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Thick-film pressure sensors
Mechatronics.Vol. 3, No. 2, pp. 167-171, 1993
0957-4.158/93 $6.00+0.00 PergamonPressLtd
Printedin GreatBritain
THICK-FILM
PRESSURE
SENSORS
GABOR HARSASYI--EMILHAH.X" Techn. Univ. Budapest, Dept. Electr. Techn.
Abstract- A new typeof pressuresensorhas beendevelopedby takingadvantageof the piezoresistiveeffectin polymer thick-filmresistorsscreenedand curedon epoxy-glassdiaphragms.The devicelookspromisingto satisfyall the requirements:low cost, high sensitivityand low thermaldrift. The piezoresistiveeffectin polymerthick-film(PTF) resistorsis discussedin comparisonwithcermetthick-films.The possibilityof makinga high-performancePTF pressure sensoris theoreticallysupported.Characteristicsof the newlydevelopedpolymerthick-filmsensorare described. In a limitedtemperaturerange, the new sensorhas parametersthat are not worsethan those of the cermetthick-film pressuresensorand at a muchreducedcost. 1. INTRODUCTION Nowadays sensors are available for measuring a lot of parameters, but they are badly matched to the microelectronics. The sensors, produced by microelectronical methods overcome this difficulty and they are compatible with microelectronics. One of the possible solutions is the application o f the thick-film technology. It is well known that metal films and semiconductor resistors are characterized by a resistance variation proportional to the applied stress, but more recently, it has been revealed that thick-film resistors show a notable sensitivity to deformation [1-4]. The good sensitivity of the thick-film resistors to deformation together with their long term stability, small temperature coefficient and their cheap production cost have suggested, to develop a new pressure sensor based on thick-film piezoresistive effect. The paper describes the sensors developed at TU Budapest Dept. Electronics Technology.
2. SOME THEORETICAL CONSIDERATIONS [5] The sensing element of the pressure sensor consists o f a circular edge - clamped ceramic or epoxy-glass diaphragm on which four thick-film resistors connected in a W e a t h s t o n e - b r i d g e configuration are screened and fired Fig. 1.
UR
1
o
Fig. 1. Structure and circuit connection o f the ttu'ck-filrn pressure sensor membranes
167
168
G. HARS,/~NYI and E. HAHN
The measured pressure induces a strain on the diaphragm and the resistors change their values. The change can be expressed by the tensor equation: Ap --= P
nT,
(1)
where O is the resistivity, rrthe piezoresistivity tensor and T the tensor of mechanical stress. If the resistors are made in the form of thick- (or thin-) film (1) can be written in very simple form: ap - -
=
(2)
Ge,
P e is the strain, G the so called gauge factor. By low deflexion of the diaphragm (2) transform to Ap
AR
. . . . p R
Sp,
(3)
where p is the pressure, S the "sensitivity" of the resistor. The bridge may be excited by constant voltage or constant current, The output-voltage of the bridge: U=
I
ZR
{R1R 2 - R 2 R 4 } .
(4)
If p = 0 and the bridge is balanced (R 1R 3 = R 2 R 4 ).
If p ,
o,
V ( 0 ) = 0.
(4a)
~i = R ° (1+ So)
(5)
and
'{(R°R° _ RoRo)+ 2 (RoRo + RoRo)
+ tR°R° - oRo) S202 }
2Ri + {(R, - R2) + (R3- R~)} so If the condition ( 4 a ) is valid andR
(6)
0 0 0 = R 2 -= R 3 = R 4 = R U ( p ) = IR ° Sp.
(7)
The S sensitivity depends on the geometry and the elasticity moduli of the diaphragm. The most important parameters of the sensors are the sensitivity, the linearity, the zero-point temperature drift. Analising the (6) relation it is shown that the linearity is depending on the uniformity of resistances and the gauge factor. (The bridge must be exactly balanced and the resistances well positioned.) For the investigation on thermal behavior of the offset voltage, (4) can be differentiated with respect to temperature [6]. If the bridge - resistances are equal, dU(0) = {0.25 (TCR 1 - TCR 2 ) + 0.25 (TCR 3 - TCR 4 )} dTU exc
(8)
Thick-film pressure sensors
169
or
dr(O)
(9)
< 0 . 5 (ATCR)max,
dTU exc where
I dRi TCR i = _ _ - - . R i dT The thermal drift can be characterised by
dU(0)
(10)
dTUrs where UFS is the full-scale output,
UFS = Uex c SPma x = Uexc (7 g p m a x , Ea K is a constant depending of the geometry of the diaphragm a the thickness o f the diaphragm. For comparison of different sensors we introduce a quality-factor QT [7]
QT =
G
Ea ( ATCR ) max
(10) can be written dU
dTU Fs
1
0.5
QT KPmax
In our department there are developed sensors from cermet and from polymer thick-film resistors. The cermet on alumina, - the polymer on FR4 glass-epoxy [8] substrate. The main parameters of the sensors are shown in Table 1. Table 1. Mainparametersof cermetand polymerthick-filmresistorsand the calculatedpressuresensorquality faclors
GF Substrate E (N/ram2) a (ram) (substratethickness) TCR (ppmPC) dTCR (ppmPC) StabilityR/R (%) 6(zlR / R) (%) Qr ( 10-3 K m/N) Q s (10-5 m / N)
Cermet thick-films
PTF (typical)
High reliability PTF of EMCA [8]
10 96% alumina 3.3 × 10~ 0.6 Jr 50 4-5 0.3 (1000 h, 150 °C) 0.03 10 16.8
10 FR4 (glass-epoxi) 2.1 × 10~ 0.2 -----500 ±50 5 (1000 h, 85 °C) 0.5 48 48
10 FR4 (glass-epoxi) 2.1 × 104 0.2 -- 200 4-20 0.5% (1000 h, 85 °C) 0.05 120 480
We can see that polymer film sensor has a much higher quality factor than the cermet one and therefore it can be made for lower nominal pressure with similar or better performance.
