I have a superficial understanding of photolithography, but I'd guess the stuff he's doing is described pretty well in undergraduate textbooks, and he got loads of equipment very cheap as it's obsolete for commercial or even academic use.

Further, capital costs will rapidly start rising as well as the required knowhow as the feature size shrinks, so I think he's already quite close to the limit a one person hobby project can achieve.

All this being said, this is an extremely impressive achievement. There's a long way from reading how stuff is supposed to work, to actually doing it your garage. My hat's off to Mr. Zeloof.