Excimer Lasers J.-P

EXCIMER LASERS J.-P. Girardeau-Montaut

To cite this version:

J.-P. Girardeau-Montaut. EXCIMER LASERS. Journal de Physique Colloques, 1987, 48 (C7), pp.C7- 225-C7-228. 10.1051/jphyscol:1987750. jpa-00227054

HAL Id: jpa-00227054 https://hal.archives-ouvertes.fr/jpa-00227054 Submitted on 1 Jan 1987

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EXCIMER LASERS

J.-P. GIRARDEAU-MONTAUT

Universitk Claude Bernard, Lyon I, Laboratoire des Interactions ~aser-~atgriau,41, Bd du 11 novembre 1918, F-69622 Villeurbame Cedex, France

A review of excimer laser systems is presented, including basic physical data, technology, performances and applications. The last few years have seen a rapid development of this new type of gas lasers and of many commercial systems pro- ducing efficient high power pulses of ultra-violet radiation. After a short recall of physical principles, we give a description of the internal structure of nanose- cond and picosecond excimer lasers, followed by the analysis of principal beam characteristics : output power, efficiency, pulse stability, beam profile and lifetime. Some cost considerations are also examined. The presentation of principal applications in various domains as photochemistry, material processing, non linear processes and medecine, shows how these lasers are definitely an useful tool.

The term "excimer laser" does not describe a single device, but a group of pulsed gas lasers that deliver optical radiation at very high peak and average po- wers. All emit pulses lasting nanoseconds or tens of nanoseconds, or picoseconds for some ones at wavelengths in or near the ultra-violet. Their comon feature is the lasing species : a diatomic molecule that is bound only in electronically excited states, while its electronic ground state is repulsive or weakly bound (Figure 1). Examples include homonuclear diatomics, e.g, He2 and Xe2 /I/. The "excimer" word originated as a contraction of "excited dimer". It is now used in a broader sense for heteronuclear and polyatomic molecules as well, in which the component atoms are only bound in the excited state. All comercially available excimer lasers operate with the rare gas-halides compounds such as : A~F*(193 nm), ~r~l*(222 nm), K~F* (244 nm), ~e~l"(308 nm) and XeF* (351 nm), which do not occur in nature. Excimer lasers can also operate with non-excimer media like N2, F2 and C02 as well.

The upper state of the excimer is formed by chemical reaction from its constituents after one or both of them have been electronically excited or ionized in a very fast high voltage discharge. When an excimer drops from the excited state to the ground state, the force between the two atoms change from attraction to repul- sion and the molecule breaks up, free constituents entering in the pumping cycle again. This energy level structure makes excimers very good laser materials, with high gain, the population inversion being as long as there are molecules in the excited states 1 I I

Figure 1 - Potential curves and laser transitions for the K~F*excimer molecule. Laser transition 1 - 249 nrn

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987750 JOURNAL DE PHYSIQUE

Active medium

In discharge excitation, excimer lasers contain a gas-mixture at total pressure usually below five atmospheres. The bulk of the mixture, higher than 88 % is a buffer gas, which mediates energy transfer : normally He or Ne, although Ar is used in some cases. The rare gas that does combine to form excimer molecules is present in much smaller concentrations 0.5 % to 12 % of the total pressure. The halogen donor is present in concentrations of 0.5 % or less. It may be either a diatomic halogen such as F2, or a halogen containing molecule such as HCl or NF3. A typical gas mixture for K~F*will contain 4 torr F2, 120 torr Kr and 2400 torr He. The opti- mum gas mixture is a complex function of gas kinetics and operating conditions. It is different for different models of lasers and there are also large difference between different excimer molecules.

Pump mechanism

The detailed dynamic processes that lead to emission from excimer lasers are complicated 12-51. However, one can get an understanding of some key features of laser design from the simplified scheme of KrF as given in Figure 1.

