Abstract: The present invention relates to a method for forming a gate of a semiconductor device that uses a cobalt silicide. The method for forming the gate of the semiconductor device can include preparing a semiconductor substrate, form a first insulation layer on the semiconductor substrate, form a doped polycrystalline silicon layer simultaneously with a deposition or after the deposition and forming a cobalt silicide layer by another deposition or by reacting a cobalt layer with the polycrystalline silicon layer. The cobalt silicide layer is selectively removed by using at least one etchant gas selected from a group of a gas including a chlorine atom group, a gas mixture of the gas including the chlorine atom group and oxygen, a gas mixture of the gas including the chlorine atom group and an inert gas, and a gas including the above-enumerated gases and a gas having a fluorine atom group. Then, the polycrystalline silicon layer is patterned.