Properties of electroless Cu films optimized for horizontal plating as a function of deposit thickness

Abstract

The properties of electroless films produced from a bath designed for horizontal plating, the preferred technology for high production volumes in printed circuit board metallization, are reported. Film thickness, substrate type and electrolyte temperature were varied. Formation of a continuous layer of copper film is correlated with a change in the visual and spectroscopic appearance. Grain orientation is random in thin films and a 〈110〉 texture develops with increasing thickness. The plating solution contains Cu and Ni ions. Nickel co-deposits in copper films in the form of Ni hydroxide, and its concentration decreases from about 6% in the vicinity of the substrate to about 1% at the film surface. Film stress and strain were measured by substrate curvature and X-ray diffraction, respectively. Both stress and strain decrease as the film thickness increases. Stress remains tensile throughout during deposition and during relaxation, promoting film adhesion by preventing blisters. After deposition, stress relaxes first towards compressive and then towards tensile. The stress, the stress relaxation and the Ni concentration are high at the base of the film. We attribute this to the higher volume fraction of grain boundaries (smaller grain size) in this region.