Investigation On Piezoresistive Properties Of Polysilicon Nanofilm And Its Application For Pressure Sensors

Polysilicon film, due to its favorable piezoresistive properties, has been widely used in piezoresistive sensors. The previous researches have shown that the polysilicon nanofilm have better piezoresistive properties than common polysilicon film, and have promising future of application.In this paper, the piezoresistive characteristics of polysilicon nanofilm are investigated, including the influence of process on piezoresistive properties and theoretical calculation of Young's modulus of the film. Based on the studies, application of the film on pressure sensor is investigated.The influences of the process on piezoresistive properties of the film were studied. The optimized process parameters for best piezoresistive properties were obtained. Here, the piezoresistive properties of the film are followed: gauge factor can reach 34 which is higher about 25% than that of common polysilicon film under same high doping concentration; temperature coefficient of resistance is less than 1×10-4/℃which is almost an order lower than that of common polysilicon film; temperature coefficient of gauge factor is about 1×10-3/℃which is about half of that of common polysilicon film. The optimized process parameters provide the necessary design rules of polysilicon nanofilm pressure sensor. The influence of process on piezoresistive nonlinearity is investigated also. The piezoresistive nonlinearity of the film is analyzed; the results show that the piezoresistive nonlinearity of the film is mainly influcened by piezoresistive nonlinearity of grain boundary. The relation between piezoresistive nonlinearity and doping concentration was studied, and the results provide the design rules of the sensor too.The Young's modulus of the film is studied. The Young's modulus is a parameter which associate piezoresistive coefficient and gauge factor, and is an important parameter in structure design of the sensor. In tunneling piezoresistive theory, the Young's modulus is obtained by Young's modulus of single silicon multiply a modified factor which is not explained reasonably. In structure design of the sensor, to make the results of finite element analyses(FEA)simulation are more close to reality, the Young's modulus is necessary as simulation parameter. The microstructures of the film were observed by scanning electron microscope and transmission electron microscope, the grain model of the film was built according to characters of growth and structure. Based on the grain model, the method to calculate the Young's modulus was presented, and the Young's modulus was theoretically calculated. The Young's modulus of the film was measured by in-situ Nano mechanical test system. The comparision between theory results and experimental results is conducted, and theory results agree with experimental results. The grain model can analyze the Young's modulus and the calculation method is valid. The studies of Young's modulus complement the tunneling piezoresistive theory, and provide simulation parameter in structure design of the sensor.Based on the studies of piezoresistive properities, application of the film on pressure sensor is investigated.The FEA of the sensor was performed; the sensors's structure is optimized designed according to the results of FEA. Utilizing the film as piezoresistors of sensor, the complete process of the sensor was described, and the key technology in sensor's fabrication was solved, finally the sensor was developed. The measured range of the sensor is 0-0.6MPa. Because of the better high temperature piezoresistive properties of the film, the input-output characteristics of the sensor were tested from 0℃to 200℃. The comaprision between polysilicon nanofilm pressure sensor and common polysilicon film pressure sensor, other high temperature pressure sensor were made, the polysilicon nanofilm pressure sensor provide advantages of high sensitivity, low temperature drift and simple fabrication process.The study of the sensor is completed, and the study results lay a foundation for application of the film to piezoresistive sensor.