Comparison of Electrical, Optical and Plasmonic

On-Chip Interconnects Based on Delay and Energy Considerations

Presented byHarish Peta – IMI2013002

Introduction

• Moore’s law is being sustained by scaling the device size.

• Because of scaling the delay, energy per bit and cross-talk among wires are the prominent problems.

• In this respect, focus is on two alternative technology options that can augment electrical interconnects on chip - Plasmonic and Optical Interconnects.

Plasmons

• Plasma oscillations, also known as "Langmuir waves" (after Irving Langmuir), are rapid oscillations of the electron density in conducting media such as plasmas or metals.

• The quasiparticle resulting from the quantization of these oscillations is the plasmon.

• A resonance condition is established when the frequency of the incident light photons matches the natural frequency of plasma oscillations against the restoring force of positive nuclei.

• Can facilitate information transport between nanoscale devices at optical frequencies and bridge the gap between the world of nanoscale electronics and microscale photonics.

• The extreme confinement of light can be useful as waveguides in interconnects.

Plasmonic Interconnects• Plasmonic Interconnects can be designed by anyone of these

structures

• Metal cylinder in dielectric• Metal-Insulator-Metal• Insulator-Metal-Insulator• Chain of metal nanoparticles

Optical microscopy image of a SiO2 substrate with an array of Au stripes attached to a large launchpad generated by electron beam lithography.

• Propagation length is not long enough to have long range surface plasmons for global interconnect applications and hence can be used for local interconnects.• The propagation length ranges from few microns (at smaller diameter and lower

wavelength) to tens of microns (at higher wavelength and larger diameters).

Propagation length through various plasmonic waveguides as a function of the free space wavelength. The propagation length is maximum at around 1.2μm where the loss through silver is minimum.

Comparison of plasmonic interconnects with electrical interconnects1. Delay Comparison:• The resistance at small dimensions is affected by the scattering due to side-walls

and line width variation.• The interconnect capacitance model including fringing effects and coupling

capacitance is taken. The RC delay of a delay-optimized RC interconnect is given as per

• The plasmonic interconnect delay is given as

L 0.89RC0.89KIKoxoL2 1

Hxox

1

WLS

Delay versus length of CMOS interconnect and plasmon interconnect. As the speed of plasmonic interconnects is comparable to the speed of light, they can be orders of magnitude faster than CMOS interconnects.

2. Energy Comparison:• The energy of CMOS interconnects is given as

• where C is the sum of the total interconnect capacitance and the load capacitance.

• For plasmon interconnects, shot noise limited transmission is considered.• Due to loss on the plasmonic interconnect, minimum energy transmitted per bit (assuming unity

quantum efficiency) is given as

• where α= 1/Lprop< m > is mean number of plasmons

Cross over length beyond which it is more energy efficient to communicate via conventional electrical interconnects rather than via SP interconnects due to attenuation of SPs

Optical Interconnects

• Due to the limitation on propagation length, plasmonic interconnects can be used only as short local interconnects.• At the global interconnect level, optics emerges as a promising technology.• Optical interconnects offer higher bandwidth compared to electrical

interconnects.

Block diagram of optical link. The optical link consists of a modulator driven by a tapered chain of electrical drivers, the optical waveguide and a photo-detector followed by a trans impedance amplifier

1. Delay of optical interconnects• The delay of an optical link topt is given as

• The design of a fast and cost efficient CMOS compatible electro-optical modulator is one of the most challenging tasks.

• The performance of optical waveguides is primarily limited by the wavelength of the utilized light and the choice of optical material.

• The limiting factor in the delay of photodetectors is the carrier transit time.

• The time delay due to global electrical interconnects is

L 0.89RC0.89KIKoxoL2 1

Hxox

1

WLS

2. Energy dissipation of optical interconnects• Energy dissipation in optical interconnects consists of three components

• Energy dissipated to charge the modulator capacitance• The energy needed at the photo-detector to generate the photocurrent proportional to the

incoming light• The static power dissipation in the transimpedance amplifier

Comparison of Electrical and Optical Interconnects by Critical Length• The metrics of delay, bandwidth-density, bandwidth-density/delay and energy per bit are compared for electrical and optical interconnects at global level.

• Critical length in this context is defined as the length beyond which optical interconnects offer superior performance compared to their electrical counterparts Critical length of optical interconnects as a

function of the width of electrical interconnects at the 2016 technology node.

Summary

• Plasmonic interconnects are suitable at short local interconnect level while optical interconnects are viable at the global interconnect level.• Issues related to integration, cost and reliability need to be addressed.

References

[1] Shaloo Rakheja and Vachan Kumar, Georgia Institute of Technology, “Comparison of Electrical, Optical and Plasmonic On-Chip Interconnects Based on Delay and Energy Considerations”, 13th Int'l Symposium on Quality Electronic Design, 978-1-4673-1036-9/12 ©2012 IEEE[2] J. A. Davis, R. Venkatesan, A. Kaloyeros, M. Beylansky, S. J. Souri, K. Banerjee, K. Saraswat, A. Rahman, R. Reif, and J. D. Meindl, “Interconnect limits on gigascale integration,” Proceedings of the IEEE, vol. 89, no. 3, March 2001.[3] E. Ozbay, “Plasmonics: Merging photonics and electronics at nanoscale dimensions,” Science, vol. 311, 2006.[4] Plasmonics: the next chip-scale technology Rashid Zia, Jon A. Schuller, Anu Chandran, and Mark L. Brongersma, Geballe Laboratory for Advanced Materials, Stanford University.[5] www.itrs.net