Performance Analysis of Optical Based VLSI Interconnects

Abstract

For more than 30 years, the performance of silicon integrated circuits has improved at an astonishing rate. The number of functions per chip has grown exponentially, dramatically bringing down the cost per function. The conventional copper interconnects are not able to fulfil different design requirements. Copper based interconnects are facing many challenges. Because of dispersion, reflections and ringing, attenuation and its variation with frequency, the high-speed signals are distorted. The performance of parallel links in conventional devices is also limited by the cross talk due to coupling from neighbouring signals. Optical Interconnects present a promising option for replacing the existing copper Interconnects. For global and semi-global wire, optical interconnects provide a better option as compared to both carbon nanotubes and copper in terms of latency, power dissipation and bandwidth density. The report includes the comparison of optical interconnects with the copper in terms of energy efficiency and latency at various sizes and technology nodes.

In this dissertation, the performance of high speed optical and electrical interconnects in terms of power vs. bandwidth have been analyzed. Results reveal that beyond a critical length, power optimized optical interconnects dissipates less power compared to high speed electrical signaling schemes. Beyond the 32nm technology node with its commensurate bandwidth, optical interconnect becomes favourable for the distances less than 10cm for inter-chip communications. We also examine two competing transmitters, the vertical cavity surface emitting laser (VCSEL) and the quantum well modulator (QWM), for optical links. The vertical cavity surface emitting laser (VCSEL) is used in the modelling of the transmitter circuit for the carrying out the SPICE simulations for calculating the delay and power dissipation in transmitter circuit.

The transimpedance types of receivers are considered here due to their high bandwidth, low noise and ease of biasing. Various transimpedance amplifiers are implemented and then, the optimum differential cascode transimpedance amplifier is used. Similarly, voltage amplifier and decision circuit are chosen. The receiver circuit is seen to dissipate more power as compared to the transmitter circuit. The optical waveguide is relatively very small so, the power dissipation in the optical waveguide is neglected. The delay in the waveguide is constant for a constant length and varies with the change in length only and has no effect of the technology node used.

In this dissertation, delay and power dissipation results are simulated for optical and copper interconnects at various technology nodes. Simulation is done using SPICE simulation tool at global interconnect level. Optical interconnects give better delay performance as compared to conventional copper interconnects at each technology node. Power dissipation of optical and copper interconnects increases with small technology nodes because of higher clock frequency and leakage current. The power dissipation in optical interconnects is more than copper interconnect at lower frequency levels but as we keep on increasing the frequency levels, the power dissipation in the optical interconnects keeps on decreasing as compared to copper interconnects, resulting more efficient operation in terms of power. Based on the comparisons made, various areas of improvement of optical fibers are concluded.