Influence of Polysilicon Deposition Conditions on Advanced 3D NAND

Abstract

As the conductive channel, undoped channel hole polysilicon (poly-Si) plays a significant role in 3D NAND. Two important properties i.e. grain size and step coverage (S/C) of undoped channel hole poly-Si have attracted great attentions in practical manufacturing. The grain size affects the mobility of conductive poly-Si film and large grain size usually leads to high mobility. The step coverage influences the subthreshold swing (SS) and improved step coverage leads to small SS. In this paper, during the growth of poly-Si, the influence of different deposition conditions, such as temperature and gas source ratio between Si2H6 and SiH4, on the grain size and step coverage is investigated comprehensively. Grain size and step coverage are characterized using by Transmission Electron Microscope (TEM) and Precession Electron Diffraction (PED). It is found that as the decrease of deposition temperature, both the grain size and step coverage are improved. Compared to gas source SiH4, the employment of Si2H6 is beneficial in achieving large grain size to some degree and no further improvement can be obtained when the flow of Si2H6 is increased to a certain value. However, as-deposited poly-Si exhibits poorer step coverage using Si2H6 in contrast to SiH4.