Porosity and thickness characterization of porous Si and oxidized porous Si layers - An ultraviolet-visible-mid infrared ellipsometry study

摘要

This paper suggests the evaluation of morphological parameters of porous silicon layers (PSL) using spectroscopic ellipsometry from UV to mid-infrared optical range. PSL were prepared by electrochemical etching of monocrystalline silicon wafers in hydrofluoric acid-based electrolyte. Measuring with an optical and an infrared ellipsometer with a wide spectral range permits an accurate characterization of PSL properties from the top surface to the bottom of the layer with thicknesses from several hundred nanometers up to a few tens of micrometers. Several different optical models for ellipsometric evaluations were developed to determine the thickness, the average porosity, the in-depth porosity gradient, the oxidation level and the surface roughness of the PSL. Porosity was modeled with multiple effective medium layers by varying ratio of crystalline silicon, void and oxidized silicon wherever needed. Thin PSL (<5 mu m) show no impact of current density on porosity and thickness. However, evaluation of thick PSL (20-50 mu m) highlights the in-depth porosity gradient. Thickness values were also cross-checked with electron microscopy confirming the proposed ellipsometric models. Additionally, different oxidation techniques have also been compared in terms of oxidation level and void content. Volume expansion during PSL oxidation follows exactly the same behavior as that during the oxidation of planar silicon wafers. (C) 2016 Elsevier Inc. All rights reserved.