A method for making a semiconductor device includes patterning at least one dielectric layer disposed over a conductive cap layer to form a via opening penetrating through the at least one dielectric layer to expose the conductive cap layer and to form a top portion of the conductive cap layer into a metal oxide layer; converting the metal oxide layer to a metal oxynitride layer by a soft ashing process using a processing gas containing nitrogen gas; removing the metal oxynitride layer from a remaining portion of the conductive cap layer; and forming a via contact in the via opening to electrically connect the remaining portion of the conductive cap layer.