Flip-chip silver joints between Si chips and Cu zubstrates made at low temperature

Abstract

Direct bonding between large silicon (Si) chips and copper (Cu) substrates using pure silver (Ag) as bonding material has been successfully developed. The Si-Ag-Cu direct bonding process was performed in two assembly structures, die attachment and flip-chip interconnect, at low process temperature of 250°C. It is a typical reflow temperature of lead-free (Pb-free) solders. In die attachment structure, the Si chip, Ag foil, and Cu substrate are stacked together and joined in one bonding step. In flip-chip interconnect, Si chips are first electroplated with Ag bumps, followed by direct bonding over Cu substrates. The resulting joints consists of only pure Ag without any intermetallic compound (IMC) layers because no molten phase is involved in bonding process. Due to lack of molten phase, it is possible to make flip-chip joints with high aspect ratio. Pure Ag joints, functioning as a strain buffer, can plastically deform without breakage to relieve shear stress caused by coefficient of thermal expansion (CTE) mismatch between Si and Cu. Because there is no IMC formation inside joints, any reliability issues associated with IMCs and IMC growth are eliminated. High quality joints are produced in die attachment structure. On one specific cross section of flip-chip interconnect, four out of ten bumps are well bonded without any voids and the other six bumps are partially bonded due to uneven applied load. Compared to conventional Sn-rich solders and silver epoxy, pure Ag joints provide better electrical and thermal performance. Ag also has high melting temperature, i. e., 961°C. This novel bonding process can be applied to a variety of electronic devices that require high thermal performance or high operating temperature.