Compact source and beam delivery system for EUV radiation using a Schwarzschild objective

In order to generate high energy densities of 13.5 nm radiation, an EUV Schwarzschild mirror objective with a numerical aperture of 0.44 and a demagnification of 10 was developed and adapted to a compact laser-based EUV source. The spherical mirror substrates were coated with Mo/Si multilayers systems. With a single mirror reflectance of more than 65% the total transmittance of the Schwarzschild objective exceeds 40 % at 13.5 nm. From the properties of the EUV source (pulse energy 3 mJ at 13.5 nm, plasma diameter approx. 300 μm), energy densities of 73 mJ/cm² at a pulse length of 6 ns can be estimated in the image plane of the objective. As a first application, the formation of color centers in lithium fluoride crystals by EUV radiation was investigated. F2, F3 and F3⁺ color centers could be identified by absorption spectroscopy. The formation dynamics was studied as a function of the EUV dose. By imaging of a pinhole positioned behind the plasma, an EUV spot of 5 μm diameter was generated, which accomplishes direct writing of color centers with μm resolution.