Page 49 - High Power Laser Handbook

20 G a s , C h e m i c a l , a n d F r e e - E l e c t r o n L a s e r s Excimer Lasers 21

discharges for preionization of the laser gas. Specially designed
excimers, as are used in fundamental laser studies, employ x-ray, or
4
creeping discharge for preionization or direct electron beam pump-
ing for the main discharge to utilize a very large gain volume that
achieves highest energies of up to hundreds of joules per pulse.
In a typical embodiment for preionization, a multitude of small
preionization pins are arranged in a row with a dielectric surface
adjacent to the discharge electrodes. Upon application of a fast volt-
age pulse, the pins act as small spark gaps, generating a surface-
guided discharge about 10 ns before the main discharge. The UV
radiation produced by the multiple discharges is sufficient to preion-
ize large cross sections of the laser gas between the electrodes with a
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homogeneous initial seed density of at least 10 electrons/cm . In
3
today’s high-energy excimer laser designs, a dielectric material is
placed between the preionization pins, and the resulting surface-
guided discharge spreads over several millimeters instead of forming
a very thin discharge channel. This design significantly reduces the
consumption of the preionization pins, thereby enabling longer gas
lifetimes and an electrode life of more than 10 billion pulses. Surface
corona preionization (SCP) is typically used when high repetition
rates using smaller discharge cross sections are required. SCP is the
preferred design for low-energy lasers and lasers used in microlithog-
raphy applications.
To remain within an optimum excitation energy density range,
the active laser volume is scaled by means of the length of the elec-
trodes, the gap of the electrodes, and the width of the discharge. In
high-pressure gas-discharge lasers, the discharge electrodes are pro-
filed to provide a highly uniform electric field distribution in the
discharge region and to avoid field concentrations near the electrode
edges, which would otherwise cause premature discharge instabili-
ties and arcing. The electrode profile determines the discharge’s
maximum energy loading, as well as its width and profile, which in
turn controls the profile of the laser beam. The electrodes must be
able to withstand the adverse effects caused by the high-current dis-
charge and must be made from a material that is chemically resistant
to the fluorine or chlorine component used in the gas. Proprietary
alloys have been developed that optimally meet the demands of flu-
orine or chlorine chemistry and thus minimize electrode erosion.
To provide high pump energy densities in a short time, as is
required for population inversion, excimer lasers are usually pumped
using a high-voltage capacitor circuit (pulser) that discharges the
stored electrical energy directly into the active medium (Fig. 2.3). The
pumping schemes involve efficient switching of this stored electrical
energy into the discharge system in a very short time. Also required
is a well-defined spatial and temporal profile, as this determines the
discharge uniformity and, in turn, influences the extracted laser