CAPACITIVE MICRO PRESSURE SENSORS WITH UNDERNEATH READOUT CIRCUIT USING A STANDARD CMOS PROCESS

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1 Journal of the Chinese Institute of Engineers, Vol. 26, No. 2, pp (2003) 237 Short Paper CAPACITIVE MICRO PRESSURE SENSORS WITH UNDERNEATH READOUT CIRCUIT USING A STANDARD CMOS PROCESS Ching-Liang Dai*, Shih-Chen Chang, Chi-Yuan Lee, Ying-Chou Cheng, Chien-Liu Chang, Jing-Hung Chiou, and Pei-Zen Chang ABSTRACT A capacitive micropressure sensor with readout circuits on a single chip is fabricated using commercial 0.35µm complementary metal oxide semiconductor (CMOS) process and post-processing. The main break through feature of the chip is the positioning of its readout circuits under the pressure sensor, allowing the chip to be smaller. Post-processing included anisotropic dry etching and wet etching to remove the sacrificial layer, and the use of plasma enhanced chemical vapor deposition (PECVD) of nitride to seal the etching holes on the pressure sensor. The readout circuit is divided into analog and digital parts, with the digital part being an alternate coupled RS flipflop with four inverters that sharpened the output wave. The analog part employed switched capacitor methodology. The pressure sensor contained an 8 8 sensing cells array, and the total area of the pressure sensor chip is 2mm 2mm. Key Words: capacitive micropressure sensor, a single chip, CMOS, readout circuit. I. INTRODUCTION Microelectromechanical system (MEMS) technology includes surface micromachining, bulk micromachining and LIGA (Lithograpie Galvanoformung Abformung). Since the system on chip (SOC) design concept appeared, the trend in related products has been miniaturization and mass production, improved performance, quality, reliability, added value, and low cost. Consequently, the integration of circuits, structures and devices is becoming increasingly important. *Corresponding author. (Tel: ext. 423; cldai@dragon.nchu.edu.tw) C. L. Dai is with the Department of Mechanical Engineering, National Chung Hsing Unviersity, Taichung, Taiwan 402, R.O.C. S. C. Chang, C. L. Chang, J. H. Chiou, and P. Z. Chang are with the Institute of Applied Mechanics, National Taiwan University, Taipei, Taiwan 106, R.O.C. C. Y. Lee and Y. C. Cheng are with the Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan 106, R.O.C. Thus standard CMOS manufacturing processes are being widely used. Applying the standard CMOS process to MEMS has several merits: allowing circuit integration, mass production and low costs. Therefore, in this study we utilized a standard CMOS process to fabricate a pressure sensor with readout circuits. Pressure sensors can be applied in the automotive industry (Dondon et al., 1993), mechanical engineering, medical fields (Kandler et al., 1992), and so on. CMOS-compatible pressure sensors are sensors and signal conditioning circuitry, which can be integrated on-chip. These devices have the advantages of small volume, high performance and low cost. Commercial 0.35 µm CMOS process technology and post-processing are used herein to fabricate a capacitive pressure sensor with readout circuits on a single chip. The main feature of the chip is that the readout circuits are situated under the pressure sensor, reducing the chip s size. The pressure sensor

