Effect of Vacuum System Base Pressure on Corrosion Resistance of Sputtered Al Thin Films

Abstract

Pure aluminum (Al) thin films were sputter deposited under conditions of variable base pressure of the deposition tool. The purpose of the study was to identify if the base pressure had a key influence on the electrochemical properties, namely corrosion resistance, of the Al deposits. The corrosion resistance was found to increase with increasingly high base pressure (i.e. lower quality vacuum). Potentiodynamic polarization experiments, pit growth studies, TEM and XPS were employed. From the results is posited that improved corrosion resistance is due to the presence of oxygen (O) being incorporated into the films during deposition resulting an O-containing (i.e. Al-O) alloy. This effect is important to identify because of the many reports in the literature of high pitting potentials for Al thin films, from studies with widely varying base pressure. Vacuum system base pressure was found to have a large effect on the corrosion resistance. A maximum base pressure in the low 10 −7 Torr range is recommended to minimize oxygen uptake into the film. Corrosion of thin film metal samples deposited by sputter deposition or evaporation has attracted considerable attention because of the technological applications of thin films in the electronics industry. From a research perspective, thin films also provide unique opportunities for studying fundamental aspects of the pitting phenomenon and also for the investigation of non-equilibrium alloy compositions. Many papers report an increased corrosion resistance of thin films relative to bulk alloy counterparts, often expressed as an increase in the pitting potential, E pit .1-11 Other researchers have found E pit for thin films to be similar to bulk alloys of a similar composition. 12,13Several different reasons for the improved corrosion resistance of thin films have been offered. Liu and coworkers proposed that the increase in pitting resistance of Ni-based alloy thin films resulted from the nanocrystalline microstructure of the films. [4][5][6][7] Regarding the role of grain size on corrosion resistance, both increases and decreases in corrosion resistance with decreasing grain size have been reported, 14 with the phenomenon being closely related to test environment.15 Because a fine grain structure has more reactive sites with respect to oxide formation and film ion conduction, the relative rates of reaction in a given environment might be altered. However, it is unlikely that grain size variation alone can explain the very noble values of E pit that have been reported for some pure metal thin film samples.Factors other than grain size have been reported to result in ennobled E pit values in thin film samples. 16 For alloys that pit at specific sites in a heterogeneous microstructure, such as MnS particles in stainless steel, improved homogeneity achieved by sputtering can certainly improve the corrosion performance. This chemical effect, as opposed to a structural effect, has been invoked by some as an important factor when evaluating sputtered stainless steel or Fe-Cr al...

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