Design of broad-band backend compatible photonic devices and vertical inter-layer couplers

Abstract

Optical interconnects are regarded as efficient replacements for conventional electrical wiring for realizing large data capacity, high-speed transmission and Silicon-based integrated photonics is the best candidate for such application. Conventional silicon photonics is typically fabricated on silicon-on-insulator (SOI) substrate using top layer crystalline silicon as the active photonic layer. Even though this platform has been very successful for standalone photonic integration, the cost of such substrates, lack of well-developed supply chain and the incompatibility with standard silicon wafer-based CMOS electronics pose open challenges. This has motivated the development of back end of line (BEOL) process integration for optical components, in which the core and cladding materials can be deposited at low temp on a pre-fabricated silicon CMOS wafer. With this motivation, we present the waveguide component design for broadband, backend compatible application with 3 set of core-cladding material, namely, amorphous silicon (a-Si) on silicon dioxide (SiO2), amorphous silicon on silicon nitride and amorphous germanium on silicon nitride. These structures can support light propagation in the near-IR to mid-IR wavelength range, making these devices suitable for both interconnect and sensing applications. The wave-guiding characteristic of ridge and channel geometry structures are also studied to achieve tunability of dispersion, a key requirement to realize active nonlinear photonic functionalities. Next, we design waveguide structures that act as vertical couplers for multilayer photonic integration. Each layer could achieve different functionality (e.g. lasers, photodetectors etc.) and hence an efficient vertical interlayer coupler for coupling light between the adjacent layers is needed. Here we propose an efficient low footprint design for heterogeneous interlayer vertical coupling using high-index inverse tapers with a medium index MMI block that helps achieve high efficiency coupling even for large interlayer separation. We demonstrate simulation results of > 90% coupling between a bottom layer a-Si waveguide to a top layer GaAs waveguide with separation as large as 2 microns. The vertical coupler designed here could lower the stringent fabrication requirement for multilayer wafer bonding for heterogeneous photonic integration.