Carbon buffer layers for smoothing substrates of EUV and X-ray multilayer mirrors

摘要

Smoothing of surfaces by thin film deposition is facilitated by methods which release hyperthermal particles on the substrate. One of these techniques is pulsed laser deposition (PLD), with high kinetic particle energies of up to several 100 eV. The concrete energy distribution of the particles can be widely influenced by the laser power density. We investigated the deposition of carbon layers by PLD on numerous substrates with mis-roughnesses between 0.15 and 0.75 nm using different laser power densities and film thicknesses. It turns out that a better smoothing can be obtained with higher laser power densities, whereby diamond-like carbon films are created. With typical thicknesses of dc = 100 nm, the rms-roughness is reduced from 0.75 nm to 0.55 nm and from 0.32 nm to 0.18 nm. Accordingly by applying smoothing carbon buffer layers, the EUV reflectance of Mo/Si multilayers on rough substrates is increased from typically 60 % to > 65 % on substrates with initial roughnesses of 0.75 nm.