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A short wavelength optical projection system might be realized
by making a somewhat larger jump in wavelength, down to 13 nm,
which lies in the extreme ultraviolet (EUV) region of the spectrum.
Absorption on transmission and lens damage precludes catadioptric systems
in the EUV regime. Instead,
all-reflective systems must be introduced. The primary reason for
the particular wavelength of 13 nm stems from
the invention of an optical coating technology using silicon/molybdenum
multi-layer films with reflectivity as high as 70%. But a system
consisting of, e.g., seven mirrors leaves
on the wafer surface only 8% of the intensity available
from the light source. Both, coatings with higher reflectivity and lower
defect density as well as brighter light sources have to be developed.
To achieve diffraction limited performance the surface figure accuracy
of the lenses has to be less than 1 nm at 13 nm wavelength.
Such high precision fabrication is well beyond the current state of the
art in terms of measurement. In spite of these difficulties, EUV lithography
is attractive because it enables printing of feature sizes less than
100 nm using a numerical aperture of only 0.1.
Heinrich Kirchauer, Institute for Microelectronics, TU Vienna