Research On SAW High Temperature Pressure Sensor Based On AlN Piezoelectric Film

There is a great demand for pressure sensors in automobile,aviation or oil exploration industries.Many devices involved in these industries usually work in high temperature environment,which requires that the pressure sensor can also withstand high temperature.The sensor based on surface acoustic wave(SAW)can be used to measure physical parameters in various extreme environments including high temperature,while Al N piezoelectric film can work normally at up to 700?,so the SAW pressure sensor based on Al N film is used in this thesis.Although SAW pressure sensor can work in high temperature environment,when the temperature increases,there is a coupling effect between pressure and temperature,which leads to the deterioration of the performance of the sensor and the inaccurate measurement results.In order to study the problem that the coupling effect of temperature and pressure disturbs the performance of SAW sensor and leads to inaccurate measurement results,the author first studies the pressure sensitivity and temperature stability of the sensor in the initial state,and then further analyzes the changes of pressure sensitivity and temperature stability caused by the coupling effect.The research work and results of this thesis are as follows:(1)A SAW pressure sensor is designed,and the sensor is fabricated by MEMS technology,and then placed in the self-built experimental system for testing.The experimental data of coupling effect leading to the change of test results are obtained to verify the accuracy of simulation research.(2)The pressure coefficient of frequency PCF(PCF indicates pressure sensitivity)and temperature coefficient of frequency TCF(TCF indicates temperature stability)of the sensor in the initial state are studied.In the simulation process,the strain is taken out from the three-dimensional model of the sensor after the pressure is applied,and then loaded into the pre-strain of the two-dimensional model to simulate the state of the sensor under pressure.Finally,the simulation results show that the PCF of the sensor at the reference temperature(20?)is 157.31ppm/MPa(tested as 101.46ppm/MPa).Similarly,the TCF of the sensor under atmospheric pressure is-17.27ppm/?(tested as-35.68ppm/?).(3)This thesis focuses on the influence of the coupling of temperature and pressure on the performance of the sensor,mainly the influence of temperature on PCF and pressure on TCF.Simulation and experiments confirm that PCF increases with temperature,because the increase of temperature leads to the decrease of Young's modulus of films and the increase of thermal mismatch(or thermal stress)between films.Finally,the frequency shift caused by the same pressure is greater,that is,PCF is greater.When the temperature rises from 20? to 150?,if only the contribution of Young's modulus is considered,the PCF increment(?PCF)obtained by simulation is0.016ppm/MPa.When the thermal mismatch is considered,the ?PCF is 23.4ppm/MPa,which is closer to the experimental test result(39.34ppm/MPa).(4)The disturbance of pressure on TCF in the coupling effect is also studied.The simulation and experimental results show that |TCF| decreases with the increase of pressure because the strain caused by high pressure can cancel part of the reduction of effective elastic modulus induced by temperature,so the frequency shift is smaller,and finally |TCF| is lower.The contribution of Young's modulus and thermal mismatch to the reduction of |TCF| is studied by simulation method.It is found that when the pressure rises from 0MPa to 2MPa,the reduction of |TCF| is 0.003ppm/? and0.49ppm/?,respectively.The test of the sensor shows that the increase of pressure leads to the decrease of |TCF| by 0.43ppm/?,which is almost equal to the simulation result when the thermal mismatch is considered.