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 oxidepassivated rear surface. The SiNTO process (Silicon Nitride Thermal Oxidation) utilizes a SiNX 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 SiNX 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 SiNX 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 Seco ndary Ion Mass Spectrometry (SIMS) profiling reveal that the SiNX-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.