Microstructure and Stress of Ni/C Multilayer Films Prepared by Reactive Sputtering

Abstract

Magnetron-sputtered Ni/C multilayers with a periodic thickness below 4 nm are difficult to produce, and reactive sputtering with nitrogen is a feasible method. The effects of nitrogen on the reflectivities of Ni/C multilayers were investigated. Pure argon and three mixing ratios of 4%, 8%, and 15% nitrogen-argon gas mixture were used as the working gas. For all Ni/C multilayer samples, each contains 40 bilayers. The nominal structure has a periodic thickness of 3.8 nm, with a ratio of the thickness of the Ni layer to the periodic thickness of Г = 0.39. The results of grazing incidence X-ray reflectivity (GIXRR) measurements indicate that reactively-sputtered Ni/C multilayers have a lower interface width and higher specular reflectance. It was shown in transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) measurements that a periodic layered structure can still be clearly observed in Ni/C multilayers with pure argon, but with quite rough interfaces between the adjacent layers. For Ni/C multilayers with the mixing ratio of 4% nitrogen-argon gas mixture, it is found that the interfaces between Ni and C layers become smoother and sharper. Additionally, nitrogen incorporation can reduce the mobility of Ni atoms, which decreases the threshold thickness that Ni layers would become continuous. This may be also a reason which accounts for the better interface quality of reactively-sputtered Ni/C multilayers. Meanwhile, Ni/C multilayers deposited with a nitrogen-argon gas mixture have lower stress due to the reduction in Ni adatom mobility.