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Temperature stable piezoelectric substrates for SAW gas sensors
B
ELSEVIER
CHEM,~CAL
Sensors and Actuators B 35 (1996) 141-145
Temperature stable piezoelectric substrates for SAW gas sensors l o s h u a J. Caron a,*, Reichl B. Haskell a, Jeffrey C. Andle b, John F. Vetelino c aSensor Research & Development Corp., P.O. Box 191, Orono, ME 04473, USA bBIODE, Inc., 20akwood Road, Cape Elizabeth, ME 0410Z USA CLaboratoryfor Su .rfaceScience and Technology, University of Maine, Orono, ME 04469, USA
Abstract A systematic search of several surface acoustic wave (SAW) substrates and crystallographic orientations has been conducted in an effort to identify temperature compensated cuts. As a result of this search, a sequence of crystallographic orientations along the rotated Y-cut (RYC) in quartz has been identified as appropriate for operation over a broad range of temperatures. Theoretical calculations, in fact, show that different orientations along RYC quartz provide compensation at temperatures ranging from -35°C to over 500°C, thereby allowing one to select an appropriate temperature stable orientation for a particular application. Experimental measurements performed on selected RYCs in quartz have confirmed these predictions of temperature stability.
Keywords: Piezoelectric substrates; SAW gas sensors
1. Introduction
Sensors for the detection of a wide variety of gases can be realized by combining SAW delay lines with chemically sensitive thin films. Often, however, these devices must be operated at elevated temperatures where the films are most sensitive, and well-known temperature stable substrates such as ST quartz can no longer compensate for temperature fluctuations. An unsensitized or passivated reference delay line is often used in an effort to eliminate this problem, but localized temperature gradients and differences in the temperature behavior of the two delay lines still cause undesirable instability. If frequency stability is required, elaborate, expensive, and bulky ovenized systems must be utilized to maintain a precise temperature around the sensor. SAW substrates which are temperature compensated at these elevated temperatures would obviously improve matters dramatically. In the present paper a systematic search of various orientations of quartz has been conducted in order to identify temperature stable substrates for SAW gas sensors. In particular, theoretical calculations have been performed * Corresponding author.
0925-4005196/$15.00 © 1996 Elsevier Science S.A. All rights reserved P l i S0925-4005(96)02028-X
followed by experimentzl investigations on selected rotated Y cuts (RYC) in quartz. 2. Theory The oscillation frequency of a SAW delay line is determined primarily by the time required for the SAW to traverse the distance from the input transducer to the output transducer. This parameter governs the closed-loop phase of the delay line, thus controlling the resonant frequency. This delay time, r, is both a function of the SAW velocity, vs, and the distance, l, that the SAW must travel. In most piezoelectric materials, however, both of these parameters are extremely sensitive to temperature. Thus, the overall temperature coefficient of delay (TCD), which is equal and opposite the temperature coefficient of oscillation frequency (TCF), is a function of the temperature coefficient of velocity (TCV) as well as the coefficient of thermal expansion, a, as shown in the following: TCD = -TCF -
-
m
l Or -
-
-
r OT
"-
I Ol
10v s
"l OT
v s OT
=a-TCV
(1)
Tim SAW velocity, vs, is determined by the elastic con-
142
J.J. Caron et al. / Sensors and Actuators B35-36 (1996) t41-145
500 E ~¢~,
0
r~
-500
t,_
.=_ -1000 ¢:
.~ .15oo ¢G ,w
.2000 r . _ . . _ 2 6 ° RYC Quartz
U,,
.2500 .100
I
0
100
200
300
400
Temperature (°C) Fig, I. Theoretical fractional change in frequency, Aft/; for ST (42,75 ° RYe) and 26 ° RYe quartz as a function of temperature.
