There are no lenses which are transparent at 13.5. This means you have to use mirrors instead, and even that's a challenge. 13.5nm was selected because it can b

There are no lenses which are transparent at 13.5. This means you have to use mirrors instead, and even that's a challenge. 13.5nm was selected because it can be reflected by special Molybdenum/Silicon mirrors, but even these mirrors only reflect about 70% of the light. This means that for every two mirrors you add to the system, you have to double the brightness of your energy source. As I understand it, the EUV light is produced by dropping little drops of tin in front of a laser, which turns it into a plasma that emits 13.5nm light. However, tin particles scatter everywhere and eventually damage the collector mirror, so it's hard to make these machines robust. I also imagine with these high energies and 30% absorption, you have to cool all the mirrors to keep them from deforming, but I'm no expert.