Research On Surface Acoustic Wave High-temperature Mechanical Sensor Based On AlN Piezoelectric Film

A mechanical sensor is a device that converts various mechanical parameters into electrical signals.High temperature mechanical sensors,especially the high temperature pressure sensors and high temperature strain sensors,play a huge role in fields like aerospace,petrochemical,weaponry and automotive energy.For a long time,many countries all over the world have attached great importance to the development of high temperature mechanical sensors,and have put a lot of scientists and money in this high-end sensor field.The research and development of high temperature mechanical sensors in our country have lagged behind.Most of high-end high-temperature mechanical sensors are still imported.The domestic sensors are only in the low-and middle-end markets,and they can not meet the increasingly demanding requirements of various industrial sites.Therefore,accelerating the development of high-end high-temperature mechanical sensors is of great significance to promoting the development of the national economy and breaking the monopoly of foreign industries.The surface acoustic wave?SAW?sensors have the characteristics of small size,high precision and passive wireless.It has long been paid great attention and are in line with the development trend of the future's sensor technology.With the development of high-temperature piezoelectric materials such as aluminum nitride?AlN?piezoelectric films and gallium silicate?LGS?piezoelectric single crystals,high-temperature surface acoustic wave sensors using these materials have also shown great prospects.In this paper,aiming at the scientific and technical problems in the research of high temperature surface acoustic wave mechanical sensors,we have developed high temperature surface acoustic wave strain sensor and pressure sensor based on aluminum nitride piezoelectric film and SOI wafer materials.These two sensors have several wave modes and thus higher strain/pressure sensitivity,and can be temperature-compensated and enable the mechanical parameters measurement with two methods.The main work of this thesis includes:?1?The research status of high temperature pressure sensor and high temperature strain sensor with various working principles and the development status of surface acoustic wave high temperature sensor based on various piezoelectric materials are reported.The development of surface acoustic wave high temperature pressure sensor and high temperature strain sensor are concluded to be feasible.?2?The theory of surface acoustic wave excitation and transmission on the surface of the medium is discussed,and it is proved that the possibility of presence of both Rayleigh waves and Lamb waves in surface acoustic wave devices by controlling the substrate thickness under the AlN film.?3?The measuring principle of strain sensor and pressure sensor are analyzed respectively.The analysis methods of strain sensitivity and pressure sensitivity are introduced.The common temperature compensation scheme is discussed and a new dual-chip temperature compensation scheme is proposed.Dual-chip temperature compensation and strain/pressure measurement methods are given,and the sensor chips for strain and pressure measurement are respectively designed.Finite element simulation of the fabricated sensor chip was carried with COMSOL software to deduce the possible excited acoustic modes.?4?The AlN piezoelectric film was successfully fabricated by magnetron sputtering deposition process and characterized.The magnetron sputtering preparation process of bismuth?Sc?doped AlN piezoelectric film were preliminarily studied.The structure of the sensor chips was adjusted according to MEMS processing technology,and the micro-machining has been completed,involving the AlN deposition process,Mo Interdigital transducer preparation process,SiO₂ anti-oxidation layer preparation process and backside deep silicon etching process.?5?A dual-chip high-temperature strain sensor using the fabricated sensor chips was fabricated,and the corresponding high-temperature tensile strain test platform was designed and demonstrated.The static characteristics test at room temperature revealed the presence of four obvious acoustic modes,i.e.the Rayleigh wave,A₄ mode Lamb wave,S₄ mode Lamb wave and S₅ mode Lamb wave.The strain responses of the four modes at room temperature were tested,and it was found that these three Lamb waves had much better strain sensitivity than Rayleigh wave,and the S₄ mode Lamb wave in chip-a shows the best sensitivity and resolution.It was found that at 250?with fluctuating temperature,the tensile strain measurement is seriously impacted by the temperature fluctuation,so two temperature compensation and strain measurement methods,are proposed.One method is to use the beat frequency calculation with the Rayleigh wave and S₄ mode Lamb wave in chip-a,and the other method is to use the beat frequency calculation with the S₄ mode Lamb waves in both chip-a and chip-b.It was theoretically and experimentally proved that both beat frequency calculation methods can achieve strain test in environment with temperature fluctuation,and also,with the strain sensitivitys improved.?6?A dual-chip high-temperature pressure sensor using the fabricated sensor chips was fabricated,and the corresponding high-temperature pressure test platform was designed and demonstrated.The static characteristics test at room temperature revealed the presence of three obvious acoustic modes,i.e.A₄ mode Lamb wave,S₄ mode Lamb wave and S₅ mode Lamb wave.The strain responses of the three modes at room temperature were tested,and it was found that the S₄ mode Lamb wave in chip-a shows the best sensitivity and resolution.It was found that at 250?with fluctuating temperature,the pressure measurement is seriously impacted by the temperature fluctuation,so the temperature compensation and strain measurement method,to use the beat frequency calculation with the S₄ mode Lamb waves in both chip-a and chip-b,are proposed.It was theoretically and experimentally proved that this beat frequency calculation method can achieve pressure test in environment with temperature fluctuation,but with the pressure sensitivity decreased.