stants, c 0, the piezoelectric constants, e~,t, the dielectric constants, ~,,,,,, and by the material density, ,o. Therefore, once the temperature dependence of each of these parameters is determined, the SAW velocity at any temperature can be calculated by adjusting each constant with a simple Taylor Series expansion as follows: oo
x(T)=
x(To)+ ~_~ I O"x(To)(T-To)" .=in! OT"
(2)
where x(To) represents any one of the material parameters measured at room temperature. The temperature dependences of the elastic, piezoelectric, dielectric, and thermal expansion coefficients are readily available for most common SAW substrate materials [I]. The temperature dependence of the density can then be calculated from the thermal expansion coefficients as follows:
0p
O-'T= -(all
+a22 +G33)P(T°)
ments. The RYC family was chosen due to the fact that previous work [4] has shown that this cut is characterized by many temperature stable orientations, provides adequate piezoelectric coupling, and zero power flow angle. Fig. 1 shows the fractional change in frequency, Aflf, as a function of temperature for two orientations in this family. ST quartz, a member of the RYC family (~, 0, Ip [2] equal 0 °, 132.75 °, and 0 °, respectively), displays its wellknown turnover temperature, where the slope of the Af/f curve is zero, at around 25°C. 26 ° RYC quartz (~, 0, ~p equal 0 °, 116 °, and 0 ° respectively), on the other hand, exhibits a turnover temperature near 200°C. Other orientations in this cut can have much higher or lower turnover temperatures. Fig. 2, in fact, shows that the calculated turnover temperatures for the entire RYC family can vary from -35°C to about 585°C. In reality, of course, aquartz devices are limited to temperatures below 565°C due to the fl phase transition which occurs at that temperature. For any desired turnover temperature below this, however, one could easily select the appropriate cut. In gas sensing applications, a dual delay line configuration is often utilized to provide temperature compensation. In this configuration, one delay line is coated with a film sensitive to the gas of interest, while the other is left bare. The adsorption of this gas onto the film changes the mass, elasticity and/or conductivity of the film, which ultimately results in a shift in the oscillation frequency of that delay line. The second delay line is used as a reference to cancel extraneous frequency shifts due to environmental effects such as temperature which, in theory, should affect both delay lines equally. In reality, however, the mass, elasticity and conductivity of the sensing film can cause a dramatic difference in the respective turnover temperatures of the two delay lines. In order to minimize this effect, a passive film, such as aluminum, can be 600 500
(3)
? Finally, the TCD (or TCF) can be calculated for an arbitrary orientatior, by performing the appropriate Euler transformations [2] to the elastic, dielectric, piezoelectric, and thermal expansion coefficient matrices and by using
Eq, (I),
...,.
2
6 ;> 0
E
400 300 200 100
3. T h e o r e t i c a l results 0
Material constants for quartz and their first, second and third order temperature coefficients were measured and tabulated by Bechmann et al. [31 in 1962. Using these constants and the necessary software associated with SAW theory, the temperature coefficient of frequency was calculated for the entire RYC family in 5 ° incre-
-100 -90
-60
-30
0
30
60
90
Rotation Angle (o) Fig. 2. Theoretical variation turnover temperature as a function of the rotation angle for RYC quartz.
143
J.J. Caron et al. / Sensors and Actuators B3S-36 (1996) 141-145
added to the reference delay line, such that it affects that delay line's temperature behavior in a similar fashion to the sensing film. Previous work in SAW gas sensors [5-7] has shown that a hydrogen sulfide (H2S) sensor can be realized by applying a thin film of tungsten trioxide (WO3) to a SAW delay line operated at 200°C. Calculations were performed to idendfy a substrate for which the turnover temperature of the delay line with the WO3 film would be 200°C. The WO3 film thickness was chosen to be 0.5% of the SAW wavelength (at 261 MHz, the operation frequency used for the experimental portion of this work, this corresponds to about 600/~). Calculations were also performed in order to identify an aluminum film of appropriate thickness to match the reference delay line's turnover temperature to that of the sensing delay line. Theoretical calculations predicated that the WO3 fihn would lower the turnover temperature of the sensing delay line by about 20°C. As predicted by the graph in Fig. 2, 24 ° RYC quartz exhibits a turnover temperature of about 220°C. Fig. 3 shows the theoretically calculated fractional change in oscillation frequency versus temperature for 24 ° RYC quartz, as well as A f l f curves for the same substrate with the WO3 film and aluminum films of various thicknesses. As the figure shows, the WO3 indeed lowers the turnover temperature by about 20°C, to just below 200°C. Likewise, an aluminum fihn of equal thickness has almost the same effect, also lowering the turnover temperature to 200°C. Thicker films lower the turnover temperature even more. Fig. 4 demonstrates, in fact, that nearly any desired turnover temperature over a broad range is achievable by varying the thickness of this film.
