Hybrid Silicon / Lithium Niobate Modulators: Design and Wafer Bonding
Optical communication has provided the exclusive means for carrying high capacity data over long distances for over three decades. As modern data storage and computing rely increasingly on high-rate sharing of information, optics-based techniques steadily penetrate towards rack-level, board-level and even chip-level communications. The future growth of both computation and communication depends, to a large extent, on the successful integration of optical communication system functionalities alongside electronic integrated circuits on the silicon material platform. Hence, the realization of photonic devices on silicon, or silicon photonics, is a research area of much interest and significance.
While the silicon-on-insulator (SOI) material platform is generally favorable for making passive devices, the properties of silicon raise several challenges to the implementation of active photonic devices. The state-of-the-art silicon-photonic light sources, amplifiers, modulators and detectors rely on the hybrid integration of additional electro-optic materials, on top of SOI waveguides. The most widely used material in electro-optic modulators is Lithium Niobate (LiNbO3). LiNbO3 has a high electro optic coefficient, and the fabrication technology of LiNbO3-based devices is mature and well established. The hybrid integration of LiNBO3 alongside silicon in a single device is therefore of much interest and potential significance. The work presented herein is part of a long-term research program, aiming to realize hybrid electro-optic modulators in LiNbO3 over SOI. More specifically, the work addresses two main challenges associated with that objective: the bonding of LiNbO3 to silicon, and the design of the hybrid waveguide structure.
There are only a few reports of bonding LiNbO3 to Silicon in the literature. A large part of the work is dedicated to a new bonding paradigm, which relies on the deposition of self-assembled monolayers (SAMs) of specially-synthesized organic materials on both surfaces and their subsequent functionalization using chemical reactions. On the design parts, two geometries are considered. First, a design with large electrodes spacing, on the order of hundreds of microns, is proposed and analyzed. Second, a more advanced device with closely spaced electrodes is also designed. Our analysis and simulations suggest that hybrid silicon-LiNbO3 modulators should be feasible.
* Research was carried out towards the M.Sc. degree in Electrical Engineering, advised by Dr. Avi Zadok, Faculty of Engineering, Bar-Ilan University.