Research On Process Of Polysilicon Nanofilm Pressure Sensor

There is a broad application prospects for Polysilicon nanofilm(PSNF) because of its favorable piezoresistive properties.PSNF pressure sensor chip is made in order to get better application of PSNF.Therefore,the main purpose of the paper is to do the researches in the process design simulation and optimization of pressure sensor chip based on PSNF, and find out the best process condition finally.Low Pressure Chemical Vapor Deposition(LPCVD) method is the core technology which applied to form PSNF with superior piezoresistive properties.The best process condition is that the temperature of deposition is 620℃;the thickness of film is between 80nm and 100nm;simultaneity,the doping concentration is about 3×10²⁰cm^-3.The strain coefficient of PNSF can reach 34 under the process condition above mentioned which is more than 25%higher than the normal polysilicon thin film.The optimal silicon cup corrosion of KOH system can be found out through the comparative analysis of a few corrosives which are anisotropic in the aspect of structure processing.The process design of insulation layer and passivation layer is particularly important for protecting the resistance of PSNF from pollution and oxidation and ensuring favorable piezoresistive properties of the film.Repeatability and linearity can be influenced and the reliability of sensor can be reduced by the residual stress which is caused by back to the room temperature and the different coefficient of thermal expansion,because of the PSNF and passivation layer is made at high temperature.To this end,SiO2,Si₃N₄ or the composite films of the both are made in the passivation layer by chemical vapor deposition for the basic structural features of the PSNF pressure sensor chip.The relationship between stress distribution and structure of the passivation layer is simulated and analyzed by finite element analysis software.The results show that the thermal stress can be effectively reduced caused by the mismatch with reasonable control of the thickness of the Si₃N₄- SiO₂-Si₃N₄ composite passivation film.Thus the design method of passivation layer structure of piezoresistive thin film is given. The chip is made according to the optimal technology condition in the paper.The test result shows that the sensitivity is 10mV/MPa.V;non-linear error is 0.006%FS; hysteresis error is 0.12%FS;repeatability error is 0.07%FS;zero temperature coefficient is 0.062%FS/℃;the sensitivity temperature coefficient is -0.1%FS/℃;the whole precision of the sensor is 0.18%FS.The performance of the PSNF pressure sensor which is made based on the optimal technology condition is so excellent.Meanwhile,the feasibility and the superiority of the technology condition discussed in the paper are conformed and the theory of polysilicon tunneling piezoresistive effect is validated.