Multi-component EUV multilayer mirrors

摘要

It is well known that molybdenum and silicon is the combination with the highest EUV reflectivity of two-component multilayers in the wavelength range λ = 12.5 ― 15nm. Using the magnetron sputter deposition method multilayers with near normal incidence reflectance of typically 69% can be prepared. A further increase to R = 70.1% at λ = 13.3 nm was demonstrated by the introduction of tiny carbon barrier layers on the Mo-on-Si interfaces, which reduce the interdiffusion of both chemical elements. This is also connected with a higher thermal stability and lower internal stress of these multilayers as compared to pure Mo/Si multilayers. However, still higher reflectance are desirable for the use of the multilayers as reflectors for EUV lithography. From model considerations we have concluded that the replacement of the Mo absorber layer by a multi-component layer consisting of two or three layers could result in an EUV reflectivity increase of up to 0.5% compared to the pure Mo/Si system assuming sharp interfaces without any roughness. Particularly Ag and Ru are promising candidates as additional elements within the absorber. Therefore we have systematically changed the thickness of the individual layers under the boundary condition of a fixed period thickness of d_p = ( 6.90 +- 0.05 ) nm. Microstructure and optical parameters of the multilayers have been investigated by HRTEM, X-ray diffractometry and Cu-Ka reflectometry. The most promising multilayers have also been characterized by EUV reflectometry.