The solid state reaction between Cu3P and silicon or germanium

The phase relations in low-phosphorus parts of the ternary systems Cu-Si-P and Cu-Ge-P have been determined for alloys annealed in evacuated capsules at 500°C. The reaction of Cu3P with silicon in diffusion couples that have been annealed in vacuum, is never faster than the reaction between copper and silicon. Cu3Si and phosphorus vapour are formed during the reaction. A peculiar morphology, that of isolated Cu3Si columns separated by large gaps, develops when the reaction is hindered. When a diffusion couple is annealed in a closed capsule, Cu3Si and presumably SiP are formed. The reaction between Cu3P and germanium takes place at a rate comparable with that of the copper-germanium reaction and leads to the formation of Cu3Ge and phosphorus vapour. Because of plastic deformation of Cu3Ge no peculiar morphology of the reaction layer was observed

The influence of phosphorus on the solid state reaction between copper and silicon

The solid state reaction between copper and silicon has been studied in the temperature range 350 to 650°C. The rate-limiting step is the diffusion of copper through the main product Cu3Si. When pure copper is used, a bulk diffusion mechanism is operative above 470°C, and the activation energy is 175 kJ/mol. Below 470°C incubation times occur. The reaction proceeds by grain-boundary diffusion, with an activation energy of 110 kJ/mol. When phosphorus-doped copper is used, bulk diffusion of copper through Cu3Si occurs above 530°C. Below this temperature grain-boundary diffusion occurs with an activation energy of 92 kJ/mol. No incubation times were observed. The segregation of phosphorus at the reaction interface removes the reaction barrier. Cu15Si4 and Cu5Si seem to be absent from the diffusion couples because of kinetic factors

The influence of impurities on the kinetics and morphology of reaction layers in diffusion couples

The growth and morphology of reaction layers in diffusion couples is probably more frequently influenced by impurities than is generally realized. We found drastic effects of impurities on the reaction between Cu and Si and on the reaction between Cu20 and Ni or Co. In the first case the always present thin Si02-film on Si plays an important role: it hampers the reaction between Cu and Si, giving rise to an incubation period. This reaction barrier can be removed completely by only a few ppm of phosphorus in the copper starting material, which enables the formation of Cu3Si at much lower temperatures than in the absence of phosphorus. In the latter example the presence of chloride ions prevents NiO or CoO, which is formed during the displacement reaction, from building up a continuous, closed and rate-limiting layer. Instead, precipitates of these oxides in a matrix of copper are formed . This results in a much higher reaction rate since then the much faster diffusion of oxygen through copper becomes rate-limiting