170
G. H A R S A N Y I a n d
E. H A H N
3. E X P E R I M E N T A L
RESULTS
In o u r experiments w e used the sensor g e o m e t r y s h o w n in Fig. 1. The m o s t important technological parameters of the cermet and the P T F s e n s o r types are s u m m a r i z e d in Table 2, Table 2. Technological parametersof the pressure sensor membranes Properties of the membrane
Sensor type
Substrate material Substrate thickness (ram) Substrate radius (ram) Conductor type Resistor type Drying Curing
Cermet
PTF
96% alumina 0.6 25,4 DP9473 Pd-Ag R H= 10 k.Q DP1441 150 °C/15 rain 850 °C/10 min
FR4 (epoxy-glass) 0.2 25.4 Tin-coated copper RI~ = 10 k.Q Hungarianproduct 120 °C/15 min 180 °C/120 min
Uo(rnv)l 250 --
iPTF
e35
200 M E M B R A N E ~ PACKING ~ ,"-1 RING ~ f~ | '~ J
U = 10V
150 - i ' J 100 ~ i
t p.. ¢q
50 o18 Fig. 2. Mechanical structure of the pressure sensors
i
0
CERMET
2
4
6
10
Fig, 3. Typical characteristics of the cermet and polymer thick-film pressure sensors
Table 3. Technical characteristicsof the thick-film pressure sensors
Full-scale pressure(Pa) Supply voltage (V) Zero output (mV) Full scale output (F. S.) (mV) Response time (ms) Non-linearity and hysteresis Working temperature(°C) Additional temperature failure Overpressure Long-term drift (1000 h, 85 °C)
8
Cermet thick-film
PTF
106 10 -+0.1 50 <5 <0.5% F.S. - 25-100 < +-0.05% F.S.PC 1.5 times F.S. < 0.5%
2 × 105 10 +0.1 250 <5 <0.1% F. S. 0-75 < +0.07% F. S./°C 1.5 times F. S, -<0.5%
Thick-film pressure sensors
171
The diaphragms were attached on screwed metal frames that were fastened in metal cases as shown in Fig. 2. Fig. 3 shows typical voltage outputs of the sensors supplied with 10 V. The main technical characteristics of the sensors are summarized in Table 3. The results correspond to our theoretical expectations.
4. C O N C L U S I O N S The theoretical analysis has shown that there is a possibility to produce PTF pressure sensors with high sensitivity and good properties even using polymer resistor parts that are not very reliable. In our experiments we have made a comparison between cermet and polymer pressure sensors using the same geometry. We found according to the theory, the PTF sensors have much higher sensitivity and with a proper choice of nominal pressure value they have as good relative thermal characteristics as the cermet ones, with much lower cost.
REFERENCES [1] F. Forlani:Proc. 4thEuropean HybridMicroelect. 1983.pp. 165-177. [2] G. Harcdmyi-S.K6ti:Proc. Int. Spring Sere. on Electronics Techn. 1987.pp. 88-101. [3] V. Zs. Illyefalvi:Proc. 8th European HybridMicroelect, 1991.pp. 17-28 [4] R. Dell"Acqua-G.Dell' Orto,P. Vicini:Proc. 3th E. H. M. 1981. pp. 121-123. [5] E. Hahn--K.Klinger:Mikrotechnika 27. 1988. pp. 132-133. [6] G. I--Iars~tnyi-M.R~zey: Actapolytech Prague 3. 1989.pp. 37-42. [7] G. Hars~nyi:Sensors and Actuators A 25--27. 1991. pp. 853-857. [8] G. Cas~lli-V. Meroni:Proc. 5th E. H. M. 1985. pp. 528-536.
