The rapid advancement of artificial intelligence (AI) applications is pushing the limits of current datacenter interconnects, particularly in terms of speed and power consumption. Optical interconnects offer a promising solution for next-generation high-performance computing systems, outperforming traditional copper interconnects in both latency and energy efficiency, especially for connections spanning distances of 10 mm or more. Transitioning to optical techniques for these connections can significantly enhance overall system performance.
This abstract discusses the potential of Optical Input/Output chips, also known as Photonic Integrated Circuits (PICs), which are packaged in close proximity to compute and memory chips in datacenters. These PICs can replace conventional heterogeneous pluggable optical components, providing substantial benefits and facilitating future scalability. The focus is on the materials and deposition techniques used in silicon-based PICs.
Photonic ICs utilize silicon and germanium to manipulate light, integrating components such as photodetectors, modulators, waveguides, multiplexers/demultiplexers, and fiber couplers into a single circuit. This integration enables energy-efficient, low-latency, and high-bandwidth interconnects. Typically manufactured on silicon-on-insulator (SOI) wafers, these PICs contain thousands of components on a chip measuring approximately 10x10 mm.
Germanium layers are used to manufacture light detectors, which convert optical signals into electrical signals. Germanium is chosen for its direct bandgap and ability to be grown epitaxially on a silicon substrate, making it suitable for integrated photonics chips. Electro-Optic Modulators, which convert electrical signals into optical signals, are made from structures etched out of silicon or epitaxially grown silicon-germanium (SiGe). These modulators change the properties of light passing through them, such as intensity, phase, or polarization. Common types include Mach-Zehnder Interferometers and Micro Ring Modulators, which are also used in multiplexer/demultiplexer devices.
Waveguides and fiber couplers require materials with a higher index of refraction compared to the cladding material, to achieve total internal reflection and minimize light pulse intensity loss. Silicon or silicon nitride (SiN) can be used for waveguides, while silicon dioxide (SiO2) serves as the cladding layer. Various deposition techniques, including Plasma-Enhanced Chemical Vapor Deposition (PECVD), Low-Pressure Chemical Vapor Deposition (LPCVD), and Physical Vapor Deposition (PVD), will be discussed and compared. Additionally, advanced packaging materials and techniques for Photonic ICs will be explored.
Current light sources for PICs are made from III-V compounds such as indium phosphide (InP), but these must integrated heterogeneously due to incompatibility of growing high-quality III-V films on Si-based materials. However, research is underway to develop epitaxial germanium-tin (GeSn) lasers, which could enable monolithic integration of lasers directly into photonic integrated circuits.
