Study on liquid-solid electromigration in Cu/Sn-9Zn/Cu interconnect using synchrotron radiation real-time in situ imaging technology

Synchrotron radiation real-time in situ imaging technology was conducted to investigate the interfacial reaction in line-type Cu/Sn-9Zn/Cu interconnect under a current density of 1.2×10
⁴
A/cm
²
at 250°C. A reverse polarity effect, i.e., the intermetallic compound (IMC) layer at the cathode grew continuously while that at the anode was restrained, was observed in Cu/Sn-9Zn/Cu interconnects. With increasing liquid-solid electromigration (L-S EM) time, the initial interfacial Cu
₅
Zn
₈
gradually transformed into Cu
₆
(Sn, Zn)
₅
at the anode, while into a mixture of Cu
₅
Zn
₈
and Cu
₆
(Sn, Zn)
₅
at the cathode. This provided a clear evidence that Zn atoms were migrating from the anode toward the cathode undergoing L-S EM, which is different from the normal diffusion behavior of atoms. Since there is no back-stress undergoing L-S EM, it is deduced that the effective charge number (Z*) of Zn atoms became positive, resulting in the directional migration of Zn atoms from the anode toward the cathode under electron current stressing.