Study On SAW Temperature Sensor Performance Based Apodization Weighting

Surface Acoustic Wave(SAW) sensor has many advantages such as small form factor and enhance sensitivity,above all,and it is easy to implement wireless and passive sensing. As a result, it can be widely applied in the sensing applications. The good frequency response characteristics make echo signal of SAW temperature sensor is easily detected. But the frequency response of existing SAW temperature sensors is generally poor. Therefore, the study of improving the SAW device frequency response characteristics has an increasingly important role. In this thesis, we focus on the work to strengthen the sensing performance of SAW temperature sensor by improving frequency response characteristics of device which include main lobe height and bandwidth.First of all, the piezoelectric crystal materials, prototype structures and reflecting grating structure are studied, respectively. Then the SAW velocities of different piezoelectric substrate materials are simulated by employing the theory of boundary condition determinant, electromechanical coupling coefficient(K2)、and temperature coefficient of frequency(TCF) are calculated using the simulation results. On that basis of the simulation and calculation, focusing on the actual needs of wireless and passive temperature sensing, 64° YX-cut lithium niobate and single-port resonator structure are selected as the designed SAW temperature sensor piezoelectric substrate material and device prototype structure, respectively.Secondly,the design method of apodization weighting is employed to improve the frequency response characteristics of SAW temperature sensor. Various SAW temperature sensors with different parameters or structures are designed. All of the designed devices are made by photolithography and lift-off process. The design method of apodization weighting brings troubles to fabrication of optical lithography mask, therefore, a novel mask design method based on MATLAB-cif-Ledit process is proposed. Mask layout files are accurately and quickly generated by employing the method.Furthermore, the network analyzer(NA) and the frequency-temperature test platform which use NA as the main equipment are described; meanwhile, the frequency characteristics at room temperature and frequency-temperature characteristics within the selected temperature range of single-port SAW resonator are tested.Finally, based on these experimental results, the impacts of design parameters on the performance of the SAW temperature sensors are analyzed and summarized:(1) The design method of apodization weighting can effectively improve the side lobe suppression capability of the SAW device, but meanwhile, it increases the bandwidth of the main lobe;(2) Wavefront is distorted when dummy fingers for interdigital transducers(IDTs) are removed. However, compared to the IDTs with dummy finger, the IDTs without dummy finger have lower side lobe level;(3) The main lobe height can be increased by Chebyshev window function weighting coefficients, but the bandwidth is increased;(4) The side lobe level falls when the finger number of IDTs and reflecting grating increase, but the extent is generally small;(5) The main lobe bandwidth decreases with the number of IDTs;(6) Metallization ratio affects the center frequency of the SAW devices, and this effect is difficult to be precisely controled.