Atomic scale interface engineering by modulated ion-assisted deposition applied to soft x-ray multilayer optics

摘要

Cr/Sc and Ni/V multilayers, intended as normal incidence soft x-ray mirrors and Brewster angle polarizers, have been synthesized by employing a novel modulated low-energy and high-flux ion assistance as a means of engineering the interfaces between the subnanometer layers on an atomic scale during magnetron sputter deposition. To reduce both roughness and intermixing, the ion energy was modulated within each layer. The flat and abrupt interfaces yielded soft x-ray mirrors with near-normal incidence reflectances of R velence 20.7percent at the Sc 2p absorption edge and R velence 2.7percent at the V 2p absorption edge. Multi-layers optimized for the Brewster angle showed a reflectance of R velence 26.7percent and an extinction ratio of R_(s)/R_(p) velence 5450 for Cr/Sc and R velence 10percent and R_(s)/R_(p) velence 4190 for Ni/V. Transmission electron microscopy investigations showed an amorphous Cr/Sc structure with an accumulating high spatial frequency roughness. For Ni/V the initial growth mode is amorphous and then turns crystalline after approx1/3 of the total thickness, with an accumulating low spatial frequency roughness as a consequence. Elastic recoil detection analyses showed that N was the major impurity in both Cr/Sc and Ni/V with concentrations of 15 at. percent and 9 at. percent, respectively, but also O (3 at. percent and 1.3 at. percent) and C (0.5 at. percent and 1.9 at. percent) were present. Simulations of the possible normal incidence reflective properties in the soft x-ray range of 100-600 eV are given, predicting that reflectivities of more than 31percent for Cr/Sc and 5.8percent for Ni/V can be achieved if better control of the impurities and the deposition process is employed. The simulations also show that Cr/Sc is a good candidate for mirrors for the photon energies between the absorption edges of B (E velence 188 eV) and Sc (E velence 398.8 eV).