Highly sensitive temperature sensor using SAW resonator oscillator

Highly sensitive temperature sensor using SAW resonator oscillator

209 Sensors and Aciuators A, 24 (1990) 209-211 Highly Sensitive Temperature Sensor Using SAW Resonator Oscillator MARTIN VIENS and J. DAVID N. CHE...

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209

Sensors and Aciuators A, 24 (1990) 209-211

Highly Sensitive Temperature Sensor Using SAW Resonator Oscillator MARTIN

VIENS and J. DAVID

N. CHEEKE

*

Dkpartement de Physique et Centre de Recherche en Micro6lectronique Sherbrooke, Sherbrooke, Que., JlK 2R1 (Canada) (Received

January

26, 1990; in revised form July 3, 1990; accepted

de I’Universit4 de Sherbrooke (CERMUS),

UniversitP de

July 30, 1990)

Abstract

series [2]:

Acoustic surface waves have been used for chemical and other sensors. In this work we explore their potential as highly sensitive temperature sensors. An acoustic resonator at 78 MHz is constructed on YZ-cut lithium niobate. A sensitivity of 80 ppm/“C is obtained over the range - 30 to + 150 “C, which is close to the theoretical value. Possible applications are discussed.

t(T) = r(To)[ 1 + a(T - T,) + b(T - T# +. . .

Introduction

A number of temperature sensors based on acoustic surface wave propagation have already been developed [l-5]. These devices are all based on the temperature variation of the sound velocity in a delay line oscillator, as for a number of substrates this dependence is significant and in some cases quasi-linear. The importance of the present work is that for the first time we have constructed such a device based on the resonator principle, which should provide better stability and resolution. Lithium niobate was chosen for the sensor material for a number of reasons; it has a high temperature coefficient of the order of 94 ppm/“C, a high electromechanical coupling coefficient (k* = 0.048) and a high thermal conductivity [6] to ensure good thermal contact with the ambient medium. The device was based on the use of frequency-selective reflectors placed at either end of the delay line, so that when the latter is placed in the feedback loop with an independent amplifier, the frequency of the resulting oscillator is proportional to the temperature. Principle of the Device

In general we can expand the temperature dependence of the device delay time in a Taylor *Address after July 1, 1990: SIRICON Inc., 1455 de Maisonneuve Ouest, Mont&l, Que., H3G lM8, Canada.

0924-4247/90/%3.50

+ u dT/dt]

(1)

where To is the reference temperature, a and b the first- and second-order sensitivity constants, t the time and u the dynamic proportionality constant related to spatial and temporal gradients of temperature involved during an external temperature fluctuation [ 7j. For the case of a temperature sensor a should be as large as possible in order to obtain a high sensitivity, while u and b should be small in order to ensure good linearity and a short response time to minimize errors due to sudden temperature changes. In addition to the sensitivity and linearity, one must also consider the signal-to-noise ratio. This can be expressed as r = a/a

(2)

where r is the intrinsic resolution and cr the shortterm stability as defined by Barnes et al. [8]. To maximize the short-term stability, and thus the resolution, we must minimize losses in the system. This can be achieved principally by minimizing the insertion loss, which allows one to work at low amplification and thus reduce the system noise. In addition, a high quality factor is necessary in order to reject all parasitic frequencies which are not on the principal resonance. A resonator configuration is particularly favorable to achieve this, as it contains the acoustic energy of the desired frequency inside the cavity and only this frequency component is present at the output of the resonator. Finally, it is also important to specify the overall resolution of the sensor taking into account the coupling into the frequency counter. This is ‘.g= (l/a)(a +.&l/U

(3)

where f, is the error introduced by the counter due to its own intrinsic resolution. 0 Elsevier Sequoia/Printed in The Netherlands

210 NESZOS

Sensor Design and Construction

The sensor was constructed on a YZ-cut lithium niobate substrate. A 2000 A gold film on 50 A Cr was deposited for the contacts for the interdigital transducers and reflectors. The transducers had 18 fingers at a 22 pm spacing, corresponding to a 44 pm wavelength and a 79 MHz resonant frequency. The reflecting structures were made of 300 isolated electrodes and had a calculated reflection coefficient of 0.998. With a 50 ohm load, the measured insertion loss was 4.6 dB with a Q factor of over 550 (Q, = 1350). The frequency response of the structure is shown in Fig. 1. The oscillator is formed by adding a feedback loop composed of a 40 dB amplifier and a wide bandpass filter, as shown in Fig. 2.

0

gy

3

g 1 $ 6 2

-20

-30 -40

50

60

70

60

90

Go

FREQUENCY (MHz)

Fig. 1. Frequency response of the resonator

&q Fig. 3. Measuring system.

