CHARACTERIZATION of vacuum micropackages developed

IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 28, NO. 4, NOVEMBER 2005 619 A Micromachined Pirani Gauge With Dual Heat Sinks Junseok Chae, Member, IEEE, Brian H. Stark, and Khalil Najafi, Fellow, IEEE Abstract This paper reports a micromachined Pirani gauge with dual heat sinks that can be integrated with microelectromechanical systems (MEMS) devices inside a vacuum package to monitor long-term pressure changes and stability inside the package. The Pirani gauge utilizes small gaps ( 1 m) between its heater and two thermal heat sinks to obtain large dynamic range (20 mtorr to 2 torr) and high sensitivity (3.5 10 5 (K/W) torr). The gauge is 2 2mm 2 in size, is fabricated using the dissolved wafer process (DWP) on a glass substrate, and utilizes dielectric bridges for signal routing. Measurements show the low end of the dynamic range can be extended by reducing the gap distance between the heater and thermal sinks, which matches well with analytical modeling. This gauge shows an uncertainty of 50 torr and a detectable leak rate of 3.1 10 16 cm 3 s, assuming a common micropackage volume of 1.6 10 5 cm 3, which represents at least four orders of magnitude improvement over traditional leak testing. Index Terms Microelectromechanical systems (MEMS), Packaging, Pirani gauge, pressure sensor. I. INTRODUCTION CHARACTERIZATION of vacuum micropackages developed for microelectromechanical systems (MEMS) devices, such as resonant sensors and RF MEMS, has utilized such techniques as Helium leak testing and factor extraction [1], [2]. These methods are limited either by high cost (Helium leak test) or sensor drift and lack of sufficient sensitivity at low pressures ( factor extraction) and, thus, cannot precisely measure minute pressure changes inside a sealed microcavity. Pirani gauges address these limitations by offering a low-cost, easy to use, and high-sensitivity device. Since the Pirani gauge was invented in 1906 [3], absolute pressure sensors that utilize thermal conductance changes, such as Pirani gauges, are widely used in vacuum systems [4], [5]. By taking advantage of micromachining technology, a number of miniaturized Pirani gauges have been reported [6], [7], which are sometimes integrated with readout electronics on a single chip to achieve high resolution [8], [9]. In addition to these stand alone devices, micromachined Pirani gauges to test the environment inside a MEMS package Manuscript received November 15, 2004; revised May 31, 2005. This work was supported in part by the Engineering Research Centers Program of the National Science Foundation under Award EEC-9986866. J. Chae was with the Department of Electrical Engineering and Computer Science, Center for Wireless Integrated Microsystems, University of Michigan, Ann Arbor, MI 48109-2122 USA. He is now with the Electrical Engineering Department, Arizona State University, Tempe, AZ 85044 USA (e-mail: junseok.chae@asu.edu). B. H. Stark and K. Najafi are with the Department of Electrical Engineering and Computer Science, Center for Wireless Integrated Microsystems, University of Michigan, Ann Arbor, MI 48109-2122 USA. Digital Object Identifier 10.1109/TADVP.2005.858316 have also been developed [9] [11]. In order to detect small leak rates in a MEMS package, the gauge should not only be compatible with the package fabrication technology but also should offer high sensitivity and large dynamic range. In this paper, we present a micromachined Pirani gauge fabricated using the dissolved wafer process (DWP) which uses heavily boron-doped (p++) silicon as its structural material. The gauge can be integrated with a variety of sensors fabricated in this technology [12], [13], and also with wafer-level vacuum packages, and can, thus, be used for in situ vacuum testing. Furthermore, we have introduced a new structure with dual thermal heat sinks with small m gaps that provides larger dynamic range and higher sensitivity than traditional devices that utilize only one thermal sink. In Sections II IV, first we present the operating principle and analytical modeling of a Pirani gauge. Next, performance improvements, including increased dynamic range and sensitivity by implementing the dual heat sink configuration is