Highly reliable DUV light sources are required for semiconductor applications such as a photomask inspection. The
mask inspection for the advanced devices requires the UV lightning wavelength beyond 200 nm. By use of dual fiber
lasers as fundamental light sources and the multi-wavelength conversion we have constructed a light source of 198nm
with more than 100 mW. The first laser is Yb doped fiber laser with the wavelength of 1064 nm; the second is Er doped
fiber laser with 1560 nm. To obtain the robustness and to simplify the configuration, the fundamental lights are run in
the pulsed operation and all wavelength conversions are made in single-pass scheme. The PRFs of more than 2 MHz
are chosen as an alternative of a CW light source; such a high PRF light is equivalent to CW light for inspection
cameras. The light source is operated described as follows. Automatic weekly maintenance within an hour is done if it is
required; automatic monthly maintenance within 4 hours is done on fixed date per month; manufacturer's maintenance is
done every 6 month. Now this 198 nm light sources are equipped in the leading edge photomask inspection machines.
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