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A versatile SAW-based sensor system for investigating gas-sensitive coatings
ELSEVIER
Sensors and Actuators
CHEMICAL
B 24-25 (1995) 54-57
A versatile SAW-based sensor system for investigating gassensitive coatings R.S. Falconer Andersen
Labomtories,
Inc., 45 Old Iron Ore Road, Blwmfield
CT 06002,
USA
Abstract Surface acoustic wave (SAW) technology is an extremely versatile technique for the development of gas sensors because of the SAW device’s sensitivity to a wide variety of material properties that can change with gas concentration. In the present work, a SAW sensor oscillator is described, which allows both of the important SAW properties, velocity and attenuation, to be easily measured. The SAW sensor can be used with a variety of gas-sensing materials to monitor any gases that interact with the sensing materials. The versatility of the sensor is demonstrated by making SAW sensors using hvo gas-sensitive films that work in different ways. A tungsten trioxide (WO,) film with electrical properties that vary with hydrogen sulfide (H,S) exposure has been used to make a SAW H2S sensor, and an organic material that sorbs solvents has been used to demonstrate a SAW acetone sensor.
1. Introduction Surface acoustic wave (SAW) devices have been examined as gas sensors since 1979 [l]. The SAW sensor is made by coating a SAW delay line with a film that reacts to the gas or gases of interest. When the film reacts with the gas, the properties of the film will change, and this variation will be translated into changes in the operation of the SAW device. The SAW sensor research performed since 1979 is reviewed in several published works [2] and will not be detailed here, apart from a brief description of the basic operation of the sensor. A general SAW sensor device is shown in Fig. 1. It includes a SAW delay line coated with a selectively sorbent film. The coated delay line is made into an oscillator using an amplifier in feedback, and the oscillating frequency is monitored using a directional coupler and a frequency counter. The frequency of
I
I
Coupler
Fig. 1. SAW sensing oscillator circuit. 092.C4005/95/$09,50 8 1995 Elsevier Science S.A. All rights reserved SSDI 0925.4005(94)01314-8
oscillation of the loop is then dependent upon the phase/time delay of the SAW device, and therefore upon the SAW velocity. The velocity, as well as the SAW attenuation, is in turn dependent upon the material properties of the coating. Frequency changes in the loop are proportional to SAW velocity changes, and the insertion loss of the sensing device is proportional to the acoustic attenuation caused by the gas-sensing film. With the appropriate choice of amplifier, this insertion loss can be monitored by measuring the gain of the feedback amplifier that maintains oscillation. The oscillating frequency of a SAW oscillator is temperature dependent because the material properties of the substrate are temperature dependent. When the oscillator is made into a sensor by the addition of a gas-sensitive coating, the temperature dependence becomes even more complicated. The frequency shift caused by the temperature-dependent variation of substrate properties can be increased or decreased by variations in the properties of the coating. In order to correct partially for sensor drift due to temperature effects, a second delay-line oscillator is sometimes used to generate a reference frequency. This reference device is identical to the sensing device except that it does not have a gas-sensitive coating. When the temperature drifts, it will create a frequency shift in the reference device, which is equal to the substrate-related shift in the sensing device. If the frequency signals are then
RX
Falconer I Sensors and Ac~untors B 24-25 (1995) 54-57
multiplied in a mixer, the output frequency will be the difference between the two individual frequencies. This difference frequency will generally be more stable with temperature, because substrate effects will be minimized. In general, the difference frequency will still be a function of temperature, because the coating properties are usually temperature dependent.
