Structure control of LA/B multilayer systems by partial nitridation
Dmitry Kuznetsov, a former PhD student in the Industrial Focus Group XUV Optics, defended his thesis Thursday November 8th 2018. His supervisor was prof.dr. F. Bijkerk from the Faculty of Science and Technology.
Nanoscale multilayer structures are employed in a wide range of analytical and imaging applications. Extreme Ultraviolet (XUV) multilayer coatings are key enablers of optical components in this wavelength range. This thesis addresses multilayers wavelengths above 6.6 nm, which have a perspective for light sources like FELs, diagnostic techniques like XRF, optical elements like beam-splitters and telescopes for space research, as well as for high resolution imaging systems. The focus of this work was on the synthesis of multilayers with high performance (reflectivity and thermal stability), insight into the growth and control of deposition at the atomic scale.
The figure of merit of the understanding of the basic physics processes in nanoscale multilayers, as well as the ability to control these, is usually expressed by its reflectivity. At the start of this thesis project, the highest reflectivity at near normal incidence of 6.7 nm wavelength amounted 57.3%. Nitridation of the La/B multilayers was shown to protect the B-on-La interface from the formation of optically unfavorable lanthanum boride compounds at that interface. Further development in this thesis opened up a way to improved layer control. This is based on the fact that La will not take more N than needed for the formation of a stoichiometric LaN compound. Excessive N2 during La growth results in the formation of optically unfavorable BN at the LaN-on-B interface. Therefore, the layer growth process was controlled by a special approach, so called partial (delayed) nitridation. The use of sub-nm but closed (~0.3 nm or thicker) interlayers of elemental La deposited on B was shown to minimize the interaction of N with B. This hybrid deposition allowed to synthesize multilayer structures with new record reflectivity: 64.1% at Î»â‰ˆ6.66 nm, AOI=1.5Â°. Further improvements can be anticipated.
In addition, the thermal stability was studied for the new multilayer structure with partial (delayed) nitridation. The observed changes under elevated temperatures were associated with interdiffusion and formation of compound(s). For both B-on-LaN and LaN-on-La-on-B interfaces no additional compoundÂ formationÂ wasÂ resolvedÂ in the temperature range up to 200Â°C. The partial (delayed) nitridation LaN-on-La-on-B interface was still not activated for interdiffusion at 200Â°C, judging by no registered compound formation. This result could be explained by the formation of a LaBx interlayer. Therefore, novel partial (delayed) nitridation demonstrated a thermal stability similar to previously developed LaN/B multilayers.
Finally, the possibility of fabricating La/B-based multilayers for ~6.7 nm radiation at grazing angles of incidence (GI) was investigated. LaN/B for GI (period 15 nm) showed deterioration accompanied by oxidation of at least two of the topmost LaN layers. A special scheme called La surface nitridation was invented, in which only top part of La layers is nitridized. It was found that the B protective properties depend on the thickness of the underlying LaN layer. The LaN thickness for which no deterioration occurs, was determined to be at least 0.4-1.0 nm. In order to obtain further insight into La and LaN growth, special in situ studies were accomplished at a synchrotron light source. Crystallographic structures of La and LaN during growth were revealed, and initial formation of textures and roughness transitions were observed. Two probable explanations for the LaN degradation were developed. Employing the special La surface nitridation scheme, a record reflectivity of 74.5% at 6.66 nm at off-normal (77Â°) incidence was achieved. These multilayers were demonstrated to be stable to storage in atmosphere during at least one year and a half. An absolute drop of reflectivity of about 0.5% was explained by contamination, accumulated by a B top layer.
The results of the research presented in this thesis demonstrate how knowledge on layer growth, combined with a high, atomic scale, degree of control of the deposition can be employed to improve the performance of nanoscale multilayer systems. The experimental and analytical approaches, employed in the study of reflective multilayers for the wavelengths above 6.6 nm, might be adapted for various stacked systems with nm- and sub-nm-thick layers.