Experimental study on fabrication technique of a Ni/Ti supermirror using the electron beam evaporation system

Basic study on the fabrication technique of a Ni/Ti supermirror using the vacuum evaporation system with an electron gun has been carried out to improve the neutron reflectivity. The substrate roughness dependency to the reflectivity was investigated using polished glass floats with different surface roughnesses using neutron reflectometry. It was concluded that the substrate roughness in the order of several sub-nanometers was very sensitive to the neutron reflectivity of the Ni/Ti supermirror. The reflectivity was improved to 80% for the Ni/Ti supermirror with an effective critical angle of 2 times that of natural nickel using fine polished glass substrate with a surface roughness of 0.51 nm rms. The effect of oxidizing Ni and Ti layers to the neutron reflectivity, which were deposited by changing the oxygen gas conditions in the chamber, was investigated. The oxidation effect prevents the deformation of the Ni/Ti supermirror structure, and the reflectivity shows an increase of almost 20% for the Ni/Ti supermirror with an effective critical angle of 2.6 times that of natural nickel by depositing in an atmosphere of oxygen gas with a partial pressure of 1x10-5 Torr. From the depth profile by X-ray photoelectron measurement (XPM) and neutron reflectometry results, it was concluded that the reflectivity increase by oxidation mentioned is caused by decreasing the interface roughness and the interfacial diffusion between Ni and Ti layer, and it is not caused by forming a NiO compound which increases the scattering potential of the Ni layer. (author)