<title>Fabrication of high-aspect ratio photonic bandgap structures on silicon-on-insulator</title>

Abstract

Silicon-on-Insulator (SOI) is an attractive platform for fabrication of photonic bandgap devices [1,2]. The large refractive index step between the silicon waveguide and the SiO 2 lower cladding layer permits realization of periodic waveguides with very large index modulation. The large refractive index modulation is obtained by deep anisotropic etching into the silicon guide region, and makes it possible to obtain strong resonances in compact periodic structures with only a few periods. In addition, the processing of these structures is highly compatible with standard silicon CMOS processing. Hence this technology is attractive for low cost, highly integrated photonic and optoelectronic circuits. Here we report on the fabrication process for photonic bandgap structures on SOI using Electron Beam Lithography (EBL) and Deep Reactive Ion Etching (DRIE). The etching was performed in time-multiplexed inductively coupled Fluorine plasma. Samples were patterned using direct electron beam writing on both positive and negative resists. We have optimized the resist process and proximity effect correction scheme to define features down to 25 nm and gratings with periods as low as 200 nm. The etch anisotropy and selectivity with respect to the resist mask have been studied as a function of source and bias power, active and passive gas pressure and flows, and time multiplexing, for a Fluorine plasma. Having optimized the process parameters, we have successfully etched periodic structures with feature sizes as low as 100 nm and to a depth of 3 micron. Examples of SOI photonic bandgap structures fabricated with EBL/DRIE technique, as well as the influence of various process parameters on the device structure are presented.