Transistor and physical properties of polycrystalline silicon films prepared by infralow-pressure chemical vapor deposition

Abstract

The infralow-pressure chemical vapor deposition (ILPCVD) system has been developed to reduce the partial pressure of silane (PSiH4) to the sub-mTorr order by increasing the pumping speed, while aiming at the improvement of as-deposited polycrystalline silicon (poly-Si) film qualities. The films prepared by the system show better physical properties than ordinary low-pressure chemical vapor deposition (LPCVD) films at fixed temperature (600 °C), so that the low-temperature processed as-deposited poly-Si thin film transistors can be easily and significantly improved, having ON/OFF current ratios of more than 108. Physical analyses have confirmed that the films deposited at temperatures as low as 555 °C by the ILPCVD system are undoubtedly polycrystalline. The fabrication of poly-Si TFT’s through a low-temperature process confirms good semiconductive behavior of the films, even when deposited at 555 °C. The superiority of the ILPCVD over other LPCVD’s is explained by the deposition kinetics. The nature of LPCVD as-deposited polysilicon film is characterized by the competition between surface reaction rate and gas-phase mass-transfer rate. High-temperature or infralow-pressure deposition is controlled by gas-phase mass transfer of the reactant. The mass-transfer controlled deposition is found to be preferable to obtaining good polycrystalline silicon film, namely high crystallinity, 〈100〉 preferred orientation, large grains, and good semiconductive properties.