Abstract: Methods of etching HAR features in a dielectric layer are described. In one embodiment, a substrate is provided into an etch chamber. The substrate has a patterned mask disposed on a dielectric layer formed thereon where the patterned mask has openings. A gas mixture is provided into the etch chamber, the gas mixture includes CO, O2, a fluorocarbon gas, and an optional inert gas. A plasma is formed from the gas mixture.