Non-destructive spatially resolved characterization of porous silicon layer stacks

Porosity and thickness of porous silicon layers, enabling the layer transfer process, are very important quantities to control reorganization of pores from the as-etched state to serving as seed layer for high quality epitaxy and predetermined breaking points for epiwafer detachment. Gravimetric determination of porosity allows only determining mean values, and those even only for single but not double layers. In addition, the method is destructive. In contrast, fitting of the optical reflectance of the porosified Si wafer allows getting these layer characteristics in a non-destructive way. We present here a characterization method based on reflectance measurements and a robust exploitation algorithm, which allows for the accurate determination of thickness and porosity of single and double porous layer stacks in a non-destructive way. Taking spatially resolved reflectance spectra allows therefore obtaining both porosity and thickness values with the same resolution. As an example, this method is used here to extract correlations between local porosity and local etching rate that are consistent with known dependencies between porosity and etching current.