- Email: [email protected]
The structures for electrostatic servo capacitive vacuum sensors
sE@RS
Aell~AfORs A
EJ-SEVIER
PHYSICAL
Sensors and Actuators A 66 ( 1998) 2 13-2 17
Abstract Two kindsof electrostaticservocapacitivevacuumsensors havebeensuccessfullyfabricatedusingP’ + siliconetch-stopandvacuum anodic-bonding techniques.In orderto maintainthe referencecavity at high vacuum,a non-evaporable getter (I\TEG) is used as a small vacuumpump.Thedynamicrangeof thesensorcanbeextendedby a servosystem.The pressure responses of the sensors aregood.Theplot of servovoltageversuspressure ismeasured andthe theoreticaldataagreewith the experimentalresults. 0 1998ElsevierScienceS.A. All rightsreserved. Keywords: Vacuum; Sensors: Servo
1. Introduction
2. The structures and fundamental
Vacuum sensorsare used for processmonitoring in vacuum technology. The required featuresin the application are high precision andwide dynamic range.Two functional principles, namely, piezoresistive andcapacitive types, have been developedfor silicon vacuum sensors.Piezoresistivevacuum sensorsthat have been commercialized have low cost, but low precision. Capacitive vacuum sensorshave high sensitivity, but a small dynamic range, becausethe gap between the capacitor platesmust be as smallaspossibleto obtain a large capacitance. Servo-type sensorshave several advantages suchashigh precisionand wide dynamic rangein comparison to passivesensors,becausethe movable electrode is fixed to the initial position by the feedback force in a servo system [ lJ1. In this paper, we describe electrostatic servo capacitive vacuum sensorsmadewith micromachining techniques.The sensorsand their fabrication technology offer several features,including: ( 1) simple bulk-silicon technology that utilizes a boron etch-stop for the formation of precise microstructures; (2) a micro vacuum pump that maintains the reference cavity at high vacuum using a non-evaporable getter (NEG) [ 3,4] ; (3) multiple isolatedelectrodeson both sidesof the movable electrode to provide protection against over-range pressureand for operation in a closed-loopservo mode.
We developed two kinds of electrostatic servo capacitive vacuum sensors.Fig. 1 showsa schematicview of the glassS&glass structure for an electrostatic servo capacitive vacuum sensor[ 51. It consistsof an upper Pyrex glass,a middle silicon wafer and a lower Pyrex glass. The glass and the middle silicon are hermetically sealedby anodic bonding. The capacitor Cs betweenthe lower glassand middle silicon movable electrode is a sensitive capacitor which detectsthe changeof pressure.By changing the applied voltage in the servo capacitor CAbetweenthe upperglassandmiddlesilicon movable electrode, the capacitor Cscan be kept constantfor any pressure,becausethe middle silicon movable electrode is fixed to the initial position by the electrostatic force. With this electrostatic servo technique, the applied voltage in capacitor C, is related to the pressure.In order to maintain the reference cavity at high vacuum, an NEG is used as a small vacuum pump. The dust filter can prevent dust getting into the gap betweenthe upper glassand middle silicon movable electrode. Fig. 3(e) shows the schematicview of the glass-Si-Si structure for an electrostatic servo capacitive vacuum sensor.Although its fundamental principle is the sameasthe glass-Si-glassstructure, its fabrication processes are simpler becauseit usedsilicon as a servo electrode. The electrostaticforce F, per unit area is given by
principle
(1) * Corresponding author. Tel.: -I- 86-57 I -795- 1706; fax: t 86-57 I -795-
1358;e-mail:[email protected] 0924-4247/98/$19.00 0 1998 Elsevier Science S.A. All rights reserved. Pllso924-4247(98)ooo37-5
where eOis the dielectric constantin vacuum, V, is the applied
211
Y. CVmg M Esrtshi/Sensors
and Acrwrors
A 66 (I 9981 213-217
If Eq. ( 1) is substitutedinto Eq. (3), the applied voltage i/b is given by i _______..___ ______
v,= \/2dZPIE(,
(4)
It can be seen from Eq. (3) that the plot of servo voltage versusapplied pressureP is V, a Pi/‘.
