Complete dielectric isolation by highly selective and self-stopping formation of oxidized porous silicon

Abstract

Complete dielectric isolation of silicon regions 45 μm wide for microcircuits on silicon substrates is demonstrated by selective formation of oxidized porous silicon in heavily doped n-type regions. Anodic etching of n-type silicon in a hydrofluoric acid electrolyte exhibits a concentration-dependent voltage threshold. This voltage dependence allows the porous silicon formation process to be selective to heavily doped regions and self-stopping on lightly doped regions. Rapid oxidation of this porous silicon yields an oxide with dielectric properties approaching those of standard thermally grown silicon oxides. This process is an improvement over previously reported processes utilizing porous silicon in that wider regions (up to 300 μm) are dielectrically isolated by an oxide whose thickness is controllable and uniform so that stress and wafer warpage are minimized.