Etchless Core-Definition Process for the Realization of Low Loss Glass Waveguides

Abstract

Optical waveguides integrated onto microchips form some of the major components that enable the optical communication system we now use for our Internet, television and communication systems around the world, and are soon to be replacing some of the electrical components in high-speed computers. Deployed optical fibers form a web that spans the globe due to the very low loss of light - less than a decibel per kilometer. Chip-scale waveguides, on the other hand, typically exhibit losses 3-5 orders of magnitude higher - a few decibels per centimeter!

In an attempt to unleash the potential of photonics in the numerous applications rendered unfeasible by these high losses, we have taken a systematic approach to understanding, modeling and improving the loss mechanisms in optical waveguides. We follow the path of optical fibers by focusing on glass-based designs, in particular the silicon dioxide/silicon oxynitride (SiO₂/SiO _xN_y) material platform. In addition to a materials study of a number of deposited glasses, and the development of simulation tools for the major loss mechanisms, we have developed a process to tackle the largest loss contributor - sidewall roughness of waveguide cores.

We demonstrate for the first time a process for the selective oxidation of silicon oxynitride, which converts high index core SiO_xN _y into cladding SiO₂ via a thermal oxidation. We thoroughly characterize the wet oxidation process, and apply this to the fabrication of optical waveguides. We discuss the underlying chemistry behind the oxidation process, which affects the shape and index of the resulting materials. We propose a model for the smoothing of waveguide sidewalls by oxidant diffusion, and present the most recent results obtained with this novel process.

Additionally, we present an iterative process for optimizing the waveguide structure for propagation loss, using a non-destructive loss measurement scheme based on Optical Frequency Domain Reflectometry (OFDR) and subsequent re-oxidations. This trimming technique, enabled by the oxidation process, is applicable to all glass waveguide foundries that use SiO_xN_y or Si ₃N₄ materials for waveguide cores.