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by mk_stjames
733 days ago
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Diffraction sets a limit on how small the features can be patterned using photolith. When the wavelength is larger than the feature size, diffraction causes the light to spread out and blur the edges of what you're trying to pattern. The Rayleigh criterion shows how the ability to separate features depends on the numerical aperture of the system and the wavelength used. The explanation under 'Resolution in projection systems' on the wikipedia for photolithography is a better explanation than what is talked about under the EUV article. https://en.wikipedia.org/wiki/Photolithography Going further and further into the UV makes the wavelength smaller and smaller and thus the feature size smaller and smaller. But making light that is controllable in a way for photolithography techniques to work that far into the UV is the difficult part. |
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The minimum feature size that a projection system can print is given approximately by:
where CD is the minimum feature size (also called the critical dimension, target design rule, or "half-pitch"), λ is the wavelength of light used, and NA is the numerical aperture of the lens as seen from the wafer.k1 (commonly called k1 factor) is a coefficient that encapsulates process-related factors and typically equals 0.4 for production. (k1 is actually a function of process factors such as the angle of incident light on a reticle and the incident light intensity distribution. It is fixed per process.) The minimum feature size can be reduced by decreasing this coefficient through computational lithography.
According to this equation, minimum feature sizes can be decreased by decreasing the wavelength, and increasing the numerical aperture (to achieve a tighter focused beam and a smaller spot size).
Thus the NA being increased in Hyper-NA (0.75) from High-NA (0.55) results in a smaller "feature size" i.e. smaller nanometers.
I need to read some more but i think i now get the basic Physics concepts involved.