introduced. Then, the fabrication process is described in detail. Finally, measurement results such as dynamic range, sensitivity, the effect of air gap distance and effective heater area on gauge performance, and the minimum detectable leak rate are presented followed by concluding remarks. II. SENSOR DESIGN A. Operating Principle The operation of a Pirani gauge is based on heat transfer from a suspended heater to a heat sink through a gas. The thermal conductance through the gas is a function of its pressure. Depending on the Knudsen number (, is mean free path of a gas, is the dimension of the domain), the gas can be modeled as in a continuum regime at high pressure or can be in a molecular regime at low pressure [Fig. 1(a)] [14], [15]. With reasonable assumptions and approximations, heat flux, which is a function of ambient pressure for all, can be modeled as [16] where is an empirical transition pressure. This has a linear dependence on at low pressure and limits to a constant at high pressure, which determines the upper limit of the dynamic range. From the simple heater and heat sink model shown in Fig. 2, can be found as [8] (1) (2) 1521-3323/$20.00 2005 IEEE

620 IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 28, NO. 4, NOVEMBER 2005 Fig. 3. Effects of dual heat sinks on the sensitivity and the dynamic range (SHS and DHS stand for single heat sink and dual heat sinks, respectively. G is ignored). (Color version available online at http://ieeexplore.ieee.org.) The sensitivity of the Pirani gauge is the slope of the total thermal conduction versus pressure as shown in Fig. 1(b). Gaseous thermal conductivity can be modeled as [16] (3) Fig. 1. Heat flux and thermal conductance versus pressure. (Color version available online at http://ieeexplore.ieee.org.) where is gaseous thermal conductivity, is thermal conductance of the gas, and is the area of the heater. The sensitivity can be obtained as (4) This parameter can be modeled with a simplified heater and heat sink model (Fig. 2) [16] Sensitivity Fig. 2. Simplified schematics of Pirani gauge. (Color version available online at http://ieeexplore.ieee.org.) where,, and are the width, thickness, and the perimeter of the heater, and is the distance between the heater and heat sink. Reducing the gap between the heater and heat sink is the most effective method of increasing the high-pressure limit of the dynamic range since typically is much smaller than 1. The lower limit of the dynamic range is determined by heat transfer through the solid support anchors (solid conduction, ). Assuming the heater is a lossless thermal conductor, the total thermal conductance of the heater is the sum of conductances due to solid conduction, gaseous conduction, gaseous convection, and radiation. Radiation can be ignored at low temperature, and gaseous convection can be neglected because the Pirani gauge is usually placed inside a package where no external forced gas convection exists [17]. Modeling constant solid conduction over pressure, the total thermal conductance at low pressure is dominated by solid conduction. Therefore, in order to obtain large dynamic range, a Pirani gauge needs to be designed to have a small gap distance and minimal solid conduction. where is the length of the heater. Typically, the width of the heater is much larger than its thickness ; thus, the sensitivity is proportional to the area of the heater. In order to increase the sensitivity and the dynamic range of the Pirani gauge, we have implemented dual heat sinks instead of the conventional single heat sink. By doing so, both gaseous conduction and effective heater area are increased to obtain high sensitivity and large dynamic range; gaseous conduction increases because heat flux can be absorbed by two heat sinks. Fig. 3 shows the effects of the dual heat sink configuration on the sensitivity and the dynamic range of a Pirani gauge. The dual heat sink configuration offers a high-performance Pirani gauge. However, implementing the dual heat sink configuration is technically challenging because it is necessary to have both heat sinks located evenly from the heater with minimal gap spacing. III. FABRICATION Two types of Pirani gauges have been developed: one is a traditional vertical heat transfer configuration, and the other (5)

