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 1. 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