Polysilicon Passivation - Tunneling Oxide Routes and Annealing Conditions Effect on Passivation

TOPCon type structures can result in exceptional wafer passivation, even surpassing 500 ms for high-resistivity n-type wafers. These passivation structures are also known to be very time stable. However, the processing is energy intensive. This structure involves a very thin tunnelling layer followed by a heavily doped polysilicon layer. The highest passivation levels are often obtained when growing this tunnelling layer by dry oxidation of the silicon wafer substrate at or above 600 °C. The doped polysilicon is then deposited by PECVD, followed by a high temperature annealing step. In this work, we have investigated several routes to the tunneling oxide in TOPCon passivation structures and how different annealing temperatures and annealing atmospheres(inert, hydrogen) affect the passivation. The tunneling oxides are divided into oxidation and deposition types. It was found that for our set of samples, oxidation through dry oxidation or nitric acid gave superior passivation than deposition of silicon oxideand silicon oxynitride through PEALD and PECVD. After the tunneling oxide, an approximately 70 nm thick layer of phosphorous doped a-Si was deposited at 200 °C. The wafers were then annealed at different temperatures from 300 to 900 °C.