Ion beam assisted etching (IBAE) of SiO2and Si3N4is described which employs inert gas ion beam and gas mixtures based on XeF2and H2. Etching rates of SiO2and Si3N4were measured as functions of various parameters such as gas pressure, mixing ratio of XeF2to H2, ion dose, ion species, and irradiation angle. Significant enhancements up to 100 and 40 times larger than physical sputtering were achieved for SiO2and Si3N4, respectively. Etching selectivity of the SiO2to Si was varied from 0.07 to 6 depending on the gas mixing ratio. These results allow etching rates as well as selectivity of SiO2to Si to be tailored to specific requirements. Simulation is performed to show the cross‐sectional profiles formed by focused ion beam and to calculate the ion dose implanted into substrates. Auger analysis and surface morphology are mentioned.