Abstract: A method for local thinning of a top silicon layer of a SOI wafer includes the consecutive steps of: providing a SOI wafer which successively includes a bottom silicon layer, a buried oxide layer and a top silicon layer; successively forming a silicon dioxide layer and a polysilicon layer over the top silicon layer; etching the silicon dioxide layer and the polysilicon layer until a top surface of the top silicon layer is exposed, such that a pattern is formed in the silicon dioxide layer and the polysilicon layer; oxidizing the silicon dioxide layer and the polysilicon layer and concurrently oxidizing the exposed portion of the top silicon layer until the polysilicon layer has been completely converted to an oxide, thereby forming a cap oxide layer; and removing the cap oxide layer, so that a locally thinned area is formed in the top surface of the top silicon layer.

Abstract: A method for forming a semiconductor device is provided, which may include: providing an interlayer dielectric layer, a metal layer formed on the interlayer dielectric layer, an etch stop layer formed on the metal layer, and a first opening extending through the etch stop layer and the metal layer, wherein the interlayer dielectric layer is exposed from the first opening; forming a protecting layer on the sidewall of the first opening to cover the metal layer; after forming the protecting layer, forming a second opening by etching a portion of the interlayer dielectric layer; and forming an isolating layer by filling up the second opening, wherein the isolating layer includes an air gap. The semiconductor device is more stable in performance.