The formation of porous silicon by chemical stain etches

Abstract

We report an investigation into the nature of the stain films formed on single crystal silicon by hydrofluoric acid solutions containing small amounts of either sodium nitrite or chromium trioxide. Cross sectional transmission electron microscopy revealed that the stain films retain the crystallinity of the substrate but contain networks of pores on the scale of several hundred angstroms. From the microstructure and electrical properties of the stain films, we deduce that they are porous silicon, conventionally produced by the anodisation of silicon in hydrofluoric acid. We propose a mechanism for the formation process of the stain films and relate it to both the anodic formation of porous silicon and to the chemical polishing of silicon in a mixture of hydrofluoric and nitric acids.