220 ,....,
210 o.. .~ 200 -,= .= 190 ~. 18o
E
~.
~7o
o 160 E 150 F-
140 ._u '130 o. 120
0
0.005
0.01
0.015
0.02
Ahuninum Film Thickness (Wavelengths) Fig. 4. Variation of the theoretically predicted turnover temperature with aluminum film thickness on 24° RYC quartz.
4. Experimental results
In an extbrt to verify the theoretical predictions, experimental measurements were performed on a 261 M H z dual delay line S A W device fabricated on a 24 ° R Y C quartz substrate with simple split-fingerintcrdigitaltransducers while temperature was varied. On one delay path a 1500 A aluminum film was deposited, while the other delay line was leftbare. The fractionalchange in oscillation frequency as a function of temperature for both of these delay lines is plotted in Fig. 5. Although the theoreticallypredicted turnover temperature was about 220°C, the actual experimentally measured turnover temperature was about 70°C higher. Keeping in mind this discrepancy between theory and experiment, an identical261 M H z dual delay line device with a similar aluminum film over the reference path, was fabricated on a 27 ° R Y C quartz substrate.This substrate was chosen after examining the error between theoreti-
500 E g
4OO
400
E 300
g, = 200
200
100
i
"=
300
r~
100
.E
0
~-100
0
--r~ -200
-100
•=O -300 100
150
200
250
Tem perat ure (°C)
Fig. 3. Theoretically predicted fractional change in frequency as a function of temperature for 24° RYC quartz, bare and with var~,ousfilm overlays. The fihn thickness is expressed in terms of a percentageof the SAW wavelength.
~" -400 150
200
250 300 Temperature (°C)
350
400
Fig. 5. Experimentally measured fractional change in frequency with temperature for bare and metallized (1500/~ of aluminum) delay lines on 24° RYC qua:tz.
144
J.J. Caron et al. / Sensors and Actuators B35-36 (1996) 141-145
205
800
E
600 400 200
•.
0
= 200 4o0
' -"-" Metalized De_~Yune
r~
~00 ~
~00
100
200
300
400
195
Temperature (°C)
- - - -
:
0
Fig. 6. Experimentally measured fractional change in frequency with temperature for bare and metallized (1500 A of aluminum) delay lines on 27° RYC quartz.
cally predicted and experimentally measured values of turnover temperature on 24 ° RYC quartz and choosing a more appropriate cut using the curve shown in Fig. 2. The theoretically predicted turnover temperature for the bare delay path was about 150°C, and it was assumed that the experimentally measured value would again be 70°C higher, or about 220°C. Experimental measurements verified that this assumption was correct, as shown in Fig. 6. These results also showed, however, that the relatively thick aluminum film (1500.~, or about 1.25% of the SAW wavelength) lowered the turnover temperature of the reference delay line to about 180°C, well below 200°C. This thick aluminum film was etched away and replaced with a much thinner aluminum film (500 .~). A
)
10
I
I
I
20 30 40 Time (minutes)
50
60
Fig. 8. The temperature variation to which the ST-cut and 27° RYC quartz substrates during stability tests.
