SCIENCE & ENGINEERING NEWS
San Diego, CALIF. — The fastest integrated circuits in the world today are unique for their technology as well as for their speed. Rather than being built out of a semiconductor material, they are made with a superconducting metal, niobium, which has no electrical resistance at temperatures near absolute zero. To reap the benefits of this so-called superconducting state, the ICs must operate below 9 K.
Superconductors have been tantalizing researchers with their promise of high speed at low power since the late 1960s. But, until recently, difficulties with the fabrication process and a cumbersome logic family have kept the technology from growing into a large-scale commercial success.
But a new manufacturing process that is rugged and reliable and a new logic family that is much faster and more compact than its predecessor promise to make commercial systems feasible within the next five years, as Darren K. Block, Elie K. Track, and John M. Rowell report in the December issue of IEEE Spectrum.
In addition to zero electrical resistance, the new logic family relies on another property of superconductors: within a closed loop, magnetic flux can exist only in discrete, or quantized, amounts that are multiples of a basic quantity called a magnetic flux quantum. In rapid single flux quantum (RSFQ) logic, as it is called, it is the presence or absence of a magnetic flux quanta that represents information bits. The quanta are sent from one logic device to another along virtually dispersionless superconducting transmission lines.
Performance of RSFQ logic is spectacular. It can operate in complex circuits at clock frequencies beyond 100 GHz. And once researchers succeed in building superconductor ICs with the same linewidths used in semiconductor manufacturing, performance should be even more awesome. In fact, operating speeds of more than 750 GHz have already been achieved experimentally for RSFQ toggle flip-flops having gates of 0.25 micrometer – roughly the size of their semiconductor counterparts.
The first RSFQ products will probably leverage superconductor superiority in performing high-speed and high-accuracy analog-to-digital conversion. Given adequate resources, the next few years are likely to see the first superconductor digital RF modules for both wireless communications base stations and high-performance instrumentation. Other potential applications include petaflops computing platforms and high-throughput and high-density network switches. In short, thanks to two key features – speed conjoined to low power, and analog-to-digital quantum accuracy – RSFQ circuits should extend digital power and flexibility directly into the RF and microwave domains.