Monitoring of Porous Silicon Layers Used for Epitaxial Wafer Production with Inline Reflectance Spectroscopy

Manufacture of photovoltaic silicon wafers through epitaxy directly from the vapor phase with the so-called sintered porous silicon method holds significant cost-saving potential compared with the traditional ingot growth and subsequent multiwire sawing because of the absence of kerf loss. In this method, the surface of a substrate wafer is porosified and sintered to provide a growth platform for the epitaxial layer and to enable its later separation. As a newly industrialized technology, kerf-free wafering requires addressing specific characterization needs. One of these requirements is a reliable quality control of as-etched porous silicon (PSi) layers. In this article, we establish a basis for mass production-scale process control of PSi layers used for epitaxial wafer fabrication by utilizing inline spectrophotometry. Specifically, we compare the reflectance spectra of the porous layers between two different inline spectrophotometers and a laboratory reference spectrophotometer. We analyze these spectra with a model-based fitting approach, relying on local optimization methods and accurate initial guess finding to maximize the speed of the fitting procedure. For model verification, we compare the fitted parameters to scanning electron microscope (SEM) images. Here, we observe a close correlation between the model-based fits and SEM in a variety of different porous structures, resulting in a mean relative deviation of 5%-11% between the two methods depending on the type of the analyzed layers. As a useful property, the inline setups inherently allow line-scan measurements across the wafers, enabling the determination of parameter profiles to evaluate the uniformity.