New materials for multilayer mirrors in the extreme ultraviolet region

Abstract

Near normal incidence multilayer mirrors are optical elements that are suitable for the extreme ultraviolet wavelength (EUV) region where applications include lithography, astronomy, and microscopy. Multilayer mirrors are made from alternating layers of two materials, called the "absorber" and "spacer," with the thickness of the layers designed such that reflections from each interface add in phase resulting in a large overall reflectivity. The criteria I used for the selection of six new material pairs included achieving the maximum theoretical reflectivity while taking into consideration the possible structural properties of the interfaces based on binary phase diagrams. The pairs were: C-Cu, B₄C-Ag, B₄C-Sn, Y-Pd, Be-Mo, and Be-Y. My experimental results on sputtered C-Cu and B₄C-Ag multilayers showed that they are not suitable as mirror materials due to the formation of discontinuous layers of Cu and Ag for small bilayer periods Λ. I also found it not possible to sputter tin films with small enough interfacial roughness values that would result in useful B₄C-Sn mirrors. My analysis of Y-Pd multilayers showed asymmetric alloying at the interfaces with an approximately 40 Å thick alloy region at the Y on Pd interface which would result in negligible mirror reflectivity. I used one of our molecular beam epitaxy (MBE) machines to attempt to grow single crystal Be-Mo mirrors. Although my attempts were unsuccessful to date, this combination cannot be excluded due to various problems with the MBE sample manipulator during the growth study. Finally I used the same MBE machine to grow Be-Y mirrors with up to 40 bilayers. These multilayers had extremely smooth interfaces (σ = 3.5-4.5 A) with a predicted mirror reflectivity larger than 65%. I found the stability of the Be-Y interfaces to be excellent under atmospheric long term storage. An X-ray photoelectron spectroscopy (XPS) annealing study I conducted also showed stable interfaces for temperatures of up to 200°C. Be-Y mirrors should be suitable for a variety of applications including EUV-lithography.