Morphology control in Ni/Ti multilayer neutron mirrors by ion-assisted interface engineering and B<sub>4</sub>C incorporation
Fredrik Eriksson, Naureen Ghafoor, Sjoerd Broekhuijsen, Grzegorz Greczyński, Norbert Schell, Jens Birch
Abstract
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 B 4 C 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 11 B isotope-enriched B 4 C combined with temporal control of the ion assistance.