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Boffins melt defects from chips

Nano state
Mon May 05 2008, 17:04

BOFFINS AT PRINCETON UNIVERSITY have come up with an innovative way of melting away microchip defects, in a move which they reckon will drastically improve chip quality, without raising costs.

As companies strive to make smaller and smaller chips, miniscule defects are becoming bigger and bigger problems. Rough edges, breaks in lines, and other tiny flaws can all damage chip performance, causing voltage leaks, interference and worse. But now electrical engineer Stephen Chou, a Princeton Professor of Engineering, along with his graduate student, Qiangfei Xia, think they have come up with something to perfect chip making technique. Because microchips perform best when all their structures are straight, thin and tall to exact proportion, Chou and Xia knew that they had to discover a better and more precise way for companies to shape the microchip components.

"What we propose is a paradigm shift: Rather than struggle to improve fabrication methods, we could simply fix the defects after fabrication," said Chou, who’s findings were published yesterday in Nature Nanotechnology.

Chou’s discovery, which is dubbed Self-Perfection by Liquefaction (SPEL), works by melting all the structures on a chip for a tiny fraction of a second, in order to take advantage of surface tension, which lets the molten liquid re-solidify in more precise, and geometrically accurate shapes.

"We are able to achieve a precision and improvement far beyond what was previously thought achievable," gushed Chou, who added that he believed the process of fixing the defects could even be made automatic, or “a process of self-perfection”, as he puts it.

Currently, chip flaws are fixed by having every single defect painstakingly measured for size and shape, in order for an expensive tailor made repair to be hacked together. But the process is a slow one, as well as being expensive, which makes Chou’s melting system a very attractive alternative indeed.

In order to melt only the top layer of the chip, and not damage the underlying structure, Chou and his team used a light pulse from an excimer laser, which is quite similar to the lasers used in corrective eye surgery. The light pulse melts down the semiconductor and metal structures for a fraction of a millionth of a second, letting the molten metal flow and change shape in an instant.

The method is perfected by placing a tiny plate over the melting chip surface, which helps the liquid flow in the right way, and stops the whole structure from expanding or warping in shape. Another advantage of the guiding plate is that, when it is placed a tiny bit above the molten structure, the melted metal rises up to touch the plate, which makes any line structures on the chip taller and narrower, which is excellent in terms of chip design. The semiconductor factories are already salivating at the prospect.

Chou notes that "regardless of the shape of each defect, it always gets fixed precisely and with no need for individual shape measurement or tailored correction".

Chou says that he and his team will now work towards demonstrating the new melting technique on big (8-inch) wafers. µ



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