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

The impact of device parameters, including AlN film thickness (h_AlN), number of interdigital transducers (N_IDT), 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 (f_r) decreased linearly, the quality factor (Q) decreased and the electromechanical coupling coefficient (Kt2) 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 h_AlN/λ favored the improvement of TCF. The acoustic propagation direction and N_IDT did not obviously impact the TCF of sensors, but they significantly influenced the Q and Kt2 of the sensors. At all temperatures measured, sensors along the a-direction exhibited higher f_r, Q and Kt2 than those along the m-direction, and sensors with N_IDT of 300 showed higher Q and Kt2 values than those with N_IDT 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 f_r 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.