Sensors mid Actuators B, 12 (1993) 53-56
SAW temperature and humidity sensor with high resolution Ling Mingfang and Li Haiguo Department of Information and Electronics Engineering, Zhepang University, Hangzhou 310008 (China)
Abstract A surface acoustic wave (SAW) temperature and humidity sensor with high resolution is presented in this paper . The sensor is based upon a SAW delay-line oscillator which is implemented in a 128° rotated Y-Z LiNbO, substrate . Two SAW interdigital transducers (lUT6) for the delay line have been developed and analysed . The experimental frequency response of the oscillator is in good agreement with the theoretical analysis . The detection sensitivities of the temperature and humidity sensor for the single SAW delay line are -6 kHz/°C and -3 kHzf RH% . The frequency response of the sensor is found to decrease nearly linearly with temperature or humidity . The sensitivity of a humidity sensor using temperature compensation is 3 kHz/RIP/..
1 . Introduction Increasing attention and interest have recently been paid and directed to surface acoustic wave (SAW) sensors, because of their small size, low cost, inherent high sensitivity, resolution, stability, frequency digital output, broad measuring range and high input impedance . A SAW has a high density of acoustic energy in the near-surface region of a crystal, making it an extremely sensitive probe of the physical characteristics of a surface . This feature has been exploited to construct a number of sensors, including pressure sensors , gas and vapour detectors , flow sensors , voltage sensors , acceleration sensors , etc. Hauden et al.  described a SAW temperature sensor on a quartz crystal with a linear temperature dependence and high sensitivity . Joshi et al.  made humidity measurements with a SAW sensor fabricated on a 128° Y-X LiNbO, substrate coated with polyimide. The purpose of this paper is to describe a SAW temperature and humidity sensor with high resolution and sensitivity based on a SAW delay-line oscillator. The SAW delay fines are fabricated on a 128° Y-Z LiNbO 3 substrate. In order to obtain a high determined resolution of the SAW sensor, the high short-term frequency stability of the SAW delay-line oscillator, which depends on its phase noise sideband spectral density, is crucial . In this paper carefully designed SAW delay-fine IDTs and an amplifier with low noise coefficient are applied to decrease the phase noise sideband spectral density of the SAW oscillator . The centre frequency of the SAW delay-line oscillator is 74.015 MHz and the untuned and unmatched insertion loss is 5 .5 dB. A thin film of polyelectrolyte humidity-sensitive material (Napss) is coated on the propagation path by a SAW. The deter-
mined sensitivities of the temperature and humidity sensor for a single SAW delay line are -6 kHz/°C and -3 kHzfRH%, respectively.
2. Principle and design The block diagram of a SAW sensor is shown in Fig . 1 . It consists of a SAW delay line connected with the feedback path of an amplifier, which constructs a delayline stabilized SAW oscillator . When the amplifier gain is greater than the circuit loss and a certain phase condition for oscillation is provided, the frequency fo of a SAW delay-line oscillator is given by  fo =n/(td+ 0
where t d is the acoustic time delay due to SAW propagation, r° is the delay due to transmission through the electric circuit and n is a mode integer which is selected by the frequency passband of the SAW transducers . Under general conditions td > tr , so that to first order eqn. (1) is well approximated by fo=n/td=nV,ld where V, is the SAW phase velocity and d is the distance between the two IDTs.
a Fig, 1 . SAW oscillator .
