Materials and Devices for Optoelectronic Packaging

Abstract

Over the last two decades, improvements in semiconductor manufacturing have allowed for the commercialization of silicon photonic integrated circuits with over 10,000 devices. These chips are critical to data and telecommunications networks where they convert and encode optical signals to electrical signals, and vice versa, in the form of transceivers. Scaling up the number of transceivers and optical fiber connections, or optical input/output (I/O), will be critical to meet the exponential rise in demand for cloud data capacity since 2010. However, the costly process of active alignment and bonding of optical fiber arrays directly to photonic chips presents a barrier to their high volume packaging and assembly. This approach limits optical I/O density to a maximum of 8 connections per millimeter since optical fibers for communications applications have cladding diameters of 125 micron.

To address this challenge, this thesis explored a new field of silicon integrated photonics involving chip-to-chip (i.e. flip-chip) optical coupling. Evanescent chip-to-chip couplers were designed, fabricated, packaged, and tested for directly connecting silicon photonic chips to other silicon photonic chips, interposers, or printed circuit boards using automated assembly. The design's compact footprint allows for coupler pitches below 10 micron, or an optical I/O density of greater than 100 connections per millimeter, to be realized - an order of magnitude improvement over fiber-to-chip connections. By designing the coupler to use silicon materials and back-end-of-line compatible packaging processes, ease of integration with existing microelectronic foundry tool sets was ensured. Results from an experimental flip-chip coupler prototype showed greater than 90% coupling efficiency with micron scale alignment tolerances when coupling from silicon nitride to silicon-on-insulator waveguides, the first demonstration of such a device.

To further improve optical flip-chip coupler performance, designs were proposed for combining the evanescent coupler with an integrated graded index lens using silicon oxynitride films. Such a device would provide a universal coupling interface in silicon photonics for both chip-to-chip or fiber-to-chip connections. Simulations showed sub-dB coupling loss across all interfaces including flip-chip coupling across a 10 micron gap. Initial fabrication processes were established to deposit, pattern, and etch greater than 10 micron thick silicon oxynitride graded index lenses on silicon and glass substrates. In showing that automated pick-and-place tools can be used for photonic chip assembly, this work represents a critical step in eliminating active alignment and sustainably scaling optical I/O in future transceiver packages.