Small features, big benefits: Next generation communications

High performance computing is advancing at such a clip that designers are turning to nanotechnology to address upcoming system requirements. But research into photonic based nano interconnects will not be confined to supercomputing systems; Qualcomm is exploring how nanotechnology can be used in future mobile devices. The processing and communication requirements of high end computers are growing tenfold every three to four years and 1000 times each decade, says Bert Offrein, photonics manager at IBM Research in Zurich. "Because the form factor – the number of racks – must remain constant, we need to increase the integration by a factor of 1000," said Offrein. The same applies for the system's communication – within a chip, between chips and between racks. The power consumption of such systems must also remain limited. Computing firms are looking to couple electrical and optical technologies closely. IBM, for example, is developing system communication technology based on optical components and waveguides, such as a 300Gbit/s optical transceiver for chip to chip communication and polymer waveguides for communication across pcbs. More generally, Sun Microsystems, Intel, H-P and IBM, as well as start ups such as Luxtera and Kotura, are developing silicon photonics, a technology that implements optical building blocks such as waveguides, photodetectors and modulators using common semiconductor technology. While silicon photonics delivers size and cost benefits compared to standard optical componentry, III-V materials are needed to make laser light sources. And coupling III-V materials with silicon is complex and costly. To deliver further optical interconnect cost and size reductions, computer firms' R&D labs, as well as universities, are exploring nanotechnology for intra system and, ultimately, intra chip communication. IBM sees its optical transceiver and polymer waveguide technology as the first stage in integrated optical communication for computing systems, to be followed by silicon photonics. Waveguides will still be used to interconnect processors, but silicon photonics brings optical integration within the processor package. "The optics are in the same technology as the logic," said Offrein. Such an approach promises devices such as a silicon based modulator several hundred nanometres in size. "Nanotechnology could help us to continue to scale silicon photonics," said Offrein. Nanotechnology, of course, refers to the manipulation and control of materials at the nanoscale. "It is material dependent," said Saif Islam, associate professor at the Integrated Nanodevices and Nanosystems Lab, Electrical and Computer Engineering at the University of California-Davis. "At less than 6nm, silicon shows exotic quantum effects while, for GaAs, it is less than 20nm." Prof Islam, whose research is backed by Qualcomm and HP, among others, says HP wants to develop optical interconnects for its servers. "It wants hundreds of optical channels between servers that cost less than $1 a connection," he said. "This means coming up with new technology, new ways of designing devices that can be grown on pcbs, glass, metal – on anything." In recent years, significant progress has been made growing III-V materials on surfaces such as silicon. Normally, thermal and crystal lattice mismatches foil such growth, but bonding at the nanoscale means light sources, modulators and photodetectors can be grown epitaxially. "[The resulting] lasers are extremely delicate," said Prof Islam. "The [light] emission looks good, but the catalysts that grow the structure are also contaminants." Photodetection is more robust, with 40GHz bandwidth performance being shown. The detectors have also been grown on silicon and quartz, but leakage current and noise remain challenges and laser modulation performance so far is poor, says Prof Islam. One of the biggest challenges is coupling light with a wavelength of 1.5µm into and out of a 100nm nanowire modulator. Two technologies look promising here: photonic bandgap crystals; and surface plasmon polaritons. Photonic bandgap crystals are regular structures that confine light. "We view photonic crystals as a way to confine light and thereby increase the interaction with a material," said Thilo Stoeferle, research staff member, optics at IBM Research Zurich. "This gives a gain in efficiency of a device; if you have more than 100,000 optical channels on a chip, for example, you will have to make such optical devices very efficient." The surface plasmon polariton is a newer approach that could restrict light within even smaller structures. "Plasmonics shrinks the guiding path considerably, from a 200nm width down to 50 or even 20nm," said Prof Islam. Plasmons rely on interaction with metals and involve photons being converted to electrons. The greater confinement comes at a price. "You are starting again to move electrons and you encounter the same problems as with electrical interconnect: you have crosstalk between channels," said Stoeferle. This will limit how many waveguides can be crammed together, but IBM says plasmonics will lead to compact modulators and photodetectors System integration issues must also be addressed and electronic circuitry must be connected to drive the modulators at Gbit/s rates. "This is going to generate a lot of heat locally," said Prof Islam. "It is not clear what the strategy will be to keep the devices unchanged physically, optically and electrically." IBM has used e-beam lithography to build prototypes, but the approach is inappropriate for mass production of nanowires and photonic devices. IBM is exploring nano patterning using scanning probe technology and, separately, how the probe technology could be scaled to produce multiple interconnects. Meanwhile, IBM is publishing a paper on a photonic crystal based optical modulator that consumes only femto joules of energy. Nano interconnect is also set for use beyond supercomputing and servers, with Qualcomm looking to use the technology within handsets. "Qualcomm is trying to see whether it can work with universities to develop technologies for stacking multiple memory chips or even stacking memory on top of the cpu, so you no longer need to carry a laptop everywhere," said Prof Islam. He admits such an application is a long way off. Qualcomm is active now, he says, because leading foundries are not considering integrating optical technology in current ic fabrication processes. All Qualcomm will say is that, while such research is showing promising results, it is too early to discuss possible commercial applications. But, a ccording to Prof Islam, HP plans to use nano interconnects within its servers by 2018. "It is very hard to talk about nanotechnology in terms of when it will be applied and in what form," said Offrein. "What is clear is that there will be massive use of optical communication." First applications will be for leading supercomputer designs before the technology will trickle down for use in other platforms. "Integrating these new technologies will make the system more complex," said Offrein. New materials will be introduced, optics and electronics will need to be matched, and the devices controlled, he says. The technology must also be reliable and manufacturable. What gives Offrein confidence is the system integration that nanotechnology will enable: "By means of integration, we can tackle tremendous complexity," he concluded.