AN ULTRA-SENSITIVE, HIGH-VACUUM
ABSOLUTE CAPACITIVE PRESSURE SENSOR
Yafan Zhang, Sonbol Massoud-Ansari, Guangqing Meng, Woojin Kim, and Nader Najafi
Integrated Sensing Systems (ISSYS), Inc.
Ypsilanti, MI 48198
fabrication technique, which allows the creation of a variety of
three-dimensional, free standing, single crystal, silicon microstructures on glass substrates. It offers a number of distinct
advantages over conventional surface or bulk micromachining
technologies. The most significant advantages of DWP are (1) it is
simple, single-sided process that reduces the cost of fabrication; (2)
it yields highly accurate and reproducible results; (3) it results in a
boron-doped single crystal microstructure for superior mechanical
properties and chemical resistance; (4) it allows the creation of
both thick and thin microstructures on the same chip; and (5) it is
capable of producing a high density of microstructures.
Figure 1 shows the top view of the ultra-sensitive, highvacuum absolute pressure sensor. The sensor consists of three
major parts. A thin silicon diaphragm acts as the sensing element.
A cavity creates a capacitive gap, and a metallized glass substrate
provides the capacitive reference plate.
ABSTRACT
This paper reports the development of a family of ultrasensitive and ultra-high-vacuum absolute capacitive pressure
sensors, fabricated using the dissolved wafer process technology
(DWP).
One type of the sensors offers the following
characteristics: full range 0-0.5 Torr, sub-µTorr resolution, five
orders of magnitude pressure response, high stability, and the
highest aspect ratio of diaphragm area to its thickness of any
MEMS capacitive sensors ever reported worldwide. This novel
pressure sensor is pushing the edge of the technology and is
capable of operating within the pressure ranges previously
monitored only by the “Cold Cathode” and “Hot Filament Ion”
techniques.
INTRODUCTION
Ultra-sensitive, high-vacuum absolute pressure sensors are
always attractive for a variety of emerging applications. Silicon
micromachined capacitive types of pressure sensors are especially
attractive for their high pressure resolution, low temperature
sensitivity, and low power consumption. There, however, exist
two major limiting facts in their massive commercialization.
These two limiting facts are (1) hermetic vacuum sealing of
capacitive cavity, and (2) electrical lead transfer between the
vacuum-sealed cavity and the outside of the world. Several
techniques have been reported to achieve the lead transfer for the
vacuum type capacitive devices [1-5]. All of these methods have
the drawback of reliability issues. A lot of work has been done to
achieve high vacuum level inside of the capacitive cavities [6-8].
With these techniques, it is still difficult to obtain vacuum level
better than mTorr range. Furthermore, it is important to maintain
the vacuum after the fabrication. ISSYS has developed a variety of
technologies with which we have successfully designed and
fabricated a family of capacitive pressure sensors having high
vacuum cavities (<0.1mTorr), ultra-hermetic electrical lead
transfer from within sealed cavities (<10-18 Torr liter/second), and
ultra-sensitive silicon diaphragms (>35pF/Torr).
Figure 1. The top view of the ultra-sensitive, vacuum absolute
pressure sensor
SENSOR DESIGN
To achieve an ultra-sensitive capacitive pressure sensor, a
very large and thin diaphragm and a very small gap are needed.
Finite element analysis and scaling theory were used as the design
tools. We are manufacturing the highest aspect ratio of diaphragm
area to its thickness of any MEMS capacitive sensors ever reported
worldwide, to the best of our knowledge. To get the same
sensitivity on a macro scale, the diaphragm would have to be one
foot thick, one foot above the ground, and cover an area of
equivalent to 70 football fields with less than one inch of height or
thickness variation.
Due to the small gap and small cavity nature of ultra-sensitive
devices, any gas trapped/generated in the small cavity will largely
The design and development of the ultra-sensitive, highvacuum absolute capacitive pressure sensors took more than four
years. It includes a variety of technologies such as creation and
maintenance of high vacuum cavities, ultra-hermetic electrical lead
transfer from within sealed cavities, and fabrication of very large
and thin diaphragms. By studying and comparing with different
micromachining technologies, the dissolved wafer process (DWP)
is adapted as the baseline for our sensor fabrication. The dissolved
wafer process is developed at the University of Michigan by
Professor Kensall Wise. ISSYS has exclusive rights to its related
six patents from the University of Michigan. DWP is a versatile
0-7803-5998-4/01/$10.00 @2001 IEEE
166
affect the performance of the sensors. ISSYS has developed
manufacturing-friendly technologies both for the production and
maintenance of high-vacuum reference cavities for absolute
pressure sensors, as well as other MEMS products. Due to the
instrumentation limitations, we were not able to accurately
measure the vacuum inside device cavities. Our test results have
shown that the cavity pressure of our sensors is below 10-4 Torr.