2. Theory The SAW velocity and attenuation in a layered structure such as the SAW sensor is a complicated function of the material properties of both the substrate and the coating, as well as the geometry of the media. The relevant material properties include the density, elastic moduli, viscosity, electrical conductivity, chargecarrier diffusion constant, dielectric permittivity and piezoelectric constants. In general, any or all of these properties within the film can change, thereby altering the SAW velocity and attenuation. In many cases the problem can be simplified when one or two effects predominate. An example of this situation is the simple mass-loading case, described by Wohltjen [3] for an isotropic film. In this case the SAW velocity can be considered to be a linear function of the mass absorbed by the film per unit area. If purely elastic changes in the film mechanical constants are appreciable, another term can be added to take this into account. Both of these effects are lossless and therefore do not affect the SAW attenuation. Another simplified situation occurs when changes in the electrical conductivity of the film create large changes in the SAW velocity and attenuation. Previous work [4] has shown that the effect of conductivity changes can lead to velocity changes much greater than those due to mass loading, as long as the sheet conductivity of the coating is within a range of approximately four orders of magnitude centered around a ‘resonant sheet conductivity’, which is a function of the substrate properties alone. The resistive losses in the film can also lead to significant attenuation changes under certain circumstances. The presence of both velocity and attenuation changes in a SAW sensor is very significant, because it supplies two pieces of information from a single sensor. In many cases a gas-sensitive film will react with more than one gas, and the sensor would not be able to differentiate between two gases if only the oscillating frequency were measured. Other work [5] has shown, however, that the attenuation changes are sometimes independent of the velocity changes, and by measuring both properties simultaneously two or more gases may be differentiated. While simplified explanations for SAW sensing effects can sometimes give the ability to work back from a SAW sensor response to calculate the fundamental
55
changes that take place in the coating, it is important to note that these efforts are by no means necessary for a researcher interested in developing a sensor for a specific gas. The fundamental physical and chemical processes that result in film property changes and hence SAW velocity and attenuation changes do not matter to the sensor designer. Regardless of what is happening within the film, the SAW sensor gives an easily measured response in frequency (velocity) and insertion loss (attenuation), which is a function of gas concentration. The response may or may not be linear, but in any case a calibration curve can be experimentally defined that correlates the sensor response to gas concentration. Once a suitable SAW sensor system is designed, the properties of the gas-sensitive film are responsible for such overall sensor properties as sensitivity, reproducibility, selectivity, stability and aging. Developing a practical SAW sensor then becomes a matter of developing a practical sensing film. Perhaps the greatest advantage of SAW sensors over competing sensor technologies is their versatility. The SAW velocity and attenuation in the layered structure of the piezoelectric substrate with a gas-sensitive coating are dependent upon all of the mechanical and electrical properties of the film. SAW sensors have been demonstrated that rely upon changes in film elastic constants, viscosity and electrical conductivity, in addition to the simple mass change caused by sorption or chemical reaction that must take place in any gas-film interaction. Because the sensor can detect changes in so many different film properties, the SAW sensor is ideally suited to the examination and testing of gas-sensitive films of all types.
3. Experimental At Andersen Laboratories a SAW sensor package is being developed to allow gas-sensing films to be easily tested in a SAW sensing configuration. The SAW sensor platform includes the oscillating circuitry for a dual-delay-line SAW sensor with outputs for the two fundamental oscillating frequencies, the difference frequency and the insertion loss of the sensing delay line. The SAW devices are held in the package with springloaded electrical contacts that allow easy removal of the devices for coating or replacement. The SAW oscillator circuit that has been developed uses two ampliier stages. A fixed amplification section ensures that the gain of the circuit is sufficient to maintain oscillation for the uncoated SAW device. A second amplifier stage uses logarithmic amplifiers, GEC Plessy model SL1613, to supply the gain necessary to overcome any added attenuation due to the application of the sensing film or the gas-film interactions. Logarithmic amplifiers are designed to amplify a sinusoidal
56
R.S. Falconer ! Sensors and Actuators B Z&25 (1995) 54-57
signal while generating a d.c. output signal proportional to the log of the input voltage level. Since the gain of the circuit is much greater than that needed to sustain oscillation, the final stage, or stages, of the logarithmic amplifier section will be in saturation, ensuring that a constant voltage level is applied to the SAW device. As the insertion loss of the SAW device increases, the signal level into the logarithmic amplifiers (i.e., the signal out of the SAW device) must therefore decrease, and the d.c. output of the circuit will change in proportion to the logarithm of the SAW losses. The output voltage is then proportional to the insertion loss, in dB, of the SAW device. The excess gain of the oscillator circuit allows stable operation of the device with as much as 55 dB of attenuation due to the gas-sensing film. The SAW devices used for this work were fabricated on ST-cut quartz with split-finger interdigital transducers (IDTs) of gold. The single-mode devices were designed to oscillate at approximately 100 MHz. The devices, with impedance-matching circuitry, were mounted on PC board, and the amplifier circuitry was on a separate PC board. The boards were connected with coaxial cable. The H,S sensor was mounted on a heater in a gas-exposure chamber without any impedance-matching network.
