Sensors and Actuators B 91 (2003) 298 302 Liquid sensor probe using reflecting SH-SAW delay line T. Nomura *, A. Saitoh, T. Miyazaki Faculty of Engineering, Shibaura Institute of Technology, 3-9-14 Shibaura, Minato-ku, Tokyo 108-8548, Japan Abstract A new liquid sensor probe has been designed by using SH-SAW device. Shear horizontal mode surface acoustic wave (SH-SAW) has a unique characteristic of complete reflection at the free edges of the substrate. The SH-SAWare excited on a 368 YX LiTaO 3 and the right angle edge of the substrate has been used to reflect the SAW. Multi-channel reflecting SAW delay lines were constructed on the substrate. Both the phase and amplitude of the SH-SAW reflected from the edge were measured as a sensor response. Several experiments were performed to verify the edge reflection type SAW sensor in liquid phase. A sensing system configuration is also suggested for the effective operation of the sensor probe. An electronic circuits system for accurately measuring the phase characteristics of the reflected wave and for detecting the output from the multi-channel SAW sensor probe was shown. # 2003 Elsevier Science B.V. All rights reserved. Keywords: SAW sensor; Surface acoustic wave; SH-SAW; Reflecting SAW delay line; Multi-channel sensor 1. Introduction Surface acoustic wave (SAW) devices are well known as electric devices. These devices have also the ability to directly respond to the mechanical and electrical properties of materials in contact with the device surface. This feature enables them to directly sense mass and mechanical properties of their environment. Therefore a potential application of SAW technique is an environment sensor. In particular, an SAW chemical micro-sensor is of interest because of its compact size and excellent sensitivity. Many studies have attempted to develop the chemical and physical SAW sensors [1 3]. The SAW has two components of particle displacement. One is parallel to the surface along the direction of the wave propagation, and the other is normal to the surface. The desire to sense the liquid phase using SAW devices is complicated by the excessive energy losses experienced at a solid and liquid interface. Displacements normal to the surface generate compressional waves, which dissipate the wave energy in the liquid. Therefore, the liquid phase sensing using the SAW is difficult. It is possible to use shear horizontal mode SAW (SH-SAW) that are not affected by the same energy loss mechanism stated above [4]. Recently, it has been shown that the SAW sensors utilizing the SH-SAW can be designed to sense the liquid properties. Many studies are attempting to develop liquid-phase sensors * Corresponding author. based on SH-SAW devices using 368 YX LiTaO 3 substrates [3,5,6]. Moreover, the SH-SAW produces a complete reflection, if a right-angled incidence is made on a perpendicular edge. This feature has several advantages in the design of the electronic devices or SAW sensors [7,8]. For example, the propagation path can be doubled in length, thereby increasing both the miniaturization and sensitivity of the sensor. This paper presents a reflecting sensor based on the SH- SAW device using 368 YX LiTaO 3 substrates. The fundamental characteristics of the reflecting sensor probe are obtained by measuring the characteristics of the SH-SAW reflection at free edge of the substrate. The properties of the sensor probe show that the reflecting SAW sensor probe is effective for sensing the liquids. A system configuration is then suggested for the effective operation of this probe. In particular, a configuration for accurately measuring the phase characteristics of a reflected wave and for separating the outputs from a multi-channel sensor is proposed. 2. Edge reflection SAW sensor probe 2.1. Structure A schematic view of a reflecting SAW sensor probe is shown in Fig. 1. The sensor probe is constructed of one pairs of interdigital transducers (IDTs) for excitation and 0925-4005/03/$ see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/s0925-4005(03)00103-5
