I believe it’s time to look again at wavelength reduction to understand its pros and cons. We do not know the best choice between 13.5nm and 1nm, so I am calling this new technology option Blue-X–something between deep blue EUV and the X-ray region.

Moving to the higher numerical aperture (NA) of 0.5 is going to be expensive. The tool cost will more than double to 235 million Euros, and the larger scanner size will require building bigger fabs with higher ceilings.

Once we have high NA working, it may make sense to consider high NA multipatterning while thinking about even higher NA at even higher cost. However, reducing wavelength can also allow reducing NA which may result in lower tool cost while increasing resolution.

Taking 0.3 as the limit of k1 for a single exposure, at a wavelength of 13.5nm, 0.33 NA gives 12nm resolution which improves to 8nm at 0.5 NA. Previously the industry looked at a 6.7nm wavelength, but this option was dropped primarily for lack of bandwidth as we were struggling with power issues.

Going to higher NA from 0.33 as compared to moving to a 6.7nm wavelength had its advantages: We could keep the infrastructure of power, ML and masks the same. We have learned that taking on too many challenges at the same time is not a good idea.

We have figured out how to scale power on laser-produced plasma (LPP), optics, contamination control and masks. We will be able to apply many of these learnings to a scanner designed for smaller wavelengths. So, I believe it is time to revisit the wavelength-reduction option. I suggest we look all the way to 1nm while considering the pros and cons of other choices.

Source and optics challenges

In the past, 11nm and 6.6 or 6.7nm light sources have been explored as potential smaller wavelengths for EUVL. Xenon could provide 11nm and for 6.X-nm Gd and Tb were considered as source material for LPP-based sources.

By increasing the atomic weight Z of the target material, we can continue to obtain photons of decreasing wavelength from LPP sources. There is not a single wavelength that these high Z materials emit, but there is a group of very close wavelengths called unidentified transition array (UTA).

Total intensity of emission will correspond to total oscillator strength of a UTA. Potential conversion efficiency will need to be evaluated for each of these potential UTAs.

This is an interesting area with several interesting features like K-edge of silicon, carbon window and water widow. The water window has seen lots of recent development efforts for microscopy applications.

However, there are several challenges in generating hundreds of watts of these shorter wavelength photons. The largest one is the power required for the drive laser. For 6.X-nm, one estimate for power required was 100kW while it is ~ 40kW for 13.5 nm.

I have seen designs for 65kW CO 2 lasers, but due to large power requirements it may be worthwhile at this point to review alternative laser technologies. A 100kW laser at 1 micron is now available for the government’s so-called Star Wars defense program.

Another option that looks attractive is the 1.2-micron laser from Lawrence Livermore National Laboratory. It can be scaled to 300kW while keeping footprints smaller than those of CO 2 lasers.

Of course, we have to see what conversion efficiency (CE) we can get at 1.2 micron. The CE for 1 micron of Nd:YAG is lower than that for 10 microns of CO 2 . So, we face a couple of things to figure out before we can pinpoint our best choices for a 100kW drive laser.

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