Arf (argon Fluoride)

ArF (argon fluoride) excimer lasers produce the 193nm deep-UV light that powers the most advanced DUV lithography systems — the workhorse behind everything from 90nm down to roughly 38nm single-exposure features. The name "excimer" is the whole story of how this laser works: it stands for "excited dimer," a molecule that exists only when excited, and falls apart the instant it isn't.

Ordinary lasers have a stable ground state the lasing medium relaxes back into after emitting light. An excimer molecule doesn't. ArF* forms only when an electrical discharge excites a gas mixture of argon, fluorine, and a neon buffer into a bound excited state — argon and fluorine genuinely bond together, but only while excited. The ground state is repulsive: there's no stable bonded configuration for Ar and F to fall back into. So the instant ArF* drops out of its excited state, it has no choice but to release a photon — at precisely 193.368nm — and dissociate back into separate, unbonded argon and fluorine atoms. That forced, no-alternative emission is exactly what "excimer" describes, and it's part of why excimer lasers are efficient, well-defined light sources rather than needing to be coaxed into emission the way many other laser media do.

Getting usable lithography light out of that raw emission takes serious additional engineering. The natural excimer linewidth is broad — roughly 400 picometers — but the projection lens's chromatic correction needs sub-picometer bandwidth to avoid introducing its own focus errors. Line-narrowing modules built from prism beam expanders and echelle diffraction gratings compress that 400pm emission down to under 0.35pm FWHM. Modern systems pair that narrowing with a MOPA (Master Oscillator Power Amplifier) architecture: a narrow-bandwidth master oscillator defines the precise wavelength, then seeds a separate high-power amplifier stage — splitting the jobs of "spectrally pure" and "high energy" between two stages instead of asking one cavity to do both.

Production-grade specs reflect just how tightly this light has to be controlled:

ArF runs in two distinct modes that trade cost for resolution. Dry ArF, with an ordinary air gap between lens and wafer, caps NA at about 0.93 and covers features down to roughly 65nm. ArF immersion (193i) replaces that air gap with ultrapure water, raising NA to as high as 1.35 and extending single-exposure resolution to roughly 38nm (see our lens-design and resolution articles for why that water layer makes such a large difference).

Switching from KrF (248nm) to ArF (193nm) wasn't just a wavelength change — it forced an entirely new resist chemistry. The phenolic-based resists that worked for 248nm absorb too strongly at 193nm to be usable; the industry moved to acrylate and methacrylate-platform chemically amplified resists specifically to get adequate transparency and sensitivity at the shorter wavelength. Cymer (now part of ASML) and Gigaphoton are the two primary ArF excimer laser manufacturers, supplying the light source inside ASML, Nikon, and Canon scanner platforms alike.