Theory of chemical sensors using surface acoustic waves

Surface acoustic waves in solids, especially Rayleigh waves, have a number of useful applications. These involve ultrasonic nondestractive testing, portable radio communication systems (including mobile telephones), and different types of sensors. A special very important application of surface acoustic waves propagating along inhomogeneous subsurface layers is chemical sensors used for detecting and measuring concentrations of surrounding gases or liquids [l-7]. The work of these devices is usually based on change in vplocities of surface acoustic waves as a result of their interaction with liquid or gas molecules adsorbed by specially deposited selective thin films. Despite a number of successful experimental investigations of surface acoustic wave chemical sensors, their current theoretical description is far from satisfactory. In the existing theoretical papers only a qualitative explanation of the sensor's sensitivity to measured gas or liquid concentrations is given, referring to the proportionality ofa sensor response to the mass loading from the deposited thin films (see, e.g., t2,3]). It is usually assumed that the influence of elastic properties of the film material can be neglected in comparison with the mass loading that, generally speaking, is not true. Besides, the influence of the adsorbed gas molecules themselves is not discussed at all. This is probably not surprising considering the fact that it can not be readily incorporated into the classical continuum theory used in [2,3]. The nonclassical, or better to say semi-classical, approach to the theory of Rayleigh wave propagation in layered structures with adsorbed molecules, which describes surface modifications in terms of integral surface parameters [8-10], is free of these limitations and, in our opinion, provides the most advanced description of surface acoustic wave chemical sensors. In the present paper we briefly describe a general approach to the theory of Rayleigh surface acoustic wave propagation along inhomogeneous surfaces modified by distributed adsorbed atoms or molecules and discuss its implications for surface acoustic wave chemical sensors. The problem of sensor sensitivity to concentrations of surrounding gases is analysed, including possible side effects, such as the influence ofgas molecules on the elastic properties of selectively adsorbing films.