500 • WO3 film was also sputtered onto the sensing delay path, and the desired turnover temperatures of 200°C were finally achieved for both delay lines, as shown in Fig. 7. Finally, temperature stability of the 27° RYC quartz SAW delay line oscillator was measured and compared to that of a more traditional SAW sensor fabricated on STquartz. Both devices utilized an identical transducer configuration and an identical WO3 film, but the ST-quartz device lacked the aluminum film over the reference delay line. Both devices were subjected to a temperature profile which varied between 198°C and 202°C every 5 min, for ! h, as shown in Fig. 8. In Figs. 9 and I0, the frequencies of the WO3-coated
150
150 I ----eemeeth
125
i
i=o
I
100
,S
75
I= J
l 30
150
175
200
225
250
Temperature (°C)
0~
o
1o
I
I
I
I
2O
3O
4O
5O
6O
Vlm(n~n) Fig, 7, Experimentally measured fractional change in frequency with t e ~ for delay lines coated with 500 A WO3 and 500 A alimunum on 27° RYC quartz.
Fig. 9. Fractional chamge in frequency of the WO3-coated and bare delay line oscillators on ST-cut quartz subjected to the temperature profile of Fig. 8.
J.J. Caron et ai. / Sensors and Actuators 835-36 (1996) 141-145
(sensing) and bare (reference) delay lines of the ST-cut and 27 ° RYC quartz devices are presented as a function of time. Results for each substrate are plotted with the same y-axis scale to emphasize the relative improvement in stability from ST-cut to 27 ° RYC quartz. Finally, Fig. 11 is a plot of the difference frequency of both dual delay line SAW devices. Again, the 27 ° RYC quartz device demonstrate superior temperature stability to the ST-cut de.vice.
145
20
[ _ :o 2~'k~'c Quartz
5. Conclusions
-,,.-ST-Quartz
Although the experimental results differed somewhat from the theoretical predictions, the main objective of this work, to identify SAW substrates which demonstrate temperature compensation at temperatures higher than room temperature, was accomplished. Inaccuracies in the theoretical calculations can be attributed to insufficient material constant data. While third-order temperature coefficients for the material constants were used, calculation of these constants at such large deviations from room temperature requires the use of fourth- or possibly even fifth-order temperature coefficients. These data are simply not available. Furthermore, the temperature coefficient data used in this work were originally compiled for the modeling of bulk waves with predominantly shear components. Because SAWs on RYC quartz have substantial longitudinal components, some accuracy may be lost. The results show that high temperature stable orientations of quartz do exist in the RYC family of cuts. For SAW gas sensors, where the sensing film thickness is usually predetermined and independent of substrate choice, a desired turnover temperature can be attained by selecting an appropriate orientation. The reference path
0
10
20
30
40
50
60
Time train)
Fig. ! !. Difference frequencies of SAW gas sensors fabricated on STcut and 27" RYC quartz substrates, subjected to the temperature profile of Fig. 8.
can then be tailored to exhibit the same turnover temperature by depositing a metal film of an appropriate thickness over its delay path. When operated at the higher temperatures, SAW devices fabricated on these cuts show dramatically improved stability over ST quartz.
Acknowledgements The authors would like to thank C.S. Lam of Vectron, Inc. for contributing the RYC quartz wafers used in this work. This work was also supported in part by NSF Grant No. EEC-953137B.
References 150
120 WOI Path
I .S
] I 0
0
I
I
I0
20
I
30 ~(n~)
~
,
[
40
50
60
Fig. 10. Fractional change in frequency of the WO3-coated and aluminum.coated delay line oscillators on 27° RYC quartz subjected to the temperature profile of Fig. 8.
