advanced micromachining processes for micro-opto-electromechanical components

This paper shows a new type of microfabrication technique which does not use the standard sacrificial layer deposited, but which uses directly the conversion of crystalline silicon by selective anodization into porous silicon structure in a predetermined p+ region. A buried p+ layer has been embedded in n-type Si wafers using either high energy (HE) B implants or low energy (LE) B implants followed by epitaxial growth of n-type Si. The samples undergo electrochemical etching in an anodization cell. Before anodization, reactive ion etching was used to define trenches that allow to expose the buried layers to the etching solution. The anodization processes transform the buried p-type layer into porous Si while leaving the n-type regions untouched. A subsequent low temperature oxidation process has been used to transform the porous Si structure into an oxide layer. This produces a fully insulated Si island consisting of the n-type layer originally above the p+ buried layer and laterally defined by the trenches. The selective dissolution of porous silicon dioxide layer, after the oxidation, or the dissolution of the porous silicon structure, before the oxidation with a KOH solution, demonstrate that this process is fully suitable for both surface and bulk micromachining processes from surface.