New porous silicon formation technology using internal current generation with galvanic elements

Abstract

Presently, two porous silicon formation technologies are published, anodization in an electrochemical cell and stain etch without external current in a hydrofluoric acid/nitride acid (HF–HNO 3) solution. For anodization an external current is necessary in order to achieve porous silicon thickness up to 100 μm. Stain etch is an electroless process, and the porous layer thickness is limited to a few micrometers. A novel porous silicon formation technique that combines the advantages of thick layer anodization and electroless stain etch will be shown. A current generated by a galvanic element of silicon and a precious metal on the backside of a silicon wafer in a hydrofluoric acid (HF)/hydrogen peroxide (H 2O 2)/ethanol electrolyte is utilised in order to generate porous silicon. In this case the silicon operates as anode and the metal as cathode for current generation. This current is similar to the external current needed for anodization. Beside the standard porous silicon etch solution HF and ethanol an oxidising agent H 2O 2 is used to support the etch process and to generate a higher etch rate. Etch rate control is given by concentration of etching solution and metallization. Different kinds of metallizations and etching solutions were investigated.