CHAE et al.: MICROMACHINED PIRANI GAUGE WITH DUAL HEAT SINKS 621 Fig. 4. Proposed Pirani gauges with dual heat sinks. (Color version available online at http://ieeexplore.ieee.org.) TABLE I DESIGN SPECIFICATIONS OF THE PROPOSING PIRANI GAUGES is a lateral configuration (Fig. 4). The vertical configuration has a thin metal resister (Cr/Pt, 200/300 ) on a dielectric membrane anchored to p++ silicon. P++ silicon and a glass substrate form the top and bottom heat sinks. Each p++ silicon island is electrically isolated and is defined by deep reactive ion etch (DRIE). The p++ silicon can be utilized as a structural material for a variety of MEMS devices that need vacuum environment for their operation. Thus, the proposed Pirani gauges can be integrated with these MEMS devices to monitor pressure changes. A metal layer on a dielectric bridge transfers signals from the substrate to the bond pads. To ensure electrical contact to p++ silicon, a contact metal is deposited over the thin resister. The distance between the heater and heat sinks is only 0.4 m. The lateral gauge employs p++ silicon as both heater and heat sinks. The two heat sinks are separated from the heater by 1 m which is defined by DRIE. The vertical gauge is made in a six-mask process, while the lateral gauge requires only two masks. Table I shows the design specifications of the proposed Pirani gauges [18]. The fabrication process for these Pirani gauges is shown in Fig. 5. The vertical gauge process starts with blanket high-concentration boron doping of a silicon wafer. Then, LPCVD Fig. 5. Fabrication process. (Color version available online at http://ieeexplore.ieee.org.) Si N, which forms a boron diffusion barrier, and sacrificial polysilicon layers are deposited and patterned. Next, a 750-nm-thick LPCVD SiO Si N SiO membrane and meandering Cr/Pt (200/300 ) resister are formed. The Cr/Pt layer also forms electrical connections between the mechanically isolated p++ islands. This is followed by DRIE to form fluidic access to the sacrificial polysilicon layer and to isolate p++ islands for subsequent anisotropic wet etch such as ethylene diamine pyrocatechol (EDP). The silicon device wafer is then bonded to a recessed glass wafer, and EDP etching releases the membrane. The lateral devices, which have minimal process complexity and reasonable performance, are manufactured in a two-mask process. Fabrication begins with a blanket boron doping followed by DRIE to isolate the heater and heat sink. Fabrication is completed after the silicon wafer is bonded to a recessed glass support wafer and etched in EDP. Fig. 6 shows fabricated Pirani gauges.

622 Fig. 6. IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 28, NO. 4, NOVEMBER 2005 Photograph of the fabricated Pirani gauges. (Color version available online at http://ieeexplore.ieee.org.) IV. TEST RESULTS A. Pirani Gauge Characterization The sensors are characterized inside a vacuum chamber using a standard four-point probe measurement to extract the pressure-dependent thermal impedance [10]. A current source with 10-nA resolution (Keithley 225 A) forces a constant current to the gauge, and the voltage drop across the gauge is measured using a standard four-point probe configuration. Temperature is determined from the measured resistance of the gauge with given temperature coefficient of resistance (TCR) while power is measured from the current and voltage product. Current is inc, creased until the gauge reaches a preset temperature and then a linear curve fit is applied to the power versus temperature data to extract thermal impedance that is a slope of the linear curve. As pressure decreases, the slope of the curve (thermal impedance) increases; thus, we can extract the pressure dependent thermal impedance of the device. Fig. 7 shows thermal impedance of a lateral device versus ambient pressure. Fig. 8 shows the thermal impedance versus ambient pressure for vertical and lateral gauges. The vertical device shows higher sensitivity and larger dynamic range due to larger heater area and smaller gap distance between the heater and heat sinks. Fig. 9 shows the variance in thermal impedance measurements. absolute This presents an uncertainty of 4 mtorr and 50 pressure at 100 mtorr for