A tungsten silicide/polycrystalline silicon composite gate structure has been fabricated in which tungsten silicide layers on polycrystalline silicon have been deposited by the low‐temperature chemical vapor deposition method (at 360 °C) using a WF6/SiH4gas mixture. The fluorine distributions in this structure have been studied by secondary ion mass spectrometry. In samples with as‐deposited tungsten silicide, it has been confirmed clearly that almost all of the fluorine resides in the silicide layer. After high‐temperature annealing (above 950 °C), however, fluorine is found to diffuse easily into the gate SiO2through the polycrystalline silicon, that is, the gate oxide has been changed into the fluorine‐doped oxide.