W etching is carried out by an Ar+ beam in XeF2 with the addition of H-2, N-2 or O-2. The energy of the beam is 400 eV and the current is 0.2 muA. The spontaneous etching rate in XeF2 with the addition of N-2 or O-2 decreases by about 0.6 times compared with that in XeF2 (2 X 10(-6) Torr), for which the total pressure of the mixed gas is 2 X 10(-5) Torr. The nitrogen or oxygen atoms attached on the W surface disturb the attachment between W and fluoride atoms. The ion-assisted etching rate increases by about 1.5 times in XeF2, by adding N-2 or O-2 compared with that in XeF2 (2 X 10(-6) Torr), for which the total pressure of the mixed gas is 2 X 10(-5) Torr. It is speculated that the addition of N-2 or O-2 gas to fluorinated gas acts upon the surface reaction to suppress the sidewall etching rate when the mask patterned W gate line is plasma etched by the same kind of gas mixture. The mixing layer is thought to be formed by W and nitrogen or by an oxygen atom by ion-assisted etching. The W-W bond in the mixing layer is thought to be weaker than that in W film both physically and chemically, which makes the etching increase. The thickness of the mixing layer is estimated to be about 10 A. But the spontaneous ion-assisted etching rate did not change in the case of H-2 addition.