4. Results The SAW oscillator circuit was tested to determine the calibration curve correlating the insertion loss of the SAW with the voltage output of the logarithmic amplifier stage. The test was accomplished by inserting a variable attenuator between the output of the SAW device and the input of the amplifier. The output voltage was recorded as the attenuation was increased in 2 dB increments. The results for the test are shown in Fig. 2, along with a straight line fit to the data. Preliminary gas-sensing measurements have been performed using the oscillator circuit and two SAW devices coated with different films for sensing different gases. The two coatings were chosen to illustrate the use of two different types of gas-sensitive materials deposited
Fig. 2. Excess insertion loss vs. the output voltage from the logarithmic amplifier
stage.
with different techniques in the SAW sensor configuration. The first film was a semiconducting tungsten trioxide (WO,) film with a conductivity that varies with exposure to hydrogen sulfide (H,S) gas. The second film was a vacuum grease (Apiezon H), which was used to sorb acetone vapour. The WOg film was reactively r.f. sputtered from a tungsten target in an argon/oxygen atmosphere at the University of Maine [4]. The film was 1000 A thick and was doped with gold and annealed at greater than 300 “C. The Apiezon grease was dissolved in toluene and deposited with a cotton swab at room temperature. These preliminary measurements were performed using a single sensing delay-line oscillator. Future measurements using the dual-delay-line configuration will be used to examine the amount of stabilization afforded by a reference oscillator. A typical response of the SAW H,S sensor is shown in Fig. 3. For this experiment the device was heated to 200 “C and allowed to stabilize. After the oscillation became stable, the device was allowed to operate in air for approximately 20 min to provide a baseline frequency. At time t=O the device was exposed to 10 ppm of H2S in air, and at t=30 min the gas feed was switched back to air. The Figure illustrates that the sensor responded to the gas within a few minutes and gave an easily measured frequency response. The initial response was significantly faster than the return to baseline after exposure. Because the physical and chemical processes that occur between the metal oxide film and H,S are not fully understood, the reason for the different rates for the forward and reverse reactions is not known. Previous experimental work [6] has shown, however, that the rise and fall times differ and depend on temperature. The insertion loss of the device did not change noticeably even though a conductivity-based SAW sensor should demonstrate attenuation changes that are intimately related to velocity changes [4]. This anomalous result can be attributed to the fact that frequency changes can be measured much more accurately than attenuation changes, and the insertionloss change in this case was probably buried in the noise of the d.c. signal. A response of the grease-coated sensor to acetone vapour is shown in Fig. 4. This device was operated
Fig. 3. Frequency response of the WO,-coated ppm of H2S.
SAW sensor to 10
RX Falconer I Senson and Actuators B 24-25 (1995) 54-57
51
and Technology at the University of Maine for their assistance with this work. In particular, the author would like to thank E.L. Wittman for the deposition of the WO, films and J.D. Galipeau for assistance with the gas-delivery and test system. 10
0
10
20
Time (min) (a)
30
10
0
10
20
30
Time (min) (h)
Fig. 4. Frequency (a) and insertion-loss (b) responses of the greasecoated SAW sensor to acetone vapour.
at room temperature and was exposed to a 50/50 mix of dry air and air saturated with acetone vapour by passing through a bubbler. The exposure ran from t=O-13 mm, and the frequency and insertion-loss changes for the device are both shown.
