Fractal analysis of Palladium hillocks generated due to oxide formation

Abstract

Understanding the controlling mechanism in growth of hillocks may lead to new processes enabling the growth of self-assembled hillocks. By processing the topographical images of the surface of Pd thin films, surface roughness was erased resulting to well-separated hillocks over the surface. As a result, strong correlation has been found between the total hillock surface area and residual stress generated in the films. Next, fractal analysis is introduced as effective mathematical approach to enhance accuracy of the aforementioned correlation when films are textured. This enables us to study hillocks geometrically and to monitor their distribution over the surface. Moreover, X-ray photoelectron spectroscopy revealed the generation of PdO selvedge layer in the interface of Pd thin films and SrTiO3 substrate as a result of lattice misfit. It could be concluded that oxygen diffusion into grain boundaries of Pd thin film leads to the thermal expansion coefficient mismatch between grains and grain boundaries resulting to hillock growth. Moreover, it has been shown using Coble creep that hillock formation is dominant stress relaxation process in thickness lower than 40nm whereas at higher thickness, selvedge layer generation becomes dominant.