The optical contrast and minimum layer thickness of Ni/Ti broadband neutron multilayer supermirrors is usually hampered by an interface width, typically 0.7 nm, caused by nanocrystallites, interdiffusion, and/or intermixing. We explore the elimination of nanocrystallites in combination with interface smoothening by modulation of ion assistance during magnetron sputter deposition of 0.8 to 6.4 nm thick Ni and Ti layers. The amorphization is achieved through incorporation of natural B₄C where B and C preferably bond to Ti. A two-stage substrate bias was applied to each layer; -30 V for the initial 1 nm followed by -100 V for the remaining part, generating multilayer mirrors with interface widths of 0.40-0.45 nm. The results predict that high performance supermirrors with m-values as high as 10 are feasible by using ¹¹B isotope-enriched B₄C combined with temporal control of the ion assistance.

The work in this thesis covers the design, growth and characterisation of neutron multilayers. The achieved reflectivity performance of a neutron multilayer depends on the achieved optical contrast between the layers as well as the achieved interface width between the layers. Because the reflectivity of a neutron multilayer depends exponentially on the square of the interface width, even a modest improvement can substantially increase the achieved reflectivity performance. It is for this reason that a large part of this work has been focused in making smoother and more abrupt multilayers, descreasing the total interface width. As multilayers are such an integral component of most neutron optical instruments, any improvement in terms of reflectivity performance has broad implications for all conducted neutron scattering experiments. 

The conventional material system of choice for neutron optical components is Ni/Ti, owing to the high contrast in scattering length density (SLD). The reflected intensity of such components is largely dependent on the interface width, primarily caused by the formation of nanocrystallites, interdiffusion, and/or intermixing. Apart from hampering the reflectivity performance, the finite interface width between the layers also limits the minimum usable layer thickness in the mirror stack. In this work, Ni/Ti based multilayers are grown using ion‐assisted magnetron sputtering. By co‐depositing B₄C in the multilayer stack, the for‐mation of nanocrystallites as well as intermetallics between the interfaces were succesfully prevented. The co‐deposition of B₄C has been combined with a modulated ion assistance scheme, where an initial buffer layer is grown at a low ion energy creating abrupt interfaces, while the remainder of the layer is grown at a higher ion energy, smoothening the interfaces. X‐ray reflectivity (XRR) measurements show significant improvements in terms of reflectivity when the multilayers are co‐deposited with B₄C . This has further been investigated using low neutron‐absorbing isotope‐enriched ¹¹B₄C . The deposited ¹¹B₄C containing multilayers have been, characterized using neutron reflectometry, X-ray reflectivity, transmission electron microscopy, elastic recoil detection analysis, X‐ray photoelectron spectroscopy and grazing incidence small angle scattering (GISAXS). Structural parameters in the growth direction such as interface width and thickness variations have been determined by combined fits on X‐ray and neutron reflectivity measurements, while the interface morphology has been investigated using GISAXS. The GISAXS measurements show that the co‐deposition of ¹¹B₄C leads to mounded interfaces with more strongly vertically correlated interface profiles, this can be attributed to a decreased adatom mobility when ¹¹B₄C is incorporated. The coupled fits to specular X‐ray and neutron reflectivity measurements suggest a significant improvement in interface width for the samples that have been co‐deposited with ¹¹B₄C using a modulated on assistance scheme during deposition, where an interface width of 2.7 Å has been achieved in a ¹¹B₄C containing multilayer. The reflectivity for such ¹¹B₄C containing multilayers have been simulated for a neutron supermirror (N = 5000) using the IMD software. The predicted reflectivity performance at the critical angle of an m = 6 supermirror for the ¹¹B₄C containing samples amounts to about 76%, which is a significant increase compared to the current state‐of‐the‐art supermirrors made with pure Ni/Ti, which have a predicted reflectivity of 65%. This results in a reflectivity increase from 1.3% to 6.6% after a total of 10 reflections from this critical angle, translating to an increase of over 500% for neutrons reflected at this angle. At lower incidence angles, corresponding to thicker periods, the current state‐of‐the‐art is expected to perform better due the higher optical contrast. By combining a material system with a higher optical contrast in the thicker layers with ¹¹B₄C containing multilayers in the thinner layers, a high reflectivity performance can be obtained over all reflected incidence angles. This has been demonstrated by a proof of concept, where thicker Ni/Ti multilayers with thin (0.15 nm) ¹¹B₄C interlayers between the interfaces show the best reflectivity at a period of 84 Å, while ¹¹B₄C deposited in both multilayer stacks has shown the best reflectivity performance at thinner periods at 48 Å and 30 Å. 