Another major hurdle in the realization of high-vacuum
absolute pressure sensors was the development of a highly
hermetic electrical lead-transfer technology, by which the electrical
leads should be able to transferred out from the vacuum-sealed
cavities while maintain the integrity of vacuum-sealing. We have
successfully designed and developed a novel (patent pending),
reproducible, wafer-level, reliable, hermetic, high-yield electrical
lead-transfer technology.
This lead transfer technology is
incorporated into the front-end and back-end fabrication processes.
The lead transfer technology together with the sealed cavities
passed all the tests, including helium leak test, high pressure test,
and accelerated leak tests. Indeed, the hermeticity of our sensors is
much higher than any experimental techniques able to detect.
Again, due to instrumentation limitations, we cannot provide the
exact leak rates, however, based on the changes in the offset
pressure we estimate that these sensors offer hermeticity of better
than 10-18 mmHg liter/second. Most military applications require
leak rates in the range of 10-9 to 10-11 mmHg liter/second.
4 5 .5
y = 3 4 .6 0 7 x + 4 1 .6 7 9
R 2 = 0 .9 9 9 7
4 5 .0
M acapacitance
in c a p a c it a n(pF)
ce
Main
4 4 .5
4 4 .0
4 3 .5
4 3 .0
4 2 .5
(a)
4 2 .0
4 1 .5
0 .0 0
0 .0 2
0 .0 4
0 .0 6
0 .0 8
0 .1 0
P re s s u re (T o rr)
42 .05
42 .00
y = 3 2 . 8 7 6 x + 4 1 .6 9 7
R 2 = 0 .9 9 9 9
M acapacitance
i n c a p a c i ta n c(pF)
e
Main
41 .95
41 .90
41 .85
41 .80
41 .75
RESULTS AND DISCUSSION
(b)
41 .70
41 .65
0.000
A family of ultra-sensitive high-vacuum capacitive pressure
sensors are fabricated, packaged, and tested. Fig. 2 shows a
picture of a packaged sensor with readout electronics. In order to
accurately test, characterize, and qualify such sensitive sensors, a
high-performance characterization system was first designed and
implemented. The system includes mass flow controllers that
manipulate the amount of pure nitrogen induced to the pressure
chamber, a down-stream throttle valve that manipulates the amount
of nitrogen that is pumped out via a high–vacuum pumping station.
LabviewTM software was used for addressing, programming,
controlling, and communicating with hardware.
This fully
automated system is capable of very low capacitance measurement
(0.01fF) at a high speed and accurately maintained set pressures.
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.010
P r e ss u r e (T o r r )
4 1 .8 1
4 1 .8 0 y = 3 2 .1 6 8 x + 4 1 .7 7
2
R = 0 .9 9 6 5
Main
M a capacitance
i n C a p a c i ta(pF)
nc
4 1 .8 0
4 1 .7 9
4 1 .7 9
4 1 .7 8
4 1 .7 8
(c)
4 1 .7 7
4 1 .7 7
0 .0 0 0 0
0 .0 0 0 2
0 .0 0 0 4
0 .0 0 0 6
0 .0 0 0 8
0 .0 0 1 0
P re s s u re (T o rr)
4 1 .7 7 7 0
4 1 .7 7 6 5
y = 0 .05 68 x + 41 .77 06
R 2 = 0.9960
M acapacitance
in C a p a c it a (pF)
nc
Main
4 1 .7 7 6 0
4 1 .7 7 5 5
4 1 .7 7 5 0
4 1 .7 7 4 5
4 1 .7 7 4 0
4 1 .7 7 3 5
Figure 2. A photograph of a packaged sensor with electronics
(d)
4 1 .7 7 3 0
Figure 3 shows the performance of one type of the absolute
pressure sensors. This type of sensors normally operates in the
pressure range of 0-100 mTorr. The pressure range can be
however extended up to 500 mTorr. As can be seen from Fig. 3(a),
the sensor has a very linear pressure response, and the pressure
4 1 .7 7 2 5
0 .0 4
0 .0 5
0 .0 6
0 .0 7
0 .0 8
0 .0 9
0 .1 0
P re s s u re (m T o rr )
Figure 3. Performance of absolute pressure sensor
167
sensitivity is as high as 35pF/Torr in the normal operating range
(0-100mTorr). Fig. 3(b)-3(d) provides the detailed measurement
results in the pressure range of 10 mTorr, 1 mTorr, and 0.1 mTorr,
respectively. Fig. 4 illustrates the linear response of the device in
the pressure range of 0-10 µTorr. All these results clearly indicate
that this type of capacitive pressure sensors can cover incredible
five orders of magnitude pressure range, and are capable of
providing sub-µTorr resolution. To the best of our knowledge, this
is the very first capacitive pressure sensor that can operate within
the pressure range previously monitored only by “Cold Cathode”
and “Hot Filament Ion” techniques. The nonlinear response shown
in Fig. 4 is most likely due to the well-known poor performance of
Cold Cathode vacuum pressure sensors (used as a reference
sensor).
