Annealing of oxide fixed charges in scaled polysilicon gate MOS structures

Annealing of oxide fixed charges (QF) under polysilicon gate in scaled MOS structures was studied. Our results indicate that, even for a gate width as small as 1.25 µm, QFunderneath the polysilicon gate is unaffected by further processing steps, including high-temperature oxidizing ambients. In other words, the QFtriangle reduces to a horizontal line, even for scaled down polysilicon gate MOS devices. This result has important practical implications, because poly-Si gate is the dominating MOS technology today. A two-dimensional oxygen diffusion model is proposed to explain this phenomenon. Numerical solution was carried out based on the finite difference method. It will be shown that the polysilicon gate not only acts as a barrier to oxygen above the gate oxide, it also keeps oxygen away from the SiO - Si-substrate interface under the gate edges, thus very effectively shielding the gate oxide from the ambient.