Development of a saw gas sensor for monitoring so2 gas

ELSEVIER Sensors and Actuators A 64 (1998) 173-178 A

PHYSICAL

Development of a SAW gas sensor for monitoring SO2 gas

Y.J. Lee a, H.B. Kim b, Y.R. Roh a,*, H.M. Cho ‘, S. Baik’ a Department of Sensor Engineering, Kymgpook National University, Snnkytrg-Dong 1370, Toegu, 702-701, Sauth Korrct

b Sensor Technology Research Center, K~~cngpook National Universiu, Sarzbtrg-Dong 1370, Taegtr, 702-701, South Korea ’ Deparrment of Muterials Science and Engineering, Pohnng Universiv of Science and Technology, San 31, HJoja-Dong, Pohang, 790-784, South Karen

Received 11 March 1997; revised 17 April 1997; accepted 23 April 1997

Abstract

We have developed a new type of SO2 gas sensor by applying a particular inorganic thin film on SAW (surface acoustic wave) devices. The sensor consists of twin SAW oscillators with a centre frequency of 54 MHz fabricated on LiTaO, piezoelectric single crystals. One delay line of the sensor is coated with a CdS thin film that selectively adsorbs and desorbs S02, while the other is uncoated for use as a stable reference. Deposition of CdS thin film has been carried out by the spray pyrolysis method using an ultrasonic nozzle. Mass loading and electric-field changes induced by the SO2 gas adsorbed onto the CdS film result in corresponding frequency shifts directly proportional to the gas concentration. The relative change in the frequency of the two oscillators is monitored with a digital signal-processing circuit. SO1 sensor properties investigated include sensitivity, response time and repeatability. The sensor shows promising performance as a microsensing tool and is capable of measuring concentrations in air less than 200 parts per billion of SO?. 0 1998 Elsevier Science S.A.

Keywords: Sutidce acoustic wave (SAW) gas sensors; Cadmium sulfide (CdS j thin fiims; Ultrasonic spray pyrolysis; Sulfur dioxide (SO,)

1. Introduction

The aim of our investigation is to develop a gas sensor for SO*, one of the most important pollutants in ambient air. This requires a sensor which has a high sensitivity and a high resolution in the sub-ppm range with a good stability over time and reproducibility for in situ measurements. Current commercially available detection methods for SO1 gas are classified into physical (flame photometric detection, UV and fluorescence measurement methods), chemical (hydrogen peroxide method) and electrochemical techniques [ 11. However, those systems are quite expensive and require com- plicated measurement equipment. In this study, we are devel- oping a new type SO:! gas sensor applying a particular inorganic thin film on an acoustic sensor which utilizes surface acoustic wave (SAW) technology. As is well known, a SAW sensor is very sensitive to its environment and thus has many merits, such as high stability, quick response and easy reproducibility. It can also be produced at a low price in a small size through photolithographic processes. The use of SAW sensors in the field of chemical sensors is quite attrac- tive because of the particular features of these devices [ 21. The properties of the reactive coating and of the acoustic

* Corresponding author. Tel.: + 82 53 950 68 28. Fax: + 82 53 950 68 21.

0924~4247/98/$19.00 0 1998 Elsevicr Science S.A. All rights reserved PUSO924-4247(97)01596-3

device itself determine how well the sensor performs as a chemical detector.

The sensor developed in this work consists of twin SAW delay lines with a centre frequency of 54 MHz fabricated on LiTaO, piezoelectric single crystals. Each delay line is con- figured as an oscillator. For the SO, reactive coating, several materials have been tried, such as triethanolamine (TEA) [3], diethanolamine (DEA) and tungsten trioxide (WO,) [4]. However, none of them has shown satisfactory results yet. In particular, the TEA shows good responses but has the fatal shortcoming of a short lifetime. In this study, we chose an inorganic material, cadmium sulfide (CdS), as a new detection material for the SO2 after extensive paperwork and preliminary experiments. Deposition of the CdS in the form of thin films has been carried out by the spray pyrolysis technique with an ultrasonic nozzle. One delay line of the sensor is coated with a CdS thin film that selectively adsorbs and desorbs SOZ, while the other is uncoated for use as a stable reference. Phase delay changes due to the mass loading and electric-field effects induced by the SO, gas adsorbed onto the CdS film result in corresponding frequency shifts of the SAW oscillators that are directly proportional to the gas concentration. The relative change in the frequency of the two oscillators is converted to a digital value and transferred to a personal computer by RS-232C protocol with a digital

171 Y. J. Lee et al. /Sensors und Actuators A 64 (1998) 173-I 78

Mini-solenoid

Pyrex Gla?SS Chamber

Mini-valve + Controller

Power Generator

C Nozzle Controller

Control Panel

I Pump s/w

Fig. 1. Schematic diagram of ultrasonic spray pyrolysis system.

microprocessor. This configuration enables continuous monitoring of the concentration of the S02. SO2 sensor properties investigated included sensitivity, response time and repeatability. In this paper, the preparation of each component of the sensor and their performance are reported and discussed.

