The various arguments for introducing optical interconnections to silicon CMOS chips are summarized, and the challenges for optical, optoelectronic, and integration technologies are discussed. Optics could solve many physical problems of interconnects, including precise clock distribution, system synchronization (allowing larger synchronous zones, both on-chip and between chips), bandwidth and density of long interconnections, and reduction of power dissipation. Optics may relieve a broad range of design problems, such as crosstalk, voltage isolation, wave reflection, impedance matching, and pin inductance. It may allow continued scaling of existing architectures and enable novel highly interconnected or high-bandwidth architectures. No physical breakthrough is required to implement dense optical interconnects to silicon chips, though substantial technological work remains. Cost is a significant barrier to practical introduction, though revolutionary approaches exist that might achieve economies of scale. An Appendix analyzes scaling of on-chip global electrical interconnects, including line inductance and the skin effect, both of which impose significant additional constraints on future interconnects. Keywords—Off-chip wiring, on-chip wiring, optical interconnects, quantum-well modulator, vertical-cavity surface-emitting laser. I.

Technologies now exist for implementing dense surface-normal optical interconnections for silicon CMOS VLSI using hybrid integration techniques. The critical factors in determining the performance of the resulting photonic chip are the yield on the transceiver device arrays, the sensitivity and power dissipation of the receiver and transmitter circuits, and the total optical power budget available. The use of GaAs--AlGaAs multiple-quantum-well p-i-n diodes for on-chip detection and modulation is one effective means of implementing the optoelectronic transceivers. We discuss a potential roadmap for the scaling of this hybrid optoelectronic VLSI technology as CMOS linewidths shrink and the characteristics of the hybrid optoelectronic tranceiver technology improve. An important general conclusion is that, unlike electrical interconnects, such dense optical interconnections directly to an electronic circuit will likely be able to scale in capacity to match the improved performance of futur...

A 1.6Gb/s receiver for optical communication has been designed and fabricated in a 0.25-m CMOS process. This receiver has no transimpedance amplifier and uses the parasitic capacitor of the flip-chip bonded photodetector as an integrating element and resolves the data with a double-sampling technique. A simple feedback loop adjusts a bias current to the average optical signal, which essentially "AC couples" the input. The resulting receiver resolves an 11A input, dissipates 3mW of power, occupies 80m x 50m of area and operates at over 1.6Gb/s.

Processor speeds continue to increase rapidly due to the scaling of CMOS line-widths, but electrical interconnect speeds have not grown at the same rate. The loss mechanisms in electrical interconnects limit their ultimate capacity. Optical interconnects have the potential to alleviate this interconnect bottleneck. At short scales such as board-to-board, chip-to-chip, and on-chip, the important requirements for these optical interconnects are low latency, high throughput, high density, high bandwidth, and simple integration with mainstream silicon technology. This thesis investigates optical interconnects designed to meet these requirements using short pulses, in conjunction with multiple quantum well (MQW) diodes filp-chip bonded to silicon CMOS chips. The use of short optical pulses (100 fs to a few ps), equivalent to a return-to-zero (RZ) format with very low duty cycle, has many potential advantages. We show that using short pulses in optical links can, a) enhance the sensitivity of the receiver

this paper. The necessity of CAD tools and models that explicitly consider optics has already been realized by other research groups.

We show that there is a limit to the total number of bits per second, B, of information that can flow in a simple digital electrical interconnection that is set only by the ratio of the length l of the interconnection to the total cross-sectional dimension p A of the interconnect wiring—the “aspect ratio ” of the interconnection. This limit is largely independent of the details of the design of the electrical lines. The limit is approximately B BoA/l2 bits/s, with Bo 1015 (bit/s) for high-performance strip lines and cables, 1016 for small on-chip lines, and 1017 –1018 for equalized lines. Because the limit is scale-invariant, neither growing nor shrinking the system substantially changes the limit. Exceeding this limit requires techniques such as repeatering, coding, and multilevel modulation. Such a limit will become a problem as machines approach Tb/s information bandwidths. The limit will particularly affect architectures in which one processor must talk reasonably directly with many others. We argue that optical interconnects can solve this problem since they avoid the resistive loss physics that gives this limit. © 1997 Academic Press 1.

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