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Xenon Difluoride is an isotropic dry etch which is increasingly being used to release structures made of polysilicon. By using a vapor-phase XeF2 pulse etching system we have investigated the effects of aperture size and thickness of polysilicon films versus etch rates. Decreasing the aperture size resulted in reduced etch rates. For a 100 μm wide structure the etch rates varied from 1.38 μm/min (1.25 μm thick polysilicon) down to 0.41 μm/min (0.1 μm thick polysilicon). Rate constants are obtained for a modified, aperture-dependent Deal-Grove model from the experimental data.