T. Nomura et al. / Sensors and Actuators B 91 (2003) 298 302 299 Fig. 1. Schematic view of the SAW sensor probe using edge reflection (six channel reflecting SAW sensor). SH-SAW shows complete reflection at free edge of the substrate. Fig. 2. Reflected pulse trains obtained by the SH-SAW sensor constructed on 368 YX LiNbO 3 substrate shown in Fig. 1. receiving, arranged on a piezoelectric substrate at an appropriate distance from and in parallel with the substrate edge. The sensor probe is consisted of six channel delay lines having short and grating surface. The feature of the sensor probe is that a twice propagation distance can be obtained to improve the sensitivity, and when impulses or RF pulse are input, the responses of the reflecting sensor are obtained at different delay times according to the propagation distance. Fig. 2 shows the response waveform reflected at the free edge when a tone burst signal having 10 cycles is input in transmitting IDT. In this figure, the three-pulse train (a) depicted in the figure are the direct through signals, and the after three-pulse train (b) are the sensor response reflected by the edge. It is found that the response is separated from the 2.2. Surface SAW guide The propagation characteristics of SH-SAWs depend on the conditions of the surface, particularly on whether the propagation surface is shorted or opened in electrically. When the surface is electrical open, a surface skimming bulk wave (SSBW) is generated and the sensor operations are influenced. However, the electrical characteristics cannot be sensed using a shorted surface alone. Therefore, in this study the surface was shorted periodically using metallic strips instead of completely shorting the surface, and the electrical properties were measured by using the grating channel. Fig. 1 also shows the reflecting sensor having three grating channel, and the other three channels having shorted surface conditions are mounted on one substrate. The sensor probe therefore consists of the three channels with shorted and grating propagation paths. In the grating channel, the electrodes with one-eighth of a wavelength wide were arranged on the SAW propagation path. Fig. 3. Measurement method of liquid property using SH-SAW sensor probe. L is the insertion length in liquid. 3. Characteristics of free edge reflection 368 YX LiTaO 3, which has a large electro-mechanical coupling factor, was used for exciting the SH-SAW. The IDTs have a center frequency of 40 MHz and the electrodes were fabricated of aluminum. The distances from the edge to each output IDT are 200, 250 and 300l (l: wavelength), respectively. Fig. 4. Changes of propagation losses due to the insertion length immersed in liquid. Viscous liquid (glycerol mixtures) is loaded on the grating surface.
300 T. Nomura et al. / Sensors and Actuators B 91 (2003) 298 302 experimental arrangement. The micrometer was attached to the sensor probe, and first inserted into the sample liquid with the perpendicular edge. The output of the reflected wave was measured as a function of insertion length. Fig. 4 shows the result to glycerol mixtures. The output voltages show a linear relationship to propagation length, L, in the viscous liquid. The result also corresponds with the case of relatively large viscosity. It is found that the gradient of the curve depends to the square root of product of the density and viscosity. The result indicates that the sensor is suitable to sense the liquid properties. Fig. 5 shows the results when a conductive liquid, sodium hydroxide solution (NaOH), was used as a sample liquid. Each of figures (a) and (b) shows the outputs for concentrations of 2, 20, and 100 (mm/l) on the short and grating channel, respectively. The results for the shorted surface show no difference between the solution concentrations, i.e., conductivities, as expected; however, the results for the grating surface show different gradients for different concentrations. The difference in gradient of two curves shown in Fig. 5(a) and (b) is due to the electrical effect, which shows a characteristic of the grating structure. The above results also suggest that a SAW gas phase sensor utilizing reflecting SAW sensor system is promising. Fig. 5. Changes of propagation losses due to the insertion length immersed in liquid. A conductive liquid (NaOH solution) is loaded on (a) short surface and (b) grating surface. environmental echoes and the other signal in the time domain. The experiment was conducted using the sensor probe shown in Fig. 1. Fig. 3 shows a representation of the 4. Implementation system In this section, we propose an electronic circuit system that can detect the phase and amplitude of the signals from an edge-reflecting SAW. The phase and amplitude of reflection SH-SAW contain a lot of information about the properties of the surrounding atmosphere. Especially, since the phase of reflecting waves contain both the changes caused in the propagation and at the reflection, accurate measurement of Fig. 6. Schematic diagram of a sensing system for the reflecting SH-SAW delay line sensor.
