In recent years, surface acoustic wave devices used in bio-sensing applications have demonstrated high sensitivity in the detection of fluid properties such as density, viscosity, stream velocity. In this paper, a more effective measurement of the SAW sensor structure is presented. | Vietnam Journal of Mechanics, VAST, Vol. 34, No. 4 (2012), pp. 225 – 236 ATTENUATION COEFFICIENT FOR SURFACE ACOUSTIC WAVES IN FLUID REGION Bui Thu Hang, Bui Duc Tung, Nguyen Tien Dat, Chu Duc Trinh University of Engineering and Technology, VNU, Vietnam Abstract. In recent years, surface acoustic wave devices used in bio-sensing applications have demonstrated high sensitivity in the detection of fluid properties such as density, viscosity, stream velocity. In this paper, a more effective measurement of the SAW sensor structure is presented. It is reported that at density of 6 g/cm3 , the amplitude of mechanical wave is excited while for electrical signal, attenuation at 3 g/cm3 reaches a peak. In our analysis, single–crystal Aluminium Nitride substrate is used. Several parameters of leaky waves including displacement, decay constant in the liquid media are analyzed. Keyword: Rayleigh – Surface Acoustic Wave sensor (R–SAW), liquid sensor, Aluminium Nitride. 1. INTRODUCTION Saw devices have been applied for sensing chemical and physical features in gas and liquid phases. A mechanical wave in the piezoelectric crystal which is generated by metal electrodes or interdigital transducers (IDTs) placed travels through substrate [1, 2]. It includes a Rayleigh and a shear mode which propagate through the surface as shown in Fig. 1. The Rayleigh mode, termed Rayleigh wave, is a combination of longitudinal Fig. 1. Acoustic wave propagation direction in Cartesian coordinate system. (a) Compressional or longitudinal; (b) Shear vertical; (c) Shear horizontal [3]. and shear vertical particle displacement while the shear mode, termed Shear Horizontal – Surface Acoustic Wave (SH–SAW), is a shear horizontal wave on the surface [4–6]. 226 Bui Thu Hang, Bui Duc Tung, Nguyen Tien Dat, Chu Duc Trinh While the shear component is very attractive for liquid sensors, the presence of the normal displacement component is a reason why Rayleigh–SAW devices are poorly suited for liquid .