[i] A.J. Slobodnik, The temperature coefficients of acoustic surface wave velocity and delay on lithium niobate, lithium tantalate, quartz, and tellurium dioxide, AFCRL-72-0082, Bedford, MA, 1972. [2] B.A. Auld, Acouslic Fields and Waves in Solids, Vol. 11, 2nd edn., Krieger, Malabar, FL, 1990, pp. 83-84. [3] R. Bechmann, A.D. Ballato and T.J. Lukaszek, Higher order temperature coefficients of the elastic stiffnesses and compliances of alpha-quartz, USAELRDL Technical Report 2261, Fort Monmouth, NJ, 1963. [4] R. Raghavan and J.F. Vetelino, Temperature compensation with metallic overlay films on quartz, Ultrasonics Symposium Proc., !979, pp. 606-61 !. [5] J. Vetelino, R.K. Lade and R.S. Falconer, Hydrogen sulfide surface acoustic wave gas detector, 2nd Mtg. on Chem. Sensors, Bordeaux, France, 1986, pp. 688-69 i. [6] J.F. Vetelino, R.K. Lade and R.S. Falconer, H2S surface acoustic wave ~:a~or, Trans. IEEE UFFC, 34 (1987) 157-161. [7] .LD ~alipea~0 R.S. Falconer, J.C. Andle, E.L. Wittman, M.G. Schweyer, J.C. Caron and J.F. Vetelino, Theory, design and operation of a surface acoustic wave hydrogen sullide microsensor, Sensors and Actuators B, 24-25 (1995) 49-53.
ELSEVIER
CHEM,~CAL
Sensors and Actuators B 35 (1996) 141-145
Temperature stable piezoelectric substrates for SAW gas sensors l o s h u a J. Caron a,*, Reichl B. Haskell a, Jeffrey C. Andle b, John F. Vetelino c aSensor Research & Development Corp., P.O. Box 191, Orono, ME 04473, USA bBIODE, Inc., 20akwood Road, Cape Elizabeth, ME 0410Z USA CLaboratoryfor Su .rfaceScience and Technology, University of Maine, Orono, ME 04469, USA
Abstract A systematic search of several surface acoustic wave (SAW) substrates and crystallographic orientations has been conducted in an effort to identify temperature compensated cuts. As a result of this search, a sequence of crystallographic orientations along the rotated Y-cut (RYC) in quartz has been identified as appropriate for operation over a broad range of temperatures. Theoretical calculations, in fact, show that different orientations along RYC quartz provide compensation at temperatures ranging from -35°C to over 500°C, thereby allowing one to select an appropriate temperature stable orientation for a particular application. Experimental measurements performed on selected RYCs in quartz have confirmed these predictions of temperature stability.
Keywords: Piezoelectric substrates; SAW gas sensors
1. Introduction
Sensors for the detection of a wide variety of gases can be realized by combining SAW delay lines with chemically sensitive thin films. Often, however, these devices must be operated at elevated temperatures where the films are most sensitive, and well-known temperature stable substrates such as ST quartz can no longer compensate for temperature fluctuations. An unsensitized or passivated reference delay line is often used in an effort to eliminate this problem, but localized temperature gradients and differences in the temperature behavior of the two delay lines still cause undesirable instability. If frequency stability is required, elaborate, expensive, and bulky ovenized systems must be utilized to maintain a precise temperature around the sensor. SAW substrates which are temperature compensated at these elevated temperatures would obviously improve matters dramatically. In the present paper a systematic search of various orientations of quartz has been conducted in order to identify temperature stable substrates for SAW gas sensors. In particular, theoretical calculations have been performed * Corresponding author.
0925-4005196/$15.00 © 1996 Elsevier Science S.A. All rights reserved P l i S0925-4005(96)02028-X
followed by experimentzl investigations on selected rotated Y cuts (RYC) in quartz. 2. Theory The oscillation frequency of a SAW delay line is determined primarily by the time required for the SAW to traverse the distance from the input transducer to the output transducer. This parameter governs the closed-loop phase of the delay line, thus controlling the resonant frequency. This delay time, r, is both a function of the SAW velocity, vs, and the distance, l, that the SAW must travel. In most piezoelectric materials, however, both of these parameters are extremely sensitive to temperature. Thus, the overall temperature coefficient of delay (TCD), which is equal and opposite the temperature coefficient of oscillation frequency (TCF), is a function of the temperature coefficient of velocity (TCV) as well as the coefficient of thermal expansion, a, as shown in the following: TCD = -TCF -
-
m
l Or -
-
-
r OT
"-
I Ol
10v s
"l OT
v s OT
=a-TCV
(1)
Tim SAW velocity, vs, is determined by the elastic con-
142
J.J. Caron et al. / Sensors and Actuators B35-36 (1996) t41-145
500 E ~¢~,
0
r~
-500
t,_
.=_ -1000 ¢:
.~ .15oo ¢G ,w
.2000 r . _ . . _ 2 6 ° RYC Quartz
U,,
.2500 .100
I
0
100
200
300
400
Temperature (°C) Fig, I. Theoretical fractional change in frequency, Aft/; for ST (42,75 ° RYe) and 26 ° RYe quartz as a function of temperature.
