Microstructure evolution during solid state reactions of Nb/Al multilayers

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, recrystall ization and rapid grain coarsening led to significant changes in film micro structure and deviations from the predictions of the model. In an effort to clarify the picture of reactive phase formation emerging from these studie s, several complementary analyses were employed to characterize the reactio n. First, thermal data obtained from constant-heating-rate differential sca nning calorimetry experiments were used to determine the annealing conditio ns for ex situ cross sectional transmission electron microscopy studies of the evolving film microstructure. Next, high-resolution chemical mapping wa s employed to track the penetration of the product phase into the At grain boundaries. It was found, for example, that the NbAl3 layer was 8-10 mit th ick when the first stage of reaction was completed. It was also found that during stage two, the NbAl3 layer consisted of columnar, somewhat faceted g rains 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 archite cture in which several bilayers with different periods comprise one multila yer. Investigations of this multi-period, multilayer sample correlated with those on single-period multilayers, indicating that the reaction reached c ompletion at lower temperatures as the bilayer thickness was reduced. (C) 2 001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights rese rved.