Alternatives for nickel as barrier coating for intermetallic migration

Abstract

The interdiffusion of electrodeposited Ni, Pd, Pd-Fe, Co, Co-W with Cu substrate were studied, at temperature ranging from 250 C to 800 C, using Energy Dispersive X-ray Spectroscopy (EDS). Chemical interdiffusion coefficients were calculated using the Boltzmann-Matano Analysis. Intermetallic phase formation was studied by X-ray Diffraction (XRD). Grain sizes of the electrodeposits were estimated by the particle size boardening method. Microstructures were observed by optical microscopy and scanning electron microscopy (SEM). The higher interdiffusion coefficients observed for Cu/lamellar Ni to that of the Cu/columnar Ni couple might be due to having smaller initial grain size and different microstructure. The grain growth rate of the Cu/Ni couple at 500 C for the first 30 min. annealing was observed to be higher than after 48 hours annealing, resulting to higher diffusivities determined. A transition range at about 700-750 C was observed from the Arrhenius plot of diffusivities for the Cu/Ni systems indicating a transition between different diffusion mechanism. Interdiffusion coefficients of the Cu/Pd system determined in the temperature range 300-700 C are higher than about one order of magnitude for those of Cu/Ni system. Interdiffusion of Cu/Pd was reduced when Fe was alloyed with Pd as 75wt%Pd-25wt%Fe alloy by electroplating. The as-deposited Pd-Fe coating consists of FePd3 and FePd phases, and PdCu3 was observed after heat treatment. No remarkable interdiffusion was observed by EDS measurement for the Cu/Co and Cu/Co-W systems up to 800 C. However, after annealing ranging from 400-800 C for the Cu/Co/Ni system, accumulation of Cu atoms was found at the Co/Ni interface in which Cu had further diffused into the Ni electrodeposit. It is suggested that Cu atoms diffuse through the Co layer by the grain-boundary diffusion mechanism. Alloying Co with W as 65wt% Co-35wt% W by electroplating reduced interdiffusion of Cu through Co to Ni. The structure of the Co-W coating has changed from a mixture of h.c.p. and f.c.c. Co-like structure to f.c.c. Co-like structure upon 1100 C annealing for 90 min. The diffusion barrier properties of Co and Co-W were compared with those of Ni. Results ascertain that Co is a more effective barrier than Ni for Cu diffusion up to 219 h at 400 C. Co-W alloy coating, of 36 wt.% W is a more effective barrier for Cu diffusion than Ni up to 72 h at 500 C. Co-W coating containing crystalline phases was found to be a more effective barrier than amorphous Co-W coating. The crystalline Co-W coating was found to be more effective as a diffusion barrier than Co coating at all investigated temperatures (400-800 C), whereas amorphous Co-W coating performs better than Co coating only at 500 C or lower. However, interpenetration of Cu through Co is more pronounced than diffusion of Cu through Ni for temperatures ranging from 500 C to 800 C. Interdiffusion of Cu through Co-36 wt.% W coating is more pronounced than diffusion of Cu through Ni for temperatures ranging from 600 C to 800 C. Results of diffusion experiments on different Cu/barrier/Au systems showed that Co-35wt% W coating was confirmed to be the most effective barrier for Cu/Au interdiffusion at 400 C or lower among other electrodeposited coatings Co, Ni (lamellar/columnar), Pd and Pd-Fe. Corrosion properties of different electrodeposited coatings Ni (lamellar/columnar), Pd, Pd-Fe, Co and Co-W were evaluated by AC impedance and D.C. polarisation measurements with 3 % NaCl electrolyte, in which polarisation resistances (Rp) were determined.