2. Preparation of CdS thin films

We selected CdS as the detection layer material for the SAW SO2 sensor after extensive paperwork [ 5,6]. The thin- film deposition is carried out via the spray pyrolysis method [7]. Spray pyrolysis is a process in which a thin film is deposited by spraying a solution onto a heated surface, where the constituents react to form a desired chemical compound. The process is particularly useful for the deposition of oxides and has long been a production method for conductive thin films such as SnOx, CdS and ZnO. A schematic of the CdS film deposition apparatus is shown in Fig. 1. An ultrasonic transducer (Sono-Tek 8700-120MS) is employed here for atomization of the solution. The spray solution is prepared

28 Fig. 2. XRD pattern variation of the CdS film with deposition temperature.

by mixing cadmium chloride (CdCl,) of 99.99% purity and thiourea ( (NH,),CS) of 99.99% purity in the ratio of 1: 1 and dissolving the mixture in distilled water. The vacuum of the reactor is maintained in the range IO-’ to low3 torr and controlled by a butterfly valve. In deposition, we change such process parameters as solution concentration, flow velocity, hot-plate temperature and pressure in the reactor, and observe corresponding structural variations of the film [ 81. The crys- talline structure and the growth rate are checked through XRD analyses, and the surface morphology through SEM analyses. The thickness of the film is also observed by an alpha-step 200 and ellipsometry. After all the experiments, optimal conditions are determined as a deposition temperature of 300°C a solution concentration of 0.010 M and a solution flow rate of 0.222 ml min - ’ . The deposited film shows a hexagonal wurtzite crystal structure withlattice constants a = 0.4135 nm and c= 0.6713 nm, and a good thickness uniformity. Fig. 2 is the XRD pattern variation of the film with the deposition temperature.

For verification of the performance of the deposited CdS thin film as the detection layer, gas chromatography and temperature-programmable desorption measurements are per- formed with the CdS thin-film samples. Fig. 3 shows results of the gas chromatography measurement. The Figure describes the variation of thermal conductivity of the gas out of the reactor when either normal air or a mixture of SO2 and air is blown into the chamber. Region A denotes the range where normal air is injected into the reactor chamber. Since the CdS thin film does not respond to normal air, the air out of the chamber shows stable thermal conductivity. At the end

desorption

I

Tiae Fig. 3. Gas chromatogram of CdS thin films: repeated adsorption and desorption of 100 ppm SO? gas on CdS thin films at 80°C.

Y.J. Lee et al. /Sensors rind Actuators A 64 (1998) 173-178 115

of the region A, a mixture of SO2 and normal air is introduced into the reactor. Inclusion of SO2 in the air should show different thermal conductivity. After some transient region (B), the gas mixture reaches a stable value (region C) , The region B is considered to be where the SO, is adsorbed by the CdS film, hence causing some fluctuation of the conductivity of the gas mixture out of the chamber. Othe_rwise, the conductivity curve should level off immediately as in the region A. Desorption of the SO, is enforced by blowing normal air into the reactor again at the end of the region C. Similarly, after some transient region (D) where the SO, on the CdS film is desorbed, the gas shows a stable thermal conductivity (region E), which is exactly the same value as that in the region A. Hence, Fig. 3 confirms the responsiveness of the CdS to SO, and its repeatability. The CdS films do not show any deterioration in responsiveness even after several tens of experiments. Basically, CdS is a semiconductor material. The semiconductive properties must have played an important role in attracting and releasing the SO, mole- cules [ 91. Once adsorbed or desorbed, the combined effect of electric-field changes and mass loading induced by the SO2 will eventually lead to phase delay changes of the SAW on the film.

3. SAW oscillator sensor system

Fig. 4 is a schematic diagram of the SAW sensor system configuration. The SAW sensor system consists of two oscillators on the same substrate, a digital frequency counter, computer interface part and Pt-coated resistive heater.

