Device fabrication with planar bragg gratings suppressing parasitic effects

Abstract

The present invention relates to various methods of fabricating Planar Bragg Gratings (PBG) in a doped waveguide in a Planar Lightwave Circuit (PLC) device, suppressing unwanted parasitic grating effects during fabrication of the device. One approach to reduce parasitic gratings is to use a hard mask before the waveguide photolithography and etch, that results in a steeper sidewall angle that reduces or eliminates the parasitic grating effect. Another method of reducing parasitic grating effect is to deposit a layer of developable Bottom Anti Reflective Coating (BARC) prior to depositing the photo resist for waveguide etch. A third method of resisting parasitic gratings comprises using a planarizing undoped silica layer as a barrier layer on top of the core. During subsequent high temperature annealing germanium outdiffuses laterally into the cladding. The net effect is an optical waveguide with improved lateral uniformity because germanium diffusion smoothes out the sidewall roughness created during the waveguide reactive ion etch process. The undoped silica (SiO2) layer on top of the grating also serves the purpose of significantly reducing germanium outdiffusion from the core in the upward direction.