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The velocity change due to different interface perturbations, such as pressure (p), temperature (T), mass loading (m), rigidity (C), permittivity (a), conductivity (e), etc ., can be expressed by
AV, - 0V,/am Am + aV /aT AT + 0V,/ao Aa +BVacAs+OVfap4p+aV,/BC AC
Because n and d are constant, any interface perturbation can change the frequency of the SAW oscillator according to
Aflf =AV,I V
We made a SAW temperature sensor on a 128° Y-Z LiNb03 crystal . The frequency shifts of the SAW sensor follow a polynomial expansion as a function of temperature, which can be expressed as follows : fo(T) =f(Tox1+ao(T-To)+bo(T - To) 2 +
where ao and b o are temperature coefficients of the first and the second order of the frequency at reference temperature To and a is the dynamic temperature coefficient of the frequency at To . If the variations are slow, the last term of relation (4) can be neglected . To make a SAW temperature sensor have large sensitivity and good linearity, ao will be as large as possible and b o null . Experimental results show the 128° Y-Z LiNbO3 crystal has a good temperature characteristic . When a thin humidity-sensitive material film is coated on the propagation path by a SAW, the magnitude of the frequency shift can be described by Af=(k,+k2)
sensor is proportional to ARH when n is approximately one. This has been confirmed by experimental results . In order to improve the stability of the SAW sensor, we developed and analysed two SAW delay-line IDTs as shown in Fig . 2(a) and (b). Based on the Taker model, their frequency responses are derived with a S function [8) as follows :
H,(f )I =Isin(R, nX)l(R, rX)I x Ism((P+R,)RX)/((P+R,)nX)I x Isin(R2nX)/(R2 rrX)I H2(f)I=Isin(R2nX)!(R 2 nflr where
X=(f-falfo fo is the acoustic synchronism frequency and f is the freqency of the external signal voltage . The other geometric parameters are shown in Fig. 2 . After optimization, the theoretical frequency responses of the two SAW delay lines are shown in Fig . 3(a) and (b), respectively .
Af = (k, + k 2
A Pa~~ -
where k, and k2 are material constants for the SAW substrate, f, is the unperturbed resonance frequency, h is the thickness of The film, p 'is the film density, V,is the unperturbed Rayleigh wave velocity, Z' is the Lame constant and p' is the modulus of the overlay film . hp' is the mass per unit area . If the chemically selective film is an elastomeric organic polymer, it is often possible to neglect the mechanical property term since the elastic modulus is relatively small . Thus eqn . (5) can be simplified to
(a) Ladder mTs; (b) homogeneous
When the environment humidity changes, the thin humidity-sensitive film absorbs the aqueous vapour and a mass per unit area (hp')'= m o"P~=hp'+m0"ARHPW
where mo is the aqueous vapour molecule coefficient absorbed per unit area, n is related to the temperature and material, ARH is the humidity change, Pw is the saturated vapour pressure at a certain temperature and P„ is the vapour pressure. From eqns . (6) and (7) we obtain that the frequency shift of the SAW humidity
(b) Fig . 3 .(a) Frequency responses with (a) ladder IDTs and (b) with homogeneous mTs.
As is well known, the minimum amount detectable by SAW sensors is limited by the short-term stability of the SAW oscillator, which is related to the phase noise band spectral density L(f)  :
L(f) = 10lg[(G2KTpFfo)/(4trn2pe Af2)] dBc/Hz where G is the power gain of the amplifier, F is the operational noise figure, p o is the saturated output power of the amplifier, n is an integer and K is the Boltzmann constant. It is apparent that the lowest noise performance will be obtained by using a low insertion loss (IL) SAW delay line and an amplifier with low noise and high power. In our design SAW delay lines with ladder IDTs and low-noise transistors are utilized, which made L(f) small and gave a high short-term frequency stability of the SAW oscillator.
3. Experimental results The two SAW delay-line IDTs shown in Fig . 2 were fabricated on a 0 .5 mm thick 128° Y-Z LiNbO 3 substrate . Their construction parameters and electrical properties are fisted in Table 1 . The experimental frequency response photographs of the two SAW delay lines are shown in Fig . 4(a) and (b), respectively . The frequency response is in good agreement with the theoretical calculation . The SAW delay line with ladder transducers connected with a low-noise amplifier is used to measure temperature and humidity when a thin film of NaPss polyelectrolyte is coated on the region between the input and output IDTs . Experiments are implemented at variable temperatures and set humidities . Figure 5 shows the temperature-sensitive response of the SAW temperature sensor . Its frequency is found to decrease linearly with temperature, with a slope of -6 kHz/°C. Figure 6 shows the humidity-sensitive response of the SAW humidity sensor . Its frequency is
TABLE 1 .
Construction parameters and electrical properties
(b) Fig . 4. and (b)
Experimental frequency response with (a) ladder homogeneous IDTs.
NJ 73 .8 S T 73 .7 V G 73 .6 P Or `~- 77 .5
73 .4 40.0
tempereture C Cent frequency (MHz) Input 1DT pairs Output IDT pairs Aperture (mm) Centre to centre distance
74 .015 30 3 (4 uumbers) 1 .0 5.7
75.003 50 20 0 .59 7 .0
3 dB bandwidth (MHz) Untuned and unmatched insertion loss (dB)
Fig . 5 .
