The surface states of molybdenum (Mo) gate metal oxide semiconductor (MOS) devices have been examined and Mo penetration into the Si-SiO2 interface during Mo film deposition has been confirmed to be one of : the causes of surface state generation. Penetration of Mo atoms occurs when the substrate is heated during deposition by sputtering or electron-beam evaporation. The mechanism responsible for the penetration is assumed to be caused by Mo ions on the SiO2 surface during the initial stages of deposition. To prevent this Mo penetration, a very thin Mo film is first deposited at room temperature to form a conductive layer, and a thick Mo film is subsequently deposited at high temperature. By applying this two-step deposition method, the penetration depth of Mo into the SiO2 can be reduced to less than 3 nm and the surface state density can be reduced to below 3 x 10(10) cm-(2) eV(-1) even in MOS devices with a gate oxide thickness of 10 nm.