Semiconductor string pressure sensor

Sensors and Acfuators A, 39 (1993) 125-128

125

Semiconductor string pressure sensor N. Bogdanova Technical Unrversuy Sofia, Lkpt KTPPME, 1756 Sofia (Bulgana)

R Balzar, V. Voronm and E. Krasnogenov Polytechnrcal Inwute Lvov, I2 Mua St, 2PO64d Lvov (Ubame) (Received

March 21, 1991, m reused

form March 30, 1993, accepted

Apnl 27, 1993)

Abstract A semiconductor pressure resonator sensor usmg a sdlcon dtaphragm and a monocrystalhne described Its prmclple of operation IS based on the change of the resonator’s natural vlbratlon result of tbe pressure apphed to the diaphragm The basic advantages of this type of sensor are Its stability and frequency output The theory and fabrication of the basic sensor element using technology are &cussed Some experImenta results are shown

1. Introduction Capacitive and plezoresntlve sensors are the most widespread types of pressure transducers [l] Plezoresistive sensors show approxnnately linear transfer charactenstlcs, but they have an analog output signal The operating temperature range IS lumted because of the strong temperature dependence of their output slgnal Capacitive pressure sensors have a wrde operating temperature range, but their characteristics are nonlinear Both devices use a slhcon diaphragm as a sensitive element The aim of our efforts IS to design a sensor which combines the advantages of both types of sensors For that purpose a mlcromechamcal resonator was developed Referring to Fig 1, the operation can easily be explained as follows The diaphragm 3 undergoes mechanical deformation m response to apphed pressure This deformation 1s transferred to the fibre 1 As a consequence the resonance frequency of the fibre changes Therefore this resonance frequency change 1s

sihcon fibre I frequency as a lugh sensltmty, nucroelectromc

a direct consequence of the pressure change applied axially to the diaphragm The development of mlcroelectromcs and micromachining allows the use of a monocIystallme fibre as a vlbratmg strmg placed on the diaphragm [2, 31 Among the vanous types of electromechanical resonators used for mechamcal measurements, fibre sensors are the ones with the lnghest sensltlvlty [3] The best results are obtained when the fibre and the diaphragm are made from the same material, so that the influence of temperature on the characteristics of the sensor 1s mmnnlzed With respect to the piezoresistlve properties, nhcon 1s the best material [4] Thus sennconductor allows the advantageous use of microelectronic technology combined with nucromachmmg We use electrostatic excitation of the fibre, whereas for known s&con resonator structures, light IS used for the excitation and detection [S]

2. Theory When an alternating slgnal u =u,, sm & IS applied between the monocrystalhne string (fibre) 1 and the exatmg electrode 4 (Fig l), an electrostatic force F IS Induced [6] F = 0 5(dC/dz)u,2sm20t

Rg 1 Prmaple of the electromechamcal stnng resonator 1, monocrystalhne fibre, 2, current ternunal, 3, diaphragm, 4. electrlcal connection

0924-4247/93/$X5 00

(1)

where C IS the stnng-dlaphragm capaaty and z 1s the distance between the string and the diaphragm, equal to the height at which the fibre IS mounted above the diaphragm

0 1993 - Elsevler Sequota

All rights reserved

As a result of the action of the harmonic excltmg force, harmonic osclllatlons with amplitude A are generated These oscdlatlons cause the stram of the monocrystalline string to be l

= (1?,4~/81~)(1-CDS 2wt)

(2)

where 1 1s the oscdlatmg length of the fibre As a result of the plezorenstive effect, the variable component of the strain causes the generation of a variable component of the resistance AR = KE= (&A2/812)R, cos 2wt

(3)

where K 1s the gauge factor and R, 1s the nominal stnng resistance If a direct current passes through the monocrystallme fibre, an alternatmg voltage 1s generated m It Its frequency 1s equal to twice the mechanical vlbratlon frequency The vlbratlon frequency 1s calculated from f=(1/2~“)((~~,(~,/4)*+B,~,l~)/p)’~

(4)

where B and B, are numerical coefficients depending on the strain, d, 1s the strmg diameter, E, IS Young’s modulus of the strmg, p the crystal density and o, the stress acting over the fibre When a pressure 1sapplied to a sensor with a circular diaphragm, o, 1s a function of the applied pressure uc=3P(1-~z)E,(z+h/2)(~-12)/16E,h3

(5)

where P IS the pressure acting axially to the diaphragm, p 1s Poisson’s coefficient, D 1s the diaphragm diameter, h the diaphragm thickness and E,,, Young’s modulus of the diaphragm Equation (5) is valid when the diaphragm bending IS much less than the diaphragm thickness For a square diaphragm the fibre 1s fixed at the geometrical centre In this case the operation of the transducer 1s slmdar to that with a circular diaphragm The expression for a, m the dire&Ion of the maximum stram is