3. Fabrication The fabrication sequenceof the servo vacuum sensorof glass-S-glass structureis shownin Fig. 2. Feedthroughholes for contact to the electrode in the upper glass are madeby sand-blasting.The electrodein the upper glassis about 7 pm thick P+ + silicon, which is madeby the P* + etch-stop technique ( Fig. 21a) >. The startingmaterial of the middle silicon structure is a double-side-polished( 100) Pi silicon wafer 200 p,rn thick. First, the backsideand topside of the middle silicon wafer areetched in about3 pm and 11 km by TMAH, respectively, to make the gap spacingfor capacitors C, and C, ( Fig. 2(b) ) . Boron is heavily diffused from a solid boron
P++
Si
(a)
Semtive
elecaode
PC+> Si
without dustfiiter
(4 Fig. 1. Schematic view of the glass-Si-glass structtm for a servo vs~uum sensor. voltage in capacitor C, and d is the gap of capacitor CA. When d= 3.6 ym, the electrostatic force F, per unit area is given by
(4 Pyrex glass Ground
F,=2,6XlO-”
torr V-’
(2)
When the applied pressureis P, the middle silicon movable electrode can be kept in the initial position by applying an electrostaticforce Fe per unit area;the force-balanceequation is given by F,=P
Sensitive elecmdc
-
(3)
NEG
PC+) silicon
Al
Fig. 2. Fabrication process of thz scnmr of glass-Si-,olass structure.
215
source at 1160°C for 3 h to make the P’ + etch-stop layer. Then oxide masks for the diaphragm are patterned on the backside of the middle silicon, and the topside of the silicon is bonded to the upper glass using anodic bonding (Fig. 2(c) ). Feedthrough holes are mctnllized by mask-sputtering with aluminium. In order to protect the electrode, the upper glass is bonded to another protective silicon wafer, and then the middle silicon wafer is etched by EPW (Fig. 2(d) ) After the protective silicon is stripped ( Fig. 2(.e) ), the backside of the middle silicon is bonded to the lower glass with an aluminium electrode by vacuum anodic bonding (Fig. 2(f) ). Lead wires were attached with conductive epoxy. The fabrication sequence of the servo vacuum sensor of glass-Si-Si structure is shown in Fig. 3. After feedthrough holes for contact to the electrode in the glass are made by sand-blasting, Pt/Ti electrodes are deposited in the glass. Then the electrode about 7 pm thick P+ + silicon is made in the glass by the P+ 1- etch-stop technique (Fig. 3(a) ). The starting material of the middle silicon structure is a doubleside-polished ( 100) P* silicon wafer 200 pm thick. First, the topside of the middle silicon wafer is etched in about 11 pm and 160 pm by TMAH, respectively, to make the gap spacing for capacitor C, and the NEG cavity (Fig. 3(b)). Then boron is heavily diffused from a solid boron source at 1160°C for 3 h to make the P’-’ etch-stop layer. After the oxide masks for the NEG cavity and insulation are patterned, the topside of the silicon is bonded to the glass using vacuum anodic bonding (Fig. 3 (c) ). Then the sample is etched by EPW (Fig. 3 (d) ). Finally, the sample is bonded to the upper Pi-Si electrode by vacuum anodic bonding (Fig. 3(e) ). The
Fig. 3. Photographs hhown in Fig. 2(e);
of the backside of the sensor of glass-Si-glass structure: (c) crab:, section of structure shown in Fig. 2(f).
(a)
(4
Sensitive elecuade~Ph
Si)
Servo elecnode(P+Si)
‘I’ Lead wire
Conductive
epoxy
(e) Fig. 3. Structure and fabrication
procestes of the sensor of glass-Si-Si
stmcture.
lead wire of the sensitive electrode was attached with conductive epoxy. We can see that this process is simpler than that for the glass-Si-glass structure.
(a) cross section of structure
shown in Fig. 2(d):
(b) cross section of structure
4. Results and discussion Photographs of the back of the device of glass-Si-glass structure are shown in Fig. 4. The diaphragm is pushed to the upper glass (Fig. 4(a) ) because the pressure of the cavity between the protective wafer and middle silicon is smaller than atmospheric pressure. After the protective wafer is removed. the diaphragm returns to the equilibrium position (Fig. 4(b) ) . The diaphragm is pushed to the lower glass (Fig. 4(c) after the backside of the middle silicon is bonded to the lower glass by vacuum anodic bonding. In order to prevent attracting the silicon boss to the lower glass when the middle silicon is bonded to lower glass, the lower electrode and silicon boss are shorted. After bonding, the short is melted down by laser (Fig. 4(c) ) . Fig. 5 shows photographs of the device of glass-Si-Si structure. We can see that the diaphragm is pushed to the glass (Fig. 5 (b) ) because the reference cavity is maintained at high vacuum. Fig. 6 shows an example of measured pressure responses of the sensor of glass-Si-glass structure under different servo voltages. The capacitance of Cs increases with the pressure, otherwise, the capacitance of CA decreases when the pressure increases. When the applied pressure increases, the middle movable electrode is near the lower glass; therefore, the gap of capacitor Cs decreases and the gap of capacitor C, increases. The range of the sensor is widened when the servo voltage V, increases.
