Experimental characterisation of roughness induced scattering loss in Si and SiC waveguide sensors

An atomic force microscope was used to directly measure the sidewall roughness of silicon on insulator (SOI) rib and silicon carbide waveguides. In order to make the vertical walls accessible, the chip containing the ribs was fixed on a 15° steel wedge and loaded onto an AFM scanner stage; this fitting ensures enough probe contact area on one of the sidewalls. The data was processed using a fully automated algorithm to extract the roughness in the direction of light propagation. The technique used allows the investigation of sensor devices at chip level without damaging the structures. The well-known smoothing process based on thermal oxidation of silicon waveguides to achieve low transmission loss was studied to demonstrate the possibilities of the method. Additionally, results on silicon carbide waveguides are presented.