In this paper, we demonstrate the feasibility of using on-chip optoelectronics within VLSI systems to address a wide range of signal distribution issues by examining the following fundamental question: how can we transmit information from one source to many destinations while minimizing propagation delay, skew, jitter, and noise in a way that is compatible with low-cost manufacturing and CMOS circuits? Example systems with such information distribution requirements include banked arrayable memories such as a DRAM or a dense imager with scanned high speed readout, or a clock distribution system. In all instances individual lines are typically connnected to thousands of gates, slowing cell access times and generating skew. We demonstrate how the use of on-chip photonics within VLSI systems can reduce delays introduced by electrical wires in system-on-a-chip interconnects, busses, caches, and control lines at distances shorter than one meter and as short as a few millimeters. We also describe and demonstrate how a simple on-chip optoelectronic system addressing these problems can be realized at low cost, with monolithic photodetectors and on-chip waveguides in a commercial CMOS process, benefiting both ultra-short and one meter link architectures. This unexplored signal distribution architecture promises high optical to electrical efficiency, low noise, and the benefits of monolithic photodetection not previously achieved in existing approaches.
In this work, we demonstrated a practical means to construct wafer-level optical interconnects in commercial BiCMOS electronics. Through modifying the layout and design of commercially available SiGe Heterojunction Bipolar Transistors (HBT) through MOSISTM foundry, we obtained high performance SiGe Heterojunction Phototransistors (HPT) that utilize the SiGe Base and Collector junction for photo-detection and the transistor action for the amplified photocurrent. Responsivities of 2.4A/W and 0.2A/W were achieved for the phototransistor detecting light of 850nm and 1060nm, respectively. The external quantum efficiency of 350% was obtained. The photocurrent gain was shown to be 78. Furthermore, we investigated the integration of optical waveguides and elements with the SiGe commercial platform to demonstrate an effective approach of the wafer-level optical interconnects. The leaky-mode waveguide routed on the chip surface can couple the light laterally from the input fiber to the buried photodetection region. A 20% coupling efficiency is obtained in the SiGe layer, and provides a response about 40 times higher than that of the vertical illumination. The integrated on-chip waveguides and photodetectors in the commercial platform offer efficient optical-to-electrical conversion and a low-loss routing scheme useful for on-chip computational architectures.