a method of forming a multiple level porous silicon substrate for semiconductor integrated circuits including anodizing non - porous silicon layers of a multi - layer silicon substrate to form multiple levels of porous silicon . at least one porous silicon layer is then oxidized to form an insulating layer and at least one other layer of porous silicon beneath the insulating layer is metallized to form a buried conductive layer . preferably the insulating layer and conductive layer are separated by an anodization barrier formed of non - porous silicon . by etching through the anodization barrier and subsequently forming a metallized conductive layer , a fully or partially insulated buried conductor may be fabricated under single crystal silicon .