A DROP of water may be all it takes to ensure microprocessors get ever more powerful and memory chips ever more dense. So say engineers who have made the first microprocessor fabricated using a technique that lets chip makers shrink transistors without having to resort to expensive new manufacturing technologies.
Microprocessors and memories are etched using a lithographic process in which ultraviolet light with a wavelength of 193 nanometres is projected through a mask that defines the shape of all the chip’s wiring and transistors. This wavelength can create chips with features as small as 65 nm, which should be enough to maintain Moore’s law – the doubling of the number of transistors on a chip every two years – for a while. But this wavelength will not allow chip makers to reach the 45 nm that will be needed to keep this rate of progress going beyond 2012.
The problem arises from the fundamental properties of light. Diffraction effects mean that light passing through a slit in the mask that is narrower than one-quarter of its wavelength starts to spread out, rather than produce a sharp image on the chip. This means 193-nm UV light cannot be used to etch features that are less than 48 nm wide.
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Chip makers are exploring alternatives to UV to reach down to 45 nm and the next target after that, which is 32 nm. These alternatives include an expensive shift away from the lenses that focus UV light to mirrors that direct soft X-rays (also known as extreme UV) with a wavelength of 15 nm. But this new technology is expected to be extremely costly, so manufacturers want to delay the change for as long as possible.
Engineers at IBM may have the answer. They have built a fully functioning 64-bit Power PC microprocessor using a technique called immersion lithography, in which a drop of water bridges the gap between the UV lens and the mask on the silicon chip’s surface (see Graphic).
Because UV light passes though water more slowly than through the air or nitrogen that normally occupies this gap, its wavelength shrinks from 190 nm to 135 nm, which reduces the width of the components that can be etched to less than 34 nm. Although this falls just short of the 32-nm target, denser liquids could eventually shrink the wavelength still further to etch even smaller features.
Immersion lithography appeared on the chip industry’s horizon some 18 months ago, but this is the first time anyone has used it to build a working chip. “It’s a significant milestone,” comments Andrew Grenville of Intel, who is devising an immersion lithography strategy for Sematech, the chip industry’s process research consortium based in Austin, Texas.
“Immersion lithography also produces chips with fewer defects, and IBM is putting it into production immediately”
Engineers had feared that bubbles in the water would block light and wreck the etching process, or that the water would seed the wafer with impurities and create chip defects. But IBM says these problems can be overcome if the water is pure enough. And in fact, immersion produces chips with fewer defects, so IBM is putting the technique into production immediately.
