Integrated silicon capacitive accelerometer with PLL servo technique

209

Sensors and Actuakm A, 39 (1993) 209-217

Integrated silicon capacitive accelerometer with PLL servo technique Y Matsumoto and M Esashl (Recexved Aprd 1, 1993, m revised form July 19, 1993, accepted JuIy 26, 1993)

Abstract An Integrated s&con capac&ve accelerometer has been developed wltb LIMOS and ~c~rnach~nIng technology The accelerometer chzp has glass-slhcon-glass structure and IS 3 7 X 4 5 X 0 9 mm3 m srze A sllrcon seismic mass IS suspended wrth slhcon-oxmltnde beams m full symmetry The sensor capacitance IS formed between the slhcon mass and a metal electrode on the upper glass A CMOS capacitance to frequency (C-F) converter IS Integrated on the silicon chip The clrcmt IS deslgned to be stable to temperature and supply voltage The clrcmt has a reference capacitor m it, and the dnft of the cxcurt 1s compensated by the rnformabon obtamed when the clrcult IS connected to the reference capacttor The output frequency of the accelerometer chip vanes hnearly wttb acceleratron The fore-baIanclng system has been reahzed usmg the accelerometer chrp and an outer phaselocked-loop (PLL) servO csrcutt The output voltage vanes linearly wtth acceleration, and the sensltmty and offset of the output voltage can be adjusted with the outer crrcmt parameter

Muuature high-performance accelerometers are required for the sophisticated control systems used m aeroplanes, advanced auLomoblles and robots The character&es reqmred m such applications are high preCEIOU,Htlde dynamic range and wide frequency range mcludmg d c response S&on technology IS best suited for the fabrication of miniature low-cost accelerometers Slhcon capacitive accelerometers have high sennuvtty, but the measurement range tends to be narrow because the gap between the capacitor plates 1s small (of the order of mlcrometres) m order to obtain large capacitance The frequency range also tends to be narrow because of the squeeze-film effect caused by air between the capacitor plates The measurement range can be extended by fixmg the mass to its mmal postlon usmg the fork-b~an~lng technrque The frequency range 1s also extended by this techntque Therefore, s&con capacltwe accelerometers with the electrostatic forcebalancing technrque have been developed in recent years [l-3] High precslon (=&, mde measurement range (- l-lg) and mde frequency range (d c -100 Hz) are achieved usmg the fork-balan~g teehmque However, the electrostatic force 1sproportional to the square of the voltage, so that the feedback voltage changes non-linearly with the acceleration The pulse-width modulatron (PWM) servo IS one technique to solve the problems, but it needs a complex clrcmt [2]

0924-4247/93/$6 00

We have previously reported an Integrated s&con capacltlve accelerometer chip usmg CMOS and mlcromachmmg technologies [4] The chip was composed of the sensor capacitor, the C-F converters and forcefeedback metal electrode m a packaged glass&con-glass structure The electrostatz force-balan~ng system has been developed usmg the chip and an outer phase-locked loop (PLL) clrcult The output voltage varies linearly with the acceleration However, the C-F converter has a large supply-voltage dependence, and has temperature shift and long-term dnft Moreover, the characteristics vary with the fabrl~tlon process vanatlons [S] Therefore, a novel C-F converter has been developed to reduce the supply-voitage dependence [6] Furthermore, another C-F converter bave been developed to compensate the drift of the arcult The circuit has the reference capacitor in it, and the tem~rature stuft and long-term drift can be compensated by the mformation obtamed when the ctrcmt IS connected to the reference capacitor A novel Integrated accelerometer chip with C-F converter has been developed m ths work An electrostatic force-balanemg system has also been developed usmg the chop and PLL arcmt The output voltage vanes hnearly with acceleratton, and the senatlvrty and offset of the output voltage can be adjusted with the outer PLL clrcult parameter This paper describes the design and characterlstlcs of the accelerometer

