Wafer Level Solid Liquid Interdiffusion Bonding: Formation and Evolution of Microstructures

Abstract

Wafer-level solid liquid interdiffusion (SLID) bonding, also known as transient liquid-phase bonding, is becoming an increasingly attractive method for industrial usage since it can provide simultaneous formation of electrical interconnections and hermetic encapsulation for microelectromechanical systems. Additionally, SLID is utilized in die-attach bonding for electronic power components. In order to ensure the functionality and reliability of the devices, a fundamental understanding of the formation and evolution of interconnection microstructures, as well as global and local stresses, is of utmost importance. In this work a low-temperature Cu-In-Sn based SLID bonding process is presented. It was discovered that by introducing In to the traditional Cu-Sn metallurgy as an additional alloying element, it is possible to significantly decrease the bonding temperature. Decreasing the bonding temperature results in lower CTE induced global residual stresses. However, there are still several open issues to be studied regarding the effects of dissolved In on the physical properties of the Cu-Sn intermetallics. Additionally, partially metastable microstructures were observed in bonded samples that did not significantly evolve during thermal annealing. This indicates the Cu-In-Sn SLID bond microstructure is extremely stable.

Demonstrating 170 °C Low-Temperature Cu–In–Sn Wafer-Level Solid Liquid Interdiffusion Bonding

“…According to the isothermal section, Cu6(Sn,In)5 can't be in local equilibrium directly with Cu and therefore either Cu3(Sn,In) (or CuIn_Delta) and Cu41Sn11 phases should nucleate between Cu6(Sn,In)5 and Cu. However, as has been previously reported, the solubility and stabilizing effect of In to Cu6(Sn,In)5 is significantly higher than that to Cu3(Sn,In) and Cu41Sn11 [35]. Therefore, even though this interface is not thermodynamically stable, the driving force for nucleation and growth of these additional phases is extremely low.…”

“…10) and isothermal (see Fig. 11) sections from Cu-In-Sn ternary system [35,39]. The black dotted line in Fig.…”

“…The objective is to study the possibilities of low-temperature wafer-level SLID bonding down to 170C utilizing ternary Cu-In-Sn metallurgy at bonding temperatures, that are below currently demonstrated solutions [33][34][35]. Additionally, the effect of bonding temperature on the bond quality, microstructure defect formation, and mechanical reliability will be investigated.…”

Low-temperature Metal Bonding for Optical Device Packaging

“…However, their processes are limited by hightemperature (>350 °C) requirement, which induces thermomechanical stress in the device due to variations in the coefficient of thermal expansion (CTE). These residual stresses exist locally, between the substrate and bond, and globally, between the bonded substrates [3], [4]. Furthermore, high-temperature processes are also incompatible with temperature-sensitive materials, such as organic-based lighting or sensors [5].…”

“…The solid-liquid interdiffusion (SLID) bonding method is a wafer-level metal bonding process that could be done at a low temperature [3], [6]. The process involves the interdiffusion of base metals to form an intermetallic compound (IMC) that results in a bonded structure with a higher remelting temperature than the bonding temperature [7], [8].…”