Defect depth profiling of sputter-deposited Cu/Nb bilayers using a positron accelerator

Abstract

Ultrathin bilayers of Cu/Nb viz. Nb/Cu/Si and Cu/Nb/Si have been deposited on Si substrate using DC and RF magnetron sputtering. The structural characterization of bilayers has been carried out using X-ray reflectivity (XRR) to determine the thickness and density of individual layers. The thickness of Nb and Cu layer in the bilayers ranges from ∼ 44–71 nm and 6–11 nm, respectively. Defect depth profiling of bilayers has been carried out using a positron accelerator. The measured depth profiles, i.e., S-parameter profiles have been fitted using VEPFIT, a program that factors the positron implantation profile and its annihilation to estimate S-parameter and positron diffusion length in individual layers. We present a detailed and systematic analysis of the measured S-profiles using VEPFIT to evaluate S-parameter and positron diffusion length in individual layers of the bilayer structures. These parameters are representative of defect characteristics of the layers. In this work we also highlight the nuances of VEPFIT analysis useful to extract defect characteristics in bilayers. The results indicate that defect microstructure of the layer depends on the thickness, deposition method and irradiation. Defect density is seen to increase with the reduction of thickness of the layer. RF sputtered layer has more defects as compared to DC sputtered layers. Positron depth profiling does not reveal distinct interface between the two metallic layers as well as layer and the substrate. A systematic characterization of defect in ultrathin structures is crucial to establish a correlation between defect microstructure and properties of nanolayer composites for advanced applications.