Integrated piezoresistive pressure sensor with both voltage and frequency output

Sensors

and Actuafors, 4 (1983)

113

113 - 120

INTEGRATED PIEZORESISTIVE PRESSURE SENSOR WITH BOTH VOLTAGE AND FREQUENCY OUTPUT* SUSUMU

Toyota

SUGIYAMA,

Central

Research

MITSUHARU

TAKIGAWA

and Development

Labs,

and ISEMI IGARASHI Inc Nagakute,

Awhr

480-11

(Japan)

Abstract An on-chip mtegrated plezoreslstlve pressure sensor Hnth both voltage and frequency outputs has been developed. The sensor chip, havmg a size of 3 X 3.8 mm2, was reahzed by the use of a standard bipolar IC process The output voltage span 1s 1 to 4 V for a pressure range of 0 to 750 mmHg. The pressure sensltlvlty of the voltage output 1s 4 mV/mmHg. The nonhneanty 1s less than 0.4 % of the full scale. The sensltlvlty of the frequency output ls about 30 kHz for a 750 mmHg change m pressure. The output 16 m the T*L level. The temperature coefficient of the sensltlvlty IS less than 0.06 %/‘C m the temperature range -20 to 110 “C

1. Introduction Slllcon plezoreslstlve pressure sensors have been widely used for mdustnal and biomedical electronics [ 1 -41. The plezoreslstlve sensors have excellent electrical and mechanical stablhty, and can be fabncated m a very small size. Sensors for automotive engme control and robot control have to detect accurate mfonnatlon m a wide temperature range and under high electromagnetic interference. However, the output voltage of a plezoreslstlve pressure sensor is too small m magnitude and must be compensated for a vvlde temperature range For that reason, the output of the pressure sensor needs to be amplified to a high level m order to increase the S/N ratio and provide dig&al output suitable for signal processmg m a microprocessor system. For this purpose, an integrated plezoreslstlve pressure sensor IS actively being developed with higher output level, higher performance and lower cost [ 5 - 71. We have developed a practical mtegrated pressure sensor havmg plezoreslstlve bmdges, temperature compensation c111cultry,high-level amplifiers and a frequency converter on a monohthlc sticon chip. This approach can *Based on a Paper presented at Sohd-State Transducers 83, Delft, The Netherlands, May 31 -June 3,1983 0250-6874/83/$3

00

0 Ebevier Sequola/Prmted

m The Netherlands

114

nnprove the sensor performance and enhance rehablhty of transmission of the detected mformatlon to a host microprocessor for automotive engme control and robot control 2. Design

An integrated pressure sensor havmg two plezoreslstlve bndges, a rectangular &aphragm and signal processmg clrcults was desrgned The plezoreslstlve bndge 1s the sensing element and was formed on a thm dlaphragm The signal processmg clrcults with voltage and frequency outputs were formed on the frame of the diaphragm In order to develop the mtegrated pressure sensor, we designed it very carefully m the light of pxevlous work and two points were especially considered (1) mmlmlzatlon of the electrical interaction between the sensing element and the processmg CIPcult, (2) exclusion of the influence of stresses m the diaphragm on the Low cost, high performance and small size are also processing circuits crltrcally needed for a mde range of mstrumentatlon systems To cope with such needs, the most promlsmg IC fabrication technique enabling a batch process was used, The substrate used was a p-type (100) s&on wafer 3 inch m diameter and about 420 pm m thickness Plezoreswtrve bridge and diaphragm Four plezoreslstors were organlzed as a full bndge Two plezoreslstors, opposrng each other xn the bndge, were onented along the crystalhne direction and other two plezoreslstors along the dlrectlon To obtam a high sensltlvlty, the plezoreslstlve bndge was arranged as near as possible to the penphery of the diaphragm A thm diaphragm with a small area was elaborately formed from a thick slhcon substrate by amsotroplc etchmg technique using NaOH etchant [ 111 Geometrical parameters of the diaphragm were determined from our expernnental results on conventional pressure sensors The side length of the square diaphragm, 2a, was 1 mm and the typical diaphragm thickness, h, was 25 E.crnThe theoretical expression for the output voltage IS as follows+ AV 1 -=zK44(1-Y)h2P, V

a2

where AV and V are the plezoreslstlve bridge output voltage and applied voltage of the bndge and v and R~ are Poisson’s ratio and the dominant prezoreslstlve coefficient m p-type slhcon, respectively [8 - lo] The senntlvlty of the bndge 1s roughly estunated as IO mV/V when the applied pressure to the diaphragm IS 750 mmHg. The layout of the diaphragm must be precisely deslgned m an integrated pressure sensor When the thickness of a silicon wafer substrate 1s t pm, the wmdow width of a photomask for the square diaphragm formation must be designed wider than the width of the necessary diaphragm by 2d = 2(t - h)/,/2

