Mechanical Reliability Assessment of Cu_6Sn_5 Intermetallic Compound and Multilayer Structures in Cu/Sn Interconnects for 3D IC Applications

In recent decades, three-dimensional integrated circuit (3D IC) design has been developed for the purpose of moving into a new era of More-than-Moore by vertical stacking and heterogeneous integration of functional chips. However, critical issues arise in the paradigm shift from conventional 2D packaging to hierarchical 3D architecture, and one of them is that solders in micron-sized joints may get totally transformed into intermetallic compounds (IMCs) within a short period of time. With the increased volume fraction of IMCs, the mechanical behaviors was not determined by solders as in conventional ball-grid-array joints anymore, and instead IMCs are anticipated to bear great responsibility of mechanical reliability in 3D IC joints. In light of this, this study put emphasis on mechanical reliability assessment of IMC-occupied micro-joints by means of uniaxial micropillar compression. At first, mechanical properties of single-crystalline micropillars of Cu
₆
Sn
₅
was characterized to investigate the effect of grain orientation. Besides, Ni element was found to significantly increase Young's modulus of Cu
₆
Sn
₅
. Furthermore, micro-compression on multilayer structures, such as Cu
₆
Sn
₅
/Sn/Cu
₆
Sn
₅
and Cu
₆
Sn
₅
/Cu
₃
Sn/Cu, was also conducted to gain insight into overall deformation behaviors of Cu-Sn interconnects. Cu
₆
Sn
₅
/Sn/Cu
₆
Sn
₅
cylinders exhibited remarkable plasticity in the manner of interface sliding at around 100 MPa, accommodating at least 10% strain and retaining its integrity without any void formation at the interface. On the other hand, multilayer micropillars of Cu
₆
Sn
₅
/Cu
₃
Sn/Cu underwent plastic deformation through slip of Cu substrate, suggesting sufficient strength of intermetallic compounds and the interfaces. These results are beneficial in evaluation of validity and reliability of 3D IC micro-joints.