5. Conclusions A SAW gas-sensor circuit has been made that allows the simple measurement of both SAW velocity and attenuation. The velocity shifts are proportional to changes in oscillating frequency, and a voltage output is proportional to the insertion loss of the device. The sensor has been demonstrated to be compatible with both a semiconducting metal oxide film and an organic film, even though the sensing mechanisms for the two devices are different. As part of a complete SAW sensor platform, the device will allow gas-sensitive materials of all kinds to be tested.
Acknowledgements The author would like to thank the members of the SAW sensor group at the Laboratory for Surface Science
References H. Wobltjen and R. Dessy, Surface acoustic wave probe for chemical analysis, Anal. Chem., 51 (1979) 1458-1475. PI See, for example, C.C. Fox and J.F. Alder, Surface acoustic wave sensors for atmospheric monitoring, Analyst, 214 (1989) 997-1004. [31 H. Wohltjen, Mechanism of operation and design considerations for surface acoustic wave device vapour sensors, Senrors and Acfuafors, 5 (1984) 307-325. [41 R. Let, J.F. Vetelino, R.S. Falconer and Z. Xu, Macroscopic theory of a surface acoustic wave gas microsensor, kc. 1988 IEEE Ultrasonics Symp., Chicago, IL, USA, 2-5 Oct., 1988, pp. 585-589. G.C. Frye and S.J. Martin, Dual output acoustic wave sensors for molecular identification, Pxx. 6th ht. Conj Solid-Stare Sensors and Actuators, (Transducers ‘91), San Fmncisco, CA, USA, 24-28 June, 1991, pp. 566-S-569. [61 D.J. Smith, A study of the sensitivity and selectivity of WO, films for sensing applications, M.S. Thesis, University of Maine, 1991.
PI
Biography Robert Falconer was born in Biddeford, Maine (USA) on March 29, 1963. He received his B.S., M.S. and Ph.D. degrees in electrical engineering from the University of Maine in 1985, 1989 and 1993, respectively. He began working for Andersen Laboratories in September 1993, where he is involved with research and development concerning SAW gas sensors.
Sensors and Actuators
CHEMICAL
B 24-25 (1995) 54-57
A versatile SAW-based sensor system for investigating gassensitive coatings R.S. Falconer Andersen
Labomtories,
Inc., 45 Old Iron Ore Road, Blwmfield
CT 06002,
USA
Abstract Surface acoustic wave (SAW) technology is an extremely versatile technique for the development of gas sensors because of the SAW device’s sensitivity to a wide variety of material properties that can change with gas concentration. In the present work, a SAW sensor oscillator is described, which allows both of the important SAW properties, velocity and attenuation, to be easily measured. The SAW sensor can be used with a variety of gas-sensing materials to monitor any gases that interact with the sensing materials. The versatility of the sensor is demonstrated by making SAW sensors using hvo gas-sensitive films that work in different ways. A tungsten trioxide (WO,) film with electrical properties that vary with hydrogen sulfide (H,S) exposure has been used to make a SAW H2S sensor, and an organic material that sorbs solvents has been used to demonstrate a SAW acetone sensor.
