Thermally stable nanomultilayer films of Cu/Mo

Abstract

Thermal decomposition of nanoscaled (5, 50, and 100 nm) multilayer films has been studied in an immiscible Cu and Mo system. While the onset of nanolayer instability is by thermal grooving at elevated temperatures, the entire nanomultilayer film decomposition can be differentiated into three distinct stages, over a range of temperatures (848 to 1073 K). Stage I is characterized by the onset of grooves, which appear as minor perturbations in otherwise flat interfaces. This is followed by the occurrence of prominent grooves in stage II. Stage III consists of a complete breakdown of the layered structure, with the microstructure composed of grains of Cu and Mo. However, a good layer stability was observed in some of these nanomultilayers (50 Mo:5 Cu and 5 Mo:5 Cu), and stage II is retained up to long times at elevated temperatures. This is attributed to the large difference in the individual layer melting temperatures, combined with unequal film thickness (and hence volume fractions), which inhibits the attainment of an equilibrium groove configuration, up to extended periods of time. Analytical models for thermal grooving in bulk polycrystalline materials were applied to the case of thin film nanomultilayers. The predicted instability kinetics were found to corroborate with the experimentally observed stability of the nanomultilayers, except at very small size ranges (5 nm). A methodology for generating stable nanomultilayer films is suggested as an outcome of this study.