A method of processing a thin film structure on a semiconductor substrate using an optically writable mask includes placing the substrate in a reactor chamber, the substrate having on its surface a target layer to be etched in accordance with a predetermined pattern, and depositing a carbon-containing hard mask layer on the substrate by (a) introducing a carbon-containing process gas into the chamber, (b) generating a reentrant toroidal RF plasma current in a reentrant path that includes a process zone overlying the workpiece by coupling plasma RF source power to an external portion of the reentrant path, and (c) coupling RF plasma bias power or bias voltage to the workpiece. The method further includes photolithographically defining the predetermined pattern in the carbon-containing hard mask layer, and etching the target layer in the presence of the hard mask layer.