System Bits: Sept. 3

Microprocessor built with carbon nanotubes
Researchers at the Massachusetts Institute of Technology were able to design a microprocessor with carbon nanotubes and fabricate the chip with traditional processes, an advance that could be used in next-generation computers.

Work on producing carbon nanotube field-effect transistors has gone on for some time. Fabricated at scale, those CNFETs often come out with significant defects, making them nearly useless.

The MIT team demonstrated a 16-bit microprocessor with more than 14,000 CNFETs, described in a paper published in the journal Nature. The paper includes more than 70 pages detailing the manufacturing methodology.

“This is by far the most advanced chip made from any emerging nanotechnology that is promising for high-performance and energy-efficient computing,” says co-author Max M. Shulaker, the Emanuel E Landsman Career Development Assistant Professor of Electrical Engineering and Computer Science (EECS) and a member of the Microsystems Technology Laboratories. “There are limits to silicon. If we want to continue to have gains in computing, carbon nanotubes represent one of the most promising ways to overcome those limits. [The paper] completely re-invents how we build chips with carbon nanotubes.”

A close-up of a modern microprocessor built from carbon nanotube field-effect transistors. Image: Felice Frankel

Joining Shulaker on the paper are: first author and postdoc Gage Hills, graduate students Christian Lau, Andrew Wright, Mindy D. Bishop, Tathagata Srimani, Pritpal Kanhaiya, Rebecca Ho, and Aya Amer, all of EECS; Arvind, the Johnson Professor of Computer Science and Engineering and a researcher in the Computer Science and Artificial Intelligence Laboratory; Anantha Chandrakasan, the dean of the School of Engineering and the Vannevar Bush Professor of Electrical Engineering and Computer Science; and Samuel Fuller, Yosi Stein, and Denis Murphy, all of Analog Devices.

The researchers came up with a technique called DREAM (an acronym for “designing resiliency against metallic CNTs”), which positions metallic CNFETs in a way that they won’t disrupt computing. In doing so, they relaxed that stringent purity requirement by around four orders of magnitude — or 10,000 times — meaning they only need carbon nanotubes at about 99.99% purity, which is currently possible.

“The ‘DREAM’ pun is very much intended, because it’s the dream solution,” Shulaker says. “This allows us to buy carbon nanotubes off the shelf, drop them onto a wafer, and just build our circuit like normal, without doing anything else special.”

The main goal is to get the chips out into the real world. To that end, the researchers have now started implementing their manufacturing techniques into a silicon chip foundry through a program by Defense Advanced Research Projects Agency, which supported the research. Although no one can say when chips made entirely from carbon nanotubes will hit the shelves, Shulaker says it could be fewer than five years. “We think it’s no longer a question of if, but when,” he says.

The work was also supported by Analog Devices, the National Science Foundation, and the Air Force Research Laboratory.

Ceramic welding without a furnace
A team of engineers led by the University of California San Diego came up with a ceramic welding technology that promises several advances in electronics.

The process, published in the Aug. 23 issue of Science, uses an ultrafast pulsed laser to melt ceramic materials along the interface and fuse them together. It works in ambient conditions and uses less than 50 watts of laser power, making it more practical than current ceramic welding methods that require heating the parts in a furnace.

Ceramics have been fundamentally challenging to weld together because they need extremely high temperatures to melt, exposing them to extreme temperature gradients that cause cracking, explained senior author Javier E. Garay, a professor of mechanical engineering and materials science and engineering at UC San Diego, who led the work in collaboration with UC Riverside professor and chair of mechanical engineering Guillermo Aguilar.

Ceramic materials are of great interest because they are biocompatible, extremely hard, and shatter-resistant, making them ideal for biomedical implants and protective casings for electronics. However, current ceramic welding procedures are not conducive to making such devices.

“Right now, there is no way to encase or seal electronic components inside ceramics because you would have to put the entire assembly in a furnace, which would end up burning the electronics,” Garay said.

Garay, Aguilar, and colleagues’ solution was to aim a series of short laser pulses along the interface between two ceramic parts so that heat builds up only at the interface and causes localized melting. They call their method ultrafast pulsed laser welding.

To make it work, the researchers had to optimize two aspects: the laser parameters (exposure time, number of laser pulses, and duration of pulses) and the transparency of the ceramic material. With the right combination, the laser energy couples strongly to the ceramic, allowing welds to be made using low laser power (less than 50 watts) at room temperature.

“The sweet spot of ultrafast pulses was two picoseconds at the high repetition rate of one megahertz, along with a moderate total number of pulses. This maximized the melt diameter, minimized material ablation, and timed cooling just right for the best weld possible,” Aguilar said.

