Characteristics Research Of Li Doped ZnO Multilayer Thin Films Acceleration Sensor

Based on the demand of environmental vibration monitoring,a wafer level Li doped ZnO multilayer thin films acceleration sensor based on MEMS process is designed and and fabricated in this paper.The ZnO Piezoelectric theoretical model is constructed,the preparation process of ZnO thin film and Li doping concentration are optimized,the basic structure of Li doped ZnO multilayer thin films acceleration sensor is given,and the theoretical model is constructed.The double piezoelectric layer structure and MOSFET series structure are used to optimize the output sensitivity.The wafer level acceleration sensor is fabricated based on MEMS technology.By optimizing the fabrication process,the ultra-thin cantilever structure is fabricated to realize the measurement of small acceleration.The Li doped ZnO multilayer acceleration sensor and its optimized structure are tested and analyzed by using a built test system.In this paper,the piezoelectric coefficient matrix and piezoelectric equation of wurtzite ZnO crystal are derived by analyzing the force changes of electric dipole moment of wurtzite ZnO crystal in xy,xz and yz planes.Aiming at the problem of n-type in the actual manufacturing process of pure ZnO thin films,Li impurities are introduced to modulate the conductivity of ZnO films.The existence mode of Li impurities in ZnO is analyzed from the lattice and energy band structure.The results show that Li impurities are easy to form Li _Zn substitution impurities in ZnO under the condition of O enrichment,which makes ZnO films p-type and increases the resistivity,so as to improve the piezoelectric properties.Based on the theoretical analysis,ZnO piezoelectric thin films were prepared by magnetron sputtering.The effects of sputtering power and oxygen argon ratio on the microstructure of ZnO piezoelectric thin films were characterized and analyzed,and the preparation process was optimized.On this basis,the microstructure of Li doped ZnO piezoelectric thin films and the valence content of Li,Zn and O elements are characterized and analyzed.The test of d₃₃ piezoelectric characteristics show that 5 wt%Li doped ZnO piezoelectric thin film has a high d₃₃ piezoelectric coefficient of 7.56 pm/V.The piezoelectric coefficient of d₃₁ is 81.17 p C/N.Using the optimized doping concentration and preparation process of Li doped ZnO piezoelectric thin film,the acceleration sensor with Li doped ZnO multilayer thin films cantilever structure is designed.The structure of piezoelectric layer is optimized,and the basic structure of single and double Li doped ZnO multilayer thin films acceleration sensor is given.Through the force analysis of multilayer thin films,the mathematical model of the sensor is constructed.The theoretical analysis shows that the output voltage sensitivity of double piezoelectric layer is twice that of single layer.In order to further improve the output voltage sensitivity of the designed acceleration sensor,a series optimized structure of MOSFET is proposed.The mechanical and electrical characteristics of single and double piezoelectric acceleration sensors are simulated by COMSOL finite element simulation software.Through the Lift-Off process,the graphic difficulty in the preparation process of multilayer thin films and the short circuit of top bottom electrode are broken through to realize wafer level chip fabrication.For the measurement of micro acceleration,an optimized structure of ultra-thin cantilever beam is proposed,and the wafer level chip is fabricated by wet etching process.Through the built test system,the transient characteristics,frequency response characteristics,dynamic characteristics and sensitivity characteristics of the acceleration sensor are tested.The results show that the transient output voltage sensitivity of single-layer Li doped ZnO acceleration sensor is 46.05 m V/N,which can realize the measurement of acceleration.The dynamic output voltage sensitivity of single and double piezoelectric layer sensors are 26.05 m V/g and 33.08 m V/g respectively,and that of double layer is 1.27 times that of single layer.The output voltage sensitivity of the structure in series with MOSFET is 2.05 V/g.Ultra-thin cantilever beam structure,beam thickness is about 3μm.The sensitivity of the resultant force in the direction of the ultra-thin beam 1 is 31.55 N/g,which shows that the structure can realize the measurement of micro acceleration.In this paper,the miniaturization and integration of Li doped ZnO multilayer thin films acceleration sensor are realized.The sensor can detect the acceleration in vibration environment,and the designed ultra-thin acceleration sensor can detect the small acceleration.