stants, c 0, the piezoelectric constants, e~,t, the dielectric constants, ~,,,,,, and by the material density, ,o. Therefore, once the temperature dependence of each of these parameters is determined, the SAW velocity at any temperature can be calculated by adjusting each constant with a simple Taylor Series expansion as follows: oo
x(T)=
x(To)+ ~_~ I O"x(To)(T-To)" .=in! OT"
(2)
where x(To) represents any one of the material parameters measured at room temperature. The temperature dependences of the elastic, piezoelectric, dielectric, and thermal expansion coefficients are readily available for most common SAW substrate materials [I]. The temperature dependence of the density can then be calculated from the thermal expansion coefficients as follows:
0p
O-'T= -(all
+a22 +G33)P(T°)
ments. The RYC family was chosen due to the fact that previous work [4] has shown that this cut is characterized by many temperature stable orientations, provides adequate piezoelectric coupling, and zero power flow angle. Fig. 1 shows the fractional change in frequency, Aflf, as a function of temperature for two orientations in this family. ST quartz, a member of the RYC family (~, 0, Ip [2] equal 0 °, 132.75 °, and 0 °, respectively), displays its wellknown turnover temperature, where the slope of the Af/f curve is zero, at around 25°C. 26 ° RYC quartz (~, 0, ~p equal 0 °, 116 °, and 0 ° respectively), on the other hand, exhibits a turnover temperature near 200°C. Other orientations in this cut can have much higher or lower turnover temperatures. Fig. 2, in fact, shows that the calculated turnover temperatures for the entire RYC family can vary from -35°C to about 585°C. In reality, of course, aquartz devices are limited to temperatures below 565°C due to the fl phase transition which occurs at that temperature. For any desired turnover temperature below this, however, one could easily select the appropriate cut. In gas sensing applications, a dual delay line configuration is often utilized to provide temperature compensation. In this configuration, one delay line is coated with a film sensitive to the gas of interest, while the other is left bare. The adsorption of this gas onto the film changes the mass, elasticity and/or conductivity of the film, which ultimately results in a shift in the oscillation frequency of that delay line. The second delay line is used as a reference to cancel extraneous frequency shifts due to environmental effects such as temperature which, in theory, should affect both delay lines equally. In reality, however, the mass, elasticity and conductivity of the sensing film can cause a dramatic difference in the respective turnover temperatures of the two delay lines. In order to minimize this effect, a passive film, such as aluminum, can be 600 500
(3)
? Finally, the TCD (or TCF) can be calculated for an arbitrary orientatior, by performing the appropriate Euler transformations [2] to the elastic, dielectric, piezoelectric, and thermal expansion coefficient matrices and by using
Eq, (I),
...,.
2
6 ;> 0
E
400 300 200 100
3. T h e o r e t i c a l results 0
Material constants for quartz and their first, second and third order temperature coefficients were measured and tabulated by Bechmann et al. [31 in 1962. Using these constants and the necessary software associated with SAW theory, the temperature coefficient of frequency was calculated for the entire RYC family in 5 ° incre-
-100 -90
-60
-30
0
30
60
90
Rotation Angle (o) Fig. 2. Theoretical variation turnover temperature as a function of the rotation angle for RYC quartz.
