Laser-induced interface reactions of copper thin films on sapphire substrates

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Abstract
The interface of a copper-sapphire couple that was irradiated with a nanosecond pulsed-exeimer 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/cm2. 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.

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