0 1993 - Elsewer Sequofa Al1 aghts reserved

210

Design Figure 1 shows the structure of the mtegrated slhcon capacitive accelerometer chip It consists on an upper Pyrex glass cover, a s&on substrate and a bottom Pyrex glass cover The upper glass cover has small glass holes for feedthrough from the circuit [7] The glass covers and the slhcon substrate are hermetically sealed by anodlc bondmg m the wafer process The chip is 3 7~ 4 5 mm2 m area and 0 9 mm thick The silicon selsmlc mass IS suspended wtth 96 beams of siliconoxymtnde @ON) m full symmetry The mass is 2X 2 mm’ 1x1size and 0 3 mm 111thickness The SlON beam size IS 55x425 pm2 and 2 pm m thickness The slhcon mass IS electncally isolated from the peripheral silicon by SION beams The alummlum wires on the beams realize the electrical connection between the outer PLL clrcult and the sIllcon mass The sensor capacitor is formed between a Tl-Pt electrode on the upper Pyrex glass cover and the slhcon mass The gap between the electrode and the slhcon mass 1s 2 pm The sensor capacitance is calculated as 18 pF The C-F converter using a Schmitt tngger circuit IS integrated on the slhcon chip A metal electrode is formed above the circuit in order to shield the circuit from hght, which causes ctrcult nolSe due to the photoelectnc current The pnncrple of the C-F converter IS shown m Fig 2 The reference capacitor C, formed by a metal-o~de-semlconductor (MOS) structure is integrated m the circuit Either the sensor capacitor C, or the reference capacitor C, 1s selected by the outer slgnal V, When the sensor capacitor C, is selected, the output frequency changes w&h acceleration, but S10tj Beam

IO

IO

Ftg 2 Prmclple of the Schmitt trqger C-F converter, which has selectron function of the sensor capacitor and the reference

Cur

I

ent

Source

the frequency also changes ullth clrcult temperature shift, and so on On the other hand, when the reference capacitor C, 1s selected, the frequency changes only with circuit temperature shift and so on, because the MOS capacitance 1s independent of acceleration, temperature, and so on Thus, the clrcult temperature shift and long-term drift can be compensated by the frequency change when the circuit is connected to the reference capacitor When the C-F converter selects the sensor capacitor C,, the output frequency fx IS given by

2cxVI

I

Fig 1 The structure of the Integrated capaclhve accelerometer chip

11

I gger

Fig 3 Cnxxnt diagram of the C-F converter

fx= -L

T, -PI

Schml Tr

(1)

where I,, IS the charge or discharge current and V,, 1s the hysteresis of the Schmitt tngger The current 1, and hysteresis V,, are deslgned III order to reduce the circuit temperature shift and clrcmt supply-voltage dependence [6, 81 Figure 3 shows the circuit diagram of the C-F converter, the current source IS formed by the depletion-type NMOS transistor and enhancement-type MOS transistor Figure 4 shows the SPICE sunulatlon result of the circuit charactenstics when the threshold voltage of the depletion-type NMOS transistor 1s - 1 V The temperature shift and the supply-voltage dependence are nearly zero at a supply voltage of 5V The Schmitt trigger hysteresis 1s centred on half of the supply voltage V,, so that the average voltage of

211

FIN 4 Simulated charactenstlcs

of the C-F converter

the upper Tl-Pt electrode becomes v,,L! The other Ti-Pt electrode is formed on the bottom Pyrex glass cover, and 1s connected to the slhcon substrate The voltage IS fixed to the supply voltage V,, because the silicon substrate 1s at the supply voltage Vdd m the CMOS clrcult If the voltage of the &con mass 1s V,, the electrostatic force F, applied to the mass is gnren as the dtierence between the upward force F. and downward force F_, The force F, IS calculated as

Hrlth the CVD gas-flow ratio of CO, and NH3 as shown m Fig 5 The compressrve stress (- 43 MPa) remams m the &ON beam when the NH, gas is equal to zero The residual stress changes from compresswe stress to tensile stress d the NH, gas ratio increases The weak tensile-stress condition NHJ(C0, + NH,) = 0 05 was chosen to prevent bucldmg of the beams The residual stress of the beam is 60 MPa The Young’s modulus of &ON 1s approxunately the same as that for SlO, (74 GPa) [lo] The first term of eqn (6) IS calculated as 41 N/m, and the second term 1s 1490 N/m The sprmg constant k is 1530 N/m and mamly depends on the second term, which 1s caused by the residual stress The mass displacement due to acceleration 1scalculated as 0 018 pm/g from eqn (5), and the mass displacement by the electrostatic force 1s calculated as 0 012 m when the slhcon mass voltage V. is changed at a supply voltage V,, of 4 V The sensor capacitor C, is given by C,= 2 Substltutmg eqn. (7) m eqn (l), the output frequency fX of the circuit is expressed as

