In a process for fabricating a semiconductor device, an DLC (diamond like carbon) film is formed on a principal surface of a semiconductor substrate, and an ashing protecting film is formed on the DLC film for protecting the DLC film from an ashing. A hard mask film having a resisting property against an etching agent for the ashing protecting film and the DLC film, is formed on the ashing protecting film. The hard mask film is patterned using a patterned photo resist film as a mask, and then, the patterned photo resist film is removed by an oxygen ashing. The ashing protecting film and the DLC film is patterned using the patterned hard mask film as a mask, and a trench is formed in the principal surface of the semiconductor substrate using the patterned hard mask film, ashing protecting film and DLC film as a mask. An insulator film is deposited on the whole surface to completely fill up the trench. The deposited insulator film, the hard mask film and the ashing protecting film are etched back by a chemical mechanical polishing, using the DLC film as an etching stopper. The DLC film is removed by the ashing, so that the deposited insulator film remains in only the trench to constitute a trench isolation structure.