Proton Beam Writing of Passive Polymer Optical Waveguides

Proton beam writing is a new direct-write micromachining technique capable of producing 3-dimensional (3D), high aspect ratio micro-structures with straight and smooth sidewalls. It uses a focused sub-micron beam of 2.0 MeV protons to direct-write on a suitable polymer, such as the photoresists: poly-methylmethacrylate (PMMA) and SU-8, a negative tone photoresist from MicroChem. In this paper, we report on the application of proton beam writing to fabricate low-loss passive polymer waveguide structures such as symmetric y-branching waveguides in SU-8. SU-8 channel waveguides are fabricated by first direct-writing the pattern using a proton beam and subsequently chemically developing the latent image formed. A UV-cured resin, Norland Optical Adhesive 88 (NOA-88) is used as the cladding layer. Being a direct-write technique, proton beam writing offers us great flexibility to fabricate waveguides of arbitrary patterns and this is an asset that can be applied to the rapid prototyping of optical circuits. With all its unique characteristics, proton beam writing is an excellent technique for waveguide fabrication.