Oxidation Kinetics of Cobalt Diffusing out of High Current Density (HCD) Plated Gold

Abstract

Auger electron spectroscopy and Auger depth profiling techniques have been used to study the growth kinetics of cobalt oxide films which form on thermally aged HCD‐plated hard gold in dry laboratory air. Films were observed to grow according to parabolic growth kinetics. At temperatures of 150° and 180°C, a large amount of potassium is observed on the film surface and may be associated with the observed recrystallization in hard gold electroplates. Film growth appears to saturate at 180°C for times greater than 48 hr. A general model was developed for film growth on dilute two‐component noble metal alloys exposed to a single reacting gas specie, assuming conservation of the reacting specie (flux continuity at the metal‐film interface) and parabolic film growth. The conditions necessary for the film growth to be controlled by diffusion from the alloy are given in terms of the material constants. For this specific case, the general solution reduces to that of an infinite sink and the diffusion constant can simply be computed from the parabolic growth rate constant. The diffusion coefficients for Co in Au were calculated between 100° and 180°C. An activation energy of 1.83 eV was obtained for this process.