We have used x-ray photoelectron spectroscopy (XPS) to investigate the composition of sidewall passivation films formed during the plasma patterning of 0.1 mum silicon gates in a high density plasma source, as a function of plasma operating conditions for HBr, Cl-2, and O-2 based chemistry. XPS analyses clearly show that sidewall passivation films SiXxOy (X = Br, Cl, x + 2y less than or equal to 4) are formed during the main step of the gate etch process and become oxidized at the early stage of the overetch step leading to an oxidelike layer after halogen desorption. Sidewall passivation films are thicker under HBr based chemistry than under chlorine containing chemistries. In addition, the film thickness seems to be highly sensitive to oxygen dilution in HBr based plasma. These results are discussed in view of the critical dimension deviation budget bearable in sub-0.1 mum gate fabrication.