The thermal stability of thin copper films deposited on TiO 2 (110) studied by scanning tunneling microscopy
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The thermal stability of thin copper films deposited on TiO2 (110) studied by scanning tunneling microscopy D. L. Carroll Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6272
M. Wagner and M. R¨uhle Max-Planck-Institut f¨ur Metallforschung, Institut f¨ur Werkstoffwissenschaft, D-70174 Stuttgart, Germany
D. A. Bonnell Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6272 (Received 21 September 1995; accepted 25 November 1996)
The morphology of Cu thin films is strongly dependent on the temperature of the substrate during and after deposition. Films grown at temperatures between 300 ±C and 400 ±C form clusters distributed in close packed domains and isolated clusters across the surface. Increased substrate temperatures result in cluster shape evolution indicative of mass flow and sintering. Deposition of Cu at substrate temperatures higher than 500 ±C results in a completely different morphology of the film and the suppression of cluster formation. Annealing these Cu films to temperatures of 700 ±C allows the system to relax into an equilibrium state characterized by large facets in the film and large areas of exposed surface. These observations are discussed in terms of basic thermodynamic data for bulk Cu oxidation and surface tensions for this system.
I. INTRODUCTION
The importance of the properties of metal/ceramic interfaces to the stability of composite systems has motivated 15 years of research.1 Frequently, applications such as high frequency microelectronics and electrooptical nanocomposites require the use of metal-oxide nanocomposites which must withstand high power densities. Thus, the thermal stability of electronic structure at a metal-oxide interface is of direct interest in component packaging, thick film circuitry, and high power device production, as well as composite material design. Oxides typically used for their excellent thermal characteristics, such as Al2 O3 (alumina), TiO2 (rutile), and various perovskites, do not interact strongly with the metals best suited for electronic applications such as Au or Cu. One approach to probing metal/ceramic interface interactions is to examine the morphology of thin films on oxide substrates. Classical treatments then describe film structure in terms of surface and interface energies. In systems where the metal of the oxide has a stronger oxygen affinity than that of the film and a single preferred oxidation state, the morphology of the film at high temperatures seems well described by thermodynamic arguments that exclude the possibility of oxidation of the film by the substrate. Cu,2 Rh,3 and Pt4 on Al2 O3 are examples of such systems showing weak metalsubstrate interactions resulting in three-dimensional island growth (Volmer–Weber, V-W) at room temperature and island coalescence at elevated temperatures. J. Mater. Res., Vol. 12, No. 4, Apr 1997
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