Controlling the microstructure from the gold-tin reaction

Abstract

The microstructures from the reaction between Au and Sn under different conditions were studied. A Sn/Au/Ni sandwich structure (2.5/3.752 µm) was deposited over the Si wafer. The overall composition of the Au and Sn layers corresponded to the Au20Sn binary eutectic (wt.%). When the reaction condition was 290°C for 2 min, the microstructure produced was a typical two-phase (Au5Sn and AuSn) eutectic microstructure over Ni. In contrast, when the reaction condition was 240°C for 2 min, a AuSn/Au5Sn/Ni layered microstructure was produced. In both microstructures, a small amount of Ni was dissolved in Au5Sn and AuSn. When the AuSn/Au5Sn/Ni layered structure was subjected to aging at 240°C, the AuSn layer gradually exchanged its position with the Au5Sn layer and eventually formed an Au5Sn/AuSn/Ni three-layer structure in less than 9 h. The driving force for Au5Sn and AuSn to exchange their positions is for the AuSn phase to seek more Ni. The dominant diffusing species for the AuSn and Au5Sn has also been identified to be Au and Sn, respectively.