Ultranarrow Silicon Inverse Taper Waveguide Fabricated with Double-Patterning Photolithography for Low-Loss Spot-Size Converter

Abstract
Using a double-patterning process of i-line photolithography that twice performs a pair of photoresist patterning and dry etching processes, we were able to form an ultranarrow silicon inverse taper waveguide with a tip end width that was much narrower than the resolution limit of photolithography. We fabricated a spot-size converter (SSC) consisting of a 50-mu m-long silicon taper waveguide with gradually decreasing width from 400 to 50 nm and a polyimide second core. The insertion loss of the SSC was 0.55 dB for the transverse electric-like mode, which was the lowest value for an SSC fabricated using photolithography. (c) 2012 The Japan Society of Applied Physics