Thermal characterization and modeling of intermediate phase formation in 20/80 nm and 10/20 nm Cu/Mg multilayers
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Thermal characterization and modeling of intermediate phase formation in 20/80 nm and 10/20 nm Cu/Mg multilayers
M.González-Silveira, J. Rodríguez-Viejo and M.T. Clavaguera-Mora Grup de Física de Materials I, Physics Department, Universitat Autònoma de Barcelona 08193 Bellaterra, Spain.
ABSTRACT The kinetics of intermediate phase formation in the Cu/Mg multilayer system is analyzed using Differential Scanning Calorimetry. Three main exothermic processes are found during the continuous DSC treatments. The first two, significantly overlapped, are related to the same process, nucleation and growth of the Mg2Cu along the interface. We interpret differences between the Mg/Cu and Cu/Mg interfaces are at the origin of this unexpected behavior. The third exothermic reaction is due to the growth of the Mg2Cu phase perpendicular to the original interface. A kinetic model is developed which yields calorimetric traces in good agreement with the experimental data. INTRODUCTION Calorimetric characterization of multilayer structures has been shown to be a useful tool to study the nucleation and growth events that takes place during heat treatments [1,2]. Previous work on the Cu/Mg system [3,4] determined that the formation of the Mg2Cu product phase is divided in nucleation and lateral growth, followed by vertical growth after coalescence is completed. The corresponding DSC traces show two exothermic reactions, one for each process. In this paper we present experimental results obtained by X-Ray diffraction and Differential Scanning Calorimetry (DSC) on two different sets of Cu/Mg multilayers and interpret the data through a model developed to fit the calorimetric traces. The results obtained so far indicate a non simultaneous formation of the intermetallic phase at the two kinds of interfaces (Mg/Cu and Cu/Mg). EXPERIMENT The multilayer thin films were prepared by alternate electron-beam evaporation. As substrate, both bare and photoresist-coated Si-wafers were used. The latest allowed us to work with free-standing films just by removing the photoresist with an acetone bath. The deposition was carried out at room temperature, though at the end of the process temperatures around 60ºC were reached. The working vacuum laid between 4 and 8·10-6 mbar during the deposition. The growth rate was controlled in-situ with a piezo-electric system. Two sets of samples were prepared in order to achieve a better understanding of the nucleation and growth processes. The first (set 1) had an overall composition ratio of 1Cu:2Mg. It was formed by 8 bilayers of 20 nm for Cu and 80 nm for Mg. The growth rate was fixed at 0.3 nm/s. The other set (set 2) had a total composition ratio of 1Cu:1Mg and was formed by 20 bilayers of 10 nm for Cu and 20 nm for Mg. In this case, the growth rate was fixed at 0.1 nm/s. An extra Cu
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layer of 20 nm was added at the end of the deposition in both sets to avoid the oxidation of the last layer of Mg. The bilayer sequence of the sample was measured with XRR measurements. A Perkin-Elmer DSC-7 diffe
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