Ellipsometry Characterization and Modelling of Porous Silicon Layers: Utilizing Different Absorption Depths to Aid Modelling

Ellipsometry is a powerful and establish characterization tool for porous silicon (PS) and other thin film materials on silicon substrates. PS is often made through electrochemical etching in an HF-containing solution. Etch rates, pore dimensions, porosity level variations in the etch direction etc, depends on the electrical current and the chemistry of the solution among other. In many cases, this leads to interface layers towards the substrate-PS and air-PS interfaces, gradients and even optical anisotropy if the pore dimensions are large enough. In this work, we show that we can utilize the large wavelength-dependent variations in absorption depths to model the different parts of such samples independently, simplifying the modelling process and allows us to solve some aspects of the sample structure without being required to solve the whole structure which can be complex. Ellipsometry data and modelling of one such sample having both porosity gradients, interface layers and a thin and denser layer towards the surface is shown below. Such complexities for PS layers are well-known in literature, but can still be challenging to solve compared to a simple homogeneous film. We show that both the UV-blue and NIR ranges are necessary for a complete solution, but also that these ranges can be used to solve the top and bulk depths independently.