Compact EUV source and Schwarzschild objective for modification and ablation of various materials

Abstract

In recent years technological developments in the area of extreme ultraviolet lithography (EUVL) have experienced great improvements. So far, intense light sources based on discharge or laser plasmas, light guiding and imaging optics, as well as detection devices are already available. Currently, the application of EUV radiation apart from microlithography, such as metrology, high-resolution microscopy, or surface analysis comes more and more into focus. The aim is to make use of the strong interaction between soft x-ray radiation and matter for surface-near probing, modification or structuring techniques. In this contribution, we demonstrate the surface-near direct structuring of different polymeric materials as well as lithium fluoride crystals using EUV radiation with a wavelength of 13.5 nm. The setup consists of a table-top EUV source based on a laser-induced plasma and a modified Schwarzschild objective with a resolution down to 130 nm. The mirrors of the employed objective were coated with Mo/Si multilayers, providing a transmittance of around 42 % (reflectivity ~65 % @ 13.5 nm per mirror). With a demagnification factor of 10 small foci are generated, leading to spot diameters of 30 mm in plasma imaging mode and down to 1 mm in mask imaging mode, respectively. The EUV energy density of ~100 mJ/cm2 obtained in the focus is sufficient to observe direct photo-etching of polymers, e.g. PMMA. Thus, material interaction studies are currently in progress. The investigations revealed already that in contrast to common excimer laser ablation there are no incubation pulses when using EUV radiation. For lower energies the ablation rate is found to be linear with respect to the applied dose, whereas for higher energies a saturation behavior is observed. By EUV irradiation of LiF samples surface-near defects within the crystal lattice are formed. These color-centers (mainly F2- and F3 +-color centers) are known to be stable at room temperature. They are able to emit characteristic radiation in the visible range after optical excitation with a wavelength around 450 nm. In the future structured areas of such color centers could be used as laser-active gain medium in distributed feedback lasers. For measuring the radiation resistence of Mo/Si mirrors, the setup was used as a top-illuminated microscope at 13.5 nm. By placing the analyzing Mo/Si mirror into the image plane of the objective, the change in reflectivity due to irradiation at a fluence of 20mJ/cm2 could be observed.