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today’s deep UV lithography systems, which depend on excimer lasers. The EUV radiation is produced from a microscopic droplet of molten tin, hit with a very high-power CO2

turned into plasma.

If the light is not bright enough, each silicon wafer has to be exposed for longer, which increases the time it takes to manufacture a chip. At the moment, the source power is around 10W, but ‘what we need is at least 100W in the short and mid-term and more than 250W in long term’, said Torsten Feigl of OptixFab, a Fraunhofer IOF spin-off company founded in 2012 to commercialise Fraunhofer IOF’s EUV optics research and development activities. To operate at 13.5nm, machines

also require sophisticated nano-size optics, which have to be positioned to tolerances measured in microns and be precise to sub-nanometres to avoid mid-spatial frequency (MSF) and high spatial frequency (HSF) errors. HSF roughness leads to lower reflectivity and conventional field aberrations. MSF errors, meanwhile, can result in flare – unwanted scatter around the area of photo-resist exposure – during the projection. To make the surfaces of its mirrors flawless, optics manufacturers use special sub-aperture polishing and coating techniques, such as computer controlled optical polishing (CCOS), magneto-rheological finishing (MRF) and ion beam finishing (IBF). Testing is usually performed at visible wavelengths of 633nm, and so has to be done to better than

Assembly of Zeiss EUV illumination system

one thousandth of a wavelength. Roughness measurements are done with an atomic force microscope. ‘We have built multiple custom

interferometers with 20 to 50pm repeatability errors to measure the figure of these optics,’ said Viswa Velur of Zygo Corporation, Middlefield, Connecticut, one of the leading manufacturers of EUVL optics. Also, according to Zeiss, the highest irregularity on a mirror used for EUV lithography is in the single nanometre range.

Reflecting badly

One of the main challenges in optics production is to design with 10 or

12 off-axis parabolas and ellipsoids, instead of simpler symmetric optics. ‘A curious feature of EUV is that reflecting light 90 degrees – off a 45-degree mirror – as is common at visible wavelengths, is bad,’ said Wallhead. ‘The reflection is highly polarising and inefficient, and this causes everything to be at odd angles. Also, a variation in angle of incidence on a mirror is bad.’ This is usually unavoidable, he said, so the coating needs to be graded to vary its thickness across the surface, which further complicates the manufacturing process. Another challenge is the

An EUV machine from ASML 16 ELECTRO OPTICS l FEBRUARY 2014

requirement to coat these optics to make them highly reflective at EUV while also preserving the wavefront quality. The coating comprises 30 to 40 bi-layers alternating between molybdenum and silicon, with each layer being only a few nanometres thick. The full stack needs to be perfect to achieve ultra-smooth interfaces with the highest possible reflectivity, which in theory is about 70 per cent for a perfect system. But currently, ‘for each “bounce” of the light in the lens there is only at best 60 per cent to 65 per cent of the light reflected’, said Kevin Cummings of Sematech, a consortium that

performs research and development to advance chip manufacturing. ‘That means for a lens necessary to reproduce sub-20nm nodes – typically six reflections – the amount of light making it to the resist is quite small.’

Patrick Naulleau, director and senior staff scientist at the Centre for X-ray Optics at Lawrence Berkeley National Laboratory, said, what is even harder than achieving the reflectivity, is that ‘these 80 layers must be deposited across the entire optic which can have diameters of many hundreds of millimetres and maintain uniformity to the l/50 level or 0.05nm – once you account for reflectivity and the fact that one optical system is comprised of four mirrors. Coating technology represents a huge accomplishment.’ A big challenge for EUV optics is scattering. ‘[It] goes as [much as] l-4

laser and


which made it a very large concern for EUV compared to deep UV optics, because of the 15 times reduction in wavelength,’ said Naulleau. ‘For scattering to stay below an acceptable level, the mirror roughness needs to be on the order of one angstrom, and this also has now been achieved by EUV optics suppliers.’ From the more conventional

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