Microstructural and mechanical characteristics of Cu-Sn intermetallic compound interconnects formed by TLPB with Cu-Sn nanocomposite

Transient liquid phase bonding (TLPB) is a promising technology for three-dimensional integration of circuits (3D IC), but it can be slow and less productive. A novel Cu-Sn nanocomposite interlayer (Cu-Sn NI) composed of Sn matrix with an embedded Cu nanowire array prepared by electrodeposition can significantly accelerate the bonding process, approximately by 20 times. Bonding time with a Cu-Sn NI can be as short as-2 min to achieve a full Cu-Sn intermetallic compound (IMC) joints, whereas it can take-60 min with a pure Sn interlayer of the same thickness under the same bonding conditions (250 degrees C). Unlike the columnar Cu6Sn5 grains commonly formed with Sn interlayer, refined equiaxed Cu6Sn5 grains with an average size of-1.6 mu m are found to be formed with Cu-Sn NI. Such grain refinement has significantly contributed to the improvement of shear strength of IMC joints formed with Cu-Sn NI (23.1 +/- 3.3 MPa), higher than those bonded with pure Sn interlayer (17.9 +/- 2.1 MPa). The underlying mechanisms of the new TLPB process and the formation of finer microstructure when bonding with Cu-Sn NIs are also illuminated and validated based on the experimental observation.