Interdiffusion and phase formation in Cu(Sn) alloy films

Abstract

The interdiffusion of Cu and Sn, and the formation and dissolution of Cu-Sn precipitates have been examined for Cu alloy films. Cu(Sn) films were deposited by electron beam evaporation either as Sn/Cu bilayers or Cu/Sn/Cu trilayers, with overall Sn concentrations from 0.1 to 5 at.%. In situ resistance, calorimetry, electron, and x-ray diffraction measurements indicate that η-Cu6Sn5 forms during film deposition. Upon heating, ε-Cu3Sn forms at 170 °C, then this phase dissolves into the Cu matrix at approximately 350°C. Finally, ζ-Cu10Sn3 forms and precipitates after thermal cycling to 500°C. The final resistivity of Cu/Sn/Cu films with more than 2 at.% Sn exceeds 3.5 μΩ cm. However, resistivities from 1.9 to 2.5 μΩ cm after annealing were reached in Cu/Sn/Cu films with less than 2 at.% Sn. Auger and Rutherford backscattering analysis of Cu/Sn bilayers (1 mm thick) showed that the homogenization of Sn in Cu requires annealing in excess of 350°C for 30 min; after annealing, the Sn concentration at the surface is approximately 20 at.%. The interdiffusion of Sn and Cu is inhibited by contamination at the Sn/Cu interface caused by air exposure. © 1998 American Institute of Physics.