Future Of Computing -- Carbon Nanotubes, Superconductors to Replace Silicon Chip

The silicon chip, which has supplied several decades’ worth of remarkable increases in computing power and speed, looks unlikely to be capable of sustaining this pace for more than another decade – in fact, in a plenary talk at the the Institute of Physics’ Condensed Matter and Materials Physics conference, Suman Datta of Pennsylvania State University, gives the conventional silicon chip no longer than four years left to run.

As silicon computer circuitry gets ever smaller in the quest to pack more components into smaller areas on a chip, eventually the miniaturized electronic devices are undermined by fundamental physical limits. They start to become leaky, making them incapable of holding onto digital information. So if the steady increases in computing capability that we have come to take for granted are to continue, some new technology will have to take over from silicon.

Replacing the chip with carbon nanotubes

At the conference, researchers at Leeds University in the UK will report an important step towards one prospective replacement. Carbon nanotubes, discovered in 1991, are tubes of pure carbon just a few nanometres wide – about the width of a typical protein molecule, and tens of thousands of times thinner than a human hair. Because they conduct electricity, they have been proposed as ready-made molecular-scale wires for making electronic circuitry.

Some nanotubes behave as semiconductors, like silicon; others carry electric currents like metal wires. Already, fundamental elements of computer circuits such as transistors have been made from individual carbon nanotubes.

But the problem is arranging nanotubes into circuit patterns. One particular difficulty is that they are typically made as mixtures of metallic and semiconducting tubes, whereas just one type or the other is needed for a specific component. These electrical properties depend on the precise arrangement of carbon atoms in the nanotube, but that’s hard to determine for single tubes.

Bryan Hickey and his coworkers at Leeds have now developed a technique that will reveal an individual nanotube’s structure (and thus its electrical properties), and then allow it to be placed in a position on a surface with an accuracy of about 100 nanometres, a fraction of the width of a human blood cell. The nanotubes are grown on a perforated ceramic grid, and tubes lying across the holes are examined in an electron microscope to deduce their atomic structures. Then the researchers use two needle-fine tips like tweezers to pick up a single tube under the microscope and put back down on another surface.

Chris Allen, one of the Leeds teams, says, “With this technique we can make carbon nanotube devices of a complexity that is not achievable by most other means.”

Boosting computer power with superconductors

Two further talks at the meeting will describe an even more dramatic way to overcome the limitations of silicon computers. Hans Mooij of the Delft University of Technology in the Netherlands and Raymond Simmons of the National Institute of Standards and Technology in Boulder, Colorado, USA, will claim that superconductors – materials that conduct electricity with zero electrical resistance – can harness the power of quantum physics to boost computer power tremendously.

So-called quantum computers have become one of the hottest items in physics over the past decade. They attempt to improve on the power of silicon not by making components smaller but by exploiting the counterintuitive principles of quantum mechanics, the theory generally used to understand how objects behave at the scale of atoms and subatomic particles.

Objects governed by quantum theory can be in several different states at once, like a light switch being simultaneously ‘on’ and ‘off’. These ‘superposition’ states don’t correspond to anything familiar from our everyday world, but countless experiments have proved that they can exist so long as the quantum objects are not disturbed by, for example, making a measurement on them.
In a quantum computer, the equivalent of ‘bits’ that hold binary information as 1’s and 0’s in today’s computers will be quantum bits or qubits, which can also exist as superpositions of 1’s and 0’s. This massively increases the amount of information that can be encoded in a quantum computer’s memory. The catch is that superpositions are extremely delicate and hard to maintain, especially in memories containing large numbers of qubits that interact with one another.

Various candidates for making qubits are being explored, such as magnetically trapped atoms or nanometre-scale blobs of semiconductors. But it has long been recognized that loops of superconducting material can also be placed in quantum superposition states, and thus act as qubits. Here the quantum states may correspond to an electric current circulating round the ring in one direction or the other. (In superconductors this circulation can continue more or less indefinitely without petering out, because there is no electrical resistance.)