For the piezoelectric substrate of the SAW device, we select LiTaO, single crystals. There are a number of piezoelectric materials available to the SAW component designer: the choice depends on the type of device to be prepared, the frequency of operation, the time delay and application of the system. In this study, the LiTaO, single crystal is chosen for

its moderate piezoelectric coupling factor and temperature coefficient. Further, the optimal SAW propagation path is determined as Y-cut crystal and Z-axis crystal direction after extensive computer simulation [ IO]. The determination is based on eight item evaluations for different crystal cuts and SAW propagation directions: SAW velocity, coupling factor, surface permittivity, frequency-temperature coefficient, air loading attenuation, pure mode propagation, beam steering and misalignment sensitivity. For temperature control of the sensor, a ceramic heater is prepared by depositing a Pt thick film in the form of wires on the surface of an Al,O, plate through electron beam evaporation. The A1,03 plate is mounted on the bottom of the LiTaO, substrate. Temperature is controlled by either changing the applied electric voltage or the thickness and length of the Pt film. For the oscillator configuration, the SAW device is fabricated as a dual-delay- line pattern, with only one line coated with the CdS film, to eliminate the effect of the external environment and noise. The IDT (interdigital transducer) has the standard single electrode pattern of a centre frequency of 54 MHz. Each IDT has 40 electrode fingers. The whole device measures 18 mm X 18 mm and the CdS film takes up an area of 5 mm X 5 mm in the middle of the IDTs of one delay line. The oscillators convert the SAW velocity change due to the reaction between the CdS thin film and the SOZ gas to frequency change. The electronic part of the oscillators is fabricated in the form of a multivibrator circuit that is widely used in timing applications.

For the processing of the two oscillator signals, we utilize a digital signal-processing circuit instead of the usual analog mixer and low-pass filter. Counting the two oscillating signals and subtracting one from the other is done with a microprocessor (89C5 1) . Processed results are sent to a personal computer every eight seconds. The RS-232C protocol is used in data communication between the RX/TX ports of the microprocessor and the PC. The merits of this digital configuration are that: ( 1) it is more robust to external noise than analog

to Computer (RS-232C)

feedback amp.

Fig. 4. Schematic diagram of the SAW SOZ gas sensor system.

176 Y.J. Lee et al. /Sensors and Actuutors A 64 (1998) 173-178

counterparts; (2) it does not need any other bulky measurement devices; (3) hence the sensor system provides better portability; (4) the signal-processing strategy for the oscillator signals can be changed easily by just modifying the program for the microprocessor; (5) the digital interface to any other system or data collection is easier and simpler, and so on.

4. Experiments and discussion

A schematic diagram of the system to evaluate the responsiveness of the SAW sensor to SO2 gas is shown in Fig. 5. The system is designed to deliver a precise and accurate concentration of gas or gases to the sensor. For initial set-up of the measurement conditions, air is injected into the reactor at 100 ml min-’ for a while. To minimize the effect of moisture, a moisture-removing trap is attached to the gas inlet. After the oscillation frequency of the sensor reaches a stable value, SO, gas is infused into the chamber. Phase delay changes due to the electric-field and mass loading effects by the SO* gas adsorbed onto the CdS film result in corresponding frequency shifts proportional to the gas concentration. Flowing SO, gas into the reactor, we measure the sensitivity and response time of the sensor while checking its stability.

Fig. 6 is a typical response of the sensor, the relative change in the frequency of the SAW oscillator with the CdS film to that without the film when exposed to SO,. A clear depend- ence of the oscillating frequency difference is observed to the existence of SOT. No matter how carefully one fabricates the two SAW oscillator channels, it is virtually unavoidable to have some initial difference of operating frequencies. Fig. 6 shows this initial frequency difference is about 29.5 kHz for our sensor. That is supposedly due to the mass loading effect of the CdS thin film itself and other subtle factors probably occurred during lithography processing. With adsorption or

f- Air

q=flz Fig. 5. Schematic diagram of the SAW sensor characterization system,

desorption of the SO? onto the CdS film, the frequency difference increases from A to B, or decreases from B to C, respectively. The frequency shift dffrom A to B or B to C corresponds to the sensitivity of the SAW sensor to SO, gas. It takes only 3 min to adsorb the SO2 gas and 5-6 min to desorb. Fig. 6 proves the possibility of the SAW SO2 sensor with the CdS thin film. However, the sensitivity of the sensor has been found to be dependent on the substrate temperature. Hence, for determination of the optimum operating temperature, the response of the sensor is investigated while varying the voltage supplied to the Pt heater so that the substrate spans the temperature range from room temperature to 180°C. Fig. 7 shows the variation of the sensitivity with the change of the substrate temperature. Afincreases with the substrate temperature, but seems to saturate at some limit. From the result, the substrate temperature is determined to be 160°C. Higher temperature may give a little better AJ but power conservation is also an important factor of consideration for our portable sensor system that is to be operated by a battery.

