The Gain Medium
The excimer gain medium is a gas mixture, typically containing a noble gas (rare gas) (e.g. argon, krypton, or xenon) and a halogen (e.g. fluorine or chlorine, e.g. as HCl), apart from helium and/or neon as buffer gas. An excimer gain medium is typically pumped with short (nanosecond) current pulses in a high-voltage electric discharge (or sometimes with an electron beam), which create so-called excimers (excited dimers) – molecules which represent a bound state of their constituents only in the excited electronic state, but not in the electronic ground state. (Strictly speaking, a dimer is a molecule consisting of two equal atoms, but the term excimer is normally understood to include asymmetric molecules such as XeCl as well. The term rare gas halide lasers would actually be more appropriate, and the term exciplex laser is sometimes used.) A key point is that after stimulated or spontaneous emission, the excimers rapidly dissociate, so that reabsorption of the generated laser radiation is avoided. This makes it possible to achieve a fairly high gain even for a moderate concentration of excimers.
As excimer lasers use molecules as the gain medium, and can therefore in principle be called molecular lasers, although the term is usually used for lasers using stable molecules like CO2, which may occasionally dissociate, but not as part of the intended laser process.
Different types of excimer lasers typically emit at wavelengths between 157 and 351 nm:
|F2 (fluorine)||157 nm|
|ArF (argon fluoride)||193 nm|
|KrF (krypton fluoride)||248 nm|
|XeBr (xenon bromide)||282 nm|
|XeCl (xenon chloride)||308 nm|
|XeF (xenon fluoride)||351 nm|
For various of those wavelengths, specialized excimer optics (ultraviolet optics) have been developed, which need to have a high optical quality and in particular a very high resistance to the intense ultraviolet light.
Pulse Parameters, Beam Quality and Power Efficiency
Continuous-wave operation is not possible with excimer lasers, partly because it is not possible to obtain a stable electric discharge with suitable properties. The pulse duration is often a few nanoseconds, but sometimes longer, of the order of 100 ns.
Typical excimer lasers emit pulses with a pulse energy between 10 mJ and 1 J. Some reach pulse repetition rates of only e.g. 10 Hz, while others reach 1 kHz or even more. Consequently, the average output power can range from less than 1 W to several hundred watts. Therefore, excimer lasers are the most powerful laser sources in the ultraviolet region, particularly for wavelengths below 300 nm.
The wall-plug efficiency normally varies between 0.2% and 5%; significantly more is possible with electron beam pumping.
The beam quality is typically quite low; it is generally difficult to reach high beam quality under the given circumstances with a short resonator, a large gain volume and the fast pulse build-up. Before sending such a beam to an application, one often needs to employ some kind of beam homogenizer.
The emission linewidth is normally of the order of 1 nm when not taking any special measures, but using a wavelength-selective elements such as a diffraction grating in the laser resonator it can be reduced to well below 1 pm.
Early excimer lasers had quite limited lifetimes due to a variety of problems, arising e.g. from the corrosive nature of the gases used, the ablation of material from the electrodes, degradation of optical materials by the strong UV light, and from contamination of the gas with chemical byproducts and dust created by the electric discharge. The latter problem is usually solved by a regular exchange of the gas mixture, for example each time after 30 million pulses. For maintaining electrodes and optics in a well-performing state, however, a lot of more or less sophisticated measures had to be developed. A lot of engineering, involving e.g. the use of corrosion-resistant materials and of advanced gas recirculating and purification systems, has mitigated challenges of the excimer laser concept to a significant extent. The lifetime of modern excimer lasers is now limited by that of the ultraviolet optics, which have to withstand high fluxes of short-wavelength radiation, to something of the order of a few billion pulses.
Another challenge is providing the very short but intense current pulses. This originally allowed the thyratron switches to last only for a couple of weeks or months. Modern power electronics (with solid-state high-voltage switches) have led to substantial advances.
Applications of Excimer Lasers
The short wavelengths in the ultraviolet spectral region make possible a number of applications:
- the generation of very fine patterns with photolithographic methods (microlithography), for example in semiconductor chip production
- laser material processing with laser ablation or laser cutting (e.g. on polymers), exploiting the very short absorption lengths of the order of a few micrometers in many materials, so that a moderate pulse fluence of a few joules per square centimeter is sufficient for ablation
- pulsed laser deposition
- laser marking and microstructuring of glasses and plastics
- laser annealing, e.g. in display fabrication
- fabrication of fiber Bragg gratings
- ophthalmology (eye surgery), particularly for vision correction by corneal reshaping with ArF lasers at 193 nm; common methods are laser in-situ keratomileusis (LASIK) and photorefractive keratectomy (PRK)
- psoriasis treatment with XeCl lasers at 308 nm
- pumping other lasers, e.g. certain dye lasers
- drivers for nuclear fusion
Photolithography in semiconductor device manufacturing is an application of major importance. Here, photoresists on processed semiconductor wafers are irradiated with high-power ultraviolet light through structured photomasks. High-power UV light, as can be generated with excimer lasers, is essential for obtaining short processing times and correspondingly high throughput, while the short wavelengths allow one to fabricate very fine structures (with optimized techniques even far below the optical wavelength). However, the latest developments in lithography require even shorter wavelengths in the extreme ultraviolet (EUV), e.g. at 13.5 nm, which can no longer be produced with excimer lasers. Certain laser-generated plasma sources are developed as the successors for excimer lasers in that area. Still, it is to be expected that excimer lasers will be used for fabricating many semiconductor chips for a long time to come, as only the most advanced computer chips require still finer structures than possible with such techniques.
Note that excimer lasers raise a variety of safety issues, related to the use of high voltages, the handling of poisonous gases (halogens), and the risk of causing skin cancer and eye damage by irradiation with ultraviolet light.
There are also excimer lamps which basically use the same kind of gas discharge with excimer generation as excimer lasers, but they do not contain a laser resonator and thus exploit only spontaneous emission. Some of them are operated in continuous-wave mode rather than with a pulsed discharge. They can be used as ultraviolet light sources, but with spatially diffuse emission instead of a well directed output beam.
The RP Photonics Buyer's Guide contains 13 suppliers for excimer lasers. Among them:
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