Applications of High-Performance MEMS Pressure Sensors Based on Dissolved Wafer Process Srinivas Tadigadapa and Sonbol Massoud-Ansari Integrated Sensing Systems (ISSYS) Inc., 387 Airport Industrial Drive, Ypsilanti, MI 48198 ISSYS’ exclusive dissolved wafer process (DWP) technology has enabled the development of a variety of the state-of-the-art, high performance pressure sensors for a wide range of applications. DWP is a simple, single-sided wafer fabrication process, which can very reproducibly and accurately fabricate both low and high aspect ratio single crystal silicon structures at high densities. In this paper, we will discuss the performance and applications of the pressure sensors fabricated by this wafer processing technique. These applications include diverse areas such as ultra-sensitive, high vacuum measurements; barometric pressure sensors; ultra-high sensitivity altimeters; miniaturized pressure sensors for biomedical applications; and multiple sensors on a single chip for applications requiring extended dynamic range and improved accuracy, as well as enhanced reliability. Introduction Miniaturization of sensors and actuators requires the ability to create accurate, three-dimensional microstructures, which are capable of performing thermoelectro-mechanical functions. Several innovative modifications and adaptations in the silicon microelectronics fabrication processes have led to the possibility of creating such accurate, three-dimensional, free-standing structures from silicon. One such fabrication technique known as the “Dissolved Wafer Process” (DWP) was developed at the University of Michigan by Professor Kensall Wise and his group. ISSYS Inc. has exclusive rights to six patents from the University of Michigan, which comprehensively deal with all aspects of the DWP fabrication technique and its applications. After a brief presentation of the DWP technology, we will demonstrate the versatility of DWP by the performance and the variety of applications of capacitive pressure sensors manufactured on the flexible platform of this technique. Background Integrated micromachining technologies can be divided into four major categories: bulk micromachining (including DWP), polysilicon surface micromachining, electroplate micromachining (also known as LIGA) and post-processed CMOS wafer micromachining. Bulk micromachining is the dominant commercial microsensor technology in which the silicon wafer is directly etched to create three-dimensional devices, with thickness up to that of the single wafer. Major disadvantages of bulk micromachining are the relatively large silicon real estate consumption and the lack of complete compatibility with standard CMOS processing. Surface micromachining manipulates materials (mainly polysilicon) that are deposited on the front surface of the silicon wafer. The bulk silicon wafer is primarily used as a substrate and acts as a structural support for the micromachined devices. Major advantages of surface micromachining are the small silicon area Industrial Pressure Sensor used for the mechanical structures, compatibility with standard microelectronics processes and capability of fabricating multilevel microstructures. The major drawback of surface micromachining is the use of polysilicon as the mechanical material and several yield and mate rial reliability issues associated with thin micromachined structures. In electroplated micromachining, microdevices with very high aspect ratio are fabricated with deep X-ray lithography, electro-forming and possible molding processes. In post-process CMOS wafer technology, standard CMOS processed wafers are further post-processed to release microstructures formed from the thin films available in this process (typically polysilicon and SiO2). A standard post process step involves a single maskless etch stop. Dissolved Wafer Process DWP is a versatile fabrication technique, which allows the creation of a variety of three-dimensional, free-standing, single-crystal, silicon-microstructures on glass substrates. DWP is a simple, singlesided wafer fabrication process, which is both reproducible and accurate. Using DWP, it is possible to simultaneously create high aspect ratio thick and/or thin microstructures on the same chip and at high densities. Single crystal silicon is an ideal micromechanical material. It has a Young’s modulus and hardness comparable to those of stainless steel, with a density only a third and yield strength three