Fig. 7 shows the measured pressure responses of the servo capacitor C, for the sensor of glass-Si-Si structure. When the pressure increases, the movable silicon electrode will leave the silicon servo electrode and the value of the servo capacitance will decrease. We can see that the capacitance value in Fig. 6 will reach its saturation value rapidly as the applied pressure increases, 60 1
-0
2
4 6 Pressure(Torr) Fig. 6. Pressure responses of the sensor of glass-Si-glass different servo voltages.
8
10
structure
under
PCTOK) Fig. 7. Pressure response ofthe sensor of glass-%Si
structure.
60 50 5 M40 2 3 30 i g
20 10 0 0
Fig. 5. Photographs (b) the backside.
of the sensor of glass-Si-Si
strwture:
(a) the topside;
2
4 6 Pressure(Torr)
8
Fig. 8. Plot of servo voltage vs. pressure of glass-Si-glass
10 structure.
after the applied pressure reaches a critical value. In fact, it embodies the stress-concentration effect of an E-shaped structure. It is the stress-concentration effect that brings the characteristics of high sensitivity, rapid saturation and smaller dynamic range. The capacitance value in Fig. 7 tends to saturation slowly as the pressure increases, without a stressconcentration effect, showing the characteristics of low sensitivity and large dynamic range, which are typical of C-shaped sensor structures. So in the sensor design, the difference between the two structures should be considered, choosing the proper structure according to the specific requirements. A plot of servo voltage versus pressure of the glass-Siglass structure is shown in Fig. 8. The servo voltage increases with the pressure, because the electrostatic force againstpressure increases with the pressure. The theoretical data are from Eq. (4). We can see they agree with the experimental results.
5. Conclusions Two kinds of electrostatic servo capacitive vacuum sensors have been successfully fabricated. The dynamic range can be extended by a closed-loop servo mode. The pressure responses of the sensors are good. The plot of servo voltage versus pressure is measured and the theoretical data agree with the experimental results.
References [ I ] F. Rudolf, A. Jornod, J. Bergqvist, H. Leuthold, Precision acceleromcters with k*fi resolution, Sensors and Actuators A A21-A23 ( 1990) 297-302.
[21
K. Jono, K. Minami, M. Esashi, An electrostatic servo-type three-axis silicon accelerometer. Mesa. Sci. Technol. 6 ( 1995) 1 I-15. [;I H. Henmi, S. Shoji, Y. Shoji, K. Yoshimi, M. Esashi, Vacuum packaging for microsensors by glass silicon anodic bonding, Sensors and Actuators A 13 ( 1991) 213-218. 131 K. Hatanaka, D.Y. Sim, K. Minami, M. Esahhi, Silicon diaphragm capacitive vacuum sensor, Tech. Digest, 13th Sensor Symp., Japan. 1995, pp. 3740. servo capacitive vacuum 151 Y. Wang, M. Esashi, A novel electrostatic sensor, Tech. Digest, 9th Int. Conf. Solid-State Sensors and Actuators (Transducers ‘97)) Chicago, USA, 16-19 June, 1997, pp. 1457-1160.
Biographies Yrfelirl Wclng was born in Nanchang, China, in 1959. He received the B.S., M.S. and Ph.D. degrees from Zhejiang University, Harbin Institute of Technology and Tsinghua University, China, in 1982, 1985 and 1989, respectively. From 199 1 to 1993, he was an associate professor at the Department of Information and Electron Engineering, Zhejiang University. Since 1993 he has been a professor at the same department. His current research interests include micromachining technologies, sensors and micromechanical optics. He is a senior member of the IEEE. Mnsnyoslzi Esashi received the B.E. degree in electronic engineering and the Ph.D. degree in engineering from Tohoku University, Japan, in 197 1 and 1976, respectively. From 1976 to 1981, he served as a research associate at the Electronic Engineering Department of Tohoku University. During this period, he worked on biomedical transducers fabricated by micromachining. From 198 1 to 1990 he was an associate professor. Since 1990 he has been a professor in the Department of Mechatronics and Precision Engineering, Tohoku University. His current research is an intelligent sensors and sensor-actuator systems fabricated by micromachining.
