A monolithically integrated surface micromachined touch mode capacitive pressure sensor
Introduction
Capacitive pressure sensors have been developed for industrial applications for many years with advantages of high sensitivity, low temperature drift, robust structure and low sensitivity to environmental effects 1, 2. However, the output capacitance of microsensor is normally very small (the order of fF or pF) and is very susceptible to parasitic effects [3]. In order to detect such small capacitance changes, a well-matched readout circuit is required and it has to be placed as close as possible to the sensor. Therefore, on-chip integration of sensors with interface circuits is important in this application. Surface micromachining is a promising technology to achieve this goal. Nonlinear characteristics is another drawback of the normal mode capacitive sensors. Much effort has been made to improve the linearity of capacitive sensor either by modifying the structure of sensors or by designing special interface circuits 4, 5, 6, 7. Among those attempts, touch mode capacitive pressure sensor is a promising design to fabricate a linear capacitive sensor with a simple structure [8]. Touch mode capacitive pressure sensors are intentionally designed to operate in the range where the diaphragm touches substrate. The change of capacitance is mainly determined by the touched area, and is proportional to the applied pressure [9]. The advantages of touch mode capacitive pressure sensor are good linearity near the operating point, large operating pressure range, large overload protection, and zero suppression possibility.
In this report, we present a prototype of a touch mode capacitive pressure sensor integrated with CMOS interface circuits. The fabrication of the prototype combines two technologies: surface micromachined sensor fabrication process using polysilicon as the diaphragm, and conventional 2.0-μm double-poly, double-metal n-well CMOS IC process for the interface circuits.
Section snippets
Design of the sensor element
The plan view and cross-sectional view schematic drawings of a circular micromachined sensor element are shown in Fig. 1. The sensor consists of a polysilicon deformable diaphragm with thickness h, referred to as the top electrode, and a polysilicon layer covered by an insulating layer, t on an isolated silicon substrate, referred to as the bottom electrode. The two electrodes are separated by an initial gap, d. The top electrode is formed using an in situ phosphorous doped polysilicon layer
Results and discussion
An optical photograph of a completely processed chip is shown in Fig. 5. The CP11 circuit, which has only frequency output, is on the top-left and the CP12 circuit, which has both frequency and DC output, is on the bottom left. A pair of circular pressure sensors in the left middle of the chip are the sensor capacitor Cx and reference capacitor C0. Both are connected to the CP11 circuit right above them in the figure. The C0 has no cavity underneath so it is not sensitive to pressure. Another
Conclusion
We have fabricated and tested a fully CMOS compatible surface micromachined touch mode capacitive pressure sensor. The CMOS interface circuit works properly after integration with the sensors. There is no obvious shift in threshold voltage observed after the additional sensor fabrication process steps are added to the CMOS process for sensor fabrication. Measurement results of the completed sensor and circuit are in good agreement with simulation results. The frequency and voltage output
Acknowledgements
This project has been partially supported by DARPA grant (Darpar-DABT63-95C-0071) and NASA grant (NAG3-2204).
Dr. Shuwen Guo received his BS degree in Semiconductor Physics from Nanchang University in 1982 and his MS and PhD degrees in EE from Shanghai Institute of Metallurgy, Chinese Academy of Sciences, China in 1987 and 1991, respectively. From 1991 to 1993, he was a faculty member in Nanchang University. He worked as a visiting scientist in the Department of Physics and Measurement Technology, Linkoping University in Sweden from 1993 to 1995 and then worked as an R&D researcher in Microsystems
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Dr. Shuwen Guo received his BS degree in Semiconductor Physics from Nanchang University in 1982 and his MS and PhD degrees in EE from Shanghai Institute of Metallurgy, Chinese Academy of Sciences, China in 1987 and 1991, respectively. From 1991 to 1993, he was a faculty member in Nanchang University. He worked as a visiting scientist in the Department of Physics and Measurement Technology, Linkoping University in Sweden from 1993 to 1995 and then worked as an R&D researcher in Microsystems group in Inter-university Micro-electronics Center (IMEC), Leuven-Belgium from 1995 to 1996. Since 1996, he has been a senior researcher in the MEMS group at Department of Electrical Engineering and Computer Sciences at Case Western Reserve University. His research interests include integrated sensors and actuators, MEMS, Si and SiC micromachining processes, and integrated circuit design. He is currently a principal engineer at Advanced Sensors Technology Center, BFGoodrich Aerospace, MN, USA.Mr. Jun Guo, was born in Hunan, China. He received his BS degree from Tsinghua University in 1993. He had worked 4 years in China's Greatwall Computer. Since 1997, he studied in the Department of Electrical Engineering and Applied Physics, Case Western Reserve University CWRU, Cleveland, OH, USA. He is currently a PhD student.Dr. Wen H. Ko was born in 1923, in Fujian, China. He received his BS in EE from Amoy (Xiamen) University of China in 1946, and his MS and PhD degrees in EE from Case Institute of Technology, Cleveland, OH, USA, in 1956 and 1959, respectively. He has been an assistant, an associate and a full professor of Electrical Engineering and Biomedical Engineering, at Case Western Reserve University CWRU, Cleveland, OH, USA since 1959, 1962 and 1967, respectively. He become a Professor Emeritus in EE of CWRU in July 1993. Dr. Ko is interested in solid state electronics, micro-sensors and actuators, MEMS, Medical and Biological Engineering. He is on the editorial board of Sensors and Actuator, Sensors and Materials, Micro-system Technologies, Telemetry and Patient Monitoring 1974–1984, and Medical. Progress Through Technology 1983–1988 and is a reviewer for journals in his fields of interest. He was the chairman of International steering committee on solid state sensors and actuators conferences from 1983 to 1987, and the general chairman of 1985 Conference in Philadelphia, USA. He also was the chairman of the international steering committee on chemical sensor meetings from 1991 to 1993. He is the president of the Transducer Research Foundation, since 1992, that sponsored the Hilton Head Work-shops on Sensors and Actuators.