115

Therefore, to exclude the influence of stresses induced m the dlaphragm on the signal processmg cu-cults, these clrcults must be kept away from the diaphragm edge by a distance d Signal processrng cmw t Figure 1 shows a schematic diagram of the on-chip mtegrated pressure sensor with both voltage output and frequency output clrcults. Two signal processing clrcults are formed on the thick frame of the diaphragm The left hand one 1s the voltage output processmg curcult and the nght hand one 1s the frequency output processmg clrcult These mdlvldual signal processing circuits independently have a temperature compensation clrcult and a sensitivity adJustment circuit The actual equivalent clrcult of the frequency output processmg CID cult IS shown m Fig 2 This circuit cons&s of a plezoreslstlve bndge, a constant current crrcultry including transistors Q1 and Q2, a differential amplifier A,, a frequency converter and an output buffer clrcultry. Each plezoreslstor had a resistance of 2 kS1 to obtan a higher voltage gam at a dlfferentlal amphfler. This amplifier gam was designed to be about 40. The frequency converter consisted of an operational amplifier AZ, a timing capacitor Ct and a resistor Rt, to form a current-controlled oscillator In this work, the nommal frequency was deslgned to be about 200 kHz The output buffer cucultry was formed m the end stage to obtam a steady output of T2L level In the case of the voltage output processing circuit, the second-stage amplifier was connected behmd the fust-stage amphfrer A,, which 1s shown m Fig 2. To obtam an output of several volts, the total gam was designed to be about 100 Each plezoreslstor was made to have a high resistance of 4 ka m order to be operated at a lower power level The excltatlon power of the mtegrated pressure sensor was roughly estimated to be only 20 mW

NdPLlFIER-1

Fig 1 Schematlc diagram of on-chip integrated pressure sensor with both voltage output and frequency output cvcults

116

Fig

2 Equwalent

cmcult of frequency

output

processmg

clrcult

The temperature sensltlvltles of the plezoreslstors cancelled each other out, so that mdlvldual plezoreslstors were organized as a full bndge Furthermore, the temperature compensation for sensltlvlty decrease of the plezoresrstlve bndge was accomplished by usmg the constant current method The temperature coefficient of the sensltlvlty of the plezoreslstlve bridge was self-compensated by the temperature coefficient of the resistance of the bndge itself The plezoreslstors and all the resistors comprlsmg clrcults were simultaneously formed by the base dlffuslon step m the standard bipolar IC process, these diffused resistors having the same temperature coefficient Therefore, the temperature dependence of the on-chip cmxut gam 1s very little The plezoreslstlve bndge sensltlvxty was deslgned to be adJusted by a tnmmer r,, and the offset of the cvcult can be adjusted by a tnmmer r,

3. Fablncatlon The mtegrated plezoreslstlve pressure sensor was fabncated by using the standard bipolar IC process The startmg maternal was a p-type {loo} onented s&con wafer havmg a thickness of 420 ,um and a reslstlvlty of 3 GZ cm An n-type epltaxlal layer 10 pm thick havmg a reslstlvlty of about 1 SL cm was grown on the silicon substrate after buned layer dlffuslon (antimony) For formation of isolated Islands, boron was diffused selectively Then base and plezoreslstor regons were formed by the boron dtifusion process As a result, the sheet resistance of the diffused layer was controlled to be 150 a/Cl, with a surface lmpunty concentration of 2 X 101’/cm3 Phosphorus was diffused for formation of the emitter reeons As an anlsotroplc etching mask for diaphragm formation, a s&on mtnde (S+N,) layer with a thickness of 2000 ii was deposlted by the LPCVD technique on the back face of the wafer A wmdow for diaphragm formation was opened m this Sl,N, layer by reactive ion etching for subsequent amsotroplc etchmg. After contact holes were opened, an alummlum layer was deposited by vacuum evaporation to make mterconnectmg leads

117

and electrodes. The final step of the bipolar IC process was the delineation of the alumlmum layer by photolithographic process Lastly, the diaphragm was formed from the back face of the LC wafer by amsotroplc etching by use of 25% NaOH etchant at 90 “C The SEM photograph of the diaphragm formed by anlsotroplc etching 16 shown m Fig. 3 We can see that the diaphragm formed 1s a precisely-shaped frustum of a pyramid with walls of four (111) crystallme planes. Many draphragms 25 pm thick were obtamed from a 3 mch diameter wafer 430 pm thick The fabncated wafer was separated into mdlvldual integrated pressure sensor chips The completed pressure sensor IS shown m Fig. 4 The chip srze of the integrated pressure sensor IS 3 X 3 8 mm2