F,=F,,-F,,=

-

If the displacement is calculated as

f.= of the mass 1s equal to zero, Fe

dV,,( -vi+ F) I

F,=F,,-F,,=

’

2d2

(3)

where l 1s the dlelectrlc constant and d and S are the gap and area of the sensor capacitor, respectively The electrostatic force F. vanes linearly Hrlth the voltage v,

y+fo(l-

y)

(8)

where C, and f. are the sensor capacitance and the frequency when the mass displacement 1s zero C, 1s expressed as C,= c S/d The output frequency vanes lmearly with the acceleration a and the electrostatic force F, From eqn (3), the electrostatic force F, vanes lmearly with the slhcon mass voltage V,, so that the output frequency vanes linearly with V, In the case of the accelerometer chip, the hysteresis of the Schmitt tngger 1s designed to be 2 V and the current I,, 1s

The force F apphed to the slhcon mass 1s gwen by

F=ma+F,

(4)

where m IS the mass and u is the acceleration displacement z of the mass is expressed as z=

&(l-

The

F _ ma+F, k

k

where k 1s the sprmg constant of the SlON beam The sprmg constant IS given by [9] nEWt’ k=--+!!

(6)

where n 1s the number of beams, and W, L, t are the width, length, thickness of the beam, respectwely E and 0 are Young’s modulus and the residual stress of the SlON beam The residual stress of SlON vanes

Flow Ratw

(NH3I(COt+NHs))

$ L E s

l%g 5 The relation between the restdual stress and gas-flow ratlo of NH3 and CO2

212

designed to be 5 PA at a supply voltage Vdd of 4 V The frequency f. IScalculated as 70 6 kHz, the frequency change due to acceleration is calculated as 630 Hz/g, and the frequency change by the sdlcon mass voltage V, IS calculated as -410 I&/V The resonance frequency of the accelerometer chip G gven by Comparator

(9) and 1s calculated to be 3 72 kHz The pressure inside the accelerometer chip 1s 0 4 tnnes atmospheric pressure when the glass and sdicon are anodlcally bonded at 400 “C m the atmosphere, so that the cut-off frequency of the accelerometer chop becomes rather low because of the squeeze-film effect ansmg from ax between the capacitor plates The viscous damping constant r, due to the squeeze-film effect 1s given by [ll] r,=

6 4/-Q/2)=’ d3

Rw=5

BkQ

Cw=15pF

Va

Ftg 6 Diagram of two-wire detectlon clrcult

-pJ (10)

I

I

vs

where p IS the vlscoslty coefficrent of the air and b 1s the size of the s&on mass The relation between the damping factor C and the viscous dampmg constant r, 1s I

(11) l IS calculated as 111, so that the accelerometer clnp works at the over-dampmg condltlon In the case of the accelerometer chip, the slhcon mass suffers the squeeze-film effect from both sides of the glass plates Hence, the vlsc~us damping factor should be doubled, and the cut-off frequency IS given by

fw,=Lm-

ccx-Y- lY1

The cut-off frequency fcu,of the accelerometer open-loop configuration 1s 7 4 Hz

(12) chip at

PLL Servo system The output frequency of the accelerometer chip IS detected by momtormg the supply current of the CMOS C-F converter Therefore, the frequency 1s detected with only two wires using the Z-V converter shown m Fig 6 The electrostatic force-balancing system has been developed wrth the accelerometer chip and an outer PLL clrcult shown m Fig 7 Either the sensor capacitor or the reference capacitor 1s selected with outer signal V, When the reference capacitor IS selected, the frequency fr IS. counter with a 16 bit counter The counted bmary data N are latched,