143
J.J. Caron et al. / Sensors and Actuators B3S-36 (1996) 141-145
added to the reference delay line, such that it affects that delay line's temperature behavior in a similar fashion to the sensing film. Previous work in SAW gas sensors [5-7] has shown that a hydrogen sulfide (H2S) sensor can be realized by applying a thin film of tungsten trioxide (WO3) to a SAW delay line operated at 200°C. Calculations were performed to idendfy a substrate for which the turnover temperature of the delay line with the WO3 film would be 200°C. The WO3 film thickness was chosen to be 0.5% of the SAW wavelength (at 261 MHz, the operation frequency used for the experimental portion of this work, this corresponds to about 600/~). Calculations were also performed in order to identify an aluminum film of appropriate thickness to match the reference delay line's turnover temperature to that of the sensing delay line. Theoretical calculations predicated that the WO3 fihn would lower the turnover temperature of the sensing delay line by about 20°C. As predicted by the graph in Fig. 2, 24 ° RYC quartz exhibits a turnover temperature of about 220°C. Fig. 3 shows the theoretically calculated fractional change in oscillation frequency versus temperature for 24 ° RYC quartz, as well as A f l f curves for the same substrate with the WO3 film and aluminum films of various thicknesses. As the figure shows, the WO3 indeed lowers the turnover temperature by about 20°C, to just below 200°C. Likewise, an aluminum fihn of equal thickness has almost the same effect, also lowering the turnover temperature to 200°C. Thicker films lower the turnover temperature even more. Fig. 4 demonstrates, in fact, that nearly any desired turnover temperature over a broad range is achievable by varying the thickness of this film.
220 ,....,
210 o.. .~ 200 -,= .= 190 ~. 18o
E
~.
~7o
o 160 E 150 F-
140 ._u '130 o. 120
0
0.005
0.01
0.015
0.02
Ahuninum Film Thickness (Wavelengths) Fig. 4. Variation of the theoretically predicted turnover temperature with aluminum film thickness on 24° RYC quartz.
4. Experimental results
In an extbrt to verify the theoretical predictions, experimental measurements were performed on a 261 M H z dual delay line S A W device fabricated on a 24 ° R Y C quartz substrate with simple split-fingerintcrdigitaltransducers while temperature was varied. On one delay path a 1500 A aluminum film was deposited, while the other delay line was leftbare. The fractionalchange in oscillation frequency as a function of temperature for both of these delay lines is plotted in Fig. 5. Although the theoreticallypredicted turnover temperature was about 220°C, the actual experimentally measured turnover temperature was about 70°C higher. Keeping in mind this discrepancy between theory and experiment, an identical261 M H z dual delay line device with a similar aluminum film over the reference path, was fabricated on a 27 ° R Y C quartz substrate.This substrate was chosen after examining the error between theoreti-
500 E g
4OO
400
E 300
g, = 200
200
100
i
"=
300
r~
100
.E
0
~-100
0
--r~ -200
-100
•=O -300 100
150
200
250
Tem perat ure (°C)
Fig. 3. Theoretically predicted fractional change in frequency as a function of temperature for 24° RYC quartz, bare and with var~,ousfilm overlays. The fihn thickness is expressed in terms of a percentageof the SAW wavelength.
~" -400 150
200
250 300 Temperature (°C)
350
400
Fig. 5. Experimentally measured fractional change in frequency with temperature for bare and metallized (1500/~ of aluminum) delay lines on 24° RYC qua:tz.
144
J.J. Caron et al. / Sensors and Actuators B35-36 (1996) 141-145
205
800
E
600 400 200
•.
0
= 200 4o0
' -"-" Metalized De_~Yune
r~
~00 ~
~00
100
200
300
400
195
Temperature (°C)
- - - -
:
0
Fig. 6. Experimentally measured fractional change in frequency with temperature for bare and metallized (1500 A of aluminum) delay lines on 27° RYC quartz.
cally predicted and experimentally measured values of turnover temperature on 24 ° RYC quartz and choosing a more appropriate cut using the curve shown in Fig. 2. The theoretically predicted turnover temperature for the bare delay path was about 150°C, and it was assumed that the experimentally measured value would again be 70°C higher, or about 220°C. Experimental measurements verified that this assumption was correct, as shown in Fig. 6. These results also showed, however, that the relatively thick aluminum film (1500.~, or about 1.25% of the SAW wavelength) lowered the turnover temperature of the reference delay line to about 180°C, well below 200°C. This thick aluminum film was etched away and replaced with a much thinner aluminum film (500 .~). A
)
10
I
I
I
20 30 40 Time (minutes)
50
60
Fig. 8. The temperature variation to which the ST-cut and 27° RYC quartz substrates during stability tests.
