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The reaction of Ta/Si/Ta/Si... multilayers under rapid thermal annealing (RTA) conditions has been examined using Auger electron spectroscopy (AES) sputter depth profiles and line shape analysis and cross‐sectional transmission electron microscopy (X‐TEM). Oxygen and carbon contamination present initially in some of the tantalum layers slows the formation of the disilicide for these layers. C remains immobile and bound to the Ta throughout the sequence of RTA temperatures. O, which is also initially bound to Ta, diffuses to the upper Si/Ta interface and forms a Si suboxide that acts as a barrier layer to the reaction of the two adjacent Si and Ta layers until the highest treatment temperatures (1150 °C). In contrast, the uncontaminated region reacts at 750 °C and a contaminated Ta layer and the underlying Si layer (with no suboxide barrier) react together at 850 °C. These results are in qualitative agreement with predictions of the Ta–Si–O ternary phase diagram, but differ from those obtained from long furnace anneals of Ta/Si bilayers. These differences are tentatively attributed in part to the differences in the kinetics of diffusion and formation of the various phases under different annealing conditions and in part to the amorphous nature of the thin Ta/Si multilayers.