Inline Characterization of Ultrathin Amorphous Silicon Stacks in Silicon Heterojunction Solar Cell Precursors with Differential Reflectance Spectroscopy

In this article, we present a characterization technique for thin-film layers on textured surfaces with random pyramids using reflectance spectroscopy and an optical model based on the transfer-matrix method and rigorous polarization ray tracing. The optical model fits the thickness of ultrathin amorphous silicon (a-Si) layers from the measured reflectance using spectrophotometry and the measured optical constants using spectral ellipsometry. The estimated a-Si layer thickness from the optical model is compared with the measured thickness from transmission electron microscopy (TEM) images. Modeling the absolute reflectance spectrum, the a-Si stack thickness is underestimated by 51% mainly due to nonidealities such as varying pyramid base angles and scattering effects that are difficult to consider in the optical model. Modeling alternatively the differential reflectance spectrum, the a-Si stack thickness is determined in accordance with TEM measurements with relative error as low as 10%. Fitting the relative change in reflectance before and after a-Si deposition to determine the layer thickness makes the optical model robust against instrumental inaccuracies and superposed nonidealities. The on-the-fly nature of the developed optical characterization technique makes it suitable for high-throughput industrial applications.