Interface evolution of Cr/Ti multilayer films during continuous to discontinuous transition of Cr layer

Abstract

Cr/Ti multilayers with moderate to low bi-layer thickness have been deposited by Ion beam sputtering system and have been investigated thoroughly by specular grazing incidence X-ray reflectivity (GIXR) and diffused X-ray scattering (both in coplanar and non-coplanar configurations) measurements. The investigation is important for development of Cr/Ti soft X-ray multilayer mirrors for “water window” applications in the wavelength regime of 2.2–4.4 nm. Initially few multilayers have been deposited and the highest reflectivity at first Bragg peak positions of the GIXR plots have been obtained for the Cr layer to bi-layer thickness ratio of 0.4. Subsequently, keeping the ratio fixed a set of five samples with bi-layer thickness varying from 3.8 nm to 2.1 nm have been prepared. For the all five ML samples reported in this communication, Ti layer thickness is greater than 1.3 nm, however, the Cr layer thickness varies from 1.5 to 0.8 nm in the above range of bi-layer thickness and it undergoes continuous-to-discontinuous transition. Cr films undergo significant change in morphology as the thickness changes and thus the interfaces are also evolved differently. Analysis of the diffused X-ray scattering data shows that interface imperfection of Ti-on-Cr interfaces is dominated by interface diffusion at low Cr thickness (discontinuous) regime and by interface roughness at high Cr thickness (continuous) regime, while in case of Cr-on-Ti interfaces, interface roughness dominates throughout the whole thickness regime. For the sample with highest bi-layer thickness of 3.8 nm high Bragg peak reflectivity value close to the ideal case is obtained manifesting high thickness uniformity, very low interface imperfection and high vertical correlation length across the interfaces for this sample which is also confirmed by cross-sectional Transmission Electron Microscopy. A 50 bi-layer sample prepared with the above bi-layer thickness shows high reflectivity of 5.6% at 2.84 nm water window soft X-ray wavelength.