Ion-assisted magnetron sputter deposition have been used to deposit Ni/Ti multilayer neutron mirrors. Improved interface widths were obtained by using B4C doping, to eliminate nanocrystallites by amorphization, and a two-stage modulated ion assistance, to obtain abrupt and smooth interfaces. In situ high-energy wide angle X-ray scattering during multilayer depositions was used to monitor the microstructure evolution and to determine the most favourable growth conditions. Post growth X-ray reflectometry in combination with high resolution transmission electron microscopy confirmed the amorhization and revealed significant improvements in interface widths and reduction of kinetic roughening upon applying B4C doping and modulated ion assistance during growth. Significant improvement of neutron supermirror performance is predicted by employing this technique.