0957-4.158/93 $6.00+0.00 PergamonPressLtd
Printedin GreatBritain
THICK-FILM
PRESSURE
SENSORS
GABOR HARSASYI--EMILHAH.X" Techn. Univ. Budapest, Dept. Electr. Techn.
Abstract- A new typeof pressuresensorhas beendevelopedby takingadvantageof the piezoresistiveeffectin polymer thick-filmresistorsscreenedand curedon epoxy-glassdiaphragms.The devicelookspromisingto satisfyall the requirements:low cost, high sensitivityand low thermaldrift. The piezoresistiveeffectin polymerthick-film(PTF) resistorsis discussedin comparisonwithcermetthick-films.The possibilityof makinga high-performancePTF pressure sensoris theoreticallysupported.Characteristicsof the newlydevelopedpolymerthick-filmsensorare described. In a limitedtemperaturerange, the new sensorhas parametersthat are not worsethan those of the cermetthick-film pressuresensorand at a muchreducedcost. 1. INTRODUCTION Nowadays sensors are available for measuring a lot of parameters, but they are badly matched to the microelectronics. The sensors, produced by microelectronical methods overcome this difficulty and they are compatible with microelectronics. One of the possible solutions is the application o f the thick-film technology. It is well known that metal films and semiconductor resistors are characterized by a resistance variation proportional to the applied stress, but more recently, it has been revealed that thick-film resistors show a notable sensitivity to deformation [1-4]. The good sensitivity of the thick-film resistors to deformation together with their long term stability, small temperature coefficient and their cheap production cost have suggested, to develop a new pressure sensor based on thick-film piezoresistive effect. The paper describes the sensors developed at TU Budapest Dept. Electronics Technology.
2. SOME THEORETICAL CONSIDERATIONS [5] The sensing element of the pressure sensor consists o f a circular edge - clamped ceramic or epoxy-glass diaphragm on which four thick-film resistors connected in a W e a t h s t o n e - b r i d g e configuration are screened and fired Fig. 1.
UR
1
o
Fig. 1. Structure and circuit connection o f the ttu'ck-filrn pressure sensor membranes
167
168
G. HARS,/~NYI and E. HAHN
The measured pressure induces a strain on the diaphragm and the resistors change their values. The change can be expressed by the tensor equation: Ap --= P
nT,
(1)
where O is the resistivity, rrthe piezoresistivity tensor and T the tensor of mechanical stress. If the resistors are made in the form of thick- (or thin-) film (1) can be written in very simple form: ap - -
=
(2)
Ge,
P e is the strain, G the so called gauge factor. By low deflexion of the diaphragm (2) transform to Ap
AR
. . . . p R
Sp,
(3)
where p is the pressure, S the "sensitivity" of the resistor. The bridge may be excited by constant voltage or constant current, The output-voltage of the bridge: U=
I
ZR
{R1R 2 - R 2 R 4 } .
(4)
If p = 0 and the bridge is balanced (R 1R 3 = R 2 R 4 ).
If p ,
o,
V ( 0 ) = 0.
(4a)
~i = R ° (1+ So)
(5)
and
'{(R°R° _ RoRo)+ 2 (RoRo + RoRo)
+ tR°R° - oRo) S202 }
2Ri + {(R, - R2) + (R3- R~)} so If the condition ( 4 a ) is valid andR
(6)
0 0 0 = R 2 -= R 3 = R 4 = R U ( p ) = IR ° Sp.
(7)
The S sensitivity depends on the geometry and the elasticity moduli of the diaphragm. The most important parameters of the sensors are the sensitivity, the linearity, the zero-point temperature drift. Analising the (6) relation it is shown that the linearity is depending on the uniformity of resistances and the gauge factor. (The bridge must be exactly balanced and the resistances well positioned.) For the investigation on thermal behavior of the offset voltage, (4) can be differentiated with respect to temperature [6]. If the bridge - resistances are equal, dU(0) = {0.25 (TCR 1 - TCR 2 ) + 0.25 (TCR 3 - TCR 4 )} dTU exc
(8)
Thick-film pressure sensors
169
or
dr(O)
(9)
< 0 . 5 (ATCR)max,
dTU exc where
I dRi TCR i = _ _ - - . R i dT The thermal drift can be characterised by
dU(0)
(10)
dTUrs where UFS is the full-scale output,
UFS = Uex c SPma x = Uexc (7 g p m a x , Ea K is a constant depending of the geometry of the diaphragm a the thickness o f the diaphragm. For comparison of different sensors we introduce a quality-factor QT [7]
QT =
G
Ea ( ATCR ) max
(10) can be written dU
dTU Fs
1
0.5
QT KPmax
In our department there are developed sensors from cermet and from polymer thick-film resistors. The cermet on alumina, - the polymer on FR4 glass-epoxy [8] substrate. The main parameters of the sensors are shown in Table 1. Table 1. Mainparametersof cermetand polymerthick-filmresistorsand the calculatedpressuresensorquality faclors
GF Substrate E (N/ram2) a (ram) (substratethickness) TCR (ppmPC) dTCR (ppmPC) StabilityR/R (%) 6(zlR / R) (%) Qr ( 10-3 K m/N) Q s (10-5 m / N)
Cermet thick-films
PTF (typical)
High reliability PTF of EMCA [8]
10 96% alumina 3.3 × 10~ 0.6 Jr 50 4-5 0.3 (1000 h, 150 °C) 0.03 10 16.8
10 FR4 (glass-epoxi) 2.1 × 10~ 0.2 -----500 ±50 5 (1000 h, 85 °C) 0.5 48 48
10 FR4 (glass-epoxi) 2.1 × 104 0.2 -- 200 4-20 0.5% (1000 h, 85 °C) 0.05 120 480
We can see that polymer film sensor has a much higher quality factor than the cermet one and therefore it can be made for lower nominal pressure with similar or better performance.
