Laser-induced interface reactions of copper thin films on sapphire substrates
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Douglas H. Lowndes Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6056
Edward A. Kenik Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6376 (Received 10 March 1989; accepted 22 May 1989)
The interface of a copper-sapphire couple that was irradiated with a nanosecond pulsed-excimer laser was studied by transmission electron microscopy. Deposited layers of 30 or 100 nm thickness were laser treated with energy densities in the range of 0.5 to 0.75 J/cm 2 . Two different atmospheres were used during these treatments, viz., air or a mixture of argon-4 vol. % hydrogen. The copper film and a thin alumina layer were melted by the laser pulse. Two well differentiated regions could be observed in the modified layer. The region closer to the unmodified substrate consisted of epitaxially regrown alumina with crystallites misoriented up to 10° relative to the substrate sapphire orientation, while precipitate particles could be seen closer to the resolidified copper. The nature of the precipitates generated in the second region was dependent on the atmosphere present during the treatment. In air a trirutile-like compound was obtained which is either oxygen or copper deficient. In an argon atmosphere a compound having a hexagonal structure closely related to sapphire was observed, where copper substituted for some aluminum. These observations are in agreement with a previously developed mathematical model that predicts melting of a thin substrate layer.
I. INTRODUCTION The mechanical strength of a metal-ceramic couple is primarily determined by the reactions that take place at the interface level. When a metallic film is deposited on a ceramic substrate, bonding forces can develop at the interface. The nature of these forces and their intensities will depend upon the characteristics of the couple as well as the deposition process. In particular, noble metals show very poor adhesion on sapphire substrates in the as-deposited condition. Also sputter-deposited (or vacuum-evaporated) copper shows poor adhesion on sapphire. This type of metal-ceramic couple is important in electronic packaging systems for information processing and in thin film technology. Baglin and Clark1 have demonstrated that interfacial strength can be increased when a copper film deposited on sapphire is bombarded by He + and Ne + ions. Baglin et al.2 deposited copper on sapphire substrates presputtered with 500 eV Ar + for different lengths of time. Their experiments indicate that the adherent interface produced by presputtering involves copper bonded in a ternary Cu-Al-0 environment. Mitchell et al.3 improved the adhesion of metal films on semiconductor and insulator substrates by electron irradiation. They found enhanced adhesion of 1202
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platinum thin films deposited on yttria stabilized zirconia substrates after 5-30 keV electron irradiation. Because the input energy was insufficient to produce collisional displacements, the authors concluded that the
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