Different diffusion behavior of Cu, Ni and Zn atoms in Cu/Sn-9Zn/Ni interconnects during liquid-solid electromigration

Abstract

Different diffusion behavior of Cu, Ni and Zn atoms in Cu/Sn-9Zn/Ni interconnects during liquid-solid electromigration (L-S EM) were investigated under a current density of 5.0 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 230 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C. When Cu atoms were under downwind diffusion, L-S EM enhanced the cross-solder diffusion of Cu atoms to the opposite Ni side compared with the liquid-solid reaction case, resulting in the formation of interfacial Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> at Ni/Sn-Zn interface, and its thickness increased at the beginning and then decreased. For the Ni atoms, L-S EM significantly enhanced the diffusion of Ni atoms to the Cu side when Ni atoms were under downwind diffusion, resulting in the formation of a large amount of (Ni, Cu) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (Sn, Zn) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> at the Cu side. Under the combined effect of chemical potential gradient and electronic wind force, the Zn atoms with positive effective charge number would directional diffuse towards Cu side under both downwind and upwind diffusion conditions, as a result, the interfacial Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> formed at the Cu side, and its thickness continuously increased.