Hybrid III-V-on-SOI optical spot size converter by self-aligned selective undercut dry etching of Si

A technology called self-aligned selective undercut dry etching processing has been demonstrated for fabricating a highly efficient hybrid optical spot size converter (SSC) on a Si-on-insulator (SOI) template. The process was based on a bonded wafer between the upper InP-based multiple quantum well heterostructure and the lower SOI substrate. After defining the mask on the upper InP-based ridge waveguide, CF₄/O₂ dry reactive ion etching was then used for selective undercut etching of the Si material from the surrounding materials, forming a vertical waveguide coupler of the optical SSC. The lower waveguide, whose dimension is even smaller than the upper one, can thus be vertically self-aligned to the top ridge via an independent processing step. A laterally tapered waveguide ranging from 0.3 to 3 µm in width on the upper InP waveguide was fabricated. The phase-matching condition of the vertical coupler leads to a length of 45 µm and extracts 88% conversion efficiency. The selective undercut etching processing in III-V/SOI material provides a vertical self-alignment scheme for realizing compact and submicron scale heterogeneous integration in a Si photonics template.