A Detailed Chemical Model for the Diffusion of Phosphorus Into the Silicon Wafer During POCl3 Diffusion

Philip Jager,Thorsten Dullweber,Ulrike Baumann,Verena Mertens

doi:10.1109/jphotov.2020.3038331

Abstract

The POCl 3 diffusion is the main technology to form the p-n junction of industrial silicon solar cells. However, the diffusion mechanism of phosphorus (P) into the silicon wafer is not fully understood. In this article, we study the P diffusion mechanism during drive-in by systematically varying the drive-in time in the oxygen (O 2 ) atmosphere and subsequently in nitrogen (N 2 ). When increasing the drive-in time in O 2 from 0 to 120 min, the sheet resistance R sheet stays constant at 485±30 Ω/sq. Hence, we demonstrate for the first time that the phosphorus diffusion can be completely suppressed in the O 2 atmosphere. When adding a drive-in in the N 2 atmosphere directly after the drive-in in O 2 , we find that the SiO 2 thickness dSiO 2 ,O 2 changes from initially 2 to 10 nm after O 2 drive-in to an equilibrium SiO 2 thickness dSiO 2,eq of 4.7 nm after N 2 drive-in. We prove for the first time that if dSiO 2 ,O 2 > dSiO 2,eq , no P diffuses into the silicon wafer even in the N 2 atmosphere. Only if dSiO 2 ,O 2 <; dSiO 2,eq , phosphorus diffuses into the silicon wafer in the N 2 atmosphere. We propose a detailed chemical model to explain our experimental results, which assumes that the diffusion of Si from the wafer through the SiO 2 interface toward the PSG plays a key role. In this model, P can only diffuse into the Si wafer if P 2 O 5 in the PSG is reduced by the Si from the wafer to P and SiO 2 .

Full Text