Spectroscopic Inline Characterisation of Partially Processed Epi Wafers after Porosification

With the potential of reduced energy consumption and kerf loss, epitaxial wafer growth is a promising technological innovation compared to the standard ingot growth and wafering techniques. Transferring the technology to mass production requires a fast and robust inline control of the relevant quality parameters in different production stages to ensure high production yield and wafer quality. In this work, we present an inline characterisation method for the quality control after porosification of the substrate wafer. We have integrated an inline spectrophotometer in our automatic wafer inspection system (AWIS) to address this aim. The installed setup measures the reflectance spectra of the wafer surface after electrochemical porosification. Since the following stages of EpiWafer fabrication crucially depend on the quality of the porous layers, monitoring their physical properties is essential and can be achieved by optical modelling of the reflectance spectra. As modelling precision strongly depends on spectrum quality, we made a measurement comparison of the integrated inline spectrophotometer with other inline and offline tools. After appropriate calibration, a qualitative as well as a quantitative correlation with a maximum mean percent error (MPE) of 8% between inline AWIS and offline spectra is found. Over a timespan of almost 11 months, reference measurements with a special porous silicon (PorSi) wafer have been performed. From these results a quality control metric has been defined to ensure automatic reliable results in production.