Direct Electroless Plating of Iron-Boron on Copper
Chemical bath deposition of Fe was first achieved by Ruscior and Croiala1; however, these investigators used a sacrificial aluminum anode which dissolved, thereby providing the electrons for Fe deposition. True electroless deposition of FeB, i.e., using a reducing agent as electron source rather than a sacrificial anode, was achieved only recently through the use of palladium (Pd) activation of the substrate2,3. The significant cost and process complexity associated with surface activation motivated us to study and develop a ‘DIRECT’ electroless process for FeB deposition on copper (Cu) without the need for Pd activation.
Our direct electroless FeB process was observed to be highly sensitive to the reducing agent, i.e., borohydride (BH) concentration. Electrochemical polarization studies were conducted to understand why low BH concentration in the bath did not yield Fe deposition. The polarization curves corresponding to the two half reactions of Fe plating and BH oxidation are shown in Figure 1. The mixed potential theory4allows us to predict theoretical plating rates corresponding to the potential at which the reduction and oxidation current densities are equal (dashed lines). It is noted that borohydride oxidation on the substrate (Cu) is cathodically shifted compared to borohydride oxidation on Fe. This implies that, at low borohydride concentrations, Fe plating on Cu proceeds rapidly but is suppressed immediately after the substrate (Cu) is covered with a thin layer of Fe. This observation led us to investigate the oxidation kinetics of borohydride and its dependence on electrolyte composition and pH. It was determined that increasing the borohydride concentration of the bath enabled a substantial cathodic shift in the polarization curve for borohydride oxidation on Fe, as shown in Figure 2. The cathodic shift was also evident in the mixed potential during electroless FeB plating from this high-BH electrolyte. The cathodic shift ensured sustained plating of FeB even after the initial coverage of the Cu substrate by Fe.
Cross-section SEM was conducted to characterize deposit properties such as the thickness and surface morphology. X-ray photoelectron spectroscopy (XPS) was used to determine film composition, which was around 69% Fe and 31% B. X-ray diffraction (XRD) confirmed a nano-crystalline structure similar to NiP. Materials characterization and electrochemical corrosion testing confirmed that the proposed direct electroless FeB plating process has the potential of enabling Ni-free coatings in a variety of applications requiring corrosion-resistant, hard, and highly solderable surfaces5.
: C. Ruscior and E. Croiala, J. Electrochem. Soc., 118, 696 (1971).
: M. A. Dinderman, W. J. Dressick, C. N. Kostelansky, R. R. Price, S. B. Qadri and P. E. Schoen, Chem. Mater., 18, 4361 (2006).
: L. Z. Li, W. C. Hu, Y. W. Zhang, X. B. Wan and L. Zhang, T. I. Met. Finish., 89, 95 (2011).
: C. Wagner and W. Traud, Z. Elektrochem., 44, 391 (1938).