A CORROSION STUDY OF COPPER THIN FILMS

Edilson M. Pintoa, A. Sofia Ramosb, M. Teresa Vieirab, Christopher M.A. Bretta aCEMUC®, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade de Coimbra, 3004-535 Coimbra, Portugal bCEMUC® - Departamento de Engenharia Mecânica, Faculdade de Ciências e Tecnologia, Universidade de Coimbra, 3030-788 Coimbra, Portugal edilson.m.pinto@gmail.com Thin nanostructured films based on the copper-nitrogen (Cu-N) system, with compact morphology, were deposited by magnetron sputtering at specified partial pressure ratios of N2 and Ar, nitrogen being incorporated into the copper thin films without formation of CuxN compounds. The nitrogen concentration plays an important role in the film formation process, particularly on the grain size and homogeneity of the films. Thin films with smaller grain size were able to be produced by controlling the nitrogen concentration [1]. In order to investigate the electrochemical properties of these films, electrochemical measurements using linear sweep voltammetry (Tafel plots) and electrochemical impedance spectroscopy (EIS) have been carried out in 0.5 M NaCl aqueous solution in the presence of dissolved O2. Samples of pure bulk copper were compared with thin films of copper of different thicknesses and different nitrogen concentrations used in the film production process: Cu2.5 - 2.5 µm thick without nitrogen, Cu2.5 (N1.5) - 2.5 µm with 1.5 % nitrogen, and Cu1.5 (N3.5) - 1.5 µm thick with 3.5% nitrogen. The Cu-N films are all nanocrystalline, with grain size decreasing down to 30 nm with the use of nitrogen (3.5 at.% of N). Sample morphology was examined by scanning electron microscopy. Analysis of Tafel plots shows similar corrosion potential values (Ecor) for all samples, 315 ± 15 mV vs SCE, but the corrosion current values (Icor) vary significantly, Cu2.5 (N1.5) films presenting the lowest Icor value of 0.12 µA cm-2 and the Cu1.5 (N3.5) films the highest of 6.99 µA cm-2. The impedance spectra present inductive loops at low frequencies, indicative of film relaxation processes. Cu1.5 (N3.5) films have a bigger charge transfer resistance of ~1.3 k? cm-2, than all the other films, the smallest being for Cu2.5 (N1.5) films at ~0.4 k? cm-2. This inversion in behaviour with respect to the corrosion currents can be explained through the surface structure. Results show that the presence of nitrogen produces copper films with very good mechanical and electrical properties, due to minimization of grain size and structural defects on the grain borders, although the electrochemical results indicate that there is a size barrier below which electrochemical properties are not necessarily improved. Films with smaller copper grain size dimensions, Cu1.5 (N3.5), have higher corrosion currents, probably due to an increase in the number of grains per unit area which leads to an increase in the electroactive surface. [1] R.Calinas, M.T.Vieira, P.J.Ferreira, J. Nanosci. Nanotechnol. (in press).

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