Low-energy ion polishing of Si in W/Si soft X-ray multilayer structures

• Published in: Journal of applied physics 2019-07, Vol.126 (4)
• Main Authors: Medvedev, R. V., Nikolaev, K. V., Zameshin, A. A., IJpes, D., Makhotkin, I. A., Yakunin, S. N., Yakshin, A. E., Bijkerk, F.
• Format: Article
• Language: English
• Subjects: Applied physics; Argon ions; Grazing incidence; Magnetron sputtering; Multilayers; Polishing; Reduction; Reflectance; Roughness; Small angle X ray scattering; Soft x rays
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.5097378

The effect of ion polishing in sputter deposited W/Si multilayer mirrors with a d-spacing of 2.5 nm was studied. 0.1 to 0.5 nm of Si were etched with 100 eV Ar+ ions. This process resulted in a pronounced reduction in diffused scattering, measured at wavelengths about 0.1 nm. However, CuKa X-ray specular reflectivity and AFM showed only a marginal reduction of the roughness amplitude in the systems. Furthermore, the soft X-ray reflectivity at 0.84 and 2.4 nm did not show any changes after the ion polishing as compared to the nonpolished structures. Grazing incidence X-ray reflectivity (GIXR) analysis revealed that there was no pure W present in the deposited multilayers, with WSi2 being formed instead. As a result, it was concluded that the initial roughness in W/Si multilayers grown by magnetron sputtering is not the major factor in the reflectivity deviation from the calculated value for an ideal system. Nevertheless, the grazing incidence small-angle X-ray scattering (GISAXS) analysis revealed that ion polishing reduces the vertical propagation of roughness from layer to layer by a factor of two, as well as favorably affecting the lateral correlation length and Hurst parameter. These improvements explain the reduction of diffused X-ray scattering at 0.1 nm by more than an order of magnitude, which is relevant for applications like high resolution XRD analysis.