Accurate characterisation of silicon nitride films on rough silicon surfaces by ellipsometry

Spectroscopic ellipsometry is the technique of choice for determining the material properties of a- SiNx:H thin films on polished crystalline silicon (c-Si) surfaces in a fast and non-destructive way. However, c-Si surfaces in the PV sector are typically roughened in order to decrease reflection losses at the front surface of silicon wafer solar cells. Spectroscopic ellipsometry on rough c-Si surfaces with random features is very challenging, due to the depolarisation of the reflected light. This depolarisation strongly affects the ellipsometry measurement and makes it almost impossible to accurately extract the thin film properties by a model-based analysis. In this work we show that, despite these difficulties, it is possible to obtain accurate data for dielectric films on rough c-Si surfaces with random features, by analysing the ellipsometry data at specific photon energies which correspond to destructive interference (i.e., minimal reflection) at the sample surface. Silicon nitride films are deposited by PECVD onto polished and as-cut Si wafers and are then analysed by the proposed method. The results are subsequently corroborated by transmission electron microscopy.