An Experimental and Theoretical Study of Impact of Device Parameters on Performance of AlN/Sapphire-Based SAW Temperature Sensors.

Hongrui Lv,Defeng Lin,Zhe Liu,Yun Zhang,Zhe Cheng,Bingliang Guo,Lifang Jia,Yinglong Huang,Yujie Ai,Boyu Dong

doi:10.3390/mi13010040

Abstract

The impact of device parameters, including AlN film thickness (hAlN), number of interdigital transducers (NIDT), and acoustic propagation direction, on the performance of c-plane AlN/sapphire-based SAW temperature sensors with an acoustic wavelength (λ) of 8 μm, was investigated. The results showed that resonant frequency (fr) decreased linearly, the quality factor (Q) decreased and the electromechanical coupling coefficient increased for all the sensors with temperature increasing from −50 to 250 °C. The temperature coefficients of frequency (TCFs) of sensors on AlN films with thicknesses of 0.8 and 1.2 μm were −65.57 and −62.49 ppm/°C, respectively, indicating that a reduction in hAlN/λ favored the improvement of TCF. The acoustic propagation direction and NIDT did not obviously impact the TCF of sensors, but they significantly influenced the Q and of the sensors. At all temperatures measured, sensors along the a-direction exhibited higher fr, Q and than those along the m-direction, and sensors with NIDT of 300 showed higher Q and values than those with NIDT of 100 and 180. Moreover, the elastic stiffness of AlN was extracted by fitting coupling of modes (COM) model simulation to the experimental results of sensors along different directions considering Euler transformation of material parameter-tensors. The higher fr of the sensor along the a-direction than that along the m-direction can be attributed to its larger elastic stiffness c11, c22, c44, and c55 values.

Highlights

AlN-based surface acoustic wave (SAW) sensors have attracted considerable attention for wireless temperature sensing, as they are well suited for harsh environments or for placement on rotating machinery [1,2,3]
We investigated the impact of device parameters including AlN film thickness, number of interdigital transducers (NIDT ), and acoustic propagation direction on the temperature coefficient of frequency (TCF), the quality factor (Q), and the effective electromechanical constant (Kt2 ) of AlN/sapphire-based SAW temperature sensors
The results indicate that a reduction in hAlN /λ favors the improvement of the TCF

Summary

Introduction

AlN-based surface acoustic wave (SAW) sensors have attracted considerable attention for wireless temperature sensing, as they are well suited for harsh environments or for placement on rotating machinery [1,2,3]. Elastic stiffness [cij ] with different acoustic propagation directions has been extracted based on finite element method simulation (FEM) without considering the direction-dependent relations of physical parameters, including [cij ], piezoelectric constants [eik ] and dielectric constants [εik ] along different directions. This extraction method is not physically reasonable. The possible physical reasons for the origin of anisotropy of [cij ] along a- and m-direction of c-plane AlN films was analyzed These experimental and theoretical investigations may be helpful for the development of AlN-based SAW temperature sensors in the future

Experimental Methods

Discussion

The resonant frequency shifted from

Conclusions