Mechanism of low-temperature polycrystalline silicon growth from a SiF4/SiH4/H2 plasma

Abstract

A model for the low-temperature growth of poly-Si by plasma-enhanced chemical-vapor deposition using SiF4/SiH4/H2 gases is presented. The model is based on the existing so-called etching model in which growth and etching take place simultaneously. In this model a set of chemical reactions are postulated. The crucial factors to obtain high-quality poly-Si films are (1) the flux of precursors, (2) the concentration of F radicals in the vicinity of the growing surface which determines the etching rate, and (3) the H-covered surface which ensures long diffusion length of precursors. The flow rate of SiH4 [factor (1)] determines whether the film becomes crystalline or amorphous, and variation in the other gas flow rates and plasma parameters affect factors (2) and (3). According to the model the electrode spacing and rf power predominantly determine the concentration of F radicals diffused to the growing surface, while the gas pressure changes the residence time of radicals which predominantly affects the etching reaction. Natural consequences of the model are that an excess supply of F radicals will in turn deteriorate the crystallinity by stripping the hydrogen covering the surface and increasing nucleation sites. The crystallinity of poly-Si films prepared by changing the above plasma parameters are characterized by x-ray diffraction, and their dependence on the above parameters are found to be consistent with the model. A high degree of hydrogen exchange between the growing surface and the plasma is observed by secondary-ion-mass spectroscopy for the film prepared with SiF4/SiH4/D2 gases.