<title>High-performance multilayer mirrors for soft x-ray projection lithography</title>

Abstract

We present a comparative study of three metal Si multilayer coatings, Mo-Si, Ru-Si, and Rh-Si, designed for optimum normal incidence reflectivity at x-ray wavelengths near 130 angstroms. The ML structures are characterized using x- ray diffraction and high-resolution electron microscopy, and the results are correlated with the normal incidence reflectivity measured using synchrotron radiation. It is found that interlayers of mixed composition are formed to various degrees in the as-deposited structures for all three material compositions. The thinnest interlayers are observed in Mo-Si and the most intermixing occurs in Rh-Si, where the pure Rh layers are completely consumed. The stoichiometry of the interlayers in all cases is most consistent with the formation of the Si rich silicide. The highest peak normal incidence reflectivity of 61% at approximately 130 angstroms is achieved with Mo-Si, whereas the highest integrated reflectivity of 4.4 angstroms is achieved with Ru-Si. In a separate study, the formation of the interlayers in as-deposited Mo-Si multilayer coatings is investigated by systematically varying the thickness of either the Mo or Si layers. We observe a contraction of the multilayer period which is consistent with formation of MoSi2 at the interfaces. The presence of the MoSi2 interlayers decreases the normal incidence reflectivity, thereby limiting the x-ray throughput in soft x-ray projection lithography applications.