2 238 Journal of the Chinese Institute of Engineers, Vol. 26, No. 2 (2003) Passivation Metaal 4 Vacuum 1.2 µm 0.93 µm 1 µm 0.64 µm Capacitance (10 15 F) Metal 2 1 µm 0.64 µm Pressure (KPa) 750 Fig. 1 Schematic cross section of a sensing cell Fig. 3 The relation of the pressure and the capacitance with different gap distance for a sensing cell fixed a contains an 8 8 sensing cells array, and the total area of the pressure sensor chip is 2mm 2mm. II. SENSING PRINCIPLE AND CIRCUIT DESIGN 1. Sensing Principle of Capacitor w P z electrodes Fig. 2 Simplified model of sensing cell, clamped circular plate The capacitive pressure sensor contains an 8 8 sensing cells array. Fig. 1 illustrates a cross section of each sensing cell. The membrane of the sensing cell is a sandwich structure formed by a silicon oxide layer,metal 4 layer, and a passivation layer. Parallel electrodes of the sensing cell are comprised of a metal 4 layer and a metal 2 layer, and a metal 2 layer is fixed on the silicon substrate. The cavity under the metal 4 layer is a vacuum; that is, the sensor is an absolute pressure sensor. Although the membrane of the sensing cell is a twenty-sided polygon, the polygonal membrane has been simplified into a circular membrane herein for convenience. A clamped, circular plate with radius a, is now considered, acted upon by a uniformly distributed transverse load p, as shown in Fig. 2. The displacement equation of equilibrium of the classical plate theorem (Reismann and Pawlik, 1980) is D 2 ( 2 w(r)=p, 0<r<a (1) r d 0 where 2 w = 1 r dr d (rdw(r) dr ), D = Eh 3 12(1 ν 2, and w(r) ) denotes the displacement of the plate, E represents the Young s modulus of the plate, h is the thickness of the plate, and ν denotes the Poisson s ratio. Solving Eq. (1), the displacement of the plate can be written as w(r)= p 64D (r2 a 2 ) 2 (2) Because the deformation of the membrane is small, the sensing cell after deformation of the membrane is approximately a parallel-plate capacitor. The capacitance of the sensing cell can thus be expressed as C = 0 a 0 2π ε rdrdθ d 0 w(r) (3) where ε denotes the dielectric constant, and d 0 represents the gap distance at rest. Substituting Eq. (2) into Eq. (3), obtains the capacitance as a function of pressure p (Dai and Chang, 1999). C =4επ D d0 p ln d 0 + a 2 p 64D d 0 a 2 p 64D (4) Assume that E=170 Gpa, h=3.12 µm, ν=0.22, and a=60 µm, and these values are substituted into Eq. (4). The relation of capacitance, C, and pressure, p, with different gap distances, d 0, is illustrated in Fig. 3. The result shows that the relation between the pressure and the capacitance of the sensing cell is nonlinear. 2. Principle of the Readout Circuit Figure 4 displays the whole readout circuit of the capacitive pressure sensor. The readout circuit is

3 C. L. Dai et al.: Capacitive Micro Pressure Sensors with Underneath Readout Circuit Using A Standard CMOS Process 239 reference capacitor clk inv nor inv inv Cr eapacitive pressure sensors nor inv inv Mn Cs OP ANP out Vaut digital analog Fig. 4 Schematic of capacitive pressure sensor and readout circuitry on chip (a) (b) (c) (d) Circuits (e) (f) 8 8 sensing cells Nitride Photoresistance Metal 3 Silicon substrate Metal 4 Metal 2 PCB Fig. 5 The layout of the capacitive micropressure sensor with readout circuits divided into analog and digital parts. The digital part is an alternate coupled RS flip-flop with four inverters that sharpens the output wave, and has a single-phase clock that is converted into two phase non-overlapping clocks. Meanwhile, the analog part is based on the switched capacitor method. The voltage across the integrating capacitor, C r, is cleared on each Φ 2, while on Φ 1 the input voltage charges C s and the charging current flows across C r simultaneously. Consequently, the change in the charge flowing across C r, Q Cr, equals the change in charge flowing across C s, Q Cs, and therefore, at the end of Φ 1, the output voltage can be expressed as (Johns and Martin, 1996) V out = C s C r V in (5) Fig. 6 The post-processing (a) Schematic cross section of sensing cell after completion of CMOS process; (b) Dry etching; (c) Photolithography; (d) Wet etching; (e) Deposition of PECVD nitride; (f) Wire bounding III. FABRICATION Figure 5 displays the overall layout of the chip with a complete measuring circuit underneath it, and with the 8 8 capacitive pressure sensing cells. After completing the layout and verification of the pressure sensor chip, TSMC (Taiwan Semiconductor Manufacture Company) carried out the standard 0.35 µm CMOS process on the chip. Figure 6(a) shows the schematic cross section of a single sensing cell after completion of the CMOS process. Post-processing was necessary to obtain the sensing cell gap, and Fig. 7 illustrates the procedure used. During post-processing, anisotropic dry etching

