Calorimetric Study of the Energetics and Kinetics of Interdiffusion in Cu/Cu 6 sn 5 Thin Film Diffusion Couples
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Mat. Res. Soc. Symp. Proc. Vol. 398 ©1996 Materials Research Society
formation of Cu 6 Sn5 . The samples are then characterized using x-ray diffraction analysis in a standard 0-20 geometry using Cu-KI radiation. This is done to ensure that all the Sn layers have been consumed. Samples are hermetically sealed in Al pans in a He atmosphere to prepare them for thermal analysis. This provides an inert atmosphere in which the sample can be heated while minimizing oxidation reactions. Thermal analysis of the diffusion couples is performed in a differential scanning calorimeter (a Perkin-Elmer DSC-2 interfaced with a microcomputer). Each sample is heated at a constant rate of 20K/min from a temperature of 290K to the desired isotherm temperature, For a full heat scan, in which the reaction is completed during heating, the sample is heated to a temperature between 550 K and 600 K The heating process is repeated three times and the data of the third anneal is subtracted from the data of the first anneal for analysis. After heating, the reacted Cu/Sn composites are again characterized by x-ray diffraction analysis to identify the products of the reaction. RESULTS Sample Characterization and Enthalpy Measurements The heat flow as a function of temperature for two full heat scans is shown in Fig. 1(a) and 1(b). The sample of Fig. 1(a) has a bilayer thickness of 29 nm and that of Fig. 1(b) a bilayer thickness of 92 nam For both scans a significant heat flow signal is first observed at a temperature of approximately 370 K Between temperatures of 380 K and 420 K, the heat flow signal levels off and remains almost constant. At higher temperatures the scans contain a large exothermic peak centered at 440 K. This corresponds to previously reported temperatures where Cu 3 Sn is observed to grow in bulk Cu/Sn diffusion couples. 1-6 0
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350
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(a) 375
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(a) 29 nm and (b) 92 nm.
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-40 350
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samples had an average stoichiometry of Cu 3 Sn and were produced by thermal evaporation.
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Figure 1. The heat flow as a function of temperature for a constant heating rate of 20 K/min, measured by means of differential scanning calorimetry. The samples corresponding to the scans were Cu/Cu 6Sn 5 multilayered composites with bilayer thicknesses of
I(b)
375
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TenMrerature (K) The growth of Cu 3Sn in the thin film samples corresponding to the scans in Figs. 1-4 was confirmed using x-ray diffraction analysis. After these anneals, all x-ray peaks were indexed to the equilibrium Cu 3Sn phase. In an effort to determine the nature of the low temperature exothermic shoulders in Fig. 1, samples were heated once to a temperature of 425 K and immediately cooled to room temperature. X-ray diffraction analysis of these samples revealed Bragg peaks corresponding to Cu and Cu6Sn5 , as before heating, with new Bragg peaks that were 314
indexed to the equilibrium Cu 3Sn phase. It is concluded that the heat flow signals of Figs. 1-4, as well as those of other calorimetry scan
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