3. Design As mentloned earher, the main parts of the resonator are the diaphragm and the monocrystalhne fibre They are fabncated by usmg semiconductor technology This permits the preparation of diaphragms with different geometrical dunenslons and shapes, but with ldentlcal deformation propertles The same holds true for the fibres The consequence 1sthat It ISpossible to fabncate unified sensors for broad ranges of pressure measurement (from some Pa to several hundred thousand Pa) The diaphragm may either have a circular or a square shape It 1s fabricated using a (100) or (110) sdlcon wafer with a specific resistance of 4-8 Cl cm Smce bilateral etching 1s used, the wafer has to be pohshed on both sides Mostly, KOH and EDP are used as etchers [7] Dependmg on the apphcatlon, diaphragms with different mechanical dlmenslons can be prepared Equations (5) and (6) are the basis for the design of the diaphragm The fibre 1sfabricated using a CVD method [3] Due to the fabrlcatlon process, it IS a perfect monocrystal and has a surface with mmunum free energy This has the advantage that the strain properties of the fibre are a hundred times better than those of strings or beams prepared from bulk silicon [8] Because of the relatively low density of the monocrystalhne fibre, the required excitation energy 1s low Also, the breakmg strength of the fibre IS relatively high The abovementioned advantages allow the reahzatlon of an oscillator system wth maxlmum Q-factor, stablhty and rehabdity The fibre 1sfirmly fixed onto the diaphragm by unng an msulatmg epoxy glue (Fig 1) The distance between the diaphragm and the fibre as well as the length of the fibre depend on the desn-ed application The assembly of the pressure resonator sensor IS shown m Fig 2 The sensitive element (resonator) IS mounted m a hermetically sealed package (1 & 2) The internal volume (side of diaphragm with the fibre) of

uc=2E,(z+h/2)(1-1*/4d) x {2G, + G2[2Z2/4u2 - (1 - E2/4u2)]}/u2

(6)

where a 1s the square side and G, and G2 are the approxlmatron coefficients for the bending function for a square diaphragm Equations (4), (5) and (6) give the relation between the apphed pressure and the output frequency for a sensor with a arcular or a square diaphragm According to the theory, the output frequency 1s inversely proportional to the length of the fibre and to the thickness and size of the diaphragm

2.

l-

t pr Fig 2 Design of the strmg transducer 1, 2, 5, package parts, 3, shon diaphragm, 4, shcon wafer, 6, 7, 8, tubes, 9, monocrystalhe strmg, 10, electrical connectIon

127

the case 1s connected to the envlromnent with pressure p, by the capillary tube 7 The second capillary tube 8 connects the other side of the sensor to the measured pressure p2 If the first tube IS closed under known condltlons, the hermetically sealed diaphragm module may be used for absolute pressure measurements The relative pressure changes can be measured when the diaphragm 1s not hermetically sealed When the difference between p1 and pz changes, a deformation of the diaphragm occurs This deformatton IS transformed by the fibre mto vanatlons of the frequency The sensor signal IS transferred to an interface cu-cult [9]

f,kHz 160

50

,

140

,

130

120

110

lO[I

9cI

4. Results

8I

Some expenments with sensors havmg different dlaphragm dlmenslons as well as dtierent fibre lengths were carried out Figure 3 shows the output signal as a function of the applied pressure The tested sensors have a square diaphragm of length a = 5 mm and thickness h = 100 pm The fibre lengths are 15, 2 0 and 2 5 mm, respectwely As can be seen from the results, the shortest fibre has the highest sensltmty Figure 4 gives the practical results for the sensltlvlty as a function of the diaphragm thickness The length of the diaphragm IS the same as above, whereas the thicknesses are 100 and 200 pm, respectively (for a fibre length of 15 mm) As can be seen, there 1s a fgkHz)

90

a0 -

71I-

61I

513

‘lI10

20

30

Fig 4 Sensor sensltwty as a function of the diaphragm 1, h=lOO /Un, 2, h=2ocl /Lm

PI05

Pa

thuzkness

proportlonahty between the thickness of the diaphragm and the pressure range For precise measurements m a limited range, It 1s necessary to use sensors with a thm diaphragm Sample sensors with a circular diaphragm are also mvestlgated The length of the fibre IS 15 mm The thickness of the diaphragm 1s 100 pm The diameters of the diaphragm are 5, 6 and 7 mm, respectively The results are shown m Fig 5 The characterlstlcs obtamed for sensors with square and circular diaphragms are nmllar, as can be expected from the theory By comparmg the results obtained from sensors of this type to those of reference sensors, the basic error 1s 0 01% of the measuring range

