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Superconductors set course for wireless

Posted: 16 May 2005 ?? ?Print Version ?Bookmark and Share

Keywords:superconductor? plasma? etch? deposition? emulate?

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Is software-defined radio the killer application for superconducting electronics? That question is on the minds of executives at Hypres Inc. as they scout markets for the specialized technology.

Low-temperature superconductors have been developed, with an eye on high-performance computing, since the 1960s. But the technology languished, pursued by only a handful of research departments, as funding proved hard to come by. Now, signs of technological maturity and the emergence of potentially lucrative applications suggest that low-temperature superconducting is turning a corner.

The rapidly expanding wireless market provides an ideal target for an electronics capability that can hook an antenna to an ADC and digitally emulate virtually any RF component in real-time. That capability has caught the eye of the military, which is enlisting Hypres in a bid to reduce the U.S. Defense Department's entire spectrum of military communications systems to a single, superconducting software radio.

"We have been doing a lot of work under SBIR [Small Business Innovation Research] grants and government projects such as the JTRS [Joint Tactical Radio Systems] program," said Hypres CEO Jack Rosa. "It's a dual-use program, so they expect commercial products to be developed as well, something that we certainly favor."

The company developed a line of high-performance oscilloscopes in the late 1980s as its first foray into commercial products. Hypres has been selling products into niche areas such as superconducting quantum interference device magnetometers for reading brain waves and standard-volt chips used in labs and by government standards organizations.

"We developed a standard for the volt with NIST [National Institute of Standards and Technology]. Superconducting devices can define the volt more accurately than any other method," said Rosa. The development has turned into a good business for Hypres, particularly since the breakup of the Soviet Union, as the former member states have been developing their own, independent standards.

Hypres' IC technology encodes binary bits in the form of individual magnetic flux quanta to form a class of digital logic circuits known as rapid single-flux quantum (RSFQ) logic. The technique was invented by Konstantine Likharev and two colleagues at the University of Moscow in the early 1980s, just as IBM Corp. was shutting down its Josephson junction research effort. A leading IBM researcher, Sadeg Faris, left IBM to found Hypres and soon recruited Likharev, now the company's chief of operations.

Likharev also set up an RSFQ lab at the State University of New York at Stony Brook. That facility is staffed by several researchers from his earlier project at the University of Moscow.

RSFQ has been the key breakthrough for building stable logic circuits based on superconducting niobium wires that conduct electricity without resistance at a temperature of around 5K. Before the advent of RSFQ, superconducting electronics schemes tried to imitate the voltage-level methods used in semiconductor electronics. But that approach was not able to take much advantage of the lack of electrical resistance in superconducting wires.

Advanced RSFQ circuits are in the 100- to 1,000-gate density range and run comfortably at 20GHz with very little power dissipationand that's at 3?m design rules. Mixed-signal devices can reach 100GHz easily, and the speed record in laboratory experiments is 750GHz.

Magnetic flux quanta can be ballistically launched down superconducting waveguides at close to the speed of light to create interconnect with almost no delay. The ballistic quanta are immune to noise, solving a major problem in mixed-signal circuit design. With semiconductors, noise from the digital portion of the circuit tends to interfere with the analog circuits. In general, a major advantage of using flux quanta to encode bits is their status as fundamental units of nature. They are indivisible and are determined by basic physical constants.

"The semiconductor people are always talking about how many gates you can get on a chip. We don't have to worry much about that because our logic is so fast," Rosa explained.

Intrinsic silicon performance cannot be improved very much without scaling process geometriesand scaling grows more complex with every process generation. But RSFQ circuits are only at the beginning of the performance scaling curve. Hypres is moving to a 1?m process and is doing research for the 0.8?m generation. And the basic thin-film process for RSFQ is much simpler than for semiconductor processesno high-temperature deposition, no doping profiles and no epitaxy.

That is the good news. The bad news is that the technology's extreme speed is also an impediment to circuit design. There is no possibility for real-time simulation of such circuits, and high-speed interconnect combined with the extreme sensitivity of the logic gate has made it difficult to create a standard design method. There is no workstation-based software, as there is in semiconductor design, that will allow a designer to lay out RSFQ circuits and then send them off to a fab. And the need to operate at such low temperatures is a further impediment for applications.

But the nascent field is addressing those problems. Researchers are getting together to create standardized logic cells and Verilog-based design tools. Cryogenics, meanwhile, has come a long way since Hypres was founded. While early cryogenic cooling systems relied on liquid helium, modern cryocoolers use an acoustic method to remove heat from the IC. The latest versions have no moving parts and excel in reliability.

"Cryogenics is our Achilles' heel, but we've really benefited from the advances in cryocooler design," said Rosa. "We excel in cryo packaging. We have developed a proprietary multichip-module technology with interconnect and I/O that operate at superconducting temperatures."

The cryocoolers have become so compact that they resemble large power supplies; indeed, today they incorporate the power supply, said Rosa. He said that the systems exhibit exceptional reliability.

The advent of high-critical-temperature superconductors has introduced interesting new facets to RSFQ electronics. Digital circuits based on these ceramic materials that become superconducting at liquid-nitrogen temperature and above are in the research stage. High-temperature research dominates the superconductor research work being done in European labs; the United States and Japan are predominantly in the low-temperature business.

High-temperature superconductors pose problems of their own. One factor is the higher temperature itself, which makes thermal noise more of a problem.

High-temperature materials are also much more complex, involving intricate crystal lattices formed from several elements. One result of that complexity is that superconduction only occurs in one lattice direction, which makes it much more difficult to design devices.

Because of those problems, commercial products using high-critical-temperature superconductors have been confined to analog components, such as the filters and low-noise amplifiers that are sold into the cellular base station market by Superconducting Technologies Inc. and to the power grid and electric motor applications that have been developed by American Superconductor Corp.

- Chappell Brown

EE Times





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