In order to further characterizing the performance of the
sensors, we tested the reliability parameters including repeatability
(drift), zero stability, and temperature dependence. The fabricated
sensors have shown negligible drift due to fatigue after being
cycled for more than 3.5 million times of 3 times the full pressure
range, as indicated in Fig. 5. The curves in the Fig. 5 are from
both main and reference sensors that are fabricated on the same
chip. These results indicate the great robustness and high
reliability of the sensors.
3.5
3.0
Capacitance (pF)
1.5
0.5
0.0
0
0.5
0.6
0.7
0.8
0.9
Drift in
in C
Compensated
(pF)(p
Drift
om p en satedCapacitance
Cap acitan ce
14.750
0 .1 0
1
10
P re s s u re (µ T o rr)
14.75
C o m p e n s a te d C a p a cita n ce (p F)
14.740
14.74
Ma in C a p a cita n ce (p F)
14.730
14.73
14.720
14.72
14.710
14.71
14.700
14.70
14.690
14.69
14.680
1.00
0 .0 0
8
0.4
T em perature C om pensation for 4m m
D ifferential Sensor
0 .2 0
6
0.3
Figure 6. Capacitance response vs. pressure under
different temperature
0 .3 0
4
0.2
Pressure (Torr)
0 .4 0
2
0.1
14.68
2001.00
4001.00
Figure 7. Compensated temperature drift
Figure 4. Sensor performance in 0-10µTorr pressure range
Temperature dependency of the sensors is another important
fact that will affect the performance of the sensors largely. We
tested different sensors at different temperatures. Fig. 6 illustrates
the measurement results of the capacitance response as a function
of applied pressure at two different temperatures, 45 °C and 55 °C.
An increase in temperature causes the capacitance to decrease,
probably due to the increasing in diaphragm stress. It is believed
that this stress increase is mainly caused by the mismatch in
thermal expansion coefficients between the silicon diaphragm and
the glass substrate. It can be seen from the figure, this sensor has a
capacitance decrease of 23fF at zero pressure, and 100fF in the full
scale range (1 Torr).
While uncompensated temperature
dependency is relatively high, fortunately, it is extremely
repeatable and predictable.
As a result, the temperature
dependency can be easily compensated. Shown in fig. 7 is a plot
of the capacitance response as a function of temperature at zero
pressure. The step-wise curve is the capacitance response under
temperature of 45 °C and 55 °C without compensation. The other
curve is the capacitance response after compensation for the same
device. As can be seen from the figure, with a simple digital
compensation, we were able to reduce the temperature drift from
50fF to 3fF over a 10°C temperature change. Based on our
3.5
PCyclingfrom0to3Torr
C-PTestingat T=40C
Capacitance (pF)
Capacitance
(pF)
3.0
2.5
2.0
0cycles
50Kcycles
100Kcycles
250Kcycles
1Mcycles
3.5Mcycles
0cycles, ref
50K ref
MainSensor
1.5
1.0
ReferenceSensor
0.5
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Pressure(torr)
Figure 5. Sensors have shown negligible drift after 3.5 million
pressure cycles of 3 times of the full-pressure range
168
M ain Cap acitan ce (p F
Capacitance Change (fF)
2.0
1.0
0 .5 0
0
T = 45C
T = 55C
2.5
numerous measurements and test results, we claim the ultrasensitive, high vacuum capacitive pressure sensors developed by
ISSYS are very stable in their performance. On an average the
sensors are able to provide the following stability characteristics:
zero temperature coefficient of 0.01% FS/°C, zero stability of
0.01% FS/24 hours, Hysteresis of 0.035% point of reading and
repeatability (after temperature compensation) of 0.01% point of
reading.
CONCLUSIONS
A family of ultra-sensitive, high-vacuum absolute capacitive
pressure sensors have been designed, fabricated, and tested. These
ultra-sensitive, high-vacuum absolute pressure sensors exhibit
outstanding performance in terms of high sensitivity (35pF/torr),
sub-µTorr resolution, high vacuum sealing (<10-4 Torr), and highhermetic electrical lead transfer (<10-18 Torr liter/second).
Repeated pressure cycling has shown that these absolute capacitive
pressure sensors are very stable and reliable with negligible drift
and fatigue. Temperature response of the sensors is very
repeatable. After digital compensation, we are able to achieve
repeatability better than 0.01% point of reading.
REFERENCE
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Capacitive Pressure Sensor”, Technical Digest of Solid-State
Sensor and Actuator Workshop, Hilton Head, S. C., pp.212-215,
June 1998.
2. M. Esashi, Y. Matsumoto, and S. Shoji, “Absolute Pressure
sensors by Air-tight Electrical Feedthrough Structure”, Sensors
and Actuators, A21-A23, 1990, pp. 1048-1052.
3. J. M. Giachino, et al., US Patents 4261086 (4/1981) and
4386453 (6/1983).
4.
Peters, at al., US Patent 4586109 (4/1986)
5.
W. Ko at al., US Patent 5528452 (6/1996)
6. A. V. Chavan and K. D. Wise, “A Batch-Processed Vacuum
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