The gate oxide thickness for tungsten (W) polycide gate processes is studied, with tungsten silicide (WSi/sub x/) deposited either by chemical vapor deposition (CVD) or sputtering. For WSi/sub x/ deposited by CVD, it is found that the effective thickness of gate oxide as determined by CV measurement increases in all cases if the annealing temperature is 900 degrees C or higher. However, high-resolution transmission electron microscopy (TEM) measurement indicates that the physical thickness does not change after a 900 degrees C anneal. In this case, the dielectric constant of the gate oxide decreases by 7%. As the annealing temperature increases to 1000 degrees C, CV and TEM measurements give the same thickness and the decrease of the dielectric constant disappears. In contrast, for WSi/sub x/ film deposited by sputtering, annealing at 900 degrees C has no effect on the gate oxide thickness as measured by CV and TEM.< &gt;