Deposition, microstructure and properties of sputtered copper films containing insoluble molybdenum

Abstract

The microstructure and properties of Cu films containing insoluble Mo in as-deposited and annealed conditions have been studied. Using magnetron sputtering, Cu films with Mo concentrations up to 25 at. % have been deposited. The Cu-Mo films consist of nonequilibrium supersaturated solid solutions of Mo in Cu and have nanocrystalline microstructures. Upon heating, most of the films studied undergo three major transition events: recovery, crystallite growth and coalescence, and grain growth. Recovery occurs at ∼220–250 °C due to the release of strain energies stored during deposition, while the growth and coalescence of crystallites at ∼490 °C are likely driven by the crystallite-boundary reduction. Upon further annealing at above ∼670 °C, the microstructure alters considerably in most of the films, resulting in extensive growth of crystallites and grains. Rather high strain energies are thought to cause the precipitation of Cu particles in the molybdenum-rich Cu-Mo films (e.g., Cu-25 at. % Mo) during annealing at a temperature as low as 200 °C. The fine structure observed in the 800 °C-annealed Cu-25 at. % Mo film suggests that the extensive crystallite/grain growth is effectively impeded by the presence of molybdenum, signifying a good property of thermal stability. The film’s resistivity and hardness properties correlate well with the microstructure and are governed by the impurity effect of molybdenum. Low-molybdenum Cu-Mo films yield relatively low resistivity, and this is attributable to the improved film microstructure. Hardness results indicate that the strengthening of the films is mainly due to the fine structure and presence of molybdenum.