1. Introduction Surface acoustic wave (SAW) devices have been examined as gas sensors since 1979 [l]. The SAW sensor is made by coating a SAW delay line with a film that reacts to the gas or gases of interest. When the film reacts with the gas, the properties of the film will change, and this variation will be translated into changes in the operation of the SAW device. The SAW sensor research performed since 1979 is reviewed in several published works [2] and will not be detailed here, apart from a brief description of the basic operation of the sensor. A general SAW sensor device is shown in Fig. 1. It includes a SAW delay line coated with a selectively sorbent film. The coated delay line is made into an oscillator using an amplifier in feedback, and the oscillating frequency is monitored using a directional coupler and a frequency counter. The frequency of
I
I
Coupler
Fig. 1. SAW sensing oscillator circuit. 092.C4005/95/$09,50 8 1995 Elsevier Science S.A. All rights reserved SSDI 0925.4005(94)01314-8
oscillation of the loop is then dependent upon the phase/time delay of the SAW device, and therefore upon the SAW velocity. The velocity, as well as the SAW attenuation, is in turn dependent upon the material properties of the coating. Frequency changes in the loop are proportional to SAW velocity changes, and the insertion loss of the sensing device is proportional to the acoustic attenuation caused by the gas-sensing film. With the appropriate choice of amplifier, this insertion loss can be monitored by measuring the gain of the feedback amplifier that maintains oscillation. The oscillating frequency of a SAW oscillator is temperature dependent because the material properties of the substrate are temperature dependent. When the oscillator is made into a sensor by the addition of a gas-sensitive coating, the temperature dependence becomes even more complicated. The frequency shift caused by the temperature-dependent variation of substrate properties can be increased or decreased by variations in the properties of the coating. In order to correct partially for sensor drift due to temperature effects, a second delay-line oscillator is sometimes used to generate a reference frequency. This reference device is identical to the sensing device except that it does not have a gas-sensitive coating. When the temperature drifts, it will create a frequency shift in the reference device, which is equal to the substrate-related shift in the sensing device. If the frequency signals are then
RX
Falconer I Sensors and Ac~untors B 24-25 (1995) 54-57
multiplied in a mixer, the output frequency will be the difference between the two individual frequencies. This difference frequency will generally be more stable with temperature, because substrate effects will be minimized. In general, the difference frequency will still be a function of temperature, because the coating properties are usually temperature dependent.
2. Theory The SAW velocity and attenuation in a layered structure such as the SAW sensor is a complicated function of the material properties of both the substrate and the coating, as well as the geometry of the media. The relevant material properties include the density, elastic moduli, viscosity, electrical conductivity, chargecarrier diffusion constant, dielectric permittivity and piezoelectric constants. In general, any or all of these properties within the film can change, thereby altering the SAW velocity and attenuation. In many cases the problem can be simplified when one or two effects predominate. An example of this situation is the simple mass-loading case, described by Wohltjen [3] for an isotropic film. In this case the SAW velocity can be considered to be a linear function of the mass absorbed by the film per unit area. If purely elastic changes in the film mechanical constants are appreciable, another term can be added to take this into account. Both of these effects are lossless and therefore do not affect the SAW attenuation. Another simplified situation occurs when changes in the electrical conductivity of the film create large changes in the SAW velocity and attenuation. Previous work [4] has shown that the effect of conductivity changes can lead to velocity changes much greater than those due to mass loading, as long as the sheet conductivity of the coating is within a range of approximately four orders of magnitude centered around a ‘resonant sheet conductivity’, which is a function of the substrate properties alone. The resistive losses in the film can also lead to significant attenuation changes under certain circumstances. The presence of both velocity and attenuation changes in a SAW sensor is very significant, because it supplies two pieces of information from a single sensor. In many cases a gas-sensitive film will react with more than one gas, and the sensor would not be able to differentiate between two gases if only the oscillating frequency were measured. Other work [5] has shown, however, that the attenuation changes are sometimes independent of the velocity changes, and by measuring both properties simultaneously two or more gases may be differentiated. While simplified explanations for SAW sensing effects can sometimes give the ability to work back from a SAW sensor response to calculate the fundamental
55
changes that take place in the coating, it is important to note that these efforts are by no means necessary for a researcher interested in developing a sensor for a specific gas. The fundamental physical and chemical processes that result in film property changes and hence SAW velocity and attenuation changes do not matter to the sensor designer. Regardless of what is happening within the film, the SAW sensor gives an easily measured response in frequency (velocity) and insertion loss (attenuation), which is a function of gas concentration. The response may or may not be linear, but in any case a calibration curve can be experimentally defined that correlates the sensor response to gas concentration. Once a suitable SAW sensor system is designed, the properties of the gas-sensitive film are responsible for such overall sensor properties as sensitivity, reproducibility, selectivity, stability and aging. Developing a practical SAW sensor then becomes a matter of developing a practical sensing film. Perhaps the greatest advantage of SAW sensors over competing sensor technologies is their versatility. The SAW velocity and attenuation in the layered structure of the piezoelectric substrate with a gas-sensitive coating are dependent upon all of the mechanical and electrical properties of the film. SAW sensors have been demonstrated that rely upon changes in film elastic constants, viscosity and electrical conductivity, in addition to the simple mass change caused by sorption or chemical reaction that must take place in any gas-film interaction. Because the sensor can detect changes in so many different film properties, the SAW sensor is ideally suited to the examination and testing of gas-sensitive films of all types.