lateral and vertical devices, respecindicates the Pitively. The uncertainty of 4 mtorr and 50 cm s rani gauges can measure leak rates as low as 2.3 10 and 3.1 10 cm s, respectively, assuming a common micropackage volume of 1.6 10 cm. Measured specifications of the Pirani gauges are summarized in Table II. B. Effect of Air Gap Distance to Performance of Pirani Gauges According to (2) in Section II, (empirical transition pressure) is inversely proportional to (distance between heater increases as and heat sink). When the distance decreases,, ), which changing Knudsen number ( results in small thermal impedance at high pressure and large

CHAE et al.: MICROMACHINED PIRANI GAUGE WITH DUAL HEAT SINKS 623 Fig. 7. Thermal impedance (slope) of a lateral device versus ambient pressure. (Color version available online at http://ieeexplore.ieee.org.) TABLE II MEASURED PIRANI GAUGES SPECIFICATIONS Fig. 8. Extracted thermal impedance of the Pirani gauges versus pressure. (Color version available online at http://ieeexplore.ieee.org.) Fig. 9. Variance of thermal impedance measurements. (Color version available online at http://ieeexplore.ieee.org.) dynamic range. We have tested identical lateral Pirani gauges except for their different gap distance to see if the gap distance changes the thermal impedance at high pressure. Table III shows thermal impedance data of the gauges with 1.1- and 2.1- m gap distances. As the gap decreases, the thermal impedance decreases by 5.1%, which matches well with calculated values based on a simple two-dimensional analytical model [8], [19]. We also compared the measured data at low pressure (10 mtorr). The measured data are a bit lower than what we expect. This might be due to nonuniform temperature distribution of the heater. For bridge-type Pirani gauges, the center of the heater is at a higher temperature than the edges due to proximity to anchor points. This nonuniform temperature distribution might result in the smaller thermal impedance at low pressure than that calculated using a simple analytical model which assumes that the temperature of the heater is constant. The small thermal impedance also could be from parasitic heat loss to the glass substrate. Although the gap between the heater and the substrate is larger (3 m) than between the heater and p++ silicon (1.1 m), some heat dissipates to the glass substrate. C. Effective Area to Performance of the Gauges The sensitivity of the Pirani gauge is approximately proportional to the area of the heater and heat sinks as shown in (5). It should be noted that the sensitivity is a strong function of the area, not the area to volume ratio. Obviously, microscale devices, including the micro-pirani gauge developed in this work, generally have very large surface area to volume ratio compared

624 IEEE TRANSACTIONS ON ADVANCED PACKAGING, VOL. 28, NO. 4, NOVEMBER 2005 TABLE III DEPENDENCE OF THE THERMAL IMPEDANCE TO THE GAP DISTANCE can be dissipated either to p++ silicon or glass substrate. This undesirable parasitic heat loss can be minimized by simply increasing the distance to the substrate. Fig. 10. Effective area versus sensitivity of the Pirani gauge. (Color version available online at http://ieeexplore.ieee.org.) to macroscale devices. However, the operating principle of the gauge is dependent on heat flux from the heater to heat sinks. This is the amount of heat that is transferred per unit area in a unit of time. Therefore, volume does not play a significant role for heat flux. The volume of the gauge contributes to the thermal capacitance and thermal mass of the heater, and these in turn affect the thermal time constant. We have tested two types of lateral design Pirani gauges which have the same gap distance but have different heater and heat sink areas. The large device (L19) has a larger heater area (not heat sink) by a factor of 2.67 than the small device (L15). However, the small device has heat sinks surrounding its entire heater, while only 80 of the large device s heater is surrounded by heat sinks. Therefore, the effective heater area of the large device decreases, and the ratio of the effective heater area becomes 2.14. Fig. 10 shows thermal conductance versus pressure for these two devices. We took four devices each for both types