Aell~AfORs A
EJ-SEVIER
PHYSICAL
Sensors and Actuators A 66 ( 1998) 2 13-2 17
Abstract Two kindsof electrostaticservocapacitivevacuumsensors havebeensuccessfullyfabricatedusingP’ + siliconetch-stopandvacuum anodic-bonding techniques.In orderto maintainthe referencecavity at high vacuum,a non-evaporable getter (I\TEG) is used as a small vacuumpump.Thedynamicrangeof thesensorcanbeextendedby a servosystem.The pressure responses of the sensors aregood.Theplot of servovoltageversuspressure ismeasured andthe theoreticaldataagreewith the experimentalresults. 0 1998ElsevierScienceS.A. All rightsreserved. Keywords: Vacuum; Sensors: Servo
1. Introduction
2. The structures and fundamental
Vacuum sensorsare used for processmonitoring in vacuum technology. The required featuresin the application are high precision andwide dynamic range.Two functional principles, namely, piezoresistive andcapacitive types, have been developedfor silicon vacuum sensors.Piezoresistivevacuum sensorsthat have been commercialized have low cost, but low precision. Capacitive vacuum sensorshave high sensitivity, but a small dynamic range, becausethe gap between the capacitor platesmust be as smallaspossibleto obtain a large capacitance. Servo-type sensorshave several advantages suchashigh precisionand wide dynamic rangein comparison to passivesensors,becausethe movable electrode is fixed to the initial position by the feedback force in a servo system [ lJ1. In this paper, we describe electrostatic servo capacitive vacuum sensorsmadewith micromachining techniques.The sensorsand their fabrication technology offer several features,including: ( 1) simple bulk-silicon technology that utilizes a boron etch-stop for the formation of precise microstructures; (2) a micro vacuum pump that maintains the reference cavity at high vacuum using a non-evaporable getter (NEG) [ 3,4] ; (3) multiple isolatedelectrodeson both sidesof the movable electrode to provide protection against over-range pressureand for operation in a closed-loopservo mode.
We developed two kinds of electrostatic servo capacitive vacuum sensors.Fig. 1 showsa schematicview of the glassS&glass structure for an electrostatic servo capacitive vacuum sensor[ 51. It consistsof an upper Pyrex glass,a middle silicon wafer and a lower Pyrex glass. The glass and the middle silicon are hermetically sealedby anodic bonding. The capacitor Cs betweenthe lower glassand middle silicon movable electrode is a sensitive capacitor which detectsthe changeof pressure.By changing the applied voltage in the servo capacitor CAbetweenthe upperglassandmiddlesilicon movable electrode, the capacitor Cscan be kept constantfor any pressure,becausethe middle silicon movable electrode is fixed to the initial position by the electrostatic force. With this electrostatic servo technique, the applied voltage in capacitor C, is related to the pressure.In order to maintain the reference cavity at high vacuum, an NEG is used as a small vacuum pump. The dust filter can prevent dust getting into the gap betweenthe upper glassand middle silicon movable electrode. Fig. 3(e) shows the schematicview of the glass-Si-Si structure for an electrostatic servo capacitive vacuum sensor.Although its fundamental principle is the sameasthe glass-Si-glassstructure, its fabrication processes are simpler becauseit usedsilicon as a servo electrode. The electrostaticforce F, per unit area is given by
principle
(1) * Corresponding author. Tel.: -I- 86-57 I -795- 1706; fax: t 86-57 I -795-
1358;e-mail:[email protected] 0924-4247/98/$19.00 0 1998 Elsevier Science S.A. All rights reserved. Pllso924-4247(98)ooo37-5
where eOis the dielectric constantin vacuum, V, is the applied
211
Y. CVmg M Esrtshi/Sensors
and Acrwrors
A 66 (I 9981 213-217
If Eq. ( 1) is substitutedinto Eq. (3), the applied voltage i/b is given by i _______..___ ______
v,= \/2dZPIE(,
(4)
It can be seen from Eq. (3) that the plot of servo voltage versusapplied pressureP is V, a Pi/‘.
