Thick porous silicon formation using implanted mask technology

Surface micromachining is an established microtechnology. The process is on ly limited by sacrificial layer thickness and sometimes a disturbing surfac e topology. This paper describes an innovative surface micromachining techn ology. Standard surface micromachining allows layer thicknesses of a few mi crometer. Using porous silicon as sacrificial layer, it is possible to crea te any layer thickness up to 100 mum. Thick porous silicon sacrificial laye rs are used to combine the advantages of standard surface micromachining wi th the advantages of bulk micromachining. The problems resulting from surfa ce topology are eliminated by using ion implanted masks. Based on different porous silicon formation mechanisms for n- and p-type silicon, it is possi ble 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 ad ditional 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 mum. The use of this sacrificial layer technology for the rmally isolated gas sensor membrane fabrication and the detailed process pa rameters will be presented. (C) 2001 Elsevier Science B.V. All rights reser ved.