The SiNTO process: Utilizing a SiNx anti-reflection layer for emitter masking during Thermal Oxidation

We present a novel method for the industrial fabrication of a silicon solar cell that features an oxide-passivated rear surface. The SiNTO process (silicon nitride thermal oxidation) utilizes a SiN_X layer for masking the front side of the solar cell during the thermal oxidation process. This masking layer prevents the oxidation of the textured and phosphorus-doped emitter surface and limits the growth of the thermal oxide to the uncoated rear surface. After oxidation the SiN_X layer remains at the front side of the cell and serves as an anti-reflection coating (ARC). In this work we investigate the impact of the thermal oxidation process on the SiN_X film and the underlying emitter and analyze the passivation quality of the thermal oxide. The oxidation process results in a sufficiently passivated rear surface with a surface recombination velocity of ~40 cm/s, measured after Al-metallization and post-metallization anneal. Measurements of the emitter sheet resistance and secondary ion mass spectrometry (SIMS) profiling reveal that the SiN_X-coated emitter reorganizes slightly during the oxidation process whereas an uncoated reference is strongly affected. The emitter dark saturation current density is affected as well. Oxide-passivated solar cells are fabricated from Czochralski (Cz) silicon using the SiNTO approach. A 136 cm² large cell fabricated using industrial processing equipment reaches an efficiency of 17.8% (stable), which demonstrates the feasibility of the SiNTO process.