3. Fabrication The fabrication sequenceof the servo vacuum sensorof glass-S-glass structureis shownin Fig. 2. Feedthroughholes for contact to the electrode in the upper glass are madeby sand-blasting.The electrodein the upper glassis about 7 pm thick P+ + silicon, which is madeby the P* + etch-stop technique ( Fig. 21a) >. The startingmaterial of the middle silicon structure is a double-side-polished( 100) Pi silicon wafer 200 p,rn thick. First, the backsideand topside of the middle silicon wafer areetched in about3 pm and 11 km by TMAH, respectively, to make the gap spacingfor capacitors C, and C, ( Fig. 2(b) ) . Boron is heavily diffused from a solid boron
P++
Si
(a)
Semtive
elecaode
PC+> Si
without dustfiiter
(4 Fig. 1. Schematic view of the glass-Si-glass structtm for a servo vs~uum sensor. voltage in capacitor C, and d is the gap of capacitor CA. When d= 3.6 ym, the electrostatic force F, per unit area is given by
(4 Pyrex glass Ground
F,=2,6XlO-”
torr V-’
(2)
When the applied pressureis P, the middle silicon movable electrode can be kept in the initial position by applying an electrostaticforce Fe per unit area;the force-balanceequation is given by F,=P
Sensitive elecmdc
-
(3)
NEG
PC+) silicon
Al
Fig. 2. Fabrication process of thz scnmr of glass-Si-,olass structure.
215
source at 1160°C for 3 h to make the P’ + etch-stop layer. Then oxide masks for the diaphragm are patterned on the backside of the middle silicon, and the topside of the silicon is bonded to the upper glass using anodic bonding (Fig. 2(c) ). Feedthrough holes are mctnllized by mask-sputtering with aluminium. In order to protect the electrode, the upper glass is bonded to another protective silicon wafer, and then the middle silicon wafer is etched by EPW (Fig. 2(d) ) After the protective silicon is stripped ( Fig. 2(.e) ), the backside of the middle silicon is bonded to the lower glass with an aluminium electrode by vacuum anodic bonding (Fig. 2(f) ). Lead wires were attached with conductive epoxy. The fabrication sequence of the servo vacuum sensor of glass-Si-Si structure is shown in Fig. 3. After feedthrough holes for contact to the electrode in the glass are made by sand-blasting, Pt/Ti electrodes are deposited in the glass. Then the electrode about 7 pm thick P+ + silicon is made in the glass by the P+ 1- etch-stop technique (Fig. 3(a) ). The starting material of the middle silicon structure is a doubleside-polished ( 100) P* silicon wafer 200 pm thick. First, the topside of the middle silicon wafer is etched in about 11 pm and 160 pm by TMAH, respectively, to make the gap spacing for capacitor C, and the NEG cavity (Fig. 3(b)). Then boron is heavily diffused from a solid boron source at 1160°C for 3 h to make the P’-’ etch-stop layer. After the oxide masks for the NEG cavity and insulation are patterned, the topside of the silicon is bonded to the glass using vacuum anodic bonding (Fig. 3 (c) ). Then the sample is etched by EPW (Fig. 3 (d) ). Finally, the sample is bonded to the upper Pi-Si electrode by vacuum anodic bonding (Fig. 3(e) ). The
Fig. 3. Photographs hhown in Fig. 2(e);
of the backside of the sensor of glass-Si-glass structure: (c) crab:, section of structure shown in Fig. 2(f).
(a)
(4
Sensitive elecuade~Ph
Si)
Servo elecnode(P+Si)
‘I’ Lead wire
Conductive
epoxy
(e) Fig. 3. Structure and fabrication
procestes of the sensor of glass-Si-Si
stmcture.
lead wire of the sensitive electrode was attached with conductive epoxy. We can see that this process is simpler than that for the glass-Si-glass structure.
(a) cross section of structure
shown in Fig. 2(d):
(b) cross section of structure
4. Results and discussion Photographs of the back of the device of glass-Si-glass structure are shown in Fig. 4. The diaphragm is pushed to the upper glass (Fig. 4(a) ) because the pressure of the cavity between the protective wafer and middle silicon is smaller than atmospheric pressure. After the protective wafer is removed. the diaphragm returns to the equilibrium position (Fig. 4(b) ) . The diaphragm is pushed to the lower glass (Fig. 4(c) after the backside of the middle silicon is bonded to the lower glass by vacuum anodic bonding. In order to prevent attracting the silicon boss to the lower glass when the middle silicon is bonded to lower glass, the lower electrode and silicon boss are shorted. After bonding, the short is melted down by laser (Fig. 4(c) ) . Fig. 5 shows photographs of the device of glass-Si-Si structure. We can see that the diaphragm is pushed to the glass (Fig. 5 (b) ) because the reference cavity is maintained at high vacuum. Fig. 6 shows an example of measured pressure responses of the sensor of glass-Si-glass structure under different servo voltages. The capacitance of Cs increases with the pressure, otherwise, the capacitance of CA decreases when the pressure increases. When the applied pressure increases, the middle movable electrode is near the lower glass; therefore, the gap of capacitor Cs decreases and the gap of capacitor C, increases. The range of the sensor is widened when the servo voltage V, increases.