I

I

I

16blt Counter

Dwder

P

Fig 7 Diagram of the PLL servo system

and the reference frequency fr,. of the PLL clrcult 1s generated from the data N The frequencyf, IS counted perlodlcally and the reference frequency fEfISrenewed at that time When the sensor capacitor 1s selected, the slhcon mass voltage V, 1s modulated m order to balance the electrostatx force with the acceleration, and the output frequency fx ISequal to the reference frequency frer Because the output frequency fx and the reference frequency frerhave the same clrcult temperature shift and so on, these clrcult dnfts do not cause a dnft of the output voltage m the PLL servo system Because the data N determme the resolution of the reference frequency, fr 1scounted with a multi-penod measurement counter to get a large amount of counted data The frequency fr ISdesigned to have the same value as fo=ZoRCoV,, = 70 kH.z, so that the frequency fr IS divided by N.,= 90 and counted wth the highfrequency clock fc of 50 MHz 6 4 X IO data points are counted and the measurement time IS 12 ms The counted data are latched m the 16 bit counter If the

213

frequency fr vanes mth process vanatlon, the dlvlded number Nd should be adjusted to get maxunum counted data m the 16 bit counter When the sensor capacitor is selected, the clockf, is dlvlded by the latched data N The generated frequency 1s nearly 780 Hz and the frequency 1s multIphed by N,,, The frequency (NJ N&= (Yfr IS applied as the reference frequency fr.* of the PLL circuit The reference frequency frcI= (Yfr ts adjustable by the parameter N,,, If a fr =fEr 1s set tofO =1,/2C,V,,, the electrostatic force Fe becomes -mu from eqn (8) Fe IS also expressed as eqn (3) Substituting F.,= -ma m eqn (3), the silicon mass voltage V, IS derived and the output voltage V, 1s gven by

(4

(13) where /3 IS the feedback ratio m the range 1&~0 The sensltlvlty (V&J) can be mcreased d the feedback ratio /!JIS reduced. The output voltage V.,vanes linearly with the acceleration a and does not depend on the beam sprmg constant k Note that the slhcon mass voltage V, must be restrlcted in the range 0 V to the supply voltage Vdd, because V, IS apphed to the CMOS clrcmt From eqn (8), the maxunum measurement acceleration a_ is given at the condition V,= Vda, and the mmmmm acceleration a,,,,,, IS given at V,=O Therefore, the measurement range of the force-balancing system IS gven by a max- am,”=

(U4) (-3Vdd/4) (2d%rll6!w,,) - (2hz/ESv,,)

ESV,,2 = 2dzm (14)

In the case of the accelerometer chip, the slhcon mass IS 2 8 mg and the gap IS 2 pm, so the measurement range 1s calculated as 2 6 g( 1 g = 9 8 m/s*) at a supply voltage Vdd of 4 V The sensitivity (VJa) is 15/p V/g From eqn (14), the range IS mversely proportional to the square of the gap d, so that If the gap is made small, the measurement range 1sextended For example, when the gap 1s 05 pm, the measurement range 1s 40g and the sensltlvlty (V,/a) IS Bven by 0 l/B V/g

(g)

I

@I Fig 8 Fabncatlon

process of the accelerometer

chip

Fabrication

The accelerometer chip is fabricated with CMOS and mlcromachmmg technology The fabncatlon process 1s shown m Fig 8 (a) A (lOO)-onented 300 pm thick n--type slhcon wafer was used for the accelerometer The areas of the mass and the clrcmt were etched to a depth of 2 pm by 2.5 wt % TMAH solution [12] The area of the beams was etched to a depth of 4 q by TMAH

(b) The cmzult was fabricated before alummmm metalhzation wth CMOS technology Note that heavily doped channel stoppers (boron, 3X 1Ol4 atoms/cm’, phosphorus, 2X lOI atoms/cm’) were ion implanted under field omde to prevent an mcrease of clrcmt leakage current durmg the followmg CVD and anodlc bonding processes

