Porous Cu3Sn formation in Cu-Sn IMC-based micro-joints

Microbumps are widely used to interconnect stacked chips with through-Si-vias (TSVs) in vertical direction to enable 3D IC integration. Several recent studies reported [1, 2] that a unique porous void formation was found in Cu-Sn microbump particularly in Cu
₃
Sn layer. This porous Cu
₃
Sn formation was only reported in microbump which has constrained joint thickness (<;20 μm) but not identified in flip-chip solder joints and ball grid array (BGA) solder balls. Nevertheless, its impact of porous void formation on mechanical properties of microbump has not been identified yet nor its detailed mechanism of porous void formation. We believe that porous Cu
₃
Sn will be a new failure mechanism in microbump, in which most of its materials constituents are composed of Cu-Sn intermetallic compounds (IMCs). The high volumetric content of Cu
₃
Sn in microbump becomes the necessary condition of this new failure mechanism to occur. In this experimental study, a prototype of chip-to-chip interconnection with an array of CuSn microbumps were fabricated from standard photolithography and thermal compression bonding process. In the microbump, the thickness of the SnAg solder is about 20 μm and the Cu column on both sides is about 20 μm thick. Upon assembly, a reflow process was conducted to induce chemical reactions between Cu-Sn inside microbump to form a layer-type Cu
₃
Sn. Upon wetting and followed by solid state annealing around 220°C for about a day, the remaining solder is reacted completely to form Cu-Sn intermetallic compounds in a multilayered sandwiching structure of Cu-Cu
₃
Sn-Cu
₆
Sn
₅
-Cu
₃
Sn-Cu. Upon further annealing, the middle layer of Cu
₆
Sn
₅
changes to porous Cu
₃
Sn, accompanying by the outer-diffusion of Sn to the side wall of the Cu column to form a coating of Cu
₃
Sn. The mechanism of chemical reactions to form a layer-type Cu
₃
Sn and a porous-type Cu
₃
Sn in microbumps are discussed. In order to evaluate its mechanical properties, parallel experiments of line-type Cu-Sn-Cu micro-joint samples were prepared from custom-machined Aluminum V-groove fixture for uniaxial tensile testing. The line-type micro-joint thickness is controlled about 20 μm to mimic the diameter of micro bump used in chip-to-chip interconnection that we observed porous Cu
₃
Sn void formation under thermal annealing. The similar porous Cu
₃
Sn microstructural evolution was obtained in our line-type CuSn
_2.3
Ag-Cu micro-joint samples. Mechanical properties evaluation of line-type micro-joints can be hence applied in microbump for real die-stacking scenario. Lastly, experimental observations of 3-dimensional volumetric distribution of porous voids in Cu
₃
Sn-based microbumps by synchrotron X-ray tomography are presented. A comparison of the 2-dimensional cross-sectional tomography view of the porous structure with SEM images is also given. The comparison of 3D and 2D identification of porous void distribution is agreeable. The mechanism of this unique porous void formation in microbump and necessary and sufficient conditions of porous Cu
₃
Sn formation are discussed.