See 2018

STRUCTURE CONTROL OF LA/B MULTILAYER SYSTEMS BY PARTIAL NITRIDATION

In this thesis Extreme Ultraviolet (XUV) multilayer coatings were studied, being a key enabler of optical components at wavelengths above 6.6 nm.  

‘The perspective of these multilayers is as promising as it is diverse,’ says Dmitry Kuznetsov. ‘To mention some application ideas: light sources, diagnostic techniques, optical elements (for example: beam-splitters, and telescopes for space research), and high resolution imaging systems.’  

The focus of this PhD work was on the synthesis of multilayers with high performance (reflectivity and thermal stability), and insight into the growth and control of deposition at the atomic scale. ‘Designing new experiments and looking into theoretical details did go hand in hand during this PhD,’ Dmitry says. ‘We achieved some record-breaking results on important performance indicators.’ 

Dmitry: ‘However, the parameters to be controlled are too diverse to fully describe them in one theoretical framework alone. At these length scales - in which the layers are up to one hundred times thinner as usual – surprising physical phenomena arise from time to time. Growth processes of these ultrathin layers are highly sensitive for: energy input, diffusion processes, roughness conditions and imperfections, to mention a few.’

At the start of this thesis project, the highest reflectivity at near normal incidence of 6.7 nm wavelength amounted 57.3%.

New record

Nitridation of the multilayers was shown, to protect the interfaces from the formation of optically unfavourable lanthanum boride compounds. ‘Further development in this PhD work opened up new ways towards improved layer control,’ Dmitry says.

‘The growth process was controlled by our special approach, so called partial (delayed) nitridation, using sub-nm interlayers. The use of sub-nm interlayers of elemental La deposited on B was shown to minimize the interaction of N with B. This hybrid deposition approach, allowed us to synthesize multilayer structures with a new record reflectivity of 64.1%. This result attracted much attention, both in industry and in the scientific community .’

Thermal stability

The thermal stability for the new multilayer structure with partial (delayed) nitridation was further studied. ‘The novel method demonstrated a thermal stability, similar to previously developed LaN/B multilayers,’ Dmitry says. ‘Also other angles of incidence were investigated. We suggested and implemented improved growth strategies for that.’ 

Synchrotron

To obtain further insight into La and LaN growth, special in situ studies were accomplished at a synchrotron light source in Germany.

‘This was a unique and exciting experience,’ Dmitry says. ‘It opened up the curtain, when we could clearly perceive: the influence of roughness conditions, see the start of crystallization processes and compare various other interaction graphs. The experience is very different from learning about the processes involved from literature. At these scale lengths, details can suddenly turn out to be decisively influential.’

Future

Dmitry thinks a career in both academics or industry is thinkable for his work and research approach.

‘At the moment I work as a post-doc within the XUV Optics Group, on some different aspects of multilayers,’ he says. ‘Industry as well as academical research may benefit from the approach of my research work. Expertise of MESA+ was quite supportive during my PhD, to discuss the use of some experimental techniques and practical experiences, including relevant examples involving layers of nm and sub-nm thickness.’