Nodule Formation in Electroless Copper Baths

Abstract

The formation rate of copper nodules in electroless copper baths, which is a measure of the bath stability, was studied by means of a nodule monitoring technique as a function of the presence of complexing agents for copper (I)‐ions and of dissolved oxygen both in the presence and absence of surfactant. In the presence of dissolved oxygen the stabilizing effect of complexing agents including cyanide is relatively small and does not correlate with the formation constants of the respective Cu(I)‐complexes. In absence of oxygen, significant stabilization is exerted by cyanide and phenanthroline‐(1,10), but not by the other investigated complexing agents. Dissolved oxygen dramatically reduces the rate of nodule formation. Bubbling of air through the bath instead of nitrogen reduces the density of nodules deposited on the monitors by more than four decades both in the presence and absence of complexing agents. This effect is further enhanced by increasing the oxygen content of the bubbled gas mixture up to 50%. The bath does not become passive, if the concentration of formaldehyde is sufficiently high. These findings support a nodule formation model in which the bivalent copper complex is adsorbed on extraneous germs, then reduced by formaldehyde in a two‐electron step to adsorbed metallic nuclei which subsequently grow to nodules. Oxygen stabilizes through reoxidation of nuclei, whereas cyanide and phenanthroline‐(1,10) in absence of oxygen may inhibit either adsorption or onset of growth. A mechanism via monovalent copper intermediate is excluded.