2.7.1 Extreme Ultraviolet

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.