Electrodeposition and Characterisation of Novel Ni-NbOx Composite Coatings as a Diffusion Barrier for Liquid Solder Interconnects - Part II: Diffusion Barrier Performance
The present study considers a novel electrochemical route to produce Ni-NbOx composite coatings of good uniformity, smoothness, compactness and purity. The electrolyte is derived from a Type IV eutectic solvent consisting of NbCl5 and propylene glycol (PG). The addition of a small amount of Ni ions (molar ratio: Ni : Nb=1 : 30) was found to enable the co-deposition of Nb species. The introduction of NaBH4 into the electrolyte has elevated the maximum deposit thickness (above 15 µm), although this led to a reduction in the co-deposited Nb content. The effects of cathodic current density, metal precursor and NaBH4 concentrations on the surface morphology, composition and thickness of the coatings were examined. Scanning transmission electron microscopy (STEM) revealed a nanocrystalline structure resembling that of a simple FCC structure of Ni, with a slightly fluctuating composition across the coating thickness and with Nb uniformly distributed (See Fig. 1). From X-ray photoelectron spectroscopy (XPS), a 0.7 eV shift to lower binding energy for the Nb (V) species signifies the partial chemical reduction of Nb2O5 due to possible Ni-Nb bonding. It is thus proposed that a Ni-NbOx deposit was formed by a two-step mechanism: the simultaneous electrochemical reduction of adherent Ni metal and inadherent niobium oxides on Cu, followed by nickelation of the NbOx. The composite coating’s electrical conductivity, solderability and diffusion barrier characteristics (the latter preventing excessive IMC growth between molten Sn-In solder and Cu substrate) were assessed and showed promising results. From diffusion studies, a Nb-enriched layer was observed at the solder/substrate interface after ageing at 200°C suggesting evident effectiveness of the diffusion barrier characteristics.