Excimer laser
A pulsed gas laser whose gain medium is a short-lived rare-gas halide molecule such as ArF or KrF, emitting in the ultraviolet: 193 nm (ArF), 248 nm (KrF), 308 nm (XeCl) or 351 nm (XeF). Pulses last about 10–30 ns at repetition rates up to several kilohertz.
An excimer laser is a pulsed gas laser that produces ultraviolet light from molecules that exist only in an excited state. A fast electrical discharge through a mixture of a rare gas (argon, krypton or xenon), a halogen donor (fluorine or hydrogen chloride) and a neon or helium buffer creates molecules such as ArF* and KrF*, which emit as they fall apart. The main lines are 193 nm for ArF (6.42 eV photon energy), 248 nm for KrF (5.00 eV), 308 nm for XeCl (4.03 eV) and 351 nm for XeF (3.53 eV). Pulses typically last 10–30 ns, pulse energies range from a few millijoules to the joule level in large industrial units, and repetition rates reach several kilohertz. Strictly, "excimer" means an excited dimer of identical atoms, and the rare-gas halides are exciplexes, but the older name has stuck.
Why inversion is automatic
The ground state of a rare-gas halide is repulsive or only weakly bound: two ground-state atoms such as Kr and F do not form a stable molecule. The excited ionic state, Kr⁺F⁻, is strongly bound. When the excited molecule emits, it lands on the repulsive curve and dissociates within about a picosecond, so the lower laser level is always nearly empty and any excited-state population is a population inversion. Because the lower state is a continuum, the gain bandwidth is broad, a few tenths of a nanometre when free-running, and a prism or grating in the cavity is needed for narrow-line operation.
The upper-state lifetime is only a few nanoseconds, and the discharge must deposit energy faster than the molecules decay. Excimer lasers therefore use high-voltage pulsed power with rise times of tens of nanoseconds, and they cannot run continuously.
Pulse and beam figures
A typical 10 mJ pulse at 193 nm contains
photons. If it lasts 20 ns, its peak power is about 0.5 MW; at 6 kHz the average power is 60 W. The output beam is rectangular, often around a centimetre by a couple of centimetres, and highly multimode. For patterning and ablation it is shaped with homogenizers into a flat-top field and imaged through a mask, which suits this low spatial coherence: a highly multimode beam produces little speckle.
Applications
Deep-ultraviolet lithography uses KrF and ArF excimer lasers as the exposure source. The ArF scanners that print leading semiconductor layers use line-narrowed lasers with sub-picometre bandwidth, because the projection lens is made from fused silica and calcium fluoride and has little freedom to correct chromatic error; immersion in water raises the numerical aperture of these systems above 1.
In refractive eye surgery, ArF pulses remove corneal tissue by photoablation: each 6.4 eV photon exceeds the energy of many molecular bonds, and absorption in a thin surface layer removes a fraction of a micrometre per pulse with little thermal damage. XeCl lasers anneal amorphous silicon into polycrystalline silicon for display backplanes, and KrF and ArF lasers are used for micromachining polymers, drilling inkjet nozzles, pulsed laser deposition and writing fiber Bragg gratings into photosensitive fiber. XeCl at 308 nm is also used in dermatology.
Optics and handling
Below about 200 nm molecular oxygen absorbs, forming ozone, so ArF beam paths are usually purged with nitrogen and 157 nm F₂ lasers need a fully purged or evacuated path. Optics are made from UV-grade fused silica, calcium fluoride and magnesium fluoride, with dielectric coatings designed for the specific line. At these ultraviolet photon energies, two-photon absorption and colour-centre formation slowly darken even good fused silica, so optics in high-dose positions are consumables.
Pitfalls
Fluorine and hydrogen chloride are toxic and corrosive, and the laser gas degrades as the halogen reacts with electrodes and walls. Output falls over millions of pulses until the gas is refilled or the halogen is topped up, so power and pulse energy should be logged against shot count. Pulse-to-pulse energy varies by a few percent, and processes that need a precise dose use energy monitors and feedback. The beam is invisible, and scattered UV is a skin and eye hazard that visible alignment beams do not reveal; fluorescent cards show where it goes.
Common questions
What does an excimer laser do in LASIK?
It reshapes the cornea after a flap is lifted. A scanned ArF beam at 193 nm ablates a computed thickness profile, removing tissue by a mix of photochemical bond breaking and rapid ejection, which leaves little thermal damage in the surrounding tissue.
Why are excimer lasers pulsed only?
The excited molecules live a few nanoseconds and the discharge becomes unstable if sustained, arcing into filaments within tens of nanoseconds. Pumping must be fast and brief, which limits operation to pulses.
Is the F₂ laser at 157 nm an excimer?
F₂ is a true molecule with a bound ground state, so strictly it is neither an excimer nor an exciplex. It uses the same discharge technology and is usually grouped with them.
References: C. A. Brau and J. J. Ewing, Appl. Phys. Lett. 27, 435 (1975); D. Basting and G. Marowsky (eds.), Excimer Laser Technology (Springer, 2005); A. E. Siegman, Lasers (University Science Books, 1986); O. Svelto, Principles of Lasers, 5th ed. (Springer, 2010).