Electrical Beats Optical for Short Distances

New simulation research that evaluated delay and power performance of both electrical and optical interconnects has shown optical interconnects are best for communication over any kind of distance — more than half a centimeter — but electrical interconnects actually outperform optical from a power consumption perspective for short distances, in the range of 3-5 mm. This is significant because it means that chips of the future will most likely include both optical and metallic wiring, and the industry's costly development of copper and low-k technologies will not have been in vain. The work, done at Stanford University (Stanford, Calif.), was presented by Professor Krishna Saraswat at the recent International Interconnect Technology Conference (IITC).

On-chip optical interconnects could be used in the relatively near future for clocking purposes, although the light would be generated by a laser off the chip, Saraswat said. "If we just look at today's technology of 0.1 µm or so, by the use of techniques such as repeaters, etc., you should be able to get by and you may not need optical interconnects. In another five years, you start running out of steam. The copper wires become too resistive, especially if you have to use barrier layers, which are highly resistive and occupy a significant cross section of the copper wire.

"Optics, for anything longer than half a centimeter, has some advantages. Whether people would use them for normal signaling wires or not remains to be seen. However, one particular application we think is imminent is the use of optics for the distribution of clock signals." In this scenario, the light is generated off-chip, brought on and distributed through optical waveguides (made of silicon and silicon dioxide). "At some centralized locations, you'll have highly efficient optical receivers, which will convert photons to electrons and then just distribute them locally. By doing that, you can more or less eliminate the jitter, minimize the skew and also minimize the consumption of power," Saraswat added. "In the longer term, I think you will see some fraction of very long on-chip interconnects also in the optics domain."

Schematic of electrical and optical system for global on-chip communication. (Source: Stanford University)

In the end, the analysis showed that, for chip-edge long interconnects, an optical link yields a much lower delay with comparable power expenditure. In fact, above a certain switching activity, for long links, optical interconnects will not only always give a lower delay but also a lower power dissipation. Even with smaller switching activity (15%), the minimum achievable power is similar for two links, but optical link is almost a factor of five faster.

For smaller link lengths, both power and delay drop proportionately in the case of electrical interconnects. For optical interconnects, since the power is dominated by the receiver, it is unaffected by length to the first order; however, the delay component of the waveguide drops and curves simply move down. Thus, for shorter wires, although optical links still may be faster, the power dissipation will be lower with repeated electrical wires. Because of a larger number of shorter wires on-chip, it is reasonable to continue with repeated electrical wires to save power at the expense of delay.

"For very long lines, if you compare a metal wire with an optical interconnect, since waveguides hardly consume any power, the power consumption of optical interconnect more or less remains constant because the receiver size really doesn't change that much," Saraswat explained. "But in a wire, the power would be proportional to the capacitance: The higher the capacitance, the higher the power consumption. For a long metal wire, the power increases with length. For a long optical wire, it more or less remains constant. The flip side of that is for a very short wire, less than a millimeter, the power consumption by an optical wire will remain the same as it was for a long wire. But the power consumption by a metallic wire will be reduced drastically. The break point is somewhere around 3-5 mm. Eventually, if you could think of an ideal system, it'll be shorter wires all made of metal and longer wires in the optics domain."

For additional information on wafer processing, go to www.semiconductor.net/wafer.