“By focusing the energy right where we want it, we avoid setting up temperature gradients throughout the ceramic, so we can encase temperature-sensitive materials without damaging them,” Garay said.

As a proof of concept, the researchers welded a transparent cylindrical cap to the inside of a ceramic tube. Tests showed that the welds are strong enough to hold vacuum.

“The vacuum tests we used on our welds are the same tests that are used in industry to validate seals on electronic and optoelectronic devices,” said first author Elias Penilla, who worked on the project as a postdoctoral researcher in Garay’s research group at UC San Diego.

The process has so far only been used to weld small ceramic parts that are less than two centimeters in size. Future plans will involve optimizing the method for larger scales, as well as for different types of materials and geometries.

Superconductivity in a nickel oxide material
Scientists at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University collaborated on producing a nickel oxide material that has the characteristics of superconductivity.

Nickel oxide, also known as nickelate, represents the first in a potential new family of unconventional superconductors that’s very similar to the copper oxides, or cuprates, whose discovery in 1986 raised hopes that superconductors could someday operate at close to room temperature and revolutionize electronic devices, power transmission, and other technologies. Those similarities have scientists wondering if nickelates could also superconduct at relatively high temperatures.

At the same time, the new material seems different from the cuprates in fundamental ways – for instance, it may not contain a type of magnetism that all the superconducting cuprates have – and this could overturn leading theories of how these unconventional superconductors work. After more than three decades of research, no one has pinned that down.

The experiments were led by Danfeng Li, a postdoctoral researcher with the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC and described in the journal Nature.

“This is a very important discovery that requires us to rethink the details of the electronic structure and possible mechanisms of superconductivity in these materials,” said George Sawatzky, a professor of physics and chemistry at the University of British Columbia who was not involved in the study but wrote a commentary that accompanied the paper in Nature. “This is going to cause an awful lot of people to jump into investigating this new class of materials, and all sorts of experimental and theoretical work will be done.”

Ever since the cuprate superconductors were discovered, scientists have dreamed of making similar oxide materials based on nickel, which is right next to copper on the periodic table of the elements.

But making nickelates with an atomic structure that’s conducive to superconductivity turned out to be unexpectedly hard.

“As far as we know, the nickelate we were trying to make is not stable at the very high temperatures – about 600 degrees Celsius – where these materials are normally grown,” Li said. “So, we needed to start out with something we can stably grow at high temperatures and then transform it at lower temperatures into the form we wanted.”

He started with a perovskite – a material defined by its unique, double-pyramid atomic structure – that contained neodymium, nickel, and oxygen. Then he doped the perovskite by adding strontium; this is a common process that adds chemicals to a material to make more of its electrons flow freely.

This stole electrons away from nickel atoms, leaving vacant “holes,” and the nickel atoms were not happy about it, Li said. The material was now unstable, making the next step – growing a thin film of it on a surface – really challenging; it took him half a year to get it to work.

Once that was done, Li cut the film into tiny pieces, loosely wrapped it in aluminum foil and sealed it in a test tube with a chemical that neatly snatched away a layer of its oxygen atoms – much like removing a stick from a wobbly tower of Jenga blocks. This flipped the film into an entirely new atomic structure – a strontium-doped nickelate.

“Each of these steps had been demonstrated before,” Li said, “but not in this combination.”

He remembers the exact moment in the laboratory, around 2 a.m., when tests indicated that the doped nickelate might be superconducting. Li was so excited that he stayed up all night, and in the morning co-opted the regular meeting of his research group to show them what he’d found. Soon, many of the group members joined him in a round-the-clock effort to improve and study this material.

Further testing would reveal that the nickelate was indeed superconducting in a temperature range from 9-15 kelvins – incredibly cold, but a first start, with possibilities of higher temperatures ahead.

Research on the new material is in a “very, very early stage, and there’s a lot of work ahead,” cautioned Harold Hwang, a SIMES investigator, professor at SLAC and Stanford, and senior author of the report. “We have just seen the first basic experiments, and now we need to do the whole battery of investigations that are still going on with cuprates.”

Among other things, he said, scientists will want to dope the nickelate material in various ways to see how this affects its superconductivity across a range of temperatures and determine whether other nickelates can become superconducting. Other studies will explore the material’s magnetic structure and its relationship to superconductivity.

SIMES researchers from the Stanford departments of Physics, Applied Physics and Materials Science, and Engineering also contributed to the study, which was funded by the DOE Office of Science and the Gordon and Betty Moore Foundation’s Emergent Phenomena in Quantum Systems Initiative.