Asymmetry of interface reactions in Ag-Sn thin film couples—In-situ synchrotron radiation study

Abstract

Synchrotron radiation grazing incidence X-ray diffraction was used to study in situ the asymmetry of the interface reactions and the first phase formation at interfaces prepared by Ag evaporation onto Sn thin films (Ag/Sn case) and by Sn evaporation onto Ag thin films (Sn/Ag case). The ordered Ag3Sn phase forms first on both types of interfaces, in agreement with empirical thermodynamic criteria. Interface asymmetry is observed in the growth mode of the Ag3Sn phase, the reaction constant for Ag3Sn formation, the delay time for the appearance of the Ag3Sn phase, and in the grain-size growth kinetics. At the Ag/Sn interfaces, Ag3Sn growth is predominantly interface controlled and forms almost instantaneously, whereas at the Sn/Ag interfaces, it is diffusion controlled and appears 2 min after the beginning of the Sn deposition. The reaction constant for Ag3Sn formation at the Ag/Sn interface (k =0.22 ± 0.02 nm/s) is much larger than the reaction constant at the Sn/Ag interface (k ≪ 0.09 nm/s). The differences in the interface reactions are attributed to the larger diffusion coefficient of Ag in Ag3Sn (DAg = 3.9 ± 0.2 × 10−17 m2/s), compared to the diffusion coefficient of Sn in Ag3Sn (DSn = 6.2 ± 0.1 × 10−19 m2/s), estimated from the in situ X-ray diffraction data. The second intermetallic phase (Ag4Sn), existing in the Ag-Sn system, forms only in the presence of excess Ag atoms at the Sn/Ag interface (realized only after the complete consumption of the Sn bottom layers), because the Ag4Sn phase has a higher nucleation barrier ΔG* and a lower growth rate than the Ag3Sn phase.