214

(c) The arcmt was covered with 500 w thick CVD St,N, and 500 ,& thick CVD SlO, The contact holes were etched with hot phosphonc acid The SI,N, layer protects the arcult from the &ON etcbant 2 pm thick CVD S&IN was deposIted and etched by buffered HF solution to form beams The residual stress of the &ON beam vanes with the gas flow ratm of CO, and NH, as shown m Fig 5 The weak ten&e-stress condition NH,/(CO, f NHJ = 0 05 was chosen to prevent bucklmg of the beams The 1 pm thick &ON stopper was formed m the same process Tbe stopper prevents stlckmg and short-clrcultmg between the mass and the metal electrode (d) 1000 A thick CVD WI, was deposlted and patterned to cover the slhoon surface of the water The SIO, layer covers the s&con surface except the areas of the beams and contact holes Alummmm was evaporated and patterned to make the mterconnectlon An alummmm contact pad for the Tl-Pt electrode was fabricated on a multi-layer of field oxide, poly SI and SIO, The height of the pad was 2 2 pm A photograph of the chip at this step IS shown m Ftg 9(a) (e) The spin-on-glass (SOG OCD type 7 16OWT from Tokyo-Oka Co ) was spin-coated at 2000 r pm and baked at 400 “C The SOG was patterned to protect the alummmm wires and contact holes from s&on etchant Then, slhcon under the beams was etched amsotroplcally m bydrawne etchant (f) The SOG and the CVD SIO, were etched in HF(9) +NHJ?(lOO) at 36 “C The alummnnn etching rate 1s slower than the etcbmg rate of SOG and CVD SK&, so that the csrcult was not damaged m the process (g) 300 w thick Pyrex glass was used for the upper glass cover Small holes about 100 pm m diameter were drtlled wtth elect~cheml~l discharge machlnlng m NaOH solutmn [13] The glass was etched to a depth of about 10 pm m concentrated HF with Cr and photoresist mask TI (200 A) and Pt (300 A) were electron-beam evaporated and patterned by the hft-off technique The sthcon wafer and the glass plates were anodlca~y bonded m a wafer process at an applied voltage of 800 V m the atmosphere at 400 “C Tbe metal electrode above the clrcult was not attached to the s&con dunng the anodlc bondmg, so that b~gh electric field was applied to the clrcmt However, the gate of the MOS transistor was connected to the sillcon m the CMOS clrcmt, furthermore heavtly doped channel stoppers were ion Implanted under the field oxide, so that the cmzult was not damage during the anodlc bonding if the process time was less than 30 mm The upper Ti-Pt electrode was pressed on the alummium pad 22 pm m height, and the sensor capacitor was connected to the C-F converter The sensor capacitor and C-F converter were hermetlcally sealed m the glass-sthcon-glass structure The pressure mslde the

(4

@> Fig 9 Photographs of the accelerometer chip (a) after CMOS cmult fabncatlon, (b) after anodx bondmg and Cr-Cu-Au evaporation

accelerometer chip becomes about 0 4 tnnes atmospheric pressure when the packaged wafer 1s cooled down to room temperature Q-C&Au was evaporated with a metal stencil mask A photograph of the accelerometer chip IS shown m Fig 9(b) (h) Tbe packaged wafer was diced mto mdtvldual chips Fmally, lead wires were attached w&hconductwe epoxy The slhcxmsubstrate and glass plates can be anodlcally bonded in the wafer process The sensor and clrcmt are packaged m the glass-&con-glass structure Thus process grves small low-cost accelerometers

Results Tbe charactenstlcs of the accelerometer cblp were measured unth the open-loop configuratton Rgure 10 shows the temperature shtft of the frequency at a supply voltage of 5 V The C-F converter was designed to be

215 I

’

1

1

I-

Vdd=SV

.

1

1

\ f4 60

20

[“cl

Temper:i”re

Rg 10 The temperature shift of the output the reference frequency frrt

t II

1

Voltageof

1

1

1

Sd:con MassVs[Vl

Fig 11 The relation between output frequency voltage at an acceleration of lg

frequency

fi and

pi 4

and sdlmn mass

stable for temperature and supply voltage However, the clrcult parameter vaned from the parameter used m the SPICE sunulator because of process variattons, so that the output frequencyf, has a temperature shift of - 240 I-WC The reference frequency f,et = (Yfr was generated by the counter cxcuit, and the temperature shift was - 260 I&X The dtierence of - 20 WC is caused by the difference of the temperature coefficient between the sensor capacitor and the reference capacitor (MOS capaator) The dtierence wdl be small d the reference capacitor 1s fabncated with the glasssilicon-glass structure Equation (8) predicts that the output frequency of the accelerometer chip vanes linearly with the acceleration and the electrostatic force At first, a 4 V supply voltage was apphed to the chip, and the sdlcon mass voltage V, was varled from 0 to 4 V at a constant acceleration of 1 g(Eartb’s gravitational field) Figure 11 shows the relation between the output frequency and the &con mass voltage V, As predicted m eqn (B), the frequency vaned linearly with V, m the range 78 3-75 4 Hz The accelerometer chip can be self-tested by varymg the sllxon mass voltage V. The frequency change caused by V, was - 725 IVV, which was higher than the designed value (- 410 Hz/V) The difference IS mainly caused by the process vanations of beam sprmg constant k The sprmg constant vaned with the

