To analyze the deformation of the WSix sidewall in a W-polycide gate, WSix(x=2.3) film was deposited by a modified dichlorosilane (SiH2Cl2, DCS) process that consisted of in situ monosilane (SiH4, MS)-based WSix nucleation, followed by DCS-based WSix deposition at 550 degreesC. Compared to the conventional DCS-based WSix film, the WSix film did not exhibit a W-rich interface facilitating the deformation and it showed similar structural property. Examination of deformation dependence on heat-treatment ambients suggested that a deformed region having a detrimental morphology was formed at the WSix sidewall during both nonoxidizing and oxidizing ambient annealings. It was revealed that the deformation in nonoxidizing and oxidizing ambient annealing resulted from the formation of a Si precipitate and W-Si-O compound at the WSix sidewall, respectively. In the case of N-2 ambient preannealing at 600 degreesC for 30 min in the process of spacer oxide deposition, the deformation could be effectively suppressed by preventing the formation of Si precipitate at the WSix sidewall. The N-2 ambient preannealing also improved the contact resistance of metal (Al/W plug with TiN/Ti barrier) on the W-polycide gate by promoting silicidation between Ti and excess Si in the WSix layer.