Thick porous silicon formation using implanted mask technology

Abstract

Surface micromachining is an established microtechnology. The process is only limited by sacrificial layer thickness and sometimes a disturbing surface topology. This paper describes an innovative surface micromachining technology. Standard surface micromachining allows layer thicknesses of a few micrometer. Using porous silicon as sacrificial layer, it is possible to create any layer thickness up to 100 μm. Thick porous silicon sacrificial layers are used to combine the advantages of standard surface micromachining with the advantages of bulk micromachining. The problems resulting from surface topology are eliminated by using ion implanted masks. Based on different porous silicon formation mechanisms for n- and p-type silicon, it is possible to use n-implanted layers as masking material during the anodization of p-type silicon, resulting in a plane surface without any steps which would be generated using a hard surface mask. For this masking technology, no additional masking layers are required. For free-standing membrane generation it is possible to deposit, e.g. a PECVD-layer on top of the porous silicon layer. A complete process flow has been developed for thick porous silicon layers up to 100 μm. The use of this sacrificial layer technology for thermally isolated gas sensor membrane fabrication and the detailed process parameters will be presented.