SU-8 protective layer in photo-resist patterning on As₂S₃ film

Amorphous chalcogenide thin films, especially arsenic tri-sulphide (As₂S₃) are emerging as an excellent platform for integrated non-linear optic devices due to their high non-linearity as well as low linear and nonlinear optical loss. Fabricating planar As₂S₃ waveguides, however, is not straightforward because As₂S₃ is dissolved in alkalis such as photo-resist developer. In this study we present the application of thin SU-8 film as a protective layer to prevent the attack of the developer on the As₂S₃ during photolithographic process. Despite excellent coating feature and simple process of SU-8 as a protective layer, the delamination between SU-8 and polymer cladding during chip cleaving hindered its application. We deposited thin Al₂O₃ on SU-8 by atomic layer deposition and this intermediate layer could solve the problem. We measured the insertion losses of fabricated waveguides as a function of device lengths and widths. The propagation loss becomes significant in a narrow guide due to the enhanced surface scattering. It was clear that extra losses were engaged in long guides containing bending structures. SU-8 layer and thin Al₂O₃ combination allowed reliable and repeatable fabrication process in that it did not induce any extra light scattering or absorption and prevented attack of the As₂S₃ film by the alkaline developer. Moreover, the device characterisation revealed that SU-8 layer is superior to PMMA/BARC in terms of propagation loss of waveguide.