on a wafer to check the uniformity of device performance characteristics. As error bars show, except at very low pressure the measurements demonstrate less than 3% deviation for most of the pressure range. Sensitivity, the slope in the figure, is proportional to the effective heater area. The large device has higher sensitivity than the small device by a factor of 1.85 which is less than what is expected from analytical modeling. This is because of parasitic heat loss to the glass substrate. Heat flux from the heater of lateral design devices V. CONCLUSION Micromachined Pirani gauges with dual heat sinks have been developed to monitor long-term pressure changes and stability inside the package. The gauges can be integrated with MEMS devices using the dissolved wafer process inside a small vacuum package. Two thermal sinks have been implemented in order to obtain high sensitivity and large dynamic range. We have developed two different designs of Pirani gauges: vertical and lateral configuration. Vertical devices requiring six masks show higher performance than lateral devices which only need two masks. The vertical device has large dynamic range (20 mtorr 2 torr) and high sensitivity (3.5 10 K/W torr) with uncertainty of 50. Assuming a common micropackage volume of 1.6 10 cm, the gauge can resolve leak rates inside a small sealed cavity as low as 3.1 10 cm s, which represents at least four orders of magnitude improvement over traditional helium leak testing with a substantially reduced cost. ACKNOWLEDGMENT The authors would like to thank Dr. H. Kulah, B. Casey, and S. Kim for wire bonding, device packaging, and device characterization. They also thank the staff at Wireless Integrated Micro-Systems (WIMS), the University of Michigan. REFERENCES [1] D. Sparks, G. Queen, R. Weston, G. Woodward, M. Putty, L. Jordan, S. Zarabadi, and K. Jayakar, Wafer-to-wafer bonding of nonplanarized MEMS surfaces using solder, J. Micromech. Microeng., vol. 11, pp. 630 634, 2001. [2] Y.-T. Cheng, W.-T. Hsu, K. Najafi, C. T.-C. Nguyen, and L. Lin, Vacuum packaging technology using localized aluminum/silicon-toglass bonding, J. Microelectromech. Syst., vol. 11, pp. 556 565, 2002. [3] Verh. Dtsch. Phys., vol. 8, pp. 686 686, 1906. [4] J. F. O Hanlon, A User s Guide to Vacuum Technology. Hoboken, N.J: Wiley-Interscience, 2003. [5] J. H. Leck, Pressure Measurement in Vacuum Systems. London, U.K.: Chapman & Hall, 1967. [6] W. J. Alvesteffer, D. C. Jacobs, and D. H. Baker, Miniaturized thin film thermal vacuum sensor, J. Vacuum Sci. Technol. A: Vacuum, Surfaces, Films, vol. 13, pp. 2980 2980, 1995. [7] J.-S. Shie, B. C. S. Chou, and Y.-M. Chen, High performance Pirani vacuum gauge, J. Vacuum Sci. Technol. A: Vacuum, Surfaces, Films, vol. 13, pp. 2972 2972, 1995.

CHAE et al.: MICROMACHINED PIRANI GAUGE WITH DUAL HEAT SINKS 625 [8] C. H. Mastrangelo and R. S. Muller, Microfabricated thermal absolutepressure sensor with on-chip digital front-end processor, IEEE J. Solid- State Circuits, vol. 26, no. 12, pp. 1998 2007, Dec. 1991. [9] M. Waelti, N. Schneeberger, O. Paul, and H. Baltes, Package quality testing using integrated pressure sensor, Int. J. Microcircuits Electron. Packag., vol. 22, pp. 49 56, 1999. [10] B. H. Stark, Y. Mei, C. Zhang, and K. Najafi, A doubly anchored surface micromachined Pirani gauge for vacuum package characterization, in Proc. IEEE 16th Annu. Int. Conf. Micro Electro Mechanical Systems, Kyoto, Japan, Jan. 19 23, 2003, pp. 506 509. [11] J. Chae, J. M. Giachino, and K. Najafi, Wafer-level vacuum package with vertical feedthroughs, in Proc. 18th IEEE Int. Conf. Microelectromechanical Systems (MEMS): MEMS Technical Dig., Miami, FL, Jan. Feb. 30 3, 2005, pp. 548 551. [12] H.-L. Chau and K. D. 