500 • WO3 film was also sputtered onto the sensing delay path, and the desired turnover temperatures of 200°C were finally achieved for both delay lines, as shown in Fig. 7. Finally, temperature stability of the 27° RYC quartz SAW delay line oscillator was measured and compared to that of a more traditional SAW sensor fabricated on STquartz. Both devices utilized an identical transducer configuration and an identical WO3 film, but the ST-quartz device lacked the aluminum film over the reference delay line. Both devices were subjected to a temperature profile which varied between 198°C and 202°C every 5 min, for ! h, as shown in Fig. 8. In Figs. 9 and I0, the frequencies of the WO3-coated
150
150 I ----eemeeth
125
i
i=o
I
100
,S
75
I= J
l 30
150
175
200
225
250
Temperature (°C)
0~
o
1o
I
I
I
I
2O
3O
4O
5O
6O
Vlm(n~n) Fig, 7, Experimentally measured fractional change in frequency with t e ~ for delay lines coated with 500 A WO3 and 500 A alimunum on 27° RYC quartz.
Fig. 9. Fractional chamge in frequency of the WO3-coated and bare delay line oscillators on ST-cut quartz subjected to the temperature profile of Fig. 8.
J.J. Caron et ai. / Sensors and Actuators 835-36 (1996) 141-145
(sensing) and bare (reference) delay lines of the ST-cut and 27 ° RYC quartz devices are presented as a function of time. Results for each substrate are plotted with the same y-axis scale to emphasize the relative improvement in stability from ST-cut to 27 ° RYC quartz. Finally, Fig. 11 is a plot of the difference frequency of both dual delay line SAW devices. Again, the 27 ° RYC quartz device demonstrate superior temperature stability to the ST-cut de.vice.
145
20
[ _ :o 2~'k~'c Quartz
5. Conclusions
-,,.-ST-Quartz
Although the experimental results differed somewhat from the theoretical predictions, the main objective of this work, to identify SAW substrates which demonstrate temperature compensation at temperatures higher than room temperature, was accomplished. Inaccuracies in the theoretical calculations can be attributed to insufficient material constant data. While third-order temperature coefficients for the material constants were used, calculation of these constants at such large deviations from room temperature requires the use of fourth- or possibly even fifth-order temperature coefficients. These data are simply not available. Furthermore, the temperature coefficient data used in this work were originally compiled for the modeling of bulk waves with predominantly shear components. Because SAWs on RYC quartz have substantial longitudinal components, some accuracy may be lost. The results show that high temperature stable orientations of quartz do exist in the RYC family of cuts. For SAW gas sensors, where the sensing film thickness is usually predetermined and independent of substrate choice, a desired turnover temperature can be attained by selecting an appropriate orientation. The reference path
0
10
20
30
40
50
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Fig. ! !. Difference frequencies of SAW gas sensors fabricated on STcut and 27" RYC quartz substrates, subjected to the temperature profile of Fig. 8.
can then be tailored to exhibit the same turnover temperature by depositing a metal film of an appropriate thickness over its delay path. When operated at the higher temperatures, SAW devices fabricated on these cuts show dramatically improved stability over ST quartz.
Acknowledgements The authors would like to thank C.S. Lam of Vectron, Inc. for contributing the RYC quartz wafers used in this work. This work was also supported in part by NSF Grant No. EEC-953137B.
References 150
120 WOI Path
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Fig. 10. Fractional change in frequency of the WO3-coated and aluminum.coated delay line oscillators on 27° RYC quartz subjected to the temperature profile of Fig. 8.
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