Chemically homogeneous B4C interference mirrors with 11B/10B isotope modulation have been investigated as well to seek new possibilities for future neutron optical components. Preliminary neutron reflectometry were performed, showing very promising results with 10% absolute reflectivity for a 128 nm thick multilayer consisting of only 20 bilayer periods with a periodicity of 33.1 Å, showing how only few layers are needed for a high reflectivity. 

The performance of multilayers in optical components, such as those used in neutron scattering instruments, is crucially dependent on the achievable interface width. We have shown how the interface width of Ni/Ti multilayers can be improved using the incorporation of B₄C to inhibit the formation of nanocrystals and limit interdiffusion and intermetallic reactions at the interfaces. A modulated ion-assistance scheme was used to prevent intermixing and roughness accumulation throughout the layer stack. In this work we investigate the incorporation of low-neutron-absorbing ¹¹B₄C for Ni/Ti neutron multilayers. Combined fitting of neutron reflectivity and X-ray reflectivity measurements shows an elimination of accumulated roughness for the ¹¹B₄C containing multilayers with a mean interface width of 4.5 Å, resulting in an increase in reflectivity at the first Bragg peak by a factor of 2.3 and 1.5 for neutron and X-ray measurements, respectively.

The work in this thesis covers the design, growth and characterisation of neutron multilayers. The performance of these multilayers is highly dependent on the obtained interface width between the layers, even a modest improvement can offer a substantial increase in reflectivity performance. As multilayers are such an integral component of many neutron optical instruments, any improvement in terms of reflectivity performance has broad implications for all neutron scattering experiments. This project has been carried out with the construction of the European Spallation Source (ESS) in mind, but the principles extend to all neutron scattering sources.

Ni/Ti is the conventional material system of choice for neutron optical components due to the high contrast in scattering length density (SLD). The reflected intensity of such components is largely dependent on the interface width, caused by the formation of nanocrystallites, interdiffusion, and/or intermixing. Apart from hampering the reflectivity performance, the finite interface width between the layers also limits the minimum usable layer thickness in the mirror stack. The formation of nanocrystallites has been eliminated by co-depositing of B₄C . This has been combined with a modulated ion assistance scheme to smoothen the interfaces. X-ray reflectivity (XRR) measurements show significantly improvements compared to pure Ni/Ti multilayers. This has further been investigated using low neutron-absorbing ¹¹B₄C instead. After deposition, the ¹¹B₄C containing films have been characterized using neutron reflectometry, X-ray reflectivity, transmission electron microscopy, elastic recoil detection analysis, X-ray photoelectron spectroscopy. A large part of his work has focused on fitting X-ray and neutron reflectivity measurements in order to obtain structural parameters.

The fits to the experimental data suggest a significant improvement in interface width for the samples that have been co-deposited with ¹¹B₄C using a modulated ion assistance scheme during deposition. Any accumulation of roughness has been eliminated, and the average initial interface width at the first bilayer has been reduced from 6.3 Å to 4.5 Å per bilayer. The respective reflectivity performance for these structural parameters have been simulated for a neutron supermirror (N = 5000) for both materials at a neutron wavelength at λ = 3 Å using the IMD software. The predicted reflectivity performance for the ¹¹B₄C containing samples amounts to about 71%, which is a significant increase compared to the pure Ni/Ti samples which have a predicted reflectivity of 62%. This results in a reflectivity increase from 0.84% to 3.3% after a total of 10 reflections, resulting in more than 400% higher neutron flux at experiment.