Modelling and wave velocity calculation of multilayer structure SAW sensors

Purpose - The purpose of this paper is to model multilayer structure surface acoustic wave (SAW) sensors, incorporated in CMOS or micro-electro-mechanical system integrated circuits, and to derive the corresponding wave velocity as an analytic expression in terms of the layers' thickness and density, which is suitable for analysis and design.Design methodology approach - The method is based on an electro-mechanical equivalent model of multilayer structure SAW sensors. A multilayered SAW device is represented by a two-port electrical equivalent circuit consisting of three parts: input transducer, output transducer, and between them the delay line, which is the sensing part. The sensing part is modelled as a mechanical two-port network. The wave velocity is calculated using analogy between the mechanical and electrical quantities and the fact that the wave motion of the SAW extends below the surface to a depth of about one wavelength.Findings - The presented model predicts very efficiently and accurately the velocity of SAW sensors with multilayer substrates in the case where the thicknesses of upper layers are much smaller than the signal wavelength. The velocity can be calculated from the formula, so that elaborate numerical computations involving partial differential equations are avoided.Research limitations implications - The model and the velocity calculation can be applied only to acoustically thin upper and middle layers where acoustically thin means that a layer is sufficiently thin and rigid (large shear modulus). The presented results provide a starting-point for further research in the analysis and design of sensors fabricated using AlGaN, GaN, AlN diamond.Practical implications - Since the majority of SAW sensors is designed with acoustically thin layers, the proposed model and calculation can be of interest for many practical material combinations. The presented model and calculation can be used in most cases of the optimal sensor design with respect to the sensor sensitivity or required area on the sensor chip.Originality value - The paper presents a new original model of multilayer structure SAW sensors and a new method of SAW velocity calculation. The method gives good results, with much simpler calculations than in the wave equation method, in cases where certain layers are acoustically thin.