system
Given the following post, respond as if you're commenting on Reddit:
human
Why is 7nm considered the limit of Moore's law?

I understand that around 7nm silicon can no longer be used, but I am unsure why. I have seen that using graphite can lower the limit to 5nm, but again unsure why.
gpt
Commercial integrated circuits today are made using projection methods. This can be grossly simplified as follows.

The wafer is coated with photoresist. A stencil of the circuit is projected onto the wafer; where the light hits the photoresist, it gets developed just like an old analog camera. This process is fundamentally limited by the wavelength of light. Because of diffraction, you can't really image features smaller than the wavelength of the light you are shining through the stencil. Most systems use deep UV light today to get the smallest wavelengths and smallest features. They also play tricks with double exposures and complicated stencils that are specially shaped to counteract the diffraction. But again, you can only get so small with projection.

After this projection, you are left with a print of the stencil on the wafer and can remove or add or dope patterns and form a chip architecture. This process is repeated dozens of times to stack up the full circuit.

Right now, we are pushing up against the limits of what is possible with UV light. It is difficult to just further shrink the wavelength because you start pushing up into x-rays which just pass through most things and cant be focused like UV.

Of course, there is a whole zoo of other specialized methods that have been demonstrated in laboratory settings, but they do not work well for large scale production. For example, using something similar to an electron microscope to scan the pattern into the wafer can give very fine features but is very slow since it must move line by line rather then an entire exposure at once.