Microstructure evolution during solid state reactions of Nb/Al multilayers Original Research Article

The microstructure produced during investigations of reactive formation of NbAl3 from Nb/Al multilayer films was found to be in general agreement with that employed in the two-stage kinetic model of Coffey et al. [Appl. Phys. Lett. 55 (1989) 852]. Upon completion of the reaction, however, recrystallization and rapid grain coarsening led to significant changes in film microstructure and deviations from the predictions of the model. In an effort to clarify the picture of reactive phase formation emerging from these studies, several complementary analyses were employed to characterize the reaction. First, thermal data obtained from constant-heating-rate differential scanning calorimetry experiments were used to determine the annealing conditions for ex situ cross sectional transmission electron microscopy studies of the evolving film microstructure. Next, high-resolution chemical mapping was employed to track the penetration of the product phase into the Al grain boundaries. It was found, for example, that the NbAl3 layer was 8–10 nm thick when the first stage of reaction was completed. It was also found that during stage two, the NbAl3 layer consisted of columnar, somewhat faceted grains and that substantial recrystallization occurred after the termination of the reaction. These observations suggested refinements of the structure type classification scheme for reacted films proposed by Barmak et al. [J. Electron. Mater. 26 (1997) 1009]. Lastly, the impact of bilayer thickness on the product microstructure was investigated using a novel sample architecture in which several bilayers with different periods comprise one multilayer. Investigations of this multi-period, multilayer sample correlated with those on single-period multilayers, indicating that the reaction reached completion at lower temperatures as the bilayer thickness was reduced.