The upper laser level is an ionically bound state formed by three-body re- combination of the Kr+ and F- ions in the presence of a collision partner. The lifetime of spontaneous deactivation depends on pressure and is approximatively 2.5 nsec. In order to acheive a sizable population (* 1015 cm-3), its ionic precursors have to be prepared on a time scale and with concentrations such that the formation reaction can produce several 1023 excimers x cm-3 x s-1. The necessary ions can be prepared from the atomic species by fast avalanche discharge, e-beam or microwave discharge. It is the avalanche discharge excitation which is used in all commercial systems.

To make efficient pumping, there are further requirements 121 : electron densities of a 1015 cm-3, current densities of lo3 ~/crn~,so that enough electtan can 'provide the14 evionization energy of Kr. To meet these conditions, a breakdown voltage of 10-15 kV/cm is need and therefore most lasers operate with electrode spacing of 2 to 3 cm and breakdown voltages of 25-35 kV. To improve energy-transfer dynamics and avoid arcing, the laser gas is normally "pre-ionized" before the main discharge is fired. Pre-ionization is normally accomplished with a pulse of W light from sparks, or by coronna effect 161. In order to the principal excitation pulse break: down homogeneously the gas mixture, the start-up electron density must be about lo7- 108 a-3.

Technology

When broken down, the excimer gas mixtures have very low impedance * 10-1 fi so that efficient excitation requires very fast low inductance, high voltage circuitry(Figure 2). Lasers are pumped by discharging a high voltage capacitor through a suitable switch over the two laser electrodes inside the laser chamber. Switching is usually with thyratrons which have jitter of 1 to 2 nsec only. The upper limit on pulse length is functionally limited to the range of tens of nanoseconds by discharge instabilities. Pulse lengths of hundred of nanoseconds can be reached only with electron beam or microwave excitation, but that's about the upper limit. Very recently 171, shortening of pulse length has been obtained by pulse com- pression in dye until %10 psec.

An excimer laser contains a tube filled with laser-gas mixture through which an excitation pulse passes. In discharge-driven lasers, the electrical pulse is perpendicular to the laser beam axis. The laser gas lies in a reservoir outside of the excitation region, and the gas may be actively flowed through the discharge zone. The laser cavity can be repeatedly refilled with different gas mixtures. This is necessary because the laser gas degradcsduring use. After the gas is spent, the cavity must be emptied and refilled. The laser cavity, optics and electrodes are designed to resist corrosion by the halogens. Figure 2 - Basic excimer laser circuitry. THY : Thyratron, : storage self, Csi, Cs2.: storage capacitors, Cp : peaking capacitor, 2: laser chamber, PC : pre~onirationcircuitry.

Optics

Excimer lasers have such high internal gain -typically g % 5 to 15 per centlcm- tha? they are virtually superradiant and require little optical feedback. The standard stable resonator consists of a plane A1/MgF2 or dielectric coated full reflector and an uncoated CaF2 or MgF2 output coupler whose 8 % reflectivity provi- des sufficient feedback. The windows are affixed directly to the laser cavity and hence are exposed directly to the laser gas. To prevent damage, reflective coatings are deposited on the outside of the rear cavity window. Laser emission in discharge pumped excimer lasers occuring for only 10 to 25 nsec, optical feedback from the optical resonator can only be of use if at least two to three optical round trips are possible in the resonator during laser emission. Thislimits resonator lengths to 'L 100 cm. Beam characteristics Wavelength output power and temporal characteristics of the major excimer laser gases are shown in Table 1. As indicates, there are major differences in output power available from different excimers. With excellent focusability of nearly diffraction -limited W radiation, allowsthe generation of very high intensities- up to 1017 ~/cmZ.

Due to discharge parameters, pulses last from a few nanoseconds to a few tens of nanoseconds. Pulse lengths vary significantly among different gases used in the same laser and individual pulses of ten have structure within them. Pulses as long as 250 ns have been obtained in the laboratory by ballasting the discharge. Using saturable absorbers it was possible to produce pulses as short as 10 ps.

