Application of porous silicon formation selectivity to impurity profiling in p-type silicon substrates

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Abstract
A new type of application of porous silicon formation is proposed, which does not deal with the material properties, but with the characteristics of the involved electrochemical silicon dissolution reaction. The anodization potential of p-type silicon in concentrated hydrofluoric acid under galvanostatic conditions has been shown to be characteristic of the silicon doping concentration at the interface and independent of the porous silicon-layer thickness. This close correlation between potential and doping level is used to determine concentration profiles of p-type implanted dopants. During anodization of silicon presenting dopant concentration variations, the anodization potential varies according to silicon doping concentration. The potential values can be converted to dopant concentration, and the electrolysis time scale to a depth scale, leading then to the doping impurity profile. This method, which is used as well for deep and shallow diffused impurities, exhibits a very good agreement with spreading resistance measurement and secondary-ion mass spectrometry analysis. Simultaneous anodization of regions of different doping concentration has also been investigated. It allows the characterization of the porous silicon formation selectivity by a selectivity coefficient, which is measured on a large range of silicon doping concentration. The selectivity is found to be strongly dependent on the doping level: it is nearly infinite for samples presenting highly doped regions (greater than 3×1018 cm−3), but decreases sharply when the doping concentration is below 1018 cm−3.