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optics perspective, one big question surrounding the EUVL optics is how to deal with high numerical aperture EUV, in particular the angular bandwidth limitations in the EUV mask Bragg reflector. These reflectors – formed from several layers of alternating materials with changing refractive index – are used in waveguides, for example in optical fibres.

Lifetime is also a challenge. As light

sources are plasma-based and not lasers, they produce particle debris that can damage collector mirrors and lead to their fast degradation. ‘Virtually any trace of hydrocarbon molecules can cause damage to the mirrors as the high energy photons crack the molecules and deposit carbon on the mirrors,’ said Wallhead. Another problem is masks that

currently have too many defects, at a factor of 10 to 100. Since the resolution is so good with EUV, any defect on the mask is reproduced

on the wafer, and therefore the EUV mask needs to be virtually defect-free. Also, photoresists need improvements to obtain resolution, sensitivity and especially reduce line edge roughness.

EUVL taking off So what are the remaining challenges for EUVL optics? ‘In the short term, where we are concerned about optics with numerical apertures below about 0.38, there are no huge challenges,’ said Naulleau. ‘As we go beyond that, however, we become concerned about optical system design issues to make sure we limit the number of mirrors in order to optimise throughput, and that we control the angular range of the coatings.’

It is in principle possible to develop broad angle coatings, he added, but it comes at the cost of reflectivity. ‘Because source power is by far the biggest challenge facing EUV, we cannot afford to do anything

to the optics that unnecessarily compromises throughput.’ Even when EUV lithography does finally take off, it is unlikely to replace all optical lithography straightaway, due to the complexity and initial high cost of the process. ‘EUVL is expected

Today the best

bet is for a change in magnification in the lenses to continue EUV evolution

to be introduced in only a limited number of critical layers, replacing the most expensive 193nm multiple patterning processes,’ said Ronse of Imec.

Even though the process is slow, one thing is certain – there are no plans to stop at what has so far been achieved. After all, the aim is to

keep up with Moore’s law, and push electronic devices to the ultimate frontier of size and power. While ASML’s high-volume tool NXE:3300 uses a six-mirror (NA=0.33) projection system, Zeiss and ASML have already discussed eight-mirror systems to increase the NA up to 0.7 at 13.5nm. ‘The second generation of EUV will add the tricks that we have learned in optical lithography to extend well into the single digit nanometre node for IC manufacturing,’ said Cummings.

‘Beyond that, there will be a tough decision to make as reflection lenses have considerable limits compared to refractive optics. Today, the best bet is for a change in magnification in the lenses to continue EUV evolution.’ There are already talks of jumping to a wavelength of 7nm, all while increasing numerical aperture to 0.6. And who knows – maybe beyond.l

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