Advances in the understanding of phosphorus silicate glass (PSG) formation for accurate process simulation of phosphorus diffusion

Abstract

The diffusion of phosphorus mediated by phosphorus oxychloride (POCl3) is extensively used in photovoltaics due to its enhanced diffusion speed and to the high controllability of the phosphorus at the wafer surface. However, the POCl3 decomposition in the gas phase in phosphorus pentoxide (P2O5) as well as the subsequent oxidation reactions induced by P2O5 and O2 at the silicon surface are not very well documented and increase the complexity of a realistic simulation of the POCl3 diffusion. It is shown in this paper that the PSG growth model of Ghoshtagore [2] should apply correctly at least in a quantitative relative way to the PSG grown during a POCl3 diffusion. This model allowed to show that the PSG contains a thin layer of 8.6 nm, presumably composed of SiO2, which acts as a diffusion barrier with a subsequent impact on the phosphorus diffusion in silicon. In order to interpret ECV profiles of emitters obtained with various POCl3 flows, a qualitative model based on the Ghoshtagore model is presented. This model emphasizes the role of O2 at the silicon/PSG interface not only as a mediator of the PSG growth, but also as a generator of self-interstitials in silicon that will enhance the extension of the ECV profile tail. It also emphasizes the role of SiP precipitates in reducing the interstitial diffusion in silicon.