FEM Modeling of the Temperature Influence on the Performance of SAW Sensors Operating at GigaHertz Frequency Range and at High Temperature Up to 500 °C

In this work, we present a two-dimensional Finite Element Method (2D-FEM) model implemented on a commercial software, COMSOL Multiphysics, that is used to predict the high temperature behavior of surface acoustic wave sensors based on layered structures. The model was validated by using a comparative study between experimental and simulated results. Here, surface acoustic wave (SAW) sensors consist in one-port synchronous resonators, based on the Pt/AlN/Sapphire structure and operating in the 2.45-GHz Industrial, scientific and medical (ISM) band. Experimental characterizations were carried out using a specific probe station that can perform calibrated measurements from room temperature to 500 °C. In our model, we consider a pre-validated set of physical constants of AlN and Sapphire and we take into account the existence of propagation losses in the studied structure. Our results show a very good agreement between the simulation and experiments in the full range of investigated temperatures, and for all key parameters of the SAW sensor such as insertion losses, resonance frequency, electromechanical factor of the structure (k²) and quality factor (Q). Our study shows that k² increases with the temperature, while Q decreases. The resonance frequency variation with temperature shows a good linearity, which is very useful for temperature sensing applications. The measured value of the temperature coefficient of frequency (TCF) is equal to -38.6 ppm/°C, which is consistent with the numerical predictions.