Manufacturing Bits: Aug. 19

28nm brain chips; enriching silicon; the ongoing mystery of silicene.

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28nm brain chips
DARPA-funded researchers have developed a 28nm chip that mimics the brain. The low-power chip is inspired by the neuronal structure of the brain.

Designed by researchers at IBM under DARPA’s Systems of Neuromorphic Adaptive Plastic Scalable Electronics (SyNAPSE) program, the chip consists of 5.4 billion transistors. Built on Samsung’s 28nm foundry process, the chip has one of the highest transistor counts of any device ever produced.

A circuit board shows 16 of the new brain-inspired chips in a 4 X 4 array along with interface hardware. (Source: DARPA/IBM)

A circuit board shows 16 of the new brain-inspired chips in a 4 X 4 array along with interface hardware. (Source: DARPA/IBM)

The chip consumes less than 100 milliWatts of power and has 256 million “synapses.” These are programmable logic points, which are analogous to the connections between neurons in the brain. But that’s still orders of magnitude fewer than the number of actual synapses in the brain, according to DARPA.

The SyNAPSE program, according to DARPA, is geared to develop low-power electronic neuromorphic computers that scale to biological levels. Today’s computers are limited by the amount of power required to process large volumes of data. In contrast, biological neural systems, such as the brain, process large volumes of information at low power.

The initial phase of SyNAPSE is to develop nanometer-scale electronic synaptic components. All told, the chip could perform difficult perception and control tasks.

“Moore’s law—the exponentially decreasing cost of constructing high-transistor-count chips—now allows computer architects to borrow an idea from nature, where energy is a more important cost than complexity, and focus on designs that gain power efficiency by sparsely employing a very large number of components to minimize the movement of data. IBM’s chip, which is by far the largest one yet made that exploits these ideas, could give unmanned aircraft or robotic ground systems with limited power budgets a more refined perception of the environment, distinguishing threats more accurately and reducing the burden on system operators,” said Gill Pratt, DARPA program manager, on the agency’s site.

“Our troops often are in austere environments and must carry heavy batteries to power mobile devices, sensors, radios and other electronic equipment. Air vehicles also have very limited power budgets because of the impact of weight. For both of these environments, the extreme energy efficiency achieved by the SyNAPSE program’s accomplishments could enable a much wider range of portable computing applications for defense,” he added.

Enriching silicon
The National Institute of Standards and Technology (NIST) has beat its own record for making highly enriched silicon. This type of material could pave the way for developing chips in quantum computers.

The highly enriched silicon material from NIST is more than 99.9999% pure silicon-28 (28Si). The material is also less than 1 part per million (ppm) of the troublesome isotope silicon-29 (29Si).

NIST broke its own record. Last year, the organization was able to enrich silicon at “five nines” (99.9998%) of pure 28Si.

NIST's silicon enrichment flow. The process begins at the left, where silicon in the form of SiH4 is ionized. The ions pass through a magnetic field (top left), which causes their paths to curve to different degrees based on their mass. The silicon-28 ions, now separated from the other silicon isotopes, are decelerated through an ultra-high vacuum chamber (top center) and into the deposition chamber (top right).  (Source: NIST)

NIST’s silicon enrichment flow. The process begins at the left, where silicon in the form of SiH4 is ionized. The ions pass through a magnetic field (top left), which causes their paths to curve to different degrees based on their mass. The silicon-28 ions, now separated from the other silicon isotopes, are decelerated through an ultra-high vacuum chamber (top center) and into the deposition chamber (top right). (Source: NIST)

Unenriched silicon contains about 92% of 28Si, but it also includes about 4.7% of 29Si. For quantum computing, the presence of 29Si is a problem. It dominates the breakdown of quantum information of the qubits, according to NIST.

“It has to do with unpaired nuclear spin states,” said Josh Pomeroy of PML’s Quantum Processes and Metrology Group, on NIST’s Web site. “(The 29Si) essentially becomes the equivalent of people talking on their cell phones in the audience of a movie theatre. It messes everything up.”

To enrich silicon, silicon atoms from silane gas (SiH4) are ionized. The ions are extracted at high voltage. Then, they are shot through a magnetic field. This, in turn, causes the trajectories of the ions to curve.

The 28Si and 29Si materials diverge into separate beams. The 28Si ions are separated from the other silicon isotopes. They are decelerated through an ultra-high vacuum chamber and into a deposition chamber. The 28Si ions are collected onto an unenriched silicon substrate about 1-square-cm.

“The real challenge now is to make this amorphous enriched silicon into a form that is equivalent to what you would get if you bought a wafer or epitaxial layer,” Pomeroy said.

The ongoing mystery of silicene
The mystery continues to unfold for silicene.

Silicene, a 2D material that is similar to graphene, is the subject of interest. Silicene is a 2D sheet of silicon atoms, which can be created by heating silicon and evaporating atoms onto a silver platform. Like graphene and other 2D materials, silicene could enable futuristic FET devices.

But recently, the U.S. Department of Energy’s Argonne National Laboratory has called into question the existence of silicene. The group basically debunked the material.

On the other hand, Consiglio Nazionale delle Ricerche and others have demonstrated the existence of silicene. Researchers have shown that multilayers of silicene can be isolated and remain intact when exposed to air for 24 hours.

Silicene must be produced in a vacuum. It must avoid any contact with oxygen. Otherwise, it will destroy the formation of the materials. In theory, silicene can be grown on the surface of silver. To create silicene, a silicon wafer is heated to high temperatures. This causes single silicon atoms to evaporate and land on the silver substrate.

This, in turn, creates single layers of silcene. More layers can be stacked on top of each other. In stacking, however, silicene may revert back to silicon, as the silicon structure is more stable, according to researchers.

In the new study, researchers made multilayers of silicene using a silver substrate. The substrate was kept at a temperature of 470 K. Some 43 monolayers of silicene were deposited onto the substrate.

Researchers also observed that a thin layer of oxidation formed on top of the multilayered stack, which acted like a protective layer. “These results are significant as we have shown that it is possible to obtain a silicon-based 2D material, which up until a couple of years ago was deemed inconceivable,” said Paola De Padova from Consiglio Nazionale delle Ricerche, on the Institute of Physics Web site.

“Our present study shows that multilayered silicene is more conductive than single-layered silicene, and therefore opens up the possibility of using it throughout the silicon microelectronics industry. In particular, we (envision) the material being used as gate in a silicene-based MOSFET, which is the most commonly used transistor in digital and analog circuits,” De Padova said. “We are currently studying the possibility of growing multilayered silicene directly onto semiconductor substrates to explore the joint superconducting properties.”