Fig

3 SEM photograph

IFig

of diaphragm formed by anlsotroplc

etchmg

3.8 mm ___1( 4 Mlcrophotograph

of completed

Integrated pressure sensor

118

4. Results The typical output voltage span of the integrated pressure sensor 1s 1 to 4 V for a pressure range of 0 to 750 mmHg, as shown m Fig 5 The pressure sensltlvlty of the voltage output IS about 4 mV/mmHg m this range at a d c supply voltage of 5 V A non-lmemty (NL) of less than 0 4% of the full scale has been obtamed, as shown m Fig. 5. The temperature dependence of the voltage output sensltlvlty IS shown m Fig 6 The temperature coefficient of the sensor sensltlvlty 1s less than O.O6%/“C for the temperature to 110 “C The temperature coefficient of the offset voltage IS range -20 less than 1 5 mmHg/“C The sensor IS found to have an excellent temperature compensation The sensltlvlty of the frequency output 1s about 30 kHz for a pressure change from 0 to 750 mmHg and the nommal zero pressure frequency 1s 210 kHz, as shown m Fig. 7. The output voltage level has been obtained at T*L level The non-lmeanty of the frequency output 1s less than 3% of the full scale for the range shown m Fig 7 supply

voltage S(V)

4

+I Fig

150 300 450 600 PRESSURE (mml-ig)

5 Voltage 6r

,

I

output I

,

750

charactenstlc I

of Integrated

pressure sensor

1

eg b-4-5: 5 c z ifi ul

32Supply

’

I -20

5(v)

I 0

20

40

TEMPERATURE

Fig

voltage

l-

6 Temperature

60

1 00

100

F’C 1

dependence

of the voltage output

sensstwlty

119

An example of the assembled sensors 1s shown m Fig 8 The mtegrated pressure sensors which are mounted m a DIE’package tiI be used for a mde range of future mstrumentatlon systems, m the same way as general purpose ICS 240

I

I

I

I

supply voltage

5(v)

2 230 Y Y 4 E! u g 220

150

300

450

600

75;0

PRESSURE (mmHg) Fig 7 Frequency output charactermtlc of integrated pressure sensor

Fig 8 Mounted Integrated pressure sensors m DIP

package

120

5. Conclusions A practical Integrated pressure sensor havmg two plezoreslstlve bndges, temperature compensation cn-cultry, high level amphfrers and a frequency converter has been realized on a 3 X 3.8 mm2 chip 430 I.trnthick The temperature coefflclent of the sensltlvlty of the voltage output has been found to be less than 0 06%/“C over a wide temperature range, from -20 to 110 “C A non-lmemty of less than 0 4% of the full scale has also been obtamed A stable output of T’L level has been obtamed at the frequency output This reahzatlon of a sensor unth two types of output ~11 promote direct connection of the sensor to a microprocessor system for automotlve engme control and rndustr& robot control References 1 T Chlku and I Igarashl, Some apphcatlons of senuconductor stram gages, Proc 20th USA, 1965 ISA Conf, No 17 1 l-3-65, Ptttsburgh, Pennsylvanta, 2 T Chlku and I Igarashl, Submmlature pressure transducer - an apphcatlon of semiconductor strain gages, ISA Tram, 10 (1971) 35 - 39 3 W H Ko, J Hynecek and S F Boettcher, Development of a mmlature pressure transducer for blomedlcal apphcatlons, IEEE Trans Electron Devmes, ED 26 (1979) 1896 - 1905 4 M Esash, H Komatsu, T Matsuo, M Takahashl, T Taklshlma, K Imabayashl and H Ozawa, Fabrxatron of catheter-tip and sldewall mlmature pressure sensors, IEEE Trans Electron Devrces, ED-29 (1982) 57 - 63 5 J M Borky and K D Wise, Integrated srgnal condltlonmg for slhcon pressure sensors, IEEE Trans Electron Devrces, ED-26 (1979) 1906 - 2016

6 R E Blckmg, R L Johnson and D B Wamstad, Totally Integrated pressure transducer, Proc SAE Meetmg, SAE-810376, Detrort, Mtchtgan, USA, 1981, pp 9 - 12 7 X P Wu, M H Bao and W X Ding, An Integrated pressure transducer for biomedical apphcatlons, Sensors and Actuators, 2 (1982) 309 - 320 8 0 N Tufte, P W Chapman and D Lond, Silicon diffused element plezoreslstlve diaphragms, J Appl Phys, 33 (1962) 3322 - 3327 9 A D Kurtz and C L Gravel, Semiconductor transducers usmg transverse and shear Chzcago, Illrnou, plezoreslstance, Proc 22nd ISA Conf , NO P4-I-PHYMMID-67, USA, Sep 1967, pp 1 - 8 10 S Tlmoshenko and S Womowsky-Krueger, Theory ofPlates and Shells, McGraw-Hill, New York, 1959 11 S K Clark and K D Wise, Pressure sensltlvlty In anlsotroplcally etched thm-dlaphragm pressure sensors, IEEE Trans Electron Devzces, ED 26 (1979) 1887 - 1896