One of the leading candidates for a technology that could make computers smaller and more powerful is based on transistors made from semiconducting nanowires. But until now, circuits made with such transistors have been impractical, because they were too power hungry and too difficult to manufacture. Now researchers at Caltech have built efficient nanowire-based circuits using a process they believe could be reliable enough for mass production.
This nanowire-based CMOS circuit (the nanowires are too small to see) could help lead to ultrasmall computers. (Credit: Dunwei Wang, Caltech)
The first applications, which could be available commercially in five years, will probably be in ultrasensitive, inexpensive sensors that could detect and measure hundreds of different cancer markers or pathogens in a small sample, such as a single drop of blood. Eventually, the nanowire-based electronics could be used in processors for computing.
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Nanowire logic is part of a growing effort to find new ways to produce computer chips after conventional methods run into physical limits. Other possibilities include carbon-nanotube transistors and molecular electronics, which would use organic molecules as transistors; but while those technologies have their own advantages, nanowires can be made of silicon, the material chip makers are used to working with. And they can more easily be made into arrays with consistent electronic properties.
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In the current work, the Caltech researchers created logic gates in which the centers of neighboring nanowire transistors were spaced at about 30 nanometers, denser than in state-of-the-art devices made with current technology. But lead researcher James Heath, a chemistry professor at Caltech, says that in these experiments, achieving the smallest possible size wasn’t the goal: they could have gone “much, much denser,” cutting the spacing at least in half. Such increase in density would allow far more transistors–and hence more computing power–to be squeezed onto a chip.
The work demonstrates for the first time the ability to exploit nanowires in CMOS, today’s standard semiconductor technology, using a process that could be adapted to mass production, Heath says. Most previous work with nanowire transistors had used older, more energy-intensive technology. And the few examples of CMOS-type circuits with nanowires were one-off prototypes, Heath says, not practical candidates for large-scale manufacturing.
CMOS relies on two kinds of transistors, n-type and p-type. One reason for the lack of reproducible nanowire CMOS devices is that it’s been difficult to make n-type nanowire transistors reliably. Even slight changes to their surfaces, caused by impurities deposited during manufacturing, can lead to wide variations in performance. Indeed, “most everything you do makes them p-type,” Heath says. So after studying how nanowires respond to surface changes, the researchers selected methods for treating the surfaces to remove impurities. This enabled them to make reliable devices.
The other challenge has been packing these two different types of nanowire transistors together on the same surface–and in a way that lends itself to mass production. The Caltech team created a checkerboard pattern of alternating p- and n-type silicon squares. Then they carved these squares into densely packed lines of nanowires, yielding p- and n-type nanowires that could easily be linked up to form transistors and circuits. Although the experimental circuits involved the use of relatively slow e-beam lithography to connect the wires, the researchers say that for mass production, similarly dense circuits could be made using much faster photolithography.
The first applications for the devices, sensors, are possible for the same reason nanowires are difficult to make reliably–their electronic properties change dramatically in response to slight changes to their surfaces. For example, a single strand of DNA could be attached to the surface of a nanowire. When a complementary strand of DNA (say, from a pathogen) in a blood sample linked to this DNA, a marked change in the resistance of the nanowire would register a hit. Hundreds of such sensors, each set to measure a different target, could easily be packed into a small chip in a handheld device. Testing each target with both types of transistors provides an automatic check against false positives, Heath says.
The Caltech work is a significant step forward for nanoelectronics, says Hongjie Dai, professor of chemistry at Stanford University, who has made similar circuits using carbon nanotubes. The move to CMOS with nanowires, he says, is “important if nanowires are to be used for future electronics applications.”
