Void formation in Cu-Sn Solid-Liquid Interdiffusion (SLID) bonding

Solid-Liquid Interdiffusion (SLID) bonding is based on intermetallics, enabling a thermal stability at temperatures far surpassing the bonding temperature. The Cu-Sn SLID process is performed at a temperature above the melting point of Sn, creating Cu-Sn intermetallic compounds (IMCs) with much higher melting points. A final bondline structure of Cu / Cu₃Sn / Cu ensures high-temperature stability. Voids can appear at different locations in aIMCs SLID bond: Along the original bond interface, in the Cu / Cu₃Sn interface, or scattered within the Cu₃Sn layer. Whereas a certain amount of voids is tolerated, an excessive amount challenges mechanical strength, hermeticity and reliability. In this work we perform wafer-level Cu-Sn bonding, patterned as bond frames on a large number of chips, with equal processing conditions for the whole wafer pair. The bondline thickness is of the order of 10 μm. The bonded wafers are singulated to chips, and selected chip pairs are cross-sectioned by Ar ion milling. The observed void area per unit length of cross-sectioned bond is measured through an image processing procedure. We observe a clear correlation that the amount of voids increases with increasing Cu₃Sn thickness. The class of voids that is subject to this relation is the voids scattered within the Cu₃Sn layer (Kirkendall voids). The same trend is found both for bonds performed at our own laboratory, at our industry partner, and for data extracted from literature. This paper contributes to understanding void formation in SLID bonds. Furthermore, it emphasizes the importance of strict process control in a wafer-bonding process.