EUV soft x-ray characterization of a FEL multilayer optics damaged by multiple shot laser beam

The mechanisms at the basis of damage process of the optics surfaces induced by XUV ultra short pulses of extremely high fluence are not completely understood [1]. However, Free Electron Laser sources (FEL), as the Fermi@Elettra in Trieste [2], requires that the spatial and temporal performances of the source optics don't drop drastically due to the very high adsorbed energy. To simulate the mechanism responsible of the coating degradation we used a multiple short pulse visible laser beam so to reach surface energy densities similar to the FEL source, although the different wavelength irradiation conditions have to been taken into account. As a test sample we used a multilayer Mo/Si, an EUV standard optics whose properties as a function of the annealing temperature have already been studied [3]. Different damaged spots have been induced in vacuum on this sample with fluences of some tens of mj/cm2, (? = 400 nm, pulse duration ~ 200 fs at 10 Hz). The specular and diffuse reflectivity of the different damage areas of the multilayer have been measured at the BEAR beamline @Elettra [4] at the wavelengths of?1.5 and 12.6 nm. The specular reflectivity at the Bragg peak at 12.6 nm shows an evident decreases at a fluence of about 100 mj/cm2, where the damage threshold can be individuated (see Fig. 1). For fluencies >150 mj/cm2 the diffuse scattering also increases, indicating the increasing of the conformal roughness. In parallel, the chemical characterization of the surface and of the first buried interface has been performed by means of XPS and standing wave enhanced XPS [5]. Measurements indicate clearly an evolution of the molybdenum silicon first interface, due to the layer intermixing with the formation of molybdenum silicide. The effects of air and vacuum irradiation conditions have been compared. AFM measurements of the different damaged regions have also been performed.