5. Conclusions

‘OL-a;--;-

pl0

5,Pa

Fig 3 Sensor sensmwty as a function of the strmg length for 1, I= 1 5 mm, 2, I= 2 mm, 3, I = 2 5 mm a square dmphragm

The basic advantages of the string pressure resonator sensor are high sensltlvlty, operatmg stab&y, linear characterlstlcs and a frequency output slgnal The theoretical mvestlgatlons and expenments show that the geometrlcal dlmenslons of the fibre and the diaphragm have a slgmficant effect on the sensor operation characterlstlcs As discussed above, the mtnnslc vlbratlon frequency of the slhcon resonator depends

128 5 K E B Thornton, D Uttamchandam and B Culshaw, A sensltlve optsally excited resonator pressure sensor, Sensors and Achcators A, 24 (1990) 15-19 6 V Voronm, R Balzar and E Krasnogenov, VIbratIonal frequency transducers on the basis of monoclystallme &con fibres, Proc 6th Int Conf ‘h4xroelecfro1~rcs ‘88: Botevgmd, Bulgana, 1988 7 G Delaplerre, Micro-machmmg a survey of the most commonly used processes, Sensors and Acfuafors, 17 (1988) 123-138 8 N Bogdanova, R Balzar, V Voronm and E Krasnogenov, Smgle-crystal sihcon resistors as sensitive elements for sensors, Proc Ann School Senuconductor Hybmi Technol, Soropol, Bulgana, May 14-18, 1990, pp 8-2 9 R Bmzar, N Bogdanova, V Voromn and E Krasnogenov, Operation of monocrystalhne slhcon resonator m measurmg CIrcmt, Sensors and Actuators A, 30 (1992) 175-178

f,kHz 100 -

90

so -

70 -

60 -

50 05

1

15

2

25

3

PlO%

Rg 5 Sensor sensltlvlty as a fun&on of the diaphragm chameter for a cucular chaphragm 1, D = 5 mm, 2, D = 6 mm, 3, D = 7 mm

on the diaphragm dlmenslons and on the length of the fibre The developed structures allow measurements to be camed out m a wide pressure range with various sensitwltles The practical measurements are m good agreement with the theory The theoretlcal and experimental results obtained are used for the design of various umversal sensors for dtierent pressure ranges, as well as for absolute and relatwe pressure measurements All types of sensors are mounted m the same type of package The mvestlgatlons m the field of semiconductor string sensors based on monocrystalhne silicon fibre and slhcon chaphragms show great posslblhtles for apphcatlons m modern measuring techmques, human medicine, chemIstry and environmental protection

References G Blasquez, P Pons and A Boukabache, Capabdltles and hnuts of sdlcon pressure sensors, Sensors and Actuators, 17 (1988) 387-407 S Mlddelhoek and S Audet, SiBcon Sensors, Academic Press, London, 1989, pp 133-140 and 287-330 V Voronm and I Manamova, The physical-chemical aspects of sermconductor wi-uskers technology and creatmg sensors on then base,Proc Ann SchoolSemrconductorHybnd Technol, Sozopol, Bulgana, May 14-18, 1990, pp 18-34 S Mlddelhoek and S Audet, S&con Sensors, Academtc Press, London, 1989, pp 105-123

Biographies N Bogdanova was born m Sofia, Bulgaria She received the D1p1-1ng m electromcs from the Technical University of Sofia, after which she worked at the Institute of Microelectromcs in Sofia In 1986 she received the Ph D degree from the Techmcal Umverslty Chemmtz, Germany Smce then she has been at the Techmcal Umverslty Sofia, where she 1s working on silicon sensors R Balzar was born m 1947 m Lvov, Ukraine He received the Dip1 -1ng m electromcs from the Polytechnical Instltnte of Lvov and the Ph D degree from the Umverslty of St Petersburg, after which he jomed the Research Laboratory on Mlcroelectromc Sensors at the Polytechmcal Institute Lvov As a head of the laboratory, he 1s currently engaged m the design and testmg of stram-gauge measurmg systems V Voromn was born m 1940 m Lvov, Ukrame He received the B SC degree m semiconductor chemistry m 1965 from the Polytechmcal Institute of Lvov After he recewed his Ph D degree m physical chemistry from the Higher Institute for Fme Chemical Technology of Moscow, he Jomed the Polytechmcal Institute of Lvov HIP current research actmtles he m the field of semiconductor technology Professor Voromn IS the head of the Department on Semiconductor Devices at the Polytechmcal Institute of Lvov E fiusnogenov was born m 1938 m Etev, Ukraine He received the B SC and Ph D degrees m semlconductor physics from the State Umverslty of Kiev Then he Joined the Research Laboratory on MIcroelectronic Sensors at the Polytechmcal Institute of Lvov He IS currently engaged m the design and testmg of stramgauge measuring systems