Adhesion mechanisms of copper films deposited onto laser-irradiated alumina
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Adhesion mechanisms of copper films deposited onto laser-irradiated alumina Jae-Won Park and Anthony J. Pedraza Department of Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee 37996-2200
Douglas H. Lowndes Solid State Division, Oak Ridge National Laboratory, P. O. Box 2008, Oak Ridge, Tennessee 37831-6056
William R. Allen Lockheed-Martin Energy Systems, Inc., Post Office Box 2009, Oak Ridge, Tennessee 37831-8084 (Received 1 June 1996; accepted 8 May 1997)
Strong adhesion between a deposited copper film and an alumina substrate takes place when the substrate is laser-irradiated prior to deposition. A post-deposition annealing is required to achieve the strong bonding. In this work, the interfacial region between the copper film and the alumina substrate was analyzed using Auger Electron Spectroscopy (AES). It was found that a transitional region is always present in couples that have a high adhesion strength, while little or no transitional region was found in weakly bonded couples. The transitional region depends on the laser irradiation atmosphere. In the case of laser irradiation in air, oxygen excess was found on the surface of the alumina substrate, and in the copper/alumina couple the transitional region consists of a copper oxide and a Cu–Al double oxide. When the laser irradiation was performed in a reducing atmosphere (Ar –4% H2 ), substoichiometric alumina and metallic aluminum were found on the surface of the substrate and also a reaction between copper and the substoichiometric aluminum oxide was detected in the subsurface. Although the substoichiometric alumina is formed on the surface irradiated in Ar–4% H2 , a stable Al2 O3 thin layer is formed on the outmost surface because the irradiated substrate is exposed to the atmosphere before deposition. This reoxidized layer remains whole at the interface of the couple upon low temperature (at least up to 300 ±C) annealing, while it is ruptured upon higher temperature annealing (500 ±C in this work). In the latter case, the copper film can contact and react with the substoichiometric alumina formed in the subsurface of the substrate irradiated in the Ar–4% H2 atmosphere. It is concluded that the Cu–Al –O interfacial compound formed in the transitional region causes the strong adhesion between the copper film and the alumina substrate.
I. INTRODUCTION
Reactions of metals and oxide ceramics at the interface are, in general, very restricted under thermodynamic equilibrium. This lack of reactivity leads to a poor bonding between the oxide and metallic films deposited on its surface. Surface modification of the ceramic is then required to promote reaction between — otherwise — mutually nonreactive materials. Pure copper cannot reduce Al2 O3 because the formation free energy of Al2 O3 (2377 Kcalymole) is less than those of copper oxides (CuO: 2 30 Kcalymole, Cu2 O: 2 35 Kcalymole).3 We have employed excimer laser irradiation to modify the near-surface region of Al2 O3 in order to
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