Page 47 - High Power Laser Handbook

Excimer Lasers 19

Figure 2.1
Schematic electron W/eV Kr* + F
transition of a
248-nm excimer
laser from the 5
excited state to
the ground state. 4 E = h*ν

3 5 eV ~ 248 nm
2

1
Kr + F
r 0 r

such as neon or helium. In the neutral channel, an excited-state rare gas
atom (Kr*) reacts with a halogen molecule (F ). These reactions take
2
place on a nanosecond (ns) timescale, with upper-level production effi-
ciencies of several tens of percent. The excited KrF* molecule remains
unstable in the upper state and decays after several nanoseconds via
emission of a photon into Kr and F. The Kr and F components that form
the ground state are then available for another excitation cycle. Because
the excitation rate must compete with fast quenching processes, colli-
sions, and nonradiative decay, it requires high pump power densities,
which can only be obtained in a pulsed system. Thus, excimer lasers
intrinsically operate in the pulsed mode with high peak power.

2.2 Technology and Performance of Excimer Lasers

2.2.1 Principal Design and Technology
After more than 30 years of engineering, excimer lasers have reached
high maturity. Several design aspects differentiate the excimer laser
construction from that of other lasers. In addition, specific technologies
have been developed to take advantage of the favorable operational
conditions offered by the excimer laser. Because the laser’s gas mixture
contains one of the halogens fluorine or chlorine in small concentra-
tions, it is of the utmost importance to select materials that will avoid
consumptive reactions. Operation with relatively large gas volumes at
gas pressures of up to 6 × 10 pascals (Pa) demands leak-tight, high-
5
strength mechanical construction. Discharge voltages of more than 40
kilovolts (kV) are used for the excitation that determines the use of effi-
cient high-dielectric-strength insulators. Performance thus relies on the
excimer laser design, choice of materials, and production techniques. 3
An important design factor of excimer lasers is the excitation
method. The technology that is almost exclusively used in high-power
industrial excimer laser systems is the high-pressure gas discharge
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