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

Abstract Amorphous chalcogenide thin films, especially arsenic tri‐sulphide (As 2 S 3 ) 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 As 2 S 3 waveguides, however, is not straightforward because As 2 S 3 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 As 2 S 3 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 Al 2 O 3 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 Al 2 O 3 combination allowed reliable and repeatable fabrication process in that it did not induce any extra light scattering or absorption and prevented attack of the As 2 S 3 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 (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)