We investigated the oxidation of crystallized tungsten silicide. We also researched the effects of preimplantation by P (or As) on the oxidation. Our results showed that the oxidation involved decomposition of the tungsten disilicide and consumption of the underlying polycrystalline silicon (polysilicon), consequently causing serious reliability problems. Cross-sectional transmission electron microscopy (XTEM) analysis showed that the oxide consisted of crystalline WO3 and amorphous SiO2, which was not removed completely using chemical solutions such as 100 : 1 HF and 7 : 1 buffered oxide etchant (BOE). However, P (or As) implantation prior to oxidation produced a SiO2 layer free of tungstem oxide, In particular, phosphorous implantation into silicide yielded a much thinner SiO2 layer on the silicide than As, which might be attributed to the presence of P2O5 in the oxide.