Impossibly thin carbon nanotubes to replace silicon and 'save' Moore's law

Future computers could run on lab-grown circuits that are thousands of times thinner than a human hair and operate on a fraction of the energy required to power today's silicon-based computer chips, extending Moore's Law for years to come.

Stanford University researchers this week reported that they have built a working computer using transistors fashioned from carbon nanotubes (CNTs), a possible breakthrough in finding a cooler alternative to the heat-generating, silicon-based circuitry used in computing today.

The team, led by Stanford professors Subhasish Mitra and H.S. Philip Wong, managed to overcome a "bedeviling" host of problems with CNTs, which "has long frustrated efforts to build complex circuits using CNTs," a Stanford media relations spokesperson said his week. The researchers reported the results of their efforts in this week's issue of Nature in a cover story written by electrical engineering doctoral student Max Shulaker and several colleagues.

For decades, Intel co-founder Gordon Moore's 1965 prediction that computer circuitry will keep getting smaller and cheaper to produce has held up. But as silicon-based integrated circuits (ICs) keep getting more densely populated with transistors, the large amounts of heat they produce has become more concentrated and difficult to dissipate, prompting concerns over whether silicon can be used for many more generations of transistor shrinkage.

Enter carbon nanotubes as a semiconductor material that could potentially succeed silicon. CNTs conduct and control energy much more efficiently than silicon, said project co-leader Wong, who suggested that we "think of it as stepping on a garden hose—the thinner the hose, the easier it is to shut off the flow."

That's important, because as the researchers noted, Moore's Law has so far worked to make computer circuitry "smaller, faster, and cheaper" every couple of years, but the increasing density of transistors on microchips has also made them "smaller, faster, and hotter."

"People have been talking about a new era of carbon nanotube electronics moving beyond silicon. But there have been few demonstrations of complete digital systems using this exciting technology. Here is the proof," Mitra said in a statement.

By producing a simple but functioning computer circuit using CNTs, the researchers accomplished something scientists around the world have been attempting for years, Prof.essor Giovanni De Micheli, director of the Institute of Electrical Engineering at Switzerland's École Polytechnique Fédérale de Lausanne, told Stanford.

"First, they put in place a process for fabricating CNT-based circuits. Second, they built a simple but effective circuit that shows that computation is doable using CNTs," De Micheli said.

It's been more than a decade since CNTs — tiny, synthesised, spherical structures composed of chains of carbon atoms — were first fashioned into on-off switches that could potentially populate a computer chip, the researchers noted. But it has proven extremely difficult to produce uniform nanotube semiconductors with the consistency needed to create a working computer chip, much less produce CNT-based processors in volume.

Unlike silicon transistors, which are printed onto computer chip substrates with highly precise lithographic tools, CNTs are "grown" using a variety of methods, much as a diamond lattice can be formed from carbon atoms in the extreme pressurised conditions of the Earth's mantle.

Scientists have been able to refine the techniques used to fashion impossibly slender CNTs in straight, uniform lines with 99.5 per cent yield rates, "[b]ut with billions of nanotubes on a chip, even a tiny degree of misaligned tubes could cause errors," the Stanford researchers explained.

Until now, no one has been able to produce a working CNT-based circuit capable of doing computational work just like silicon-based transistor arrays. The Stanford team developed what they call an "imperfection-immune design" for growing CNTs to create a simple computer circuit with 178 transistors capable of running a barebones operating system to perform counting and number sorting tasks.

The challenge was two-fold: fabrication resulted in a certain percentage of misaligned nanotubes, and a certain percentage of electricity conducting CNTs that could be switched on but not off.

To deal with the first issue, the researchers created a "powerful algorithm that maps out a circuit layout that is guaranteed to work no matter whether or where CNTs might be askew." The second problem was handled with a brute force approach — the team switched off the functioning CNT semiconductors on their circuit, and then "pumped the semiconductor circuit full of electricity" to burn up and vaporise the bad ones.

Voila — a simple, CNT-based computer chip fashioned with a technique that could pave the way for volume production of such extremely energy efficient electronics circuitry in the future.

Scientists working on the problem around the world commended the Stanford team for its accomplishment and said it marked a major milestone in making CNTs a potentially practical alternative material to silicon for fabricating the integrated circuits used on computer chips.

"This 'imperfection-immune design' makes this discovery truly exemplary," Sankar Basu, a program director at the National Science Foundation, told Stanford.