Study On Piezoresistive Principle And Properties Of Polysilicon Nanofilms

Polysilicon films, due to its favourable piezoresistive properties, have been widely applied in MEMS piezoresistive sensors. In this dissertation, research findings have shown that polysilicon nanofilms (PNSFs) are a promising material with more predominant piezoresistive properties. However, up to now, the piezoresistive properties of PSNF have not attracted the attentions of researchers inland and overseas, so that the fundamental researches on this aspect are almost vacant. Therefore, the PSNFs with different microstructure were deposited by low-pressure chemical vapour deposition (LPCVD) technology in order to investigate the piezoresistive properties of PSNF systematically. Based on the experimental findings, the theoretical model of poly-Si piezoresistive properties was established, which laid the groundwork for application of this nanostructured functional material.The existing pizoresistive theories of poly-Si, proved by the experiments and applications on piezoresistive properties of common polysilicon films (thickness is approximately from several hundred nanometers to several micrometers), were built up based on a large number of experiments at the back end of last century. In existing theories, the larger the grain is, the higher the gauge factor (GF) of poly-Si films is; in the case of heavy doping concentration, the higher the doping concentration is, the lower the GF is. In general, the thinner films are, the smaller the grain is. Hence, according to the existing piezoresistive theories of poly-Si, PSNF (around or less than 100nm in thickness) should possess a low GF. In contrast, it was found experimentally that GF increased with increasing doping concentration in the case of ultrahigh doping concentration (above 10²⁰cm^-3) for PSNF and that GF increased with decreasing grain size. Although these phenomena cannot be explained reasonably by the existing poly-Si piezoresistive theories, it indicated that PSNF possessed favourable piezoresistive properties in ultrahigh doping concentration. Depending on this property, the problem that temperature characteristics are improved leading GF to reduce for common poly-Si films can be resolved perfectly. The microstructure of PSNF was characterized by scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD), and the relationship of piezoresistive properties of PSNF and film microstructure was presented. Meanwhile, PSNF samples with different doping concentration were prepared by ion-implant technology, and then they were tested to obtain the dependence of piezoresistive properties on doping concentration. The test results provided reliable data for fabrication of PSNF with optimized piezoresistive properties and laid the groundwork for theoretical investigation.In order to reveal complex piezoresistive properties of PSNF, the conception of tunneling piezoresistive effect was presented. The mechanism was illuminated using quantum tunneling effect and energy band split decoupling theory, consequently, a new model of poly-Si piezoresistive properties–– tunneling piezoresistive model (TPM) was established. The experimental demonstrated that the tunneling piezoresistive model was not only suited for PSNF, but also suited for common poly-Si films–– it is a more comprehensive poly-Si piezoresistive model. Finally, based on the tunneling piezoresistive model, the experimental results of PSNF were analyzed completely. The conclusions indicated that as the deposition temperature, film thickness and doping concentration were approximately 620℃, 80-100nm and 3×10²⁰cm^-3, respectively, PSNF possessed optimal piezoresistive properties. Under the conditions, GF can reach 34 (25% higher than that of common poly-Si films); the resistance temperature coefficient (TCR) can be less than 10^-4/℃(almost an order lower than TCR of common poly-Si films); the gauge factor temperature coefficient (TCGF) can be less than 10^-3/℃(lower than half TCGF of common poly-Si films).