A method of opening a carbon-based hardmask layer composed of amorphous carbon containing preferably at least 60% carbon and between 10 and 40% hydrogen. The hardmask is opened by plasma etching using an etching gas composed of H.sub.2, N.sub.2, and CO. The etching is preferably performed in a plasma etch reactor having an HF biased pedestal electrode and a capacitively VHF biased showerhead.