Interface modification of Cr/Ti multilayers with C barrier layer for enhanced reflectivity in the water window regime

Abstract

The influence of a carbon barrier layer to improve the reflectivity of Cr/Ti multilayers, intended to be used in the water window wavelength regime, is investigated. Specular grazing-incidence X-ray reflectivity results of Cr/Ti multilayers with 10 bilayers show that interface widths are reduced to ∼0.24 nm upon introduction of a ∼0.3 nm C barrier layer at each Cr-on-Ti interface. As the number of bilayers increases to 75, a multilayer with C barrier layers maintains almost the same interface widths with no cumulative increase in interface imperfections. Using such interface-engineered Cr/C/Ti multilayers, a remarkably high soft X-ray reflectivity of ∼31.6% is achieved at a wavelength of 2.77 nm and at a grazing angle of incidence of 16.2°, which is the highest reflectivity reported so far in the literature in this wavelength regime. Further investigation of the multilayers by diffused grazing-incidence X-ray reflectivity and grazing-incidence extended X-ray absorption fine-structure measurements using synchrotron radiation suggests that the improvement in interface microstructure can be attributed to significant suppression of inter-diffusion at Cr/Ti interfaces by the introduction of C barrier layers and also due to the smoothing effect of the C layer promoting two-dimensional growth of the multilayer.

Role of C and B4C barrier layers in controlling diffusion propagation across the interface of Cr/Sc multilayers

“…5,6 In all the above applications, diffusion at the interface has a detrimental effect on the performance of an ML, as it results in the intermixing of two materials, leading to a reduction in contrast and sometimes even to the formation of a compound at the boundary. Improvement in the performance of ML devices by limiting interface diffusion has been demonstrated earlier by us [7][8][9] and others. 10,11 However, in some other applications of metallic MLs, diffusion may be beneficial and desirable.…”

“…60 The detailed procedure of data conversation and analysis has been described in our earlier communication. 8…”

“…This trend is similar to that in ref. 8, in which C was used as a barrier layer. In absence of the B 4 C BL, diffusion of Cr atoms into Sc layers is significant, and also, due to the non-wetting conditions of surface energy, Cr adatoms exhibit island-type growth on Sc.…”

“…60 The detailed procedure of data conversation and analysis has been described in our earlier communication. 8 Fourier-transformed EXAFS (FT-EXAFS) data i. indicates that the Cr local structure at the interface is similar to that of metallic Cr, and that there is no sign of any compound formation. We fitted the FT-EXAFS data of the Cr/B 4 C/Sc sample using the Cr crystal structure from 1 Å to 2.5 Å.…”

“…For a wide range of material combinations, a thin layer of C or B 4 C has been found to improve the reflectivity due to their low surface energy, and in the literature, there are several reports on the improvement of the microstructure of metallic MLs due to introduction of either of these two BLs for different material combinations such as Mo/Si, 20,21 Cr/Ti, 8 Co/Mg, 22 Si/Sc 23 and Cr/V. 18 In contrast, there are also some reports in which a high surface energy material such as Co or W is used as the BL to improve the optical performance of an ML.…”