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

Abstract

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. Show more