Interface state generation in oxynitride gate dielectric devices with tungsten‐polycide (W‐polycide) and polycrystalline silicon (poly‐Si) gate structures were investigated. A significant amount of fluorine‐induced interface states were detected in W‐polycide gate device with nitridation of thermal gate oxide by both the charge pumping technique and capacitance/gate voltage (C/V) measurement. Re‐oxidation of oxynitride gate oxide results in partial annealing of these interface states. A model of the fluorine induced strain gradient between silicon dioxide and silicon (SiO2/Si) interface was proposed to explain the extra interface state generation mechanism. ©1995 American Institute of Physics.