3. Experimental At Andersen Laboratories a SAW sensor package is being developed to allow gas-sensing films to be easily tested in a SAW sensing configuration. The SAW sensor platform includes the oscillating circuitry for a dual-delay-line SAW sensor with outputs for the two fundamental oscillating frequencies, the difference frequency and the insertion loss of the sensing delay line. The SAW devices are held in the package with springloaded electrical contacts that allow easy removal of the devices for coating or replacement. The SAW oscillator circuit that has been developed uses two ampliier stages. A fixed amplification section ensures that the gain of the circuit is sufficient to maintain oscillation for the uncoated SAW device. A second amplifier stage uses logarithmic amplifiers, GEC Plessy model SL1613, to supply the gain necessary to overcome any added attenuation due to the application of the sensing film or the gas-film interactions. Logarithmic amplifiers are designed to amplify a sinusoidal
56
R.S. Falconer ! Sensors and Actuators B Z&25 (1995) 54-57
signal while generating a d.c. output signal proportional to the log of the input voltage level. Since the gain of the circuit is much greater than that needed to sustain oscillation, the final stage, or stages, of the logarithmic amplifier section will be in saturation, ensuring that a constant voltage level is applied to the SAW device. As the insertion loss of the SAW device increases, the signal level into the logarithmic amplifiers (i.e., the signal out of the SAW device) must therefore decrease, and the d.c. output of the circuit will change in proportion to the logarithm of the SAW losses. The output voltage is then proportional to the insertion loss, in dB, of the SAW device. The excess gain of the oscillator circuit allows stable operation of the device with as much as 55 dB of attenuation due to the gas-sensing film. The SAW devices used for this work were fabricated on ST-cut quartz with split-finger interdigital transducers (IDTs) of gold. The single-mode devices were designed to oscillate at approximately 100 MHz. The devices, with impedance-matching circuitry, were mounted on PC board, and the amplifier circuitry was on a separate PC board. The boards were connected with coaxial cable. The H,S sensor was mounted on a heater in a gas-exposure chamber without any impedance-matching network.
4. Results The SAW oscillator circuit was tested to determine the calibration curve correlating the insertion loss of the SAW with the voltage output of the logarithmic amplifier stage. The test was accomplished by inserting a variable attenuator between the output of the SAW device and the input of the amplifier. The output voltage was recorded as the attenuation was increased in 2 dB increments. The results for the test are shown in Fig. 2, along with a straight line fit to the data. Preliminary gas-sensing measurements have been performed using the oscillator circuit and two SAW devices coated with different films for sensing different gases. The two coatings were chosen to illustrate the use of two different types of gas-sensitive materials deposited
Fig. 2. Excess insertion loss vs. the output voltage from the logarithmic amplifier
stage.
