., PA,
USA) with the softener (part C: plasticizer), which is
vulcanized to very soft (<20 Shore A) and robust rubber.
The softness of the rubber can be adjusted by changing the
proportion of the softener to be mixed. This effort used a
formulation of part A:B:C=1:1:1. After applying the mixed
solution to the upper surface of the base plate, the top plate
was placed on it. In this step, the top plate is self-aligned to
the base due to surface tension of the solution as shown in
Fig. 5a.
After curing, the device was released by
mechanically breaking the tethers (Fig. 5b). An L-C tank
was formed by winding an enamel-coated copper wire
(~127 µm, 40 turns) on the sensor (Fig. 6) and bonding
the terminals on separate stainless steel plates with
conductive adhesive. Although this effort used serial
micro-EDM with single electrodes and manual assembly of
the plates, the process can potentially be performed in a
batch manner by a combinational use of batch-mode
EDM [13] for cutting the plates and screen printing or
splay coating for the liquid rubber layer formation.
Fig. 4: Base (lower) and top (upper) plates for the pressure
sensor fabricated by EDM (step 1 in Fig. 2).
Fig. 2: Fabrication process flow to fabricate the pressure
sensor (step 1-4) and the L-C tank (step 5).
Fig. 3: Dimensions of the top and base plates
Fig. 5: (a: upper) Two plates are self-aligned together
through a polyurethane liquid layer (step 3 in Fig. 2), and
(b: lower) the device released from the original sheet of the
base plate (step 4 in Fig. 2).
Fig. 6: Fabricated L-C tank (step 5 in Fig. 2).
III. MEASUREMENT RESULTS
Young’
s Modulus of Polyurethane Rubber
To characterize Young’
s modulus of the polyurethane
rubber with the formulation mentioned above, a loading
test was performed with a 3-mm-cubic sample of the rubber.
The measurement was performed with a force gauge (DPS1, Imada Inc., IL, USA) that provided 1-mN resolution.
Figure 7 plots measured pressure with varying strain up to
0.33 in the test, showing the modulus of 67-267 KPa,
which is 15-59 % of the modulus reported in [9].
Fig. 9: Frequency shift plotted from the measurement result
in Fig. 8.
Fig. 7: Pressure vs. strain measured with a polyurethane
rubber sample for Young’
s modulus estimation.
Pressure Measurement with the L-C tank
The fabricated L-C tank shown in Fig. 6 was
measured to have nominal capacitance of 6.3 pF and
inductance of 640 nH. Measured resonant frequency and
quality factor of the tank, which were probed via test leads
shown in Fig. 6, were 106 MHz and 1.9 respectively. The
tank was placed in a pressurized chamber. The variation of
its reactance peak with applied pressure was monitored by
a spectrum analyzer (HP4195) using the test leads
transferred through the chamber wall. Figure 8 shows
measured shifts of these reactance peaks due to gauge
pressure change in 69 KPa steps up to 345 KPa. This
reactance is an output from DSP based on a series
capacitor-resistor model of the analyzer and exhibited the
most distinct shift in this set-up. The result is plotted in Fig.
9, indicating the response of 2.6-9.6 Hz/Pa and sensitivity
of 10.5-39.0 ppm/KPa in this pressure range.
The measured resonant frequency of the tank is close
Fig. 8: Measured frequency response of the L-C tank with
DSP due to gauge pressure change from zero to 345 KPa in
68.9 KPa steps.
the theoretical frequency of about 80 MHz that is obtained
from the measured capacitance and inductance of the tank.
The saturation of the frequency response observed in Fig. 9
is consistent with the variation of the Young’
s modulus of
the polyurethane rubber, i.e., the layer becomes stiffer as it
is squeezed by increased pressure, resulting in the reduced
response. The fabrication method used for bonding
between the stainless steel plates and copper leads of the
tank did not offer sufficient electrical contact between them,
resulting in the low quality factor. This significantly
limited wireless capability of the device. Achieving good
electrical contacts in the tank promises wireless
measurement with the sensor.
Temperature Dependence
The variation of the peak frequency with varying
temperature, from 20 to 60 C, was evaluated outside of the
chamber at atmosphere pressure (Fig. 10). Temperature
was controlled by changing the distance between the device
and a source of heat. Temperature of the device was
measured by an infrared thermometer (Fisher Scientific
International Inc., NH, USA). As seen in Fig. 10, the
variation shows a linear dependence on temperature within
the tested range. Measured temperature dependence of the
peak frequency and its coefficient are 392.5 KHz/C and
1629 ppm/C, respectively. Figure 11 shows the response
of the peak frequency shift with varying pressure at
increased temperature of 40C. Although the frequency
initially dropped as the pressure was increased, as shown in
Fig. 11, it started to increase once the pressure exceeded
about 80 KPa.
Fig. 10: Peak frequency of the tank vs. temperature.
Fig. 11: Frequency shift due to pressure change at
increased temperature.
The high temperature coefficient of this device may
limit its applications. However, this may be addressed by
tailoring the choice of the polymeric filler. Although
temperature dependence of material properties of the
rubber heavily depends on particular formulations, the
following two events can occur at increased temperature:
(1) Increase of volume of the rubber layer, and (2)
softening of the layer.
The dielectric constant of
polyurethane elastomer was reported to be stable at the
temperature range used in this experiment [14]. Since the
sensor of the tank was tightly wrapped with a copper wire,
(3) the parallel plates may be not only immovable outward
but also pushed inward by the wound wire. With the
situations (1)-(3), the following two circumstances may
occur at increased temperature: (A) a combination of (1)
and (3) generates pressure applied from the rubber layer to
the inner surfaces of the parallel plates, and (B) a
combination of (2) and (3) thins the layer. Either makes
the rubber layer stiffer. This hypothesis is consistent with
the reduced response of 3.6 Hz/Pa at the increased
temperature in Fig. 11 compared to that of 9.6 Hz/Pa at
room temperature for their early pressurizing stages. In
addition, since the top plate is relatively thin, and only the
center part is physically constrained by the coil but other
parts are free to move, the plates may deform at increased
temperature. Such structural factor may have partially
caused the reversed response in Fig. 11.
IV. CONCLUSION
This research has explored a micromachined
capacitive pressure sensor that eliminated both a diaphragm
and a cavity from its construction. The sensor consists of
two metal plates and an intermediate polymer, which
potentially offers mechanical robustness and high
reliability.
The device was constructed with
micromachined stainless steel plates fabricated by batchcompatible EDM technique and polyurethane liquid
rubber as the polymer layer that permitted self-aligning of
the micromachined plates in the assembly process. This
material combination can offer good corrosion resistance or
biocompatibility. An L-C tank was formed with the sensor
of size 42.4 mm2 and a 40-turn copper coil to demonstrate
the capability of pressure measurement by resonant
frequency readout. The measured sensitivity was 10-40
ppm/KPa for gauge pressure ranging from 0-340 KPa. The
temperature dependence of this L-C tank was relatively
high, but may be reduced by an appropriate choice of
materials and further structural optimization.
The main concept demonstrated in this effort may be
extrapolated for use in other contexts. Of course, the use of
an L-C tank permits wireless monitoring, as demonstrated
by other researchers in the past. The idea of using a bulk
metal electrode and a soft material instead of a cavity for
the capacitive sensor may be useful in miniaturizing the
packaging and housing of certain industrial and automotive
pressure sensor assemblies. It may be extended further to
other applications in which the sensor must be
mechanically robust, such as tactile sensors or load cells.
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