I think historically it's been a challenge mostly related to precision optics, given that semi production is more or less like exposing photographic paper from

I think historically it's been a challenge mostly related to precision optics, given that semi production is more or less like exposing photographic paper from a negative, through a lens using a light source. With EUV the challenge is the crazy difficulty of generating the required light source at the right intensity and focusing it. At that wavelength things behave differently, to say the least.