with different techniques in the SAW sensor configuration. The first film was a semiconducting tungsten trioxide (WO,) film with a conductivity that varies with exposure to hydrogen sulfide (H,S) gas. The second film was a vacuum grease (Apiezon H), which was used to sorb acetone vapour. The WOg film was reactively r.f. sputtered from a tungsten target in an argon/oxygen atmosphere at the University of Maine [4]. The film was 1000 A thick and was doped with gold and annealed at greater than 300 “C. The Apiezon grease was dissolved in toluene and deposited with a cotton swab at room temperature. These preliminary measurements were performed using a single sensing delay-line oscillator. Future measurements using the dual-delay-line configuration will be used to examine the amount of stabilization afforded by a reference oscillator. A typical response of the SAW H,S sensor is shown in Fig. 3. For this experiment the device was heated to 200 “C and allowed to stabilize. After the oscillation became stable, the device was allowed to operate in air for approximately 20 min to provide a baseline frequency. At time t=O the device was exposed to 10 ppm of H2S in air, and at t=30 min the gas feed was switched back to air. The Figure illustrates that the sensor responded to the gas within a few minutes and gave an easily measured frequency response. The initial response was significantly faster than the return to baseline after exposure. Because the physical and chemical processes that occur between the metal oxide film and H,S are not fully understood, the reason for the different rates for the forward and reverse reactions is not known. Previous experimental work [6] has shown, however, that the rise and fall times differ and depend on temperature. The insertion loss of the device did not change noticeably even though a conductivity-based SAW sensor should demonstrate attenuation changes that are intimately related to velocity changes [4]. This anomalous result can be attributed to the fact that frequency changes can be measured much more accurately than attenuation changes, and the insertionloss change in this case was probably buried in the noise of the d.c. signal. A response of the grease-coated sensor to acetone vapour is shown in Fig. 4. This device was operated
Fig. 3. Frequency response of the WO,-coated ppm of H2S.
SAW sensor to 10
RX Falconer I Senson and Actuators B 24-25 (1995) 54-57
51
and Technology at the University of Maine for their assistance with this work. In particular, the author would like to thank E.L. Wittman for the deposition of the WO, films and J.D. Galipeau for assistance with the gas-delivery and test system. 10
0
10
20
Time (min) (a)
30
10
0
10
20
30
Time (min) (h)
Fig. 4. Frequency (a) and insertion-loss (b) responses of the greasecoated SAW sensor to acetone vapour.
at room temperature and was exposed to a 50/50 mix of dry air and air saturated with acetone vapour by passing through a bubbler. The exposure ran from t=O-13 mm, and the frequency and insertion-loss changes for the device are both shown.
5. Conclusions A SAW gas-sensor circuit has been made that allows the simple measurement of both SAW velocity and attenuation. The velocity shifts are proportional to changes in oscillating frequency, and a voltage output is proportional to the insertion loss of the device. The sensor has been demonstrated to be compatible with both a semiconducting metal oxide film and an organic film, even though the sensing mechanisms for the two devices are different. As part of a complete SAW sensor platform, the device will allow gas-sensitive materials of all kinds to be tested.
Acknowledgements The author would like to thank the members of the SAW sensor group at the Laboratory for Surface Science
References H. Wobltjen and R. Dessy, Surface acoustic wave probe for chemical analysis, Anal. Chem., 51 (1979) 1458-1475. PI See, for example, C.C. Fox and J.F. Alder, Surface acoustic wave sensors for atmospheric monitoring, Analyst, 214 (1989) 997-1004. [31 H. Wohltjen, Mechanism of operation and design considerations for surface acoustic wave device vapour sensors, Senrors and Acfuafors, 5 (1984) 307-325. [41 R. Let, J.F. Vetelino, R.S. Falconer and Z. Xu, Macroscopic theory of a surface acoustic wave gas microsensor, kc. 1988 IEEE Ultrasonics Symp., Chicago, IL, USA, 2-5 Oct., 1988, pp. 585-589. G.C. Frye and S.J. Martin, Dual output acoustic wave sensors for molecular identification, Pxx. 6th ht. Conj Solid-Stare Sensors and Actuators, (Transducers ‘91), San Fmncisco, CA, USA, 24-28 June, 1991, pp. 566-S-569. [61 D.J. Smith, A study of the sensitivity and selectivity of WO, films for sensing applications, M.S. Thesis, University of Maine, 1991.
PI
Biography Robert Falconer was born in Biddeford, Maine (USA) on March 29, 1963. He received his B.S., M.S. and Ph.D. degrees in electrical engineering from the University of Maine in 1985, 1989 and 1993, respectively. He began working for Andersen Laboratories in September 1993, where he is involved with research and development concerning SAW gas sensors.