EUV exposure tools are the leading contenders for patterning critical layers at the 22nm technology node.
Operating at the wavelength of 13.5nm, with modest projection optics numerical aperture (NA), EUV projectors allow
less stringent image formation conditions. On the other hand, the imaging performance requirements will place high
demands on the mechanical and optical properties of these imaging systems.
A key characteristic of EUV projection optics is the application of a reflective mask, which consists of a reflective
multilayer stack on which the IC layout is represented by the reflectivity discontinuities1. Several mask concepts can
provide such characteristics, such as thick absorbers on top of a reflective multi-layer stack, masks with embedded
absorbers, or absorber-free masks with patterns etched in a reflective multilayer.
This report analyzes imaging performance and tradeoffs of such new mask designs. Various mask types and
geometries are evaluated through imaging simulations. The applied mask models take into account the topographic
nature of the mask structures, as well as the fundamental, vectorial characteristics of the EUV imaging process.
Resulting EUV images are compared in terms of their process stability as well as their sensitivities to the EUV-specific
effects, such as pattern shift and image tilt, driven by the reflective design of the exposure system and the mask
The simulations of images formed in EUV exposure tools are analyzed from the point of view of the EUV mask
users. The fundamental requirements of EUV mask technologies are discussed. These investigations spotlight the
tradeoffs of each mask concept and could serve as guidelines for EUV mask engineering.