A manufacturing process for a semiconductor integrated circuit device prevents occurrence of reaction between metal wiring and a boron-doped silicon plug over it in heat treatment for a MOS transistor to be formed over them and reduces the possibility of rise in contact resistance. Metal boride is formed on an exposed metal surface in the bottom of an opening made in an interlayer insulating film over the metal wiring. In order to facilitate formation of such metal boride, metal oxide remaining on the metal surface is removed with an aqueous ammonia solution. The metal surface is irradiated with high energy ultraviolet light in order to remove organic matter remaining in the opening and facilitate removal of the metal oxide with the aqueous ammonia solution.