580 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 10, NO. 4, DECEMBER 2001Batch-Processed Vacuum-Sealed Capacitive PressureSensorsAbhijeet V. Chavan, Senior Member, IEEE, and Kensall D. Wise, Fellow, IEEEAbstract—This paper reports two multitransducer vacuum-sealed capacitive barometric pressure sensors, one using single-lead and the other using multiple-leads to transfer the electricalsignal out of the vacuum-sealed reference cavity. The first deviceoperates with a resolution of 37 mtorr over a pressure range from600 to 800 torr. The sensitivity is 27 fF/torr (3000 ppm/torr). TheTCO at 750 torr is 3900 ppm/C and the TCS is 1000 ppm/ C.The second device has a resolution of 25 mtorr over a range from500 to 800 torr, with individual transducer sensitivity of 39 fF/torr.The TCO at 750 torr is 1350 ppm/C and TCS is 1000 ppm/ C.Both devices have an on-chip compensation capacitorand are readout using an electronically-trimmed switched-capacitor chargeintegrator. [611]Index Terms—Capacitive sensor, MEMS, pressure, vacuumsealing.I. INTRODUCTIONWITH an established marketof$2.5B [1], pressure sensorsare among the most important MEMS devices, with ap-plicationsinareassuchasautomotivesystems,industrialprocesscontrol, medical diagnostics, and environmental monitoring (in-cluding distributed weather forecasting networks). While a ma-jorityof thesiliconpressure transducers inuse todayarepiezore-sistive, capacitive devices have become the focus for most newdevelopments to achieve higher pressure sensitivity, lower tem-perature sensitivity, and reduced power consumption. The sen-sors described here are multitransducer capacitive devices suit-able for barometric applications [2]. They have a wide dynamicrange (500–800 torr) as well as very high resolution (25 mtorr,equivalentto analtitudedifferenceofaboutone footatsea level).Theserequirements are especiallychallengingbecause thisreso-lutionmustbemaintainedoveratemperaturerangefrom25 Cto 85C. To meet this temperature range requirement it is essen-tial that the effects of trapped gas expansion be eliminated [3]throughtheuseofavacuum-sealed referencecavity.Thisalsore-sults in a device with wider bandwidth by avoiding the dampingeffects associated with a gas-filled cavity [4]. The first devicetransfers a single lead (the electrode on the glass) from inside theManuscript received August 2, 2000; revised March 31, 2001. This work wassupported by DARPA under Contract DABT63-95-C-0111. The work of A. V.Chavan was supported by a fellowship from Delphi Delco Electronics Corpora-tion. Subject Editor D. J. Harrison.A. V. Chavan was with the Engineering Research Center for Wireless In-tegrated Micro-Systems, Department of Electrical Engineering and ComputerScience, The University of Michigan, Ann Arbor, MI 48109-2122 USA. Heis now with Delphi Microelectronics Center, Delphi Delco Electronic SystemsCorporation, Kokomo, IN 46904 USA.K. D. Wise is with the Engineering Research Center for Wireless IntegratedMicro-Systems, Department of Electrical Engineering and Computer Science,The University of Michigan, Ann Arbor, MI 48109-2122 USA.Publisher Item Identifier S 1057-7157(01)10507-X.Fig. 1. Cross sections of the capacitive sensor using a single transfer lead fromthe sealed cavity. (a) The cut is shown through the lead transferring the glasselectrode out of the cavity. A tab used to contact the wafer bulk during bondingis also shown. (b) The cut is shown along a device diagonal. The inner ringforms the vacuum seal; the outer ring provides a permanent contact to the siliconelectrode.cavityto the outside world. Parasitic capacitance in parallel withthe sensor capacitance has a different thermal behavior than theactual sensing element and thus complicates temperature com-pensation. The second device has multiple signal transfer leadsand significantly reduces such parasitics. During the develop-mentofthese sensors, specificemphasiswasputonmakingthemsuitable for planar batch processing and low cost applications.The devices described here are bonded at wafer level and avoiduse of glass drilling, epoxy seals, or special metal seals [6], [7].II. SINGLE-LEAD TRANSFER SENSORThe sensor cross-section is shown in Fig. 1 and consists ofvacuum-sealed capacitors realized in a silicon-on-glass dis-solved-wafer process. In bringing the glass electrode out fromthe reference cavity and sealing that cavity in vacuum, oneapproach is to bring the lead out using polysilicon embedded ina planarizing low temperature oxide (LTO) layer; a second level1057–7157/01$10.00 © 2001 IEEEAuthorized licensed use limited to: University of Michigan Library. Downloaded on February 25,2010 at 12:12:44 EST from IEEE Xplore. Restrictions apply.CHAVAN AND WISE: BATCH-PROCESSED VACUUM-SEALED CAPACITIVE PRESSURE SENSORS 581Fig. 2. FEA results for one quarter of a circular bossed diaphragm, showing center deflection for applied atmospheric pressure of 101 kPa.of polysilicon could then be bonded to the glass substrate. Suchseals are known to be hermetic [5]. In the device describedhere, these two polysilicon layers are collapsed into one layerthat functions both as a lead transfer and as a sealing base forthe cavity. As shown in Fig. 1(a), the internal sensing lead onthe glass is transferred to the polysilicon and then back to metalexternally using Pt–Pt bonds formed during the electrostaticbonding operation. Since the polysilicon must be electricallyconnected to the silicon bulk during bonding but must be wellisolated later, lateral polysilicon tabs are used to contact to thelightly-doped portion of the bulk. This area is automaticallyremoved during the etch-back process to provide the requiredisolation. Permanent contacts to the top silicon electrode aremade via similar transfers to the pbody region as shownin Fig. 1(b). Using this approach, the process flow is muchsimplified and wafer-level bonds can be made to both 7740 andHoya SD2 glasses. In addition, since the vacuum seal is doneat wafer level before etch-back, the cavity is never exposed tothe etch and stiction problems are avoided.The device has multiple transducers to segment the overallpressure range [2]. The diaphragm diameters vary from 920 to1100m with a gap separation of 10 m and a diaphragmthickness of3 m. The device is designed to operate witha resultant diaphragm tensile stress of about 25 MPa. The di-aphragm design is done using the ANSYS finite element anal-ysis tool. The center deflection,, for a circular diaphragm witha