Page 58 - High Power Laser Handbook

30 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 31

100000000

10000000
1000000
Occurrence (log. scale) 100000 3% p2p energy stability

10000
99.999% of laser pulses
62 Mpulses in histogram
1000
100

10
1
95 96 97 98 99 100 101 102 103 104 105
Relative energy (%)
Figure 2.11 Histogram of peak-to-peak energy stability measured at 600-Hz,
540-W output power at 308-nm (XeCl) wavelength.

production conditions over a two-day working period to enable the
uninterrupted production of the system. In the context of an anneal-
ing application, which is sensitive to the exact energy of a single
laser pulse, the achievement of this very tight energy distribution is
one of the key requirements to reach a high yield.
Since the first commercial introduction of the excimer laser in
1975 by Lambda Physik, its performance has been rapidly improved.
Pulse energies have been increased, such that special lasers today
deliver up to 10 J per pulse. In addition, repetition rates, as used in
advanced microlithography, have increased up to 6 kHz. The tremen-
dous improvement of excimer laser performance is best seen in the
development of the available output power over time. In 1976,
the first commercial products delivered up to 2 W of UV output using
the most efficient excimer gas (KrF) with emission at 248 nm. Until
1985, these lasers were used solely for scientific applications, such as
photo-ionization, UV chemistry, and spectrometry. In 1985, the power
level of 100 W became commercially available; these lasers used the
XeCl gas mixture with emission at 308 nm and started the industrial
application of the excimer for polymer ablation and other materials
processing.
Soon after power levels were further increased and stabilized.
The 300-W power level, which became available in 2000, led to wide
adoption of the excimer in the flat panel display industry. Only part
of the widespread industrial applications of the excimer demand
very high power in excess of 100 W. Figure 2.12 shows the roadmap
of the output power of a commercial excimer laser. In 2010, the