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Shrinking transistors Microprocessors are made using lithography, to shape and layer the material that makes a microchip. The wavelength of the laser light used for the process is the obvious limitation to how small the chip’s components can get.

Over the years, lithography has evolved from ultraviolet light with wavelengths of 436nm (‘g-line’), 405nm (‘h-line’) and 365nm (‘i-line’) to deep ultraviolet light of 248nm, and, not that long ago, 193nm, improving the resolution at every step. The past decade saw transistors shrink from 100nm to 22nm, while the wavelength stayed constant at 193nm.

It is this ability to pattern smaller

features on silicon chips that has kept Moore’s law valid, a law that ‘the entire economic model of the

Higher NA and

shorter wavelengths make lithography tools more difficult to make, as smaller chips mean higher precision

semiconductor industry relies on’, commented physicist Shannon Hill of the Ultraviolet Radiation Group at the National Institute of Standards and Technology (NIST), a measurement standards laboratory in Maryland, USA.

‘Optical 193nm immersion lithography is gradually running out of steam and requires multiple patterning steps to further improve the resolution,’ said Kurt Ronse of Imec, a research centre headquartered in Leuven, Belgium. ‘Multiple patterning is an extremely complex and expensive exercise, which significantly degrades the cycle time of the process.’ EUV lithography, also known as soft X-ray lithography, uses 13.5nm radiation. At this wavelength, the light source can be used to pattern much finer features than traditional 193nm | @electrooptics

optical lithography. ‘Shortening the wavelength of the light means improved resolution and smaller features,’ said Niclas Mika of ASML, a lithography machine manufacturer headquartered in Veldhoven, Netherlands. ‘EUV is the next step. It’s like using a smaller brush to paint finer details.’ EUVL technology will be able to produce features on electronic circuits at just 10nm in size. With the wavelength getting

shorter, numerical aperture (NA) has been getting higher. But higher NA and shorter wavelengths make lithography tools more difficult to make, as smaller chips mean higher precision. In particular, the depth of focus reduces rapidly. So, with the jump by a factor of 14, from 193nm to 13.5nm, it was also generally decided to reduce the NA in order to improve the resolution and depth of focus. This made ‘chips easier to manufacture, at least from the focus point of view’, said Ian Wallhead of Hyperion Development, a small EUV optics manufacturer headquartered in San Francisco.

The maximum available NA today is 0.33, but Ronse is certain it can, in the future, be increased to 0.45 and even 0.6 with different optical designs. ‘The wavelength is 15 times smaller than 193nm. The ratio of wavelength to NA expresses the resolution limit in optical as well as EUV lithography: 13.5nm to 0.33 is much smaller than 193nm to 1.35,’ he said.

From lab to industry But how long will it take before commercial EUV systems become operational? Ronse believes that it will be another two to three years before EUVL can take over the industry. At ASML, the researchers are a bit more optimistic; EUVL might be ready for commercial use by 2015, they say. To be successful on an industrial scale, however, a number of critical focus points have to be addressed, said Ronse. And the main stumbling block on the way to mass production is currently source power – it must increase at least by a factor of 10. EUV needs a different light source than



Trademarked in 1921, the Phoropter vision test equipment contains different lenses developed from the simple trial frame. It is used by eye care professionals to measure an individual’s refractive error and determine a precise eyeglass prescription.

Made for Accuracy Designed to Measure

Your laser Our sensor

• Custom designed • Quick reaction • Fast prototyping available • Several measurement technologies

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