Thickness dependent effect of swift heavy ion irradiation in W/Ni superlattice multilayers

Abstract

This study reports on the layer thickness dependent effect of swift heavy ion irradiation (SHI) in W/Ni multilayer structures (MLS) having bilayer compositions of [W(25 Å)/Ni(25 Å)], [W(40 Å)/Ni(40 Å)] and [W(34 Å)/Ni(57 Å)] of 10-bilayers. These were grown on [1 0 0]-silicon using ion-beam sputtering. The pristine MLS were subjected to 120 MeV Au9+ ion irradiation up to a fluence of 1 × 1014 ions cm−2. Structural characterization of pristine and irradiated MLS was carried out employing wide-angle x-ray diffraction (WAXD), x-ray reflectivity (XRR) and cross-sectional transmission electron microscopy (X-TEM). The XRR studies reveal a decreasing trend in the SHI-induced interface roughness of MLS with thicker Ni layers. This effect is vividly clear in the WAXD and X-TEM studies. The X-TEM studies reveal thickness dependent microstructural changes in the MLS. The MLS with thinner bilayers undergo intra-layer microstructure changes from textured to nano-crystalline on irradiation whereas the MLS with thicker bilayers remain intact although the irradiation fluence is the same. This effect has been cross-verified, beyond all doubts, by studying the SHI irradiation effect on a MLS having a composite bilayer structure of [W(50 Å)/Ni(50 Å)]5BL/W(25 Å)/Ni(25 Å)]5BL]/Si grown over each other. The observed nano-crystallization has been attributed to thermal spike caused by large electronic energy deposition. Our analysis clearly reveals that the effect of SHI strongly depends on the layer thickness of the multilayers.