W-based multilayer soft X-ray Bragg optics: progressing towards sub-nanometer scale layer thicknesses
In a sober on-line ceremony held on 22 April, Roman Medvedev, a former PhD student in the XUV Optics group at Twente, obtained his degree from his promotor prof. Fred Bijkerk.
Roman dedicated his studies to thin film reflective multilayer optics for spectroscopy in the energy range between the O Ka and the Al Ka absorption edges (525 eV-1486 eV). The main requirements here are a high peak reflectance and a narrow bandwidth so that characteristic emission from these elements can be properly analysed. The periodicity of the multilayer system for this energy range is only 2.5 nm with individual layers being thinner than 1 nm! This makes the performance of such systems extremely sensitive to any imperfection at the interfaces. Analysis of such short period structures is very challenging due to the limitations of conventional methods that are commonly used for multilayer systems with larger periodicities. Therefore, Roman’s research faced challenges both on engineering and analysis.
Roughness at interfaces is one of the main factors that normally lead to reduced reflectivity, and ion beam polishing was applied to a W/Si multilayer system in order to reduce the roughness. It improved the diffuse scattering of hard X-rays, but did not reduce the interfacial roughness, already at the atomic scale. But analysis of the GIXR data revealed a lack of bulk W in the W/Si system, indicating a reduced overall optical contrast. Improvement of the contrast became the next goal.
In the magnetron deposition system used for this study, two working gases, Ar and Kr, were compared. Since no difference was found in the standard asymmetry of the interlayers due to the different gases, it was concluded that the asymmetry was caused by direct atomic interaction between W and Si rather than by a ballistic effect of the working gas. Research was further focused on the improvement of the W-on-Si interface, trying to reduce direct W-Si interaction.
To this end, nitridation of Si was applied on every Si layer in order to form a thin Si3N4 layer which could reduce W-Si interaction. A gradual increase of the passivation time indeed showed an improvement of the interfaces. However, this effect was counterbalanced by the formation of a relatively thick Si3N4 layer which reduced the optical contract. This result suggested using lower ion energies in order to reduce the nitrogen penetration in the Si layers.
To gain contrast, boron was chosen as an alternative to silicon, and W/B multilayers were successfully synthesized, though GIXR analysis showed the formation of tungsten boride. Ion polishing of the boron layers resulted in a reduction of the diffuse scattering of hard X-rays but did not improve the reflectivity for soft X-rays. These results indicated that B is not a favourable material to substitute Si in these multilayer systems.
In conclusion, a comprehensive analysis of short-period W/Si and W/B multilayer systems was carried out in Roman’s work. A combination of various state-of-the-art analysis techniques was used to obtain information about the internal structure of the multilayers. Understanding the physics of thin-film growth at the atomic scale helped to define the next steps in improving the performance of such systems for future valorization. Roman’s thesis thus forms a basis for further studies on this type of multilayer mirrors.