3-D Integrated Free-Space Optical Interconnect for Multi-Core Systems

State of the art microprocessors and SoCs experience a degradation in the performance and signal integrity of on-chip metal interconnects with each successive technology generation due to increased resistance and signal bandwidth demands. These systems have therefore become increasingly communications limited. We proposed a fully-distributed intra-chip interconnect based on free-space optics, 3-D integrated photonic devices, and 3-D integrated analog and digital device planes. The main motivation was to construct a communication network with a large bandwidth density and without switching nodes. This free-space optical interconnect (FSOI) system consists of a photonics layer and a free-space optical guiding medium constructed using micro-mirrors and micro-lenses, which are stacked on top of two CMOS electronics layer by 3-D chip integration. The light source generated by an electrically modulated vertical-cavity surface emitting laser (VCSEL) is focused by a microlens at the backside of the GaAs substrate. After bouncing from the mirrors on the package multiple times, it is focused by another micro-lens onto a photodetector (PD), where it is converted back into an electrical signal and then processed by the CMOS transceiver located directly below the PD and connected through TSVs. This multi-technology project merging photonics devices with traditional CMOS technology is but a single example of the system level complexity that can be achieved with 3-D integrated circuits.