Structural coherence of sputtered Ni₃Al/Ni multilayers

Ni₃Al/Ni multilayers have been grown by magnetron sputtering at room temperature. Structural coherence and interfacial properties were investigated as a function of deposition conditions by means of high angle x-ray diffraction, as well as specular and nonspecular x-ray reflection. High vertical coherence over 3.5 bilayer thickness is obtained in favorable conditions. It is shown that the structural coherence results from a highly textured growth and from a rather constant bilayer thickness over the whole multilayer stack. Surprisingly, the vertical coherence length does not decrease in samples sputtered at high Ar pressure despite a higher mosaic spread of the grains and rougher interfaces. In contrast, a high base pressure prior to sputtering is found to alter the coherence and to favor sharp and smooth interfaces. In all samples the roughness profiles are partially correlated from one interface to the other. The corresponding vertical correlation length is strongly influenced by the sputtering Ar pressure. In a large lateral length scale range (10³-10⁴ Å), a relatively slow decrease of the correlation length with decreasing length scale of the roughness is measured with a correlation length significantly smaller for the sample grown at high Ar pressure.