NESZOS

El~~S”~l”g

0”tp”l

6

Fig. 2. Electric circuit in the feedback loop of the oscillator.

The measurements were made in a computercontrolled oven, as shown in Fig. 3. The sensor was kept in an environment of 3 psi of argon to ensure temperature homogeneity. The furnace temperature was controlled by a Fluke 2400B interface coupled to a platinum resistance thermometer and a heating element. The temperature sweeps were made by a ramp sufficiently slow (about 1OOmin per sweep over the full temperature range) that thermal lag effects were negligible and in any case constant to first order. The actual measurement was done in a difference mode to increase the measurement sensitivity. This was done by mixing the oscillator output with a stable

c

h -50-10

__._.______________

MICROCOMPUTER SYSTEM

211

Ackaowledgemeats Thanks are due to Dr L. Paquin for useful suggestions and to Chantal Julien for technical assistance. This work was supported by the Natural Sciences and Engineering Research Council of Canada. 0.2

t’ 0.01 -30

j

-10

10





3

30

50

70

TEMPERATURE

Fig. 4. Temperature shift.

dependence



90

110



5

130

I1

150

(“C)

of the oscillator

frequency

reference frequency from a Fluke 6060A synthesizer. The reference frequency was chosen to be slightly higher than the oscillator frequency at the lowest usable temperature. Typical results are shown in Fig. 4 and give a sensitivity of about 80 ppm/“C. This result can be advantageously compared to that of 33 ppm/“C previously published for a LST-quartz temperature sensor [3]. The difference from the theoretical value is mainly due to loading effects of the electrodes. The characteristic of the device is to a very good approximation linear over the whole usable temperature range from -30 to +150°c. We can estimate the overall sensitivity of the device by using the value u = lop9 found by Hauden et al. [3] for quartz temperature sensors over a 10 s period. This gives a value of r = lop5 “C, which would represent a significant improvement over existing devices. Detailed studies of 0 and r as a function of the design parameters are planned for the next phase of this work.

Conclusions The present work shows that the acoustic resonator concept has considerable promise for making very sensitive temperature sensors. Such a device would be of interest for the temperature regulation of ovens for stable oscillators and for use in microcalorimeters for studying chemical reactions. This device would probably not be cost competitive with others for routine temperature measurements, but we believe it has definite promise for those applications where a very high temperature sensitivity is required.

References D. Hauden, Mesures de grandeurs physiques par les variations de ftiquence d’un oscillateur, Onde E/e&r., 68 (4) (1988) 40-44. D. Hauden, G. Jaillet and R. Coquerel, Temperature sensor using SAW delay tine, Proc. IEEE Ultrasonic Symp., Chicago, IL, U.S.A., 1981, pp. 148-151. D. Hauden, Miniaturized bulk and surface acoustic wave quartz oscillators used as sensors, IEEE Trans. Ulfruson., Ferroelectr. Freq. Control, UFFC-3q2) (1987) 253-258. J:Neumeister, R. Thum and E. Liider, A SAW delay line oscillator as a high-resolution temperature sensor, Sensors and Actuators, A21-A23 (1990) 670-672. T. M. Reeder and D. E. Cullen, Surface-acoustic-wave pressure and temperature sensors, Proc. IEEE, q5) (1976) 754-756. V. V. Zhdanova, V. P. Klyuev, V. V. Lemanov, I. A. Smimov and V. V. Tikhonov, Thermal properties of lithium niobate crystals, Son Phys. Solid Stare, 1q6) (1968) 1960-1962. D. Hauden and G. Theobald, Dynamic thermal sensitivity of SAW quartz oscillators, Proc. IEEE Symp., Boston, MA, U.S.A. 1980, pp. 264-266. J. A. Barnes, A. R. Chi, L. S. Cutler, D. J. Healey, D. B. Leeson, T. E. McGunigal, J. A. Mullen Jr., W. L. Smith, R. L. Snydor, R. F. C. Vessot and G. M. R. Winkler, Characterization of frequency stability, IEEE Tram. Instrum. Meas., IM-20(2) (1971) 105-120.

Biographies Martin View was born in Mont&al, Qu&ec, Canada, in 1964. He received the B.Sc.A. degree in electrical engineering in 1987 from Sherbrooke University, Sherbrooke, Qu&ec, Canada. He is presently studying for the Ph.D. degree in electrical engineering. Since 1988, his major areas of research have been sensor applications of acoustic wave devices. He is also interested in microelectronics and its applications in sensor devices. David Cheeke received his Ph.D. in solid-state physics at Nottingham University in 1965. He is director of the microelectronics research laboratory at Sherbrooke University. His research activities are in the area of ultrasonic microscopy and acoustic sensors. As of July 1, 1990, he will be Operations Director of SIRICON Lt&e, Montrkal.