* F. Bellucci, C. R. Tomachuk, L. De Rosa, S. Saiello, Dipartimento di Ingegneria dei Materiali e della Produzione, Universita ` degli Studi di Napoli “Federico II”, P.le Tecchio 80, 80125, Napoli (Italia) J. Springer, Zentrum fu ¨r Sonnenenergie und Wasserstoff-Forschung, Baden- Wu ¨rttemberg, Industriestr. 6, 70565 Stuttgart (Germany) D. B. Mitton, H. H. Uhlig Corrosion Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139 (USA) The wet corrosion of molybdenum thin film – Part II: Behavior at 85 8C C. R. Tomachuk, L. De Rosa, J. Springer, D. B. Mitton, S. Saiello and F. Bellucci* In the past few years there has been increased interest in molyb- denum thin films, which are commonly prepared by magnetron sputtering. There is a variety of novel applications of molybdenum such as, for example, components for soft X-ray optics based on Mo/Si multi-layers, the back contact in thin film solar cells, NO gas detection, and microelectronics. Molybdenum is, also, widely used as an alloying addition in stainless steels to facilitate the for- mation of the passive film and to improve resistance to pitting at- tack. Its corrosion behaviour is complex and many aspects still need to be clarified. During this study, the corrosion behaviour of the PVD-Mo thin film immersed in aerated sulfate and chloride solu- tions at 85 8C was investigated with both polarization and electro- chemical impedance spectroscopy (EIS) measurements. It is appar- ent that the Mo thin film exhibits increased susceptibility to corro- sion in more alkaline environments. 1 Introduction Molybdenum and its alloys are largely utilized because of their high strength (up to 2000 8C), low coefficient of expan- sion combined with good thermal and electrical conductivity; high resistance to corrosion by molten glass, salts and metals; and good wear resistance in thin coatings. Mo is also suitable for sealing to hard glass since it has approximately the same coefficient of expansion and a transition temperature below 700 8C. The adhesion between glass and this metal is very sa- tisfactory and gives an absolutely tight seal [1]. The role of Mo in improving the pitting resistance of stain- less steels in chloride containing media is well known [2, 3]. And the electrochemical behavior of molybdenum has been extensively studied [4 – 6]. The optical, electrical, magnetic and physical properties of metallic oxide thin films are of increasing technological im- portance [7 – 10]. Areas of interest and application include im- proved ductility, bandpass filters and electronic interconnects. It is important to remember that the mechanical properties of thin films are not the same as those of the bulk material because of their microstructure, large surface-to-volume ratio, reduced dimensions, and the constraints caused by the substrate [11]. Over the past few years there has been an increase in the interest in molybdenum thin films, which have a variety of applications. They may be used, for example, in soft X-ray optics based on Mo/Si multi-layers [12], the back contact in thin film solar cells [13, 14], in NO gas detection [15] and in microelectronics [16, 17]. In general, Mo thin film is deposited by magnetron sputtering [7, 8, 18] but chemical vapour deposition and physical vapour deposition have also been examined [6, 9, 10, 19 – 21]. Physical Vapour Deposition (PVD) was utilized to obtain the Mo thin films studied in this work. During PVD, a thin film is grown with the species of interest deposited from the vapour phase. Among the numerous PVD techniques available: thermal evaporation, magnetron sputtering and pulsed laser deposition are some of the most frequently used [22]. In magnetron sputtering, the magnetron is placed in a chamber with an inert gas at a pressure of about 0.3 Pa. A negative bias is applied to the target mounted on the magnetron, whereby a plasma is formed. This plasma is amplified by a magnetic field with a permanent magnet in the magnetron. An electrical field is formed near the target accelerating positive ions towards it. This ion bombardment of the cathode results in sputtering of target atoms, which eventually hit the substrate with energies typically of a few eV. The flux of species hitting the cathode is proportional to the electrical power by which the magnetron is driven. In- ert-gas ion bombardment of the substrate during growth, which frequently is beneficial, can be obtained by applying a negative bias to the substrate [23]. As reported in Part I [24] of this research work, molybde- num thin films are employed in different industrial domains for the production of high value added devices. The corrosion resistance of these thin films is a major concern when devices are designed. In many standard tests for electronic devices corrosion resistance in humid environment has to be tested up to 85 8C. For various reasons, including the potential of an anode/cathode inversion as a function of temperature, ac- curate corrosion resistance at a higher temperature cannot ne- cessarily be evaluated by corrosion tests carried out at room temperature. Therefore, to assess the endurance of devices at a higher operating temperature, there was a need to evaluate the corrosion resistance of Mo thin films at 85 8C. In the first part of this work [24], the corrosion behavior of Mo thin film produced by Physical Vapour Deposition was Materials and Corrosion 2004, 55, No. 9 Wet corrosion of molybdenum thin film 665 F 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/maco.200303786