Fig. 7 shows the measured pressure responses of the servo capacitor C, for the sensor of glass-Si-Si structure. When the pressure increases, the movable silicon electrode will leave the silicon servo electrode and the value of the servo capacitance will decrease. We can see that the capacitance value in Fig. 6 will reach its saturation value rapidly as the applied pressure increases, 60 1
-0
2
4 6 Pressure(Torr) Fig. 6. Pressure responses of the sensor of glass-Si-glass different servo voltages.
8
10
structure
under
PCTOK) Fig. 7. Pressure response ofthe sensor of glass-%Si
structure.
60 50 5 M40 2 3 30 i g
20 10 0 0
Fig. 5. Photographs (b) the backside.
of the sensor of glass-Si-Si
strwture:
(a) the topside;
2
4 6 Pressure(Torr)
8
Fig. 8. Plot of servo voltage vs. pressure of glass-Si-glass
10 structure.
after the applied pressure reaches a critical value. In fact, it embodies the stress-concentration effect of an E-shaped structure. It is the stress-concentration effect that brings the characteristics of high sensitivity, rapid saturation and smaller dynamic range. The capacitance value in Fig. 7 tends to saturation slowly as the pressure increases, without a stressconcentration effect, showing the characteristics of low sensitivity and large dynamic range, which are typical of C-shaped sensor structures. So in the sensor design, the difference between the two structures should be considered, choosing the proper structure according to the specific requirements. A plot of servo voltage versus pressure of the glass-Siglass structure is shown in Fig. 8. The servo voltage increases with the pressure, because the electrostatic force againstpressure increases with the pressure. The theoretical data are from Eq. (4). We can see they agree with the experimental results.
5. Conclusions Two kinds of electrostatic servo capacitive vacuum sensors have been successfully fabricated. The dynamic range can be extended by a closed-loop servo mode. The pressure responses of the sensors are good. The plot of servo voltage versus pressure is measured and the theoretical data agree with the experimental results.
References [ I ] F. Rudolf, A. Jornod, J. Bergqvist, H. Leuthold, Precision acceleromcters with k*fi resolution, Sensors and Actuators A A21-A23 ( 1990) 297-302.
[21
K. Jono, K. Minami, M. Esashi, An electrostatic servo-type three-axis silicon accelerometer. Mesa. Sci. Technol. 6 ( 1995) 1 I-15. [;I H. Henmi, S. Shoji, Y. Shoji, K. Yoshimi, M. Esashi, Vacuum packaging for microsensors by glass silicon anodic bonding, Sensors and Actuators A 13 ( 1991) 213-218. 131 K. Hatanaka, D.Y. Sim, K. Minami, M. Esahhi, Silicon diaphragm capacitive vacuum sensor, Tech. Digest, 13th Sensor Symp., Japan. 1995, pp. 3740. servo capacitive vacuum 151 Y. Wang, M. Esashi, A novel electrostatic sensor, Tech. Digest, 9th Int. Conf. Solid-State Sensors and Actuators (Transducers ‘97)) Chicago, USA, 16-19 June, 1997, pp. 1457-1160.
Biographies Yrfelirl Wclng was born in Nanchang, China, in 1959. He received the B.S., M.S. and Ph.D. degrees from Zhejiang University, Harbin Institute of Technology and Tsinghua University, China, in 1982, 1985 and 1989, respectively. From 199 1 to 1993, he was an associate professor at the Department of Information and Electron Engineering, Zhejiang University. Since 1993 he has been a professor at the same department. His current research interests include micromachining technologies, sensors and micromechanical optics. He is a senior member of the IEEE. Mnsnyoslzi Esashi received the B.E. degree in electronic engineering and the Ph.D. degree in engineering from Tohoku University, Japan, in 197 1 and 1976, respectively. From 1976 to 1981, he served as a research associate at the Electronic Engineering Department of Tohoku University. During this period, he worked on biomedical transducers fabricated by micromachining. From 198 1 to 1990 he was an associate professor. Since 1990 he has been a professor in the Department of Mechatronics and Precision Engineering, Tohoku University. His current research is an intelligent sensors and sensor-actuator systems fabricated by micromachining.