Color variations of silicon-on-insulator wafers with silicon device layer thickness

The perceived colors of silicon-on-insulator (SOI) wafers with etched Si surface layers of thickness 90 nm to 30 nm vary from turquoise to purple to golden. Measured reflectance curves spanning ultraviolet, visible, and near infrared wavelengths have an amplitude modulated oscillatory pattern. Multilayer reflectance calculations indicate the oscillatory pattern results from the 2 µm thick buried SiO₂ layer which functions as a nearly lossless reflective Fabry-Perot etalon in the near infrared where SiO₂ and Si are transparent. The amplitude modulation of the oscillatory pattern in the visible region where Si is absorbing results from thin-film effects in the Si surface layer. The changing modulation with Si thickness and wavelength results in the observed color variations. It is therefore possible to estimate the Si thickness based on the etched SOI wafer color observed visually. This ability is useful when initially optimizing the etching process to produce a specific thin Si layer, for example when fabricating photodiodes or other photonic devices on SOI wafers. Accurate measurements of the Si thickness and the buried SiO₂ thickness can be performed by analyzing the modulated oscillatory reflectance curve using multilayer reflectance calculations.