times that of stainless steel. As a crystalline material, it is devoid of the plastic deformation phase and is practically fatigue and hysteresis free. Figure 1 shows the simplified process flow in fabricating a diaphragm using DWP. The fabrication sequence consists of silicon processing, glass processing, electrostatic bonding, and wafer dissolution. Typical silicon processing starts with a 550 µm thick, p-type (100) oriented silicon wafer. A KOH etch is first performed to define a recess that will later provide the gap between the silicon diaphragm and the glass substrate (Fig. 1a). This is followed by a patterned deep-boron diffusion step, which defines the supporting rim for the diaphragm (Fig. 1b). Typical thickness of deep-boron diffusion varies between 10µm-15µm. The diaphragm itself is formed by using shallow-boron diffusion (2µm-5µm) (Fig. 1c). Glass processing consists of metallization to form lead transfers, etching and/or formation of holes as required. Once the silicon and glass processing are completed, the silicon and glass wafers are electrostatically bonded together. The silicon wafer is then etched (dissolved) away in Ethylene Diamine Pyrocatechol (EDP), leaving only heavily boron-doped areas on the glass wafer (Fig. 1d). It can be seen that DWP is a versatile process in providing silicon microstructures with a wide range of shapes and dimensions. The fabrication process allows for the implementation of many variations in shapes and structures without increasing either the number of mask steps or process complexity. Therefore, DWP offers a very flexible fabrication platform, which can be used to manufacture a wide range of micromachined sensors and actuators. In Figure 1. Simplified process flow for Dissolved Wafer Process (DWP). Industrial Pressure Sensor addition, interfacing with circuit chips can be achieved using a simple hybrid flip chip approach. sensors, the gap between the diaphragm and the glass acts as the reference cavity ISSYS has successfully fabricated and tested a wide range of pressure sensor prototypes using DWP. The basic structure of the proposed pressure sensor is shown in Figure 1d. The diaphragm and the bottom electrode on the glass act as a parallel plate capacitor. In response to pressure variations the diaphragm deflects, causing a variation in the capacitance. Figure 2 shows a multielement sensor with 9 different pressure sensors. These elements measure full-scale pressure in the range of 0-1 Torr to 0-2200 Torr. Sensitivity (pF/Torr) 1000 Pressure Sensors manufactured using DWP and their Applications Theory Experiment 100 10 1 0.1 0.01 0.001 0 2 4 6 8 Diaphragm Diameter (mm) Figure 3 Sensitivity of pressure sensors as a function of the diaphragm diameter. and is sealed up at ultra-high vacuum. ISSYS has patent pending technique by which the electrical leads are transferred from the the vacuum sealed cavity to the outside. The multi-sensor chip also incorporates temperature sensors with a resolution of 2000 ppm/ºC. In addition to the measurement of vacuum, the differential pressure sensors along with the temperature sensors can be used for the measurement Figure 2 Sensor die containing nine pressure sensor covering a pressure measurement range from 0–2200 Torr to 0–1 Torr. Figure 3 illustrates the theoretical and experimentally measured sensitivity of these capacitive pressure sensors as a function of the diaphragm size. Capacitive pressure sensors can be fabricated for the measurement of absolute or differential pressure. In case of absolute pressure Figure 4 ISSYS’ most sensitive pressure sensor with a sensitivity of 120 pF/Torr . of fluid flow. Industrial Pressure Sensor Figure 5 Performance of the 0-250 mTorr range. This pressure sensor is capable of resolving sub-? Torr vacuum pressures. possible to measure pressures in the sub? Torr range. Figure 5 shows the measured performance of this device. High performance vacuum pressure sensors find applications in several fields such as semiconductor equipment industry, vacuum instrumentation, and flow measurement applications. Vacuum pressure sensors can also be used as barometric pressure sensors. However, the typical sensitivity of capacitive pressure sensors increases exponentially with the decrease in the full operating range. ISSYS has developed a proprietary technology, which overcomes this shortcoming and allows for the operating range and the sensitivity to be independently designed. For example, one of our