Page 54 - High Power Laser Handbook

26 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 27

RM OC

Electrodes Laser gas

Laser window
Figure 2.6 Planar resonator. RM: rear mirror; OC: output coupler.

using 248 nm or 308 nm or up to 50 percent for small lasers that
operate at a lower energy regime.
The acceptance angle of the planar resonator is given by the
geometry of the resonator; due to the short pulse length of typically
5 to 25 ns, there are only few roundtrips in the resonator. This leads to
a multimode beam with a large beam cross section and a reasonably
large beam divergence. Using the typical planar resonator, the exci-
mer provides a beam divergence of 1 to 3 milliradians (mrad) and a
large beam parameter product, which is calculated by beam size
times beam divergence. For high-power excimer lasers, the beam
parameter product is typically 50 mm·mrad. Although this is very
different from other types of lasers, it has proven to be a fundamental
advantage for many industrial large-area processing applications; the
laser beam is considered to be a low-coherence source that avoids
speckle and interference.
Figure 2.7 shows the beam profile of a high-energy excimer laser
using the planar resonator for a typical excimer laser operating at
248 nm (KrF) and with a pulse energy of 1 joule (J). The measure-
ment was taken with a standard beam profiler using beam attenua-
tion and a charge-coupled device (CCD) camera. The beam cross
section is 35 mm × 12 mm; typically the larger dimension is deter-
mined by the laser’s electrode distance.
The energy distribution of the beam in this axis is a top-hat pro-
file, which shows a plateau with high uniformity and symmetry. For
many applications, this flat-top energy profile turns out to be very
beneficial and yields uniformity in the working field without further
beam homogenization. The profile in the orthogonal axis results from
the discharge profile, which is mainly determined by the electrode
gap, the electrode profile, and the operating parameters, such as gas
composition and pressure. In particular, the electrode profile has
evolved over the years to optimize the performance and lifetime of
the different gases and the parameter range. This axis is approxi-
mated by a Gaussian beam shape.
For high-brightness applications of the excimer laser, the laser
beam divergence may be reduced; for this, the acceptance angle within
the resonator must be limited. For these high-brightness applications,