Immersion lithography
Beginner
Printing chip patterns with a thin layer of water between the lens and the wafer, which lets the machine print finer detail.
Novice
193 nm lithography with ultrapure water (refractive index 1.44 at that wavelength) filling the gap under the last lens. That raises the numerical aperture above 1, up to about 1.35.
Expert
Single exposure reaches roughly 75–80 nm pitch; tighter layers need multiple patterning. Still the workhorse for non-critical layers and for spacer-patterned gratings even where EUV is used.
Explained in Making them (Transistors).
See also: Numerical aperture (NA), Multiple patterning, EUV lithography.