T. Nomura et al. / Sensors and Actuators B 91 (2003) 298 302 301 Fig. 7. Linearity of phase detector (DBM). Output voltage versus phase deference between the short and grating channels. the phase is very important. In order to obtain the responses at separate times and to measure the phase precisely, tone burst mode is used in this system. Fig. 6 shows a block diagram of a system configuration suitable for accurately measuring the phase and amplitude characteristics of reflected waves. A tone burst signal generated by the pulse oscillator and RF oscillator is feed to the transmitting IDTs. The excited tone burst SH-SAW on each channel is reflected at the free edge and extracted from the output IDTs of each channel. Fig. 2 shows the typical waveform of the output obtained by reflecting SAW sensor. The phase difference between two of these outputs is detected by feeding to a phase detector (double-balanced mixer: DBM). Specifically, the phase of one channel is delayed 908 by the phase shifter to improve the linearity of the DBM. Moreover, the levels of the two signals are then uniformed via the automatic gain-control (AGC) amplifiers before being input to the DBM. These circuits allow for the constant and accurate measurement of small phase changes. Fig. 7 shows the relationship between the DBM phase difference and the output. The output of the Fig. 9. Variation of the phase difference between the short and grating channel as a function of immersed length in NaOH solutions. phase detector changes linearly to the phase difference by up to 508. 5. Characteristics of the liquid sensing system In order to verify the performance of the sensor probe and the electronic circuits system as a liquid phase sensor, fundamental experiments were conducted for liquid solution. Fig. 8 shows a typical output form obtained by the DBM between the short channel (ch. 1) and the grating channel (ch. 6). The length immersed in liquid, L, is 2 mm and the sample is a solution of NaOH. The concentration of the liquid is determined by the response. The variation of the phase difference between the grating and short channel as a function of length is shown in Fig. 9. The variation of the phase (sensor output) varies with a gradient of about 178 per immersed length (8/mm) for the NaOH solution having the concentration of 100 mm/l. The results clearly indicate that it is possible to design the reflecting SAW a priori so as to achieve a desired Fig. 8. Waveform of the reflecting response obtained by the phase deference between the short (ch. 1) and grating channel (ch. 6).
302 T. Nomura et al. / Sensors and Actuators B 91 (2003) 298 302 linear response in the SAW sensor. In this case, the variations of the attenuation are very small in each channel. 6. Conclusion An edge-reflecting SAW sensor based on the SH-SAW device and a tentative implementation system for the sensor probe has been proposed. The SH-SAW are excited on a 368 YX LiTaO 3 and the right angle edge of the substrate has been used to reflect these waves. Multi-channel reflecting SH- SAW delay lines were constructed on the substrate. The fundamental characteristics of the reflecting sensor probe are obtained by measuring the characteristics of the SH-SAW reflection at free edge of the substrate. Both the phase and amplitude of the SH-SAW reflected from the edge were measured as a sensor response. The properties of the sensor probe show that the reflecting SAW sensor probe is effective for sensing the liquids. A sensing system configuration is also suggested for the effective operation of the sensor probe. In particular, an electronic circuits system for accurately measuring the phase characteristics of the reflected wave and for measuring the output from the multi-channel was shown. References [1] D.S. Ballantine, R.M. White, S.J. Martin, A.J. Ricco, G.C. Frye, E.T. Zellers, H. Wohltjen, Acoustic Wave Sensor: Theory, Design, and Physico-Chemical Applications, Academic Press, New York, 1997. [2] R.M. White, Surface acoustic wave sensors, in: Proceedings of the 1985 IEEE Ultrasonics Symposium, 1985, pp. 490 494. [3] S. Shiokawa, T. Moriizumi, Chemical sensor using acoustic wave devices, Trans. IEEJ-C 111 (9) (1991) 357 363 (in Japanese). [4] K. Nakamura, M. Kazumi, H. Shimizu, SH-type and Rayleigh-type surface waves on rotated Y-cut LiTaO 3, in: Proceedings of the 1977 IEEE Ultrasonics Symposium, 1977, pp. 37 40. [5] T. Nomura, A. Saitoh, Y. Horikoshi, Measurement of acoustic properties of liquid using liquid flow SH-SAW sensor system, Sens. Actuat. B 76 (2001) 69 73. [6] T. Nomura, A. Saitoh, S. Furukawa, Liquid sensing system based on two port SH-SAW resonator, in: Proceedings of the 1999 IEEE Ultrasonics Symposium, 1999, pp. 477 480. [7] M. Kadota, J. Ago, H. Horiuchi, H. Morii, Transversely coupled resonator filter utilizing reflection of BGS wave at free edges of substrate, Jpn. J. Appl. Phys. 39 (2000) 3045 3048. [8] T. Nomura, A. Saitoh, T. Miyazaki, Liquid sensor probe using complete reflection at free edge of shear horizontal mode surface acoustic wave, in: Proceedings of the 2001 IEEE International Frequency Control Symposium and PDA Exhibition, 2001, pp. 482 488.