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

Modeling and Electrical Characterization of a Bilayer Pt/AlN/Sapphire One Port Resonator for Sensor Applications

This paper presents a two-dimensional FEM (Finite Element Method) modeling and simulation of a surface acoustic wave (SAW) resonator based on a layered Pt/AlN/Sapphire structure. Such structure that exploits the electromechanical coupling of piezoelectric film is of high interest for harsh environments. By harsh environment we mean any environment that could hinder the operation of the device. Hardness can come from a variety of sources, and examples include the following: High pressure, High temperature, Shock/high vibration, Radiation, Harsh chemicals, etc. As part of this work, we are looking for high temperature sensor applications and only operating drifts due to temperature will be studied. SAW resonator is made from piezoelectric thin film Aluminum Nitride (AlN) layer on Sapphire substrate. Modal analysis is used to determine the eigen mode and the eigenfrequency of the system and the study of the frequency domain is used to determine the response of the model under influence of a harmonic excitation for one or more frequencies. In the FEM modeling, various parameters of the surface waves in the films, such as the surface velocity, the displacement of the piezoelectric thin film, the electrical potential, the electromechanical coefficient (k2), and the quality factor (Q) were studied. A comparative study between modeled and experimental curves showed a good agreement and allowed to validate our simulation method. Finally, a FEM study of the influence of normalized thickness of AlN thin film on resonator performances was carried out and compared with theorical results of literature