In an apparatus for producing an electromagnetically coupled planar plasma comprising a chamber having a dielectric shield in a wall thereof and a planar coil outside of said chamber and adjacent to said window coupled to a radio frequency source, the improvement whereby a scavenger for fluorine is mounted in or added to said chamber. When a silicon oxide is etched with a plasma of a fluorohydrocarbon gas, the fluorine scavenger reduces the free fluorine radicals, thereby improving the selectivity and anisotropy of etching and improving the etch rate while reducing particle formation.