At the conference, Simmonds will describe the first demonstration of information being transmitted between two such superconducting qubits. This shows that elements of this kind can act as a quantum-computing memory and a “bus” for qubits to communicate with one another, an essential requirement of any working computer.

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Advances Pave Way for Powerful Carbon-Based Electronics

Bypassing decades-old conventions in making computer chips, Princeton engineers developed a novel way to replace silicon with carbon on large surfaces, clearing the way for new generations of faster, more powerful cell phones, computers and other electronics.

The electronics industry has pushed the capabilities of silicon -- the material at the heart of all computer chips -- to its limit, and one intriguing replacement has been carbon, says Stephen Chou, professor of electrical engineering. A material called graphene -- a single layer of carbon atoms arranged in a honeycomb lattice -- could allow electronics to process information and produce radio transmissions 10 times better than silicon-based devices.

Until now, however, switching from silicon to carbon has not been possible because technologists believed they needed graphene material in the same form as the silicon used to make chips: a single crystal of material eight or 12-inches wide. The largest single-crystal graphene sheets made to date have been no wider than a couple millimeters, not big enough for a single chip. Chou and researchers in his lab realized that a big graphene wafer is not necessary, as long they could place small crystals of graphene only in the active areas of the chip. They developed a novel method to achieve this goal and demonstrated it by making high-performance working graphene transistors.

“Our approach is to completely abandon the classical methods that industry has been using for silicon integrated circuits,” Chou says.

Chou, along with graduate student Xiaogan Liang and materials engineer Zengli Fu, published their findings in the December 2007 issue of Nano Letters, a leading journal in the field. The research was funded in part by the Office of Naval Research.

In their new method, the researchers make a special stamp consisting of an array of tiny flat-topped pillars, each one-tenth of a millimeter wide. They press the pillars against a block of graphite (pure carbon), cutting thin carbon sheets, which stick to the pillars. The stamp is then removed, peeling away a few atomic layers of graphene. Finally, the stamp is aligned with and pressed against a larger wafer, leaving the patches of graphene precisely where transistors will be built.

The technique is like printing, Chou says. By repeating the process and using variously shaped stamps (the researchers also made strips instead of round pillars), all the active areas for transistors are covered with single crystals of graphene.

“Previously, scientists have been able to peel graphene sheets from graphite blocks, but they had no control over the size and location of the pieces when placing them on a surface,” Chou says.

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Researchers Create Cell-Sized Memory Device

Researchers say they have created a memory device the size of a white blood cell in an advance of nanotechnology that should help manufacturer's keep up the pace of Moore's Law.

The tiny ultra-dense memory device has enough capacity to store the Declaration of Independence and still have room to spare, they say. It is called a major advance over current silicon computing technology.

"Using molecular components for memory or computation or to replace other electronic components holds tremendous promise," says J. Fraser Stoddart, director of the California NanoSystems Institute.

The cell-sized device was created by arranging 160,000 memory bits like a large tic-tac-toe board: 400 silicon wires crossed by 400 titanium wires, each 16 nanometers wide, with a layer of dumbbell-shaped molecular switches sandwiched between the crossing wires. A nanometer is one-billionth of a meter and is a common measurement in nanotechnology, which is engineering at the atomic scale.

"This research is one of the only examples of building large molecular memory in a chip at an extremely high density, testing it, and working in an architecture that is practical, where it is obvious how information can be written and read," Stoddart says.

Moore's law has long been a basic tenet of computing technology that says the complexity of an integrated circuit, or computer chip, will double every year based on increased miniaturization. Manufacturers, however, see no way to extend the miniaturization beyond the year 2013. Nanotechnology, such as the cell-sized memory device, respresents one potential solution.

"Our goal was not to demonstrate a robust technology; the memory circuit we have reported on is hardly that," says James Heath of the California Institute of Technology and a collaborator on the project. "Instead, our goal was to demonstrate that large-scale, working electronic circuits could be constructed at a density that is well-beyond (10-15 years) where many of the most optimistic projections say is possible."


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