Modelling and simulation of SAW delay line sensors with COMSOL Multiphysics

Surface acoustic waves (SAW) devices are used in several domains varying from telecommunication and mechanics to chemical and biological sensing. Their ability to function in either liquid or gaseous environments make them devices of choice for the detection of several types of analytes (DNA, antibodies/antigens, gases, pesticides, heavy metals …) in real time and without labeling. Compared to other chemical and biological sensors, the SAW ones are highly sensitive and can achieve very low limits of detection. Among the large variety of acoustic devices, SAW sensors are the most promising ones, as their operating frequencies can be varied from several hundred MHz to a few GHz. However, they are still the subject of constant development; therefore, simulation becomes a powerful tool to design optimised structure for highly sensitive devices. This study concerns 3D Finite Element Method (FEM) simulation of SAW sensors using COMSOL Multiphysics software. A SAW delay line was designed on 36°lithium tantalate (LiTaO₃) piezoelectric substrate. Interdigital transducers (IDTs) were patterned in Cr/Au (20 nm/80 nm) layers with the same periodicity of λ = 40 μm. A metallized Cr/Au (20 nm/80 nm) sensing area of 80 µm length separates the input and output IDTs. The corresponding frequency is thus of order of 104 MHz. Simulations were made on well-known structure to define a model that can account for and predict the electrical behaviour of SAW transducers for the future optimisations. The results show good agreement between COMSOL numerical simulation and experimental S21 spectra. Accordingly, we can use the built model for simulations intended to optimize the structure of devices, mainly for increasing their sensitivity.