Control of defects in EUV-multilayers and demonstration of at-wavelength defect characterization by EUV-microscopy.

In EUV optical elements microscopic imperfections (defects) in multilayer coatings give rise to increased scatter and reduced integrated reflectivity. If however, the coating is in the object plane of an imaging system, the defects call be imaged. This may give rise to undesirable dark spots in the image. In this study, experimental work is presented on characterization of the defect density and the nature of the defects in molybdenum/silicon-multilayer coatings produced by e-beam evaporation in combination with ion-beam smoothening of the interfaces. To determine in which process steps defects are created, and how these defects can be avoided, several process parameters have been varied during single- and multilayer deposition and ion-polishing. The samples have been characterized by means of an optical particle counter (opc) and electron microscopy (SEM) with energy dispersive (EDX) x-ray analysis measurement capabilities. Both ope and SEM inspection only yield information about defects at the top surface of the multilayer. To carry out in-depth inspection of the coating, we developed a method to measure the total integrated scatter for EUV radiation from a 1 mu m spot. Varying the photon energy around the Si-absorption edge enabled us to distinguish between surface defects and in-depth defects that cannot be seen at the multilayer surface.