Temperature-sensitive curve .
found to decrease nearly linearly with humidity, with a slope of -3 kHz/RH% . The linearity of the response seems to be not very good because of the influence of temperature . This influence can be reduced by a dual delay-fine configura-
56 4. Conclusions
73 .5 -
SAW temperaturee and humidity sensors coated with a thin film of NaPss polyelectrolyte are implemented on a 128° Y-Z LiNbO3 crystal . The resolution of the SAW temperature sensoris -6 kHz/°C and that of the humidity sensor is 3 kHzJRH% after temperature compensation .
N 73 .4 I X73 .4 v c 73 .3 v N -73 .3
e06-7'd6"80' 90.0 tO .o 40 .0 50.0 .6' relative humidity RH%
The authors are grateful to Mr Xionggiang Tao, electronic engineer, for his help in the manufacture of the devices .
Fig . 6. Humidity-sensitive curve. References I M . R . Risch, Precision pressure sensor using quartz SAW sensors, Sensors and Actuators, 6 (1984) 127-133. 2 H . Wohltjen .. The selective detection of vapors using SAW device, IEEE UltrasonicsSymp. Proc ., Son Francisco, CA, USA, 1985, pp . 66-70 . 3 N . Ahmad, Surface acoustic waveflow sensor, IEEE Ulirasonics Symp . Proc ., San Francisco, CA, USA, 1985, pp. 483-485. 4 R . Inaba, Y . Kasahara and K. Wass, An electrostatic voltage sensor using SAW, IEEE Ultrasonics Symp. Proc ., San Francisco, C4 . USA, 1982, pp . 312-316. 5 T. B . Bonbrake, C . A . Erikson and D . Throw, SAW accelerometers : integration of thick and thin film technologies . IEEE Ultrasonics Symp . Proc ., San Francisco, CA, USA, 1985, pp . 591-594 . 6 D . Hauden, G. Jaillet and R . Coquerel, Temperature sensor using SAW delay-line, IEEE Ultrasonics Symp . Proc., Chicago, IL, USA, 1981, pp . 148-151 . 7 S . G. Joshi and J . G . Brace, Measurement of humidity using SAW, IEEE Ultrasonics Symp . Proc ., San Francisco, CA, USA, 1985, pp . 600-602 8 W. R . Smith, H . M . Gerard, J . H . Collins, T. M . Reeder and H . J. Shaw, Design of SAW delay lines with interdigital transducers, IEEE Trans. Microwave Theory Technol., MTT-17 (1969) 865-873 . 9 S. K. Salmon, Practical aspects of SAW oscillators, IEEE Trans . Microwave Theory Technol. . MTT-27 (1979) 1012-1018 .
reference cecillator Fig . 7 . Dual delay-time configuration .
900 .0 =
700 .0 N I Y 500.0 tL d 300.0
, 50 .0 60.0 70.0 80 .0 90 .0 relative humidity RH%
Fig, 8 . Humidity-sensitive curve of the dual delay-line configuration .
Lion as shown in Fig. 7, where one delay line is used for reference and the other is coated with a thin film of NaPss polyelectrolyte . The signal output from the two oscillators is mixed in a double equilibrium mixer . Their difference frequencies are then obtained through the low-pass filter. The frequency difference of the dual delay line versus the relative humidity is shown in Fig. 8 . The linearity seems very satisfactory: 1% change in the relative humidity could produce a 3 kHz frequency variation .
Biographies Ling Mmgfang was born in Shanghai, China, in 1943 . She graduated from the Department of Electronic Engineering of Jiao-Tong University, Xi'an, in 1965 . From 1986 to 1987, she was a visiting scholar at the Electrical Engineering and Applied Computer Department, Toronto University, Toronto, Canada. She has published a number of research papers . She is an associate professor at Zhejiang University . Her current research is in SAW devices and thin-film sensors, Li Haiguo was boa in Anhui Province, China, in 1965 . He graduated from the Electromagnetic Department of Van Electric Scienceand Technology University in 1987. He is a graduate student in the Information and Electronic Engineering Department of Zhejiang University .