Page 44 - High Power Laser Handbook

CHAPTER 2

Excimer Lasers

Rainer Paetzel
Coherent GmbH, Dieburg, Germany

2.1 Introduction and Principle of Operation
The excimer laser is today’s most powerful, cost-effective, and
dependable pulsed ultraviolet (UV) laser source. Since its first
experimental realization in 1970 by Nikolai Basov et al. at
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Moscow’s Lebedew Institute of Physics, it has undergone rapid
development. The unique output characteristics of excimer lasers
enable innovations in growth industries as diverse as the medical,
microelectronics, flat panel display, automotive, biomedical devices,
and alternative energy markets.
Excimer lasers are gas lasers that, by nature, emit pulsed UV light.
The term excimer is short for excited dimer, a class of molecules formed
by the combination of two identical constituents in the excited state.
The excimer laser’s active medium is a combination of a rare gas, such
as argon (Ar), krypton (Kr), or xenon (Xe), with a halide, such as fluo-
rine (F ) or chlorine (Cl ). Under the appropriate conditions of electrical
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stimulation, an excited molecule is created that exists only in an ener-
gized state and that gives rise to laser light in the UV range. The exact
wavelength of the laser light depends on the gas mixture used. 2
Of the listed excimer wavelengths in Table 2.1, five are commer-
cially relevant—xenon fluoride (XeF) at 351 nm, xenon chloride (XeCl)
at 308 nm, krypton fluoride (KrF) at 248 nm, argon fluoride (ArF) at
193 nm, and fluorine (F ) at 157 nm. Of these wavelengths, the 308 nm, 248
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nm, and 193 nm cover the vast majority of products and applications.
The fundamental principle of operation of an excimer laser is illus-
trated by a simplified reaction scheme for KrF, as shown in Fig. 2.1.
The formation of the rare gas halogen molecule is dominated by
two reaction channels. In the ion channel, a positive rare gas ion (Kr+)
and a negative halide ion (F–) recombine in the presence of a buffer gas,
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