A method of processing a thin film structure on a semiconductor substrate using an optically writable mask, the method includes placing the substrate in a reactor chamber, the substrate having on its surface a target layer to be exposed to a light source in accordance with a predetermined pattern, depositing an optically writable carbon-containing 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, (c) coupling RF plasma bias power or bias voltage to the workpiece. The method further includes optically writing on the carbon-containing mask layer in accordance with the predetermined pattern with writing light of a characteristic suitable for transforming the transparency or opacity of the optically writable mask layer and exposing through the mask layer the target layer with reading light of a characteristic different from that of the writing light.