4 240 Journal of the Chinese Institute of Engineers, Vol. 26, No. 2 (2003) Relief valve Value N 2 gas Filters Valve Calibration pressure sensor Oscilloscope Chamber Sample on PCB Fig. 8 Schematic experimental setup of the capacitive pressure sensor Tek Run: 100kS/s Sample Fig. 7 SEM micrograph (a) Single pressure sensing cell; (b) Etching holes C1 High 5.2 V (reactive ion etch, RIE) was used first to remove silicon oxide, shown in Fig. 6(b). CF 4 was the optimal gas choice. The etching rate was 600Å/min. As Fig. 6(c) shows, photoresist was used during photolithography to protect all the exposed aluminum areas except the access holes. Metal 3 layer was a sacrificial layer made of aluminum. Wet etching solution (16 H 3 PO HNO CH 3 COOH + 2 H 2 O) was applied through the etching holes to remove the Metal 3 layer at 60 C, with a rate of 1.5um/min and an air gap of 0.64um, as shown in Fig. 6(d). Fig. 7(a) shows the SEM diagram of a single sensing cell, while Fig. 7(b) shows that of the etching holes. Then, a nitride layer was deposited by PECVD until the etching holes were sealed, shown in Fig. 6(e). Finally, wire-bounding technology was used for preliminary measuring, as shown in Fig. 6(f). IV. RESULTS The experimental setup for the capacitive pressure sensor is illustrated in Fig. 8. Nitrogen gas supplies the pressure source for the capacitive pressure sensor, and a tuning valve can change the gas pressure. When no pressure was applied to the capacitive pressure sensor, the output voltage of the capacitive pressure sensor measured by the oscilloscope is shown in Fig. 9. The result shows that the 1 2 Fig. 9 Ch1 5.00V Ch2 5.00V M 500 µs Ch1 2.4V Ch3 5.00V the output voltage of capacitive pressure sensor without gas pressure functions of circuits on the chip after post-processing are normal. The variation of output voltage was 8mv when applying 100 KPa gas pressure to the capacitive pressure sensor. To keep increasing gas pressure to the capacitive pressure sensor resulted in no further change in output voltage. Authors are trying to improve the capacitive pressure sensor with circuits. V. CONCLUSIONS C1 Low 200mV C1 Freq Hz Low signal amplitude A capacitive pressure sensor with readout circuits on a single chip has been implemented herein using the commercial 0.35 µm CMOS process and

5 C. L. Dai et al.: Capacitive Micro Pressure Sensors with Underneath Readout Circuit Using A Standard CMOS Process 241 post-processing. The main feature of the chip is that its readout circuits are located under the pressure sensor, reducing chip size. The pressure sensor contains an 8 8 sensing cells array, and its total chip area is 2mm 2mm. ACKNOWLEDGEMENTS The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under Contract No. NSC E The National Chip Implementation Center is also appreciated for their foundry support. We would, finally, like to thank the NSC Northern Region MEMS Research Center for kindly making their complete research facilities available. NOMENCLATURE a radius of circular plate, m C capacitance, F D plate flexural rigidity, N. m d 0 gap distance of parallel-plate capacitor, m E Young s modulus of the circular plate, N/m 2 h thickness of the circular plate, m p uniform pressure load, N/m 2 r r-coordinates of circular plate, m V in input voltage, V V out output voltage, V w(r) displacement of circular plate, m Greek Letters 2 Laplacian ε ν the dielectric constant, F/m Poisson s ratio REFERENCES Dondon, P., Zardini, C., and Aucouturier, J. L., 1993, BiCMOS Integrated Circuit for Capacitive Pressure Sensors in Automotive Applications, Sensors and Actuators A, Vol , pp Dai, C. L., and Chang, P. Z., 1999, A COMS Surface Micromachined Pressure Sensor, Journal of the Chinese Institute of Engineers, Vol. 22, pp Johns, D., and Martin, K., 1996, Analog Integrated Circuit Design, John Wiley & Sons, New York. Kandler, M., Manoli, Y., Mokwa, W., Spiegel, E., and Vogt, H., 1992, A Miniature Single-chip Pressure and Temperature Sensor, Journal of Micromechanics and Microengineering, Vol. 2, pp Reismann, H., and Pawlik, P. S., 1980, Elasticity Theory and Applications, John Wiley & Sons, New York. Sharpe, W., Yuan, B., Vaidyanathan, R., and Edwards, R., 1997, Measurements of Young s Modulus, Poission s Ratio, and Tensile Strength of Polysilicon, Proceedings of the 10th MEMS Workshop, Nagoya, Japan, pp Shackelford, J. F., Alexander, W., and Park, J. S., 1992, CRC Materials Science and Engineering Handbook, CRC Press, Boca Raton. Manuscript Received: Jan. 14, 2002 Revision Received: May 12, 2002 and Accepted: Jun. 24, 2002