The oxidation on nickel silicide (NiSix) during plasma etching and oxygen ashing is investigated for stable contact resistance on NiSix. NiSix exposed by various processes is observed by X-ray photoelectron spectroscopy. The oxidation on NiSix is promoted by the fluorine that remains during etching and the oxide thickness on n(+) NiSix is greater than that on p(+) NiSix. The remaining fluorine after etching can be decreased by in-situ nitrogen plasma treatment during the post-etching process. Therefore, the oxidation progress with exposure to air and the difference in oxidation on NiSix between n(+) and p(+) can be suppressed. (c) 2006 Elsevier B.V. All rights reserved.