Table 1 - Wavelength, maximum output power and pulse length of excimer lasers

Laser gas F2 ArF KrCl KrF XeCl XeF Wavelength (nm) 157 193 222 249 308 351 Pulse energy (mJ) 12 1000 25 1500 1000 400 Ang. power (W) % 1 100 2.5 200 100 50 Rep. rate (Hz) 100 250 200 500 250 200 Pulse length (ns) 6 8-14 4-9 12-16 8-40 8-18 (0.01 JOURNAL DE PHYSIQUE

Typical wall plug efficiency for a discharge driven KrF laser is around 1.5 % to 2 %.Other excimer gases are less than KrF.

The laser transitions in excimer laser are broad due to the lack of well- defined energy levels in the ground state. Typical line width is about 0.3 nm with output wavelength tunable accross about 3 nm. The broad fluorescence bandwith of ArF* and K~F*makes them suitable for generation or amplification of subpicosecond pulses.

Due to the high gain, low Q cavity and low number of round trips in excimer oscillators, mode competition is low and the output consists of very large number (% 107) of spatial modes. Therefore, the spatial and temporal coherences of the radiation are very small. Normally output is unpolarized, but polarizing optics can always be added.

With standard stable resonator optics, excimer lasers typically have rec- tangular beams roughly 1 x 2 cm. Beam divergence is 2 x 3 milliradians. Unstable resonator generate beams of similar size but much smaller divergence (% 0.5 mrad) .

At last, pulse-to-pulse variations can be significant in pulsed excimer lasers, with specified values averaging i 5 % typically. With a pulse stabilization option it is possible to reduce variations to around 1 % on strong lines. In the long term, output energy declines as the gas ages.

Cost comparison

From a recent study /8/, it appears that real cost per kilowatt-hour, including fuel gas, maintenance and capital is minimum for lasers with average power between 20 W to 50 W, values which correspond to nmerous applications.

Applications

Excimer laser applications in science, medecine and industry have become far too numerous to discussthoroughlyhere 191. The majority of these applications are in research, but a number of these lasers are now being used in industrial pro- duction or clinical action as well. The typical applications of excimer lasers include : photochemistry /lo/, medecine Ill/, dye laser pumping, material processing /12/, non linear processes, remote sensing and Raman shifting, showing how these lasers are an useful tool by now.

References

/I/ BASOV, N.G., DANILYCHEV, V.A., PROPOV, Yu.M., KHODKEVICH, D.D., aTP Lett. 11 (1970) 329. /2/ RHODES, Ch.K., Excimer Lasers, Topics in Applied Physics, vol. 30, Springer (1984) 2nd ed. /3/ HUTCHINSON, M.H.R., Applied Optics, 19 (1980) 3883. /4/ HUTCHINSON, M.H.R., Applied Physics, 21 (1980) 95. /5/ SZE, R.C., IEEE J. Quant. Electro. Q.E. 15 (1979) 1338. /6/ GIRARDEAU-MONTAUT, J.P. and MOREAU, G., Les lasers excimsres : technologie et applications, Conf. Opto (1982) Paris, p. 189. /7/ BURGHART, B. and NIKOLAUS, B., A new picosecond excimer laser system, Doc. Lambda Physics (1986). /8/ KLAUMINZER, G.K., Laser Focus/Electro Optics, dec. (1985) 109. /9/ HOMES, L., Laser Focus/Electro-Optics, july (1986) 72. /lo/ LAZARE, S. and SRINIVASAN, R., J. Phys. Chem. 90 (1986) 2124. /11/ MULLER, D., Lasers and Applications, may (1986) 85. /12/ ZNOTINS, T.A., POULIN, D. and REID, J., Laser Focus/Electro-Optics, may (1987) 54.