Selective Etching of Doped Polysilicon Layers

The implementation of p-type passivating contacts is a promising approach to reduce overall recombination in tunnel oxide passivating contact (TOPCon) or back-contact solar cells. Due to the high absorption coefficient in highly doped silicon layers, the polysilicon layer should be confined to the regions near the contact, which requires a structuring process of full area deposited layers. We report on a co-diffusion process using doped phosphosilicate glass layers from APCVD, with a drive-in during BBr3 diffusion to generate differently doped polysilicon layers in a single thermal process. By exploiting the lower etching rate of highly boron-doped polysilicon layers, we demonstrate the ability to selectively remove n-type polysilicon while only marginally affecting the p-type polysilicon layer, using either alkaline texturing or atmospheric dry etching (ADE) with F2 gas. This doping-type selective etching is more pronounced in the ADE process. Symmetric samples show a very low level of surface recombination of 5fA/cm² for boron-doped polysilicon layers after passivation.