thickness and residual stress of the SlON beams However, the vanatlon is not a serious problem because the output voltage of the PLL servo system does not depend on the beam spnng constant Figure 12 shows the output frequency versus the acceleration at a constant sihcon mass voltage of 2 V The frequency varled linearly from 75 6 to 77 2 kHz when the acceleration changed from - 1 g to 1 g The sensltlvlty was 800 Hz/g which was higher than the designed value (-630 Hz/g) The accelerometer can be used as an open-loop-type accelerometer The dynamic response of the accelerometer chip was measured (Fig 13) The cut-off frequency of the accelerometer chip was about 2 Hz with the open-loop configuration Tlus was lower than the calculated value (7 7 Hz) The cut-off frequency will be extended up to the resonance frequency cf,_, = 3 9 kHz) if the cavity mslde the accelerometer chip IS sealed at low enough pressure to neglect the squeeze-film effect The electrostatic force-balancmg system has been developed mth the chip and a PLL cncult, and the acceleration changed from - 1 g to 1 g Figure 14 shows the static response of the output voltage at a supply voltage of 4 V When the reference frequency frerwas 76.7 kHz, the output voltage V, vaned linearly from 19 to 4 1 V wth the acceleration change from - lg

Fig 12 The relation between output frequency efatlon at a srhcon maas voltage of 2 V

and the accel-

I-

Frequency

Fig 13 Dynanuc loop configuration ratlon

[Hz]

response of the output frequency for openand output voltage for closed-loop configu-

216

1

1

I4

system has been realized with the chip and a PLL cmxut The output voltage varres linearly with the acceleration, and arcult drift of the C-F converter 1s compensated usmg the novel PLL servo system The sensltlvlty and offset of the output voltage can be changed with the outer circuit parameter The temperature shift of the PLL servo system will be reduced by fabrlcatmg the reference capacitor wth a glassslhcon-glass structure The dynamic response will be Increased by the vacuum packagmg method

i 4

01

d I

-1

I

I

I

I

/

h

0

Acceleration

1

L

1

L 1

Acknowledgements This work has been partly supported by the Japanese Mm&-y of Education Science and Culture under a Grant-m-Ad No 03102001 and also by the Tatelshl Foundation

a[gl

Fig 14 Statx response of the electrostatIc force-balancmg system No&e that the output voltage IS shifted down when the reference frequency IS mcreased, and the sensltlvlty 1s doubled at a feedback rat10 /3=05

to 1 g at a feedback ratlo /3 of unity The output voltage was shlfted down when the reference frequency was increased If the reference frequency 1s not fO=I,/ 2C,V,, the slhcon mass displacement z IS not zero Therefore, the electrostatic force Fe cannot be expressed as eqn (3), so that the electrostatic force IS also proportional to the square of the sillcon mass voltage V, However, because the V,* component is much smaller than the V, component, the non-hneanty caused by V;” can be neglected The sensltlvlty (V&) was Increase if the feedback ratio p was reduced The senslttvlty (VO/a) was doubled by makmg the feedback ratio /3 half, as predicted m eqn (13) The dynamic response with the electrostatic forcebalancing system was measured as Fig 13 The cut-off frequency was 6 Hz Though the cut-off frequency IS greater than the value for the open-loop configuration, 6 Hz IS low for commercial use One method to increase the cut-off frequency IS for the cavity mslde the accelerometer chip to be sealed at low enough pressure to neglect the squeeze-film effect The vacuum packaging method using glass-sihcon anodlc bondmg has been developed recently [14, 151 Another method 1s makmg grooves on the &con mass, m spite of the decrease of sensor capacttance [16] Conclusions An Integrated slhcon capacltlve accelerometer chip has been developed, and an electrostatic force-balancmg

