An improvement of porous silicon technology for local thermal isolation on bulk crystalline silicon is presented. The technique consists of forming an air cavity below the porous layer to increase the thermal isolation efficiency. Both porous silicon and the cavity underneath are formed during the same electrochemical process in two steps: in step 1 the current density used is below a critical value, and in step 2 it is switched to a value above the critical current for electropolishing. In this way, porous silicon is formed first, followed by the formation of the cavity underneath. Experimental results and simulations are shown together with an application of this process in a thermal silicon flow sensor. (C) 2004 The Electrochemical Society.