Investigation of the doping and thickness effects of polysilicon oxide by rapid thermal N2O oxidation

In this paper, polyoxides were grown on n+ doped polysilicon by using rapid thermal N₂O processing. The doping level of the lower polysilicon layer and the polyoxide thickness effect were investigated. Results showed that N₂O oxide grown on medium-doped polysilicon layer exhibited better characteristics than that grown on heavily-doped polysilicon layer. The polyoxide/polysilicon interface of the polyoxide grown on the heavily doped bottom poly-1 layer is smoother than that of the medium doped polyoxide, apparently due to the phosphorus concentration which facilitates SiO₂ viscous flow and prevents oxide thinning and horn formation. However, despite the smoother interface, a large amount of phosphorus via out-diffusion after subsequent oxidation process accumulates at the polysilicon/polyoxide interface and is incorporated into the polyoxide, degrading the oxide quality. Therefore, to obtain better characteristics from N₂O polyoxide grown on a medium-doped polysilicon layer, an appropriate amount of phosphorus and nitrogen should be incorporated. Further, the thicker the oxide, the worse the characteristics, due to the longer oxidation time, which results in a rougher interface, leading to larger charge trapping and smaller Qbd.