Microstructural and mechanical characterization of Cu/Sn SLID bonding utilizing Co as contact metallization layer

Most micro-electro-mechanical systems (MEMS) devices contain fragile moving parts, which poses challenges in process integration of interconnection methods requiring wet-chemistry, such as solid-liquid interdiffusion bonding (SLID). These sensitive MEMS structures can be protected from either the wet-chemistry or plated metals during chemical/electro-chemical plating of SLID interconnection materials; however, this is a complex process. Hence, our previous research has investigated employing a physically deposited contact metallization on the wafers containing functional devices instead of chemically deposited layers (such as electrochemical Cu). Co is a plausible contact metallization layer for Cu-Sn SLID bonding, as it is chemically compatible with Cu–Sn systems. Furthermore, it can positively impact the mechanical reliability of the intermetallic compounds (IMCs) due to the stabilizing of the HT-hexagonal Cu₆Sn₅ phase down to room temperature and suppressing the Cu₃Sn phase formation and subsequent void formation. However, it is critical to control Co thickness to achieve a stable bond based on our previous research on Co bulk in contact with Cu-Sn electroplated silicon chips. To utilize Co as a contact metallization layer for wafer-level Cu-Sn SLID bonding, it is necessary to define appropriate metal layers in the contact metallization stack. Consequently, the present study investigated four different contact metallization stacks including A) 40nmTi/100 nm Co, B) 40 nm Ti/200 nm Mo/100 nm Co, C) 40 nm Ti/500 nm Co, and D) 40 nm Ti/200 nm Mo/500 nm Co. More specifically, we evaluated the microstructural formation and evolution and mechanical performance of the joints. Our study revealed that the Ti/Mo/100 nm Co contact metallization stack for (4 µm)Cu/(2 µm)Sn SLID bonding is composed of IMCs (Cu,2.5at%Co)₆Sn₅ and Cu₃Sn without remaining Sn. Moreover, the joint contained a negligible number of voids even after long-time annealing at 150 °C. Our analysis of the mechanical properties of the joint showed that 1) the tensile fracture surface exhibited a mixture of ductile and brittle fractures, and 2) the Young's modulus of (Cu,2.5at%Co)₆Sn₅ was higher than Cu₆Sn₅, while hardness of (Cu,2.5at%Co)₆Sn₅ and Cu₆Sn₅ were comparable. By employing a Ti/Mo/100 nm Co contact metallization stack, the current study was able to produce 25 µm and 10 µm void free (4 µm Cu/2 µm Sn) microbumps.