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Microelectromechanical Systems (MEMS): Maastricht MEMS Technical Dig., Maastricht, The Netherlands, Jan. 25 29, 2004, pp. 532 535. [19] B. Stark, Thin Film Technologies for Hermetic and Vacuum Packaging of MEMS, Ph.D. dissertation, Dept. Elect. Eng. Comp. Sci, Univ. Michigan, Ann Arbor, 2004. Junseok Chae (M 03) received the B.S. degree in metallurgical engineering from Korea University, Seoul, in 1998, and the M.S. and Ph.D. degrees in electrical engineering and computer science from the University of Michigan, Ann Arbor, in 2000 and 2003, respectively. From 2000 to 2005, he was a Postdoctoral Research Fellow at Wireless Integrated MicroSystems (WIMS), University of Michigan. He joined the faculty of Arizona State University, Tempe, in August 2005, where he is currently an Assistant Professor in electrical engineering. His areas of interests are MEMS sensors, mixed-signal interface electronics, MEMS packaging, ultrafast pulse (femto-second) laser for micro-/nanostructures, and cell-on-a-chip bio-mems. He had an invited talk at Microsoft, Inc. regarding MEMS technology for consumer electronic applications and holds a couple of U.S. patents. Dr. Chae received the first place prize and the Best Paper Award at the Design Automation Conference (DAC) student design contest in 2001 with the paper entitled Two-dimensional position detection system with MEMS accelerometer for mouse application. Brian H. Stark was born in Boston, MA, in 1977. He received the B.S. degree in electrical engineering (cum laude) from Cornell University, Ithaca, NY, in 1999 and the M.S. and Ph.D. degrees in electrical engineering with a major in solid-state theory and a minor in circuits and microsystems from the University of Michigan, Ann Arbor, in 2002 and 2004, respectively. During his undergraduate career, he interned at the Jet Propulsion Laboratory, where he worked on processes related to MEMS reliability. His work there culminated with his authorship of a MEMS reliability guideline, which remains the only published book on MEMS reliability. From 1997 to the present, he has also presided as the CEO of Stark Software, a small company that has created software packages for the medical community. He has had over 20 refereed publications since 1997. Khalil Najafi (S 84 M 86 SM 97 F 00) received the B.S., M.S., and Ph.D. degrees in electrical engineering from the Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, in 1980, 1981, and 1986 respectively. From 1986 to 1988, he was a Research Fellow, from 1988 to 1990 as an Assistant Research Scientist, from 1990 to 1993 as an Assistant Professor, from 1993 to 1998 as an Associate Professor, and since September 1998 as a Professor and the Director of the Solid-State Electronics Laboratory, Department of Electrical Engineering and Computer Science, University of Michigan. His research interests include: micromachining technologies, micromachined sensors, actuators, and MEMS; analog integrated circuits; implantable biomedical microsystems; micropackaging; and low-power wireless sensing/actuating systems. Dr. Najafi was awarded a National Science Foundation Young Investigator Award from 1992 to 1997, was the recipient of the Beatrice Winner Award for Editorial Excellence at the 1986 International Solid-State Circuits Conference, of the Paul Rappaport Award for coauthoring the Best Paper published in the IEEE TRANSACTIONS ON ELECTRON DEVICES, and of the Best Paper Award at ISSCC 1999. In 2003, he received the EECS Outstanding Achievement Award, in 2001 he received the Faculty recognition Award, and in 1994 the University of Michigan s Henry Russel Award for outstanding achievement and scholarship, and was selected as the Professor of the Year in 1993. In 1998, he was named the Arhtur F. Thurnau Professor for outstanding contributions to teaching and research, and received the College of Engineering s Research Excellence Award. He has been active in the field of solid-state sensors and actuators for more than 20 years, and has been involved in several conferences and workshops dealing with solid-state sensors and actuators, including the International Conference on Solid-State Sensors and Actuators, the Hilton-Head Solid- State Sensors and Actuators Workshop, and the IEEE/ASME Microelectromechanical Systems (MEMS) Conference. He is the Editor for Solid-State Sensors for the IEEE TRANSACTIONS ON ELECTRON DEVICES, an Associate Editor for the Journal of Micromechanics and Microengineering, Institute of Physics Publishing, and an Editor for the Journal of Sensors and Materials. He also served as the Associate Editor for the IEEE JOURNAL OF SOLID-STATE CIRCUITS from 2000 to 2004, and the Associate Editor for the IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING from 1999 to 2000.