Hillock growth kinetics in thin Pb-In-Au films

Abstract

Hillock growth has been studied in films 100–500 nm thick of Pb-12wt %In-4wt%Au deposited onto oxidized Si substrates at room temperature. Overlying SiO layers were used to suppress hillock formation everywhere except within 4μ × 4μm size openings in the SiO where hillock growth could be observed. Hillock growth was initiated in these openings by heating the samples in a scanning electron microscope. The dependence of hillock growth on time, temperature and sample geometry were investigated. In addition, an X-ray technique was used to determine the elastic strain in the Pb-alloy films. Analysis of the results indicates that the driving force for hillock formation is the compressive stress generated in the film during heat treatment produced by the thermal expansion coefficient difference between the Pb-alloy film and the Si substrate. A one-dimensional stressdriven diffusion model has been developed to analyse the hillock growth behaviour. By fitting the model to the data, values of an effective diffusivity were obtained, which characterize the hillock growth kinetics. Lattice, interface, dislocation and grain boundary paths have been considered as possible contributors to the effective diffusivity. It is concluded that hillock growth in Pb-In-Au films most probably occurs by diffusion along grain boundaries, with δD_b = (2–8)× 10⁻¹⁵ cm³s⁻¹ in the temperature range 35–110°C. These values for δD_b fall between those reported for bulk Pb and Pb-12wt%In alloys, supporting both the correctness of the model and grain boundary diffusion as the mechanism for hillock growth.