170
G. H A R S A N Y I a n d
E. H A H N
3. E X P E R I M E N T A L
RESULTS
In o u r experiments w e used the sensor g e o m e t r y s h o w n in Fig. 1. The m o s t important technological parameters of the cermet and the P T F s e n s o r types are s u m m a r i z e d in Table 2, Table 2. Technological parametersof the pressure sensor membranes Properties of the membrane
Sensor type
Substrate material Substrate thickness (ram) Substrate radius (ram) Conductor type Resistor type Drying Curing
Cermet
PTF
96% alumina 0.6 25,4 DP9473 Pd-Ag R H= 10 k.Q DP1441 150 °C/15 rain 850 °C/10 min
FR4 (epoxy-glass) 0.2 25.4 Tin-coated copper RI~ = 10 k.Q Hungarianproduct 120 °C/15 min 180 °C/120 min
Uo(rnv)l 250 --
iPTF
e35
200 M E M B R A N E ~ PACKING ~ ,"-1 RING ~ f~ | '~ J
U = 10V
150 - i ' J 100 ~ i
t p.. ¢q
50 o18 Fig. 2. Mechanical structure of the pressure sensors
i
0
CERMET
2
4
6
10
Fig, 3. Typical characteristics of the cermet and polymer thick-film pressure sensors
Table 3. Technical characteristicsof the thick-film pressure sensors
Full-scale pressure(Pa) Supply voltage (V) Zero output (mV) Full scale output (F. S.) (mV) Response time (ms) Non-linearity and hysteresis Working temperature(°C) Additional temperature failure Overpressure Long-term drift (1000 h, 85 °C)
8
Cermet thick-film
PTF
106 10 -+0.1 50 <5 <0.5% F.S. - 25-100 < +-0.05% F.S.PC 1.5 times F.S. < 0.5%
2 × 105 10 +0.1 250 <5 <0.1% F. S. 0-75 < +0.07% F. S./°C 1.5 times F. S, -<0.5%
Thick-film pressure sensors
171
The diaphragms were attached on screwed metal frames that were fastened in metal cases as shown in Fig. 2. Fig. 3 shows typical voltage outputs of the sensors supplied with 10 V. The main technical characteristics of the sensors are summarized in Table 3. The results correspond to our theoretical expectations.
4. C O N C L U S I O N S The theoretical analysis has shown that there is a possibility to produce PTF pressure sensors with high sensitivity and good properties even using polymer resistor parts that are not very reliable. In our experiments we have made a comparison between cermet and polymer pressure sensors using the same geometry. We found according to the theory, the PTF sensors have much higher sensitivity and with a proper choice of nominal pressure value they have as good relative thermal characteristics as the cermet ones, with much lower cost.
REFERENCES [1] F. Forlani:Proc. 4thEuropean HybridMicroelect. 1983.pp. 165-177. [2] G. Harcdmyi-S.K6ti:Proc. Int. Spring Sere. on Electronics Techn. 1987.pp. 88-101. [3] V. Zs. Illyefalvi:Proc. 8th European HybridMicroelect, 1991.pp. 17-28 [4] R. Dell"Acqua-G.Dell' Orto,P. Vicini:Proc. 3th E. H. M. 1981. pp. 121-123. [5] E. Hahn--K.Klinger:Mikrotechnika 27. 1988. pp. 132-133. [6] G. I--Iars~tnyi-M.R~zey: Actapolytech Prague 3. 1989.pp. 37-42. [7] G. Hars~nyi:Sensors and Actuators A 25--27. 1991. pp. 853-857. [8] G. Cas~lli-V. Meroni:Proc. 5th E. H. M. 1985. pp. 528-536.