To evaluate the sensitivity of the sensor quantitatively, we expose the sensor to various SOa concentrations such as 500,

29760

0 10 20 30 40 Tline @in)

Fig. 6. Output of the SAW sensor to 500 ppm. 2 1 min-’ SO? gas flow at a substrate temperature of 160°C. Vertical axis denotes relative change in the frequency of the SAW oscillator with the Cd.5 film to that without the film.

Y.J. Lee et ai. /2%mors rind Acnmors A 64 (lY98) 173-178 177

80 100 120 140 180 180

Temperature (“cl) Fig. 7. Frequency shift Afvs. substrate temperature under 500 ppm, 2 I min- ’ SO2 gas flow.

0 2000 4000 6ooo fJoo0

Gas Concentration (ppm) Fig. 8. Frequency shift Aj’vs. SO2 gas concentration.

1000, 3000, 5000 and 10 000 ppm. The measured result is shown in Fig. 8. Our sensor shows good repeatability and each dot in Fig. 8 is an averaged value of five measurements. It shows a fairly linear relationship between the SAW sensor responses and the SO* concentrations, but the sensitivity in the low SO2 concentration range is a little poorer than that in the high range. This is considered to be partly due to better reactivity of the CdS film with high concentration of SOz, and also partly due to less chances of experimental errors at the high concentration range. Our oscillators have a very stable resolution down to 1 Hz. Hence, the overall result in Fig. 8 means that the sensor is capable of measuring concentrations in air less than 200 parts per billion of SOZ. This performance is somewhat poorer than that in Ref. [ 31 with the TEA film. However, CdS has the significant merit over the TEA film in that it has a much longer lifetime, as proved in Fig. 3. For our system, the sensitivity can be improved by enhancing the reactivity of the CdS thin film through appropriate dopant addition, like indium. Further, increasing the

centre frequency of the SAW sensor will give better resolution.

5. Conclusions

We developed a new type of SO, gas sensor applying a particular inorganic thin film on SAW devices. The sensor consisted of twin SAW delay-line oscillators with a centre frequency of 54 MHz fabricated on LiTaO, piezoelectric single crystals. A gas-sensitive thin film of CdS was deposited by the spray pyrolysis method using an ultrasonic nozzle. The relative change in the frequency of the two oscillators was monitored with a digital signal-processing circuit to measure the concentration of the gas. The sensor showed promising performance as a microsensing tool and was capable of measuring concentrations in air of less than 200 parts per billion of SO2 at the substrate temperature of 160°C with a recovery time of less than 6 min after each measurement. Further work

178 Y.J. Lee et nl. /Sensors and ACIIKTIO~S A 64 (1998) 173-l 78

required to improve the performance of the sensor includes enhancing the reactivity of the CdS thin film with SO, through appropriate dopant addition, and increasing the centre frequency of the SAW device.

References

[l] R. Perry and R.J. Young, Handbook of Air Poliufion An&is, Chapman and Hall, New York, 1987.

[2] H. Wohltjen and R. Dessy, Surface acoustic wave probe for chemical analysis. I. introduction and instrument description, Anal. Chem., 51 (1979) 1458-1464.

[3] A. Bryant, D.L. Lee and J.F. Vetelini, A surface acoustic wave gas detector, Proc. IEEE Ultrasonics Symp., Chicago, IL, USA, 1981, pp. 171-174.

[?I D.J. McAllister, Surface acoustic wave gas microsensor for SOS, Notional Science Formdotion report ISF-87088. 1987, pp. l-29.

[5] V.V. Golovanov, A.I. Gudis and V.A. Smyntyna, A cadmium sulfide gas analyzer sensor, Zh. Ann/. Khim., 46 ( 199 1) 2374-2379.

[6] A. D’ Amico and E. Verima, SAW sensors, Sensors and Actuators, I7 (1989) 55-66.

171 D.S. Albin and S.H. Risbud, Spray pyrolysis processing of optoelectronic materials, Advanced Ceramic Materink, 2 ( 1987) 243- 252.

[8] R.R. Chamberlin and J.S. Skarman, Chemical spray deposition process for inorganic films, J. Electrochem. Sot., II3 ( 1966) 86-89.

[91 N. Golovan and V. Smyntyna, The sensitization of semiconductor gas sensors, Sensors rind Actuators, 6 ( 1992) 289-295.

[ 101 Y.R. Roh, D.S. Chung, Y.H. Bae and Y.S. Kim, Properties of LiTaO, single crystals for SAW device applications, Sensors Marer., 7 ( 1995) 75-84.

Documents

Development of a saw gas sensor for monitoring so2 gas