Interface engineered ultrashort period soft X-ray multilayers
Dennis IJpes is a PhD student in the department XUV Optics. (Co)Promotors are prof.dr. M.D. Ackermann and dr. A. Yakshin from the faculty of Science & Technology.
Soft X-ray mirrors play a crucial role as optical components in various industrial and research applications, spanning from space astronomy and X-ray fluorescence (XRF) to synchrotron research, among others. In all these applications, high reflectance of the mirror is critical in order to improve the performance of the optical system. This can be expressed through faster detection of elements in XRF analysis, increased sensitivity when observing X-ray emission of space phenomena, or increased throughput during synchrotron measurements.
These mirrors, technically known as multilayers, need to be extremely thin to efficiently reflect incident soft X-ray radiation. This requires a period of the multilayer on the order of ~1-3 nanometers (nm). At this scale, the individual layers are only a few tenths of nm thick, approaching the limit of layers to grow a closed and continuous film. Furthermore, the reflectance of these mirrors depends strongly on the absence of imperfections, such as layer mixing, rough interfaces, and the formation of optically unfavorable compounds. As a result, the reflectance of such multilayers has been far from optimal.
The research described in this thesis aims to increase the reflectance of soft X-ray multilayers with periods from 2.5 nm down to 1.0 nm by gaining a deeper understanding of how these ultra-thin layers grow and interact. This involves using a combination of cutting-edge analytical techniques to obtain reliable information about the internal structure of these nanoscale multilayers. Additionally, the study uses techniques collectively referred to as "interface engineering" to reach a reflectance which is state-of-the-art. These techniques include the use of diffusion barrier layers, ion beam polishing, and seed layers.
The first issue tackled in this thesis was the low reflectance of the tungsten/silicon (W/Si) multilayer system at a period of 2.5 nm – caused by the formation of optically unfavorable tungsten-silicide (WxSiy). In order to prevent W-Si interaction, 0.3 nm boron carbide (B4C) diffusion barriers were applied at the W-Si interfaces. This lead to a record 45% peak reflectance, measured at λ=0.834 nm. Chemical analysis indicated a partial replacement of W silicide bonds with W carbide/boride bonds, reducing W-Si interaction and increasing reflectance.
The same concept was extended to even thinner multilayers with a period of 1.0 nm, resulting in a 3.4 times increase in peak reflectance at λ=0.834 nm with the application of 0.1 nm B4C barriers. Similar to 2.5 nm W/Si, the B4C barriers reduced formation of optically unfavorable WxSiy through partial substitution of W-silicide bonds with W-carbide/boride bonds. This modified structure offers a promising alternative at the ultrashort scale, outperforming established multilayer configurations.
Furthermore, the less-explored tungsten/aluminium (W/Al) multilayer system was investigated at a period of 2.5 nm. The findings revealed that this system reflected poorly due to high interface roughness and strong intermixing between the W and Al layers, likely caused by discontinuous layer growth and/or island growth. However, adding 0.5 nm Si layers at the W-Al interfaces improved layer growth. Additional ion beam polishing of the Si layer reduced interface roughness further, yielding reflectance comparable to standard W/Si at λ=0.154 nm.
The final issue addressed in this thesis is the high interface roughness in 2.5 nm tungsten/boron carbide (W/B4C) multilayers. In order to tackle this, low-energy neon ion beam polishing (IBP) was applied to the B4C layers using 200 eV and 50 eV energies. While both energies reduced roughness, W/B4C with 200 eV IBP showed strong intermixing and a broad B4C-on-W interface, likely due to penetrating Ne+ ions modifying the buried interface. In contrast, W/B4C with 50 eV IBP resulted in sharp, symmetric interfaces, achieving a 43% peak reflectance at λ=0.834 nm.
In conclusion, this comprehensive research provides valuable insights into the synthesis and analysis of interface-engineered W-based multilayers with periods of 1.0 and 2.5 nm. Cutting-edge metrology tools were used to study the internal structure of the multilayers and resulted in an increased understanding of thin-film growth and layer interaction at the atomic scale. This research serves as a foundation for further studies on ultrashort period multilayers and offers a blueprint for scientists and engineers involved in the design and fabrication of soft X-ray optics.