Photolithographic patterning of bihelical tracks onto conical substrates

We demonstrate the direct photolithographic patterning of a grossly non-planar substrate by creating 62 micron helical tracks on a 22 mm high cone. The projection of focused light on to the three dimensional surface is achieved using a computer generated hologram (CGH) suitably illuminated so as to create the required pattern on the photoresist coated surface. The approach adopted forms the basis of a novel method for the patterning of non-planar structures. We address the key challenges encountered for the implementation of holographic photolithography in three dimensions, including mask design and manufacture, exposure compensation, mask alignment and chemical processing. Control of line width and resolution over the non-planar surface is critical. We describe the methods adopted and critically assess the structures created by this process. The bi-helical cone is representative of a broadband, high frequency coil like structure, known in wireless communications as a log-periodic antenna.