References 1 F Rudolf, A Jomod, J Bergqvlst and H Leuthold, Preclslon 2

3

4

5

6

7

8

accelerometers with M resolution, Sensors and Actuators, A21-A23 (1990) 297-302 S Suzuki, S Tuchltam, K. Sato, S Ueno, Y Yokota. M Sato and M Esashl, SemIconductor capaatance-type accelerometer with PWM electrostatic servo tcchmque, Sensors and Acmators, A21-A23 (1990) 316-319 HF Schlaak, F Amdt, A Steckenbom, HI Gevatter, L Lesewetter and H Grethen, Mechanical capacltlve acceleratlon sensor with force compensation, MUXJ System Technologws ‘90, pp 617-622 Y Matsumoto and M Esashl, Integrated capacltlve accelerometer with novel electrostatic force balancmg, Tech Dges& llth Sensor Symp, Japan, 1992, pp 47-50 Y Matsumoto and M Esash~, An mtegrated capacitive absolute pressure sensor, Tmns IEICE, J75-C-2 (8) (1992) 451-461 G Yamaguchl, Y Matsumoto and M Esash~, Supply voltage and temperature unsusceptible C-F converter, Tmns IEKE, Vol J74-C-2 (11) (1992) 763-765 M Esashl, Y Matsumoto and S Shop, Absolute pressure sensors by atr-tight electrical feedtbrough structure, Senrors and Achcatom, A21-AU (1990) 1048-1052 Y Matsumoto, S Shop and M Esashl, Fabncatlon of C-F converter CMOS IC for capacmve sensors, Tmns IEKE, 573-C-2 (31 (1990) 194-202 T Shlral: 6 -Ura and M Esashl, Two-wires s&on capacitive accelerometer, Tmns IEICE, J75-C-2 (10) (1992) 554-562 W Rlethmuller and W Benecke, Thermally exclted srhcon mrcroactuators, IEEE Tram h&&on Dewces, ED-35(6) (1988) 758-763 J B Starr, Squeeze-film dampmg m sohd-state accelerometers, Lhgest, IEEE Solrd State Sand Actuator Workhop, 1990 PP 44-47 0 Tabata, R Asaht, H Funabash] and S Susyama, Amsotropic s~hcon etchmg usmg (CH&NOH solutions, Proc 6th Int Conf Sold-State Senrors and Achutom (Tmnsducem9I), San Franctsco, CA, USA, June 24-28, 1991, pp 811-814

217 13 S ShoJi and M Esashl, Photoetchmg and electrochemrcal dtscharge dnlbng of Pyrex glass, Tech Dgest, 9th Smsor Symp , Japan, 1990, pp 27-30 14 M Esasht, N Ura and Y Matsumoto, Anodx bondmg for integrated capacttwe sensors, 5th Znt Wddop on Mmo Eketm Mechamcal System, 1992, pp 43-48 15 H Henm, S Shop, K Yoshlml and M Esashi, Vacuum packagmg for micro sensors by glass-&con anodtc bondmg, Proc 7th Int Conf Sold-State Sensors and Actuators (Transduced93), Yolwhama, Japan, June 7-10, 1993, pp 584-587 16 S Suzuki, M M&I, M Matsumoto, B Kloeck, S Tsuchttam, S Kuragakt, K Sato and A Kolde, Semtconductorcapaatancetype crash sensor for atrbag systems, MUZKJSystem Technobgaes ‘92, October, 1992, pp 383-392

Biographies Yoshmon Mafsunwfo was born m Kosal, Japan, on March 3, 1965 He received B S and Ph D degrees 111

electronic engmeermg m 1988 and 1993, respectively, at Tohoku Umverslty Since 1993, he has been a research associate at the Department of Electrical and Electromc Engmeermg, Toyohasi Umverslty of Technology Masayoshz Esashz was born m Sendal, Japan, on January 30, 1949 He received the BE degree m electromc engmeermg m 1971 and the Doctor of Engmeenng degree m 1976 at Tohoku Umverslty From 1976 to 1981, he served as a research associate at the Department of Electronic Engneermg, Tohoku Umverslty and he was an associate professor from 1981 to 1990 Since 1990, he has been a professor m the Department of Mechatromcs and Precision Engmeenng m Tohoku Unwers@ He has been studymg microsensors and integrated mlcrosystems fabricated with mlcromachmmg Dr Esashl IS a member of the IEE and the IECE of Japan