sensor designs, with an overall dimension of 8mm X 8mm, exhibits a resolution of better than 2 mTorr at 800 Torr pressure range (a resolution of 1/400,000). If this sensor is used as an altimeter, it will have an outstanding resolution of better than one inch at the sea level, and a resolution of about 1-foot at 30,000 feet, as shown in Figure 6. The simulated performance shown in Figure 6, however, has not been compensated for temperature variations. Barometric pressure sensors can also be used in wind tunnels; for very accurate measurement of pressure and pressure distribution on a variety of airfoils and aircraft models; barometric pressure measurement for weather monitoring applications; and in multi-sensor, microinstrumentation clusters requiring low-power barometric pressure sensors. ISSYS is currently developing ultraminiature pressure sensors for biomedical applications. Typical size of these pressure sensors is 300 ? m X 4 mm. This size is perfectly suited for several biomedical applications such as single/multi-point catheters, measurement of intracranial pressure, pacemaker applications and other 30 Resolution (inch) In early 1998, ISSYS fabricated the first prototype of one of the most sensitive capacitance diaphragm vacuum pressure sensors known. This sensor is shown in Figure 4. The full-scale range of this sensor is from 0 – 0.25 Torr and has a sensitivity of over 100 pF/Torr. Using this sensor, it is 25 20 15 10 5 0 0 10000 20000 30000 40000 50000 Altitude (feet) Figure 6 Simulated performance of an altimeter using ISSYS’ proprietary barometric pressure sensor. implanted coronary pressure measurements. Figure 7 shows a typical biomedical pressure sensor prototype in the eye of a needle. The performance of the sensor is shown in Figure 8. The sensitivity of this device was measured to be 3 fF/Torr in the 500 – 1000 Torr pressure range. This pressure range adequately covers the very versatile micromachining technology and offers a flexible manufacturing platform for a variety of micromachined sensors and actuators. In addition, ISSYS has several patents covering all aspects of manufacturing such silicon micromachined pressure sensors. Current patent pending areas include: sensor design, electrical lead transfer from sealed cavities, packaging, and corrosion resistant technology. References K.D. Wise and Samaun, “Methods for forming regions of predetermined thickness in silicon”, U.S. Patent 3,888,708, June 10, 1975. Figure 7 Multi-site pressure sensing catheter using ISSYS’ 300 um X 4mm pressure sensor. The device is shown in the eye of a needle to give a perspective of its size. K.D. Wise and H.L. Chau, “Ultraminiature pressure sensor and method of making same”, U.S. Patent 4,881,410, November 21, 1989. 5.50 Sensitivity = 3.2 fF/Torr Capacitance (pF) 5.00 4.50 K.D. Wise and H.L. Chau, “Method of making an unltraminiature pressure sensor”, U.S. Patent 5,013,396, May 7, 1991. 4.00 3.50 3.00 500 600 700 800 900 1000 Pressure (Torr) Figure 8 Performance of the pressure sensor designed for biomedical applications pressure measurement requirements for biomedical applications and provides a measurement sensitivity of better than 1 Torr. ISSYS is currently working on the biocompatible, corrosion-resistant packaging of these sensors and their integration into long-term implantable pressure monitoring systems. Conclusions In conclusion, ISSYS Inc. has developed capacitive pressure sensors for a variety of applications. These pressure sensors have been fabricated using ISSYS’ exclusive dissolved wafer process. DWP is a L.J. Spangler and K.D. Wise, “Fullyintegrated single-crystal silicon on insulator process, sensors, and circuits” U.S. Patent 5,343,064, August 30, 1994. Y. Gianchandani and K. Najafi, A bulk silicon dissolved wafer process for microelectromechanical devices, IEEE/ASME J. MEMS, Vol. 1, No. 2, pp. 7785, 1992. K. Najafi, Integrated sensors in biological environments, Invited Paper, Sensors and Actuators, Vol. B1, 1990. Y Zhang and K.D. Wise, An Ultra-sensitive capacitive pressure sensor with bossed dielectric diaphragm, Digest Solid-State Sensor and Actuator Workshop, Hilton Head, pp. 205-208, June 1994. S.T. Cho and K.D. Wise, Technical Digest, Intl. Conf. On Solid State Sensors and Actuators, pp. 400, June 1991. Y. Zhang and K.D. Wise, A high-accuracy multi-element silicon barometric pressure sensor, Digest Intl. Conf. On Solid State Sensors and Actuators, Stockholm, pp. 608611, June 1995.