Multilayer supermirrors play important roles as neutron optical devices used for neutron guide tubes, polarizers, focusing mirrors, beam benders, etc., on many neutron sources. Supermirrors, whose period spacing is gradually changed, consist of a bilayer having a different refractive index for neutrons. Supermirrors function as an artificial crystal lattice, and the reflection angle and wavelength range can be increased from a few times, to as much as the total reflection of the monolayer. There are some difficulties in developing a multilayer mirror with high reflectivity. The reflectivity of multilayers is reduced by surface and interface roughness caused by crystal grains and interdiffusion between layers. An ion-polishing technique to remove the very small interface roughness of the multilayer has been successfully developed.
The ion-beam sputtering method has the advantage of being able to deposit a dense film and to control the deposition conditions more precisely than other plasma methods, such as magnetron sputtering, since the substrate and plasma are separated. In the ion-polishing process, particles with the lowest binding energy, and that are in a valley of the surface, are removed by recoil effects following ion bombardment (Fig. 4-11). To create the ultimate sharp interface, the energy of the irradiating atomic beam, the irradiation time, and the incident angle had to be scrutinized and optimized.
In this study, nickel (Ni)/titanium (Ti), Ni/manganese (Mn), and nickel carbide (NiC)/Ti multilayers were deposited and ion-polished using the ion-beam sputtering system. Ion polishing was applied immediately after the deposition of a layer to smooth the layer. Multilayers with d=2 to 10 nm and N=10 to 300 pairs were deposited and ion-polished. The d-spacing and the interfacial roughness of the multilayers were examined with X-ray grazing angle reflectivity measurements. For the ion-polished Ni layers, the interface roughness, sigma, reached a minimum value of 0.3 nm at an ion-polishing time of 69 s, an ion acceleration energy of 100 eV, and an incident angle of 10 degrees. The interfaces were very sharp and the interdiffusion between the Ni and the Ti layers was decreased in comparison with the unpolished sample, as shown in Fig. 4-10, and it was 0.3 nm rms in the case of d=3 nm even when the number of layers reached 300 pairs (Fig. 4-12). On the other hand, the interface roughness was 0.7 nm rms without ion-polishing. Based on the optimal ion-polishing condition, Ni/Ti supermirrors with three times the total reflection (3Qc) of Ni were coated and characterized. The reflectivity was increased remarkably-it reached 90% at the critical angle of total reflection. Applying this method will produce higher m-Qc supermirrors, which can increase the available neutron intensity one order of magnitude greater than that of existing neutron sources.
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