W etching is carried out by an Ar+ beam in XeF2 with the addition of H2, N2 or O2. The energy of the beam is 400 eV and the current is 0.2 μA. The spontaneous etching rate in XeF2 with the addition of N2 or O2 decreases by about 0.6 times compared with that in XeF2 (2×10-6 Torr), for which the total pressure of the mixed gas is 2×10-5 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 N2 or O2 compared with that in XeF2 (2×10-6 Torr), for which the total pressure of the mixed gas is 2×10-5 Torr. It is speculated that the addition of N2 or O2 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 Å. But the spontaneous ion-assisted etching rate did not change in the case of H2 addition. © 2001 American Vacuum Society.