Formation, processing and characterization of Co–Sn intermetallic compounds for potential integration in 3D interconnects

Abstract

Cobalt exhibits good wetting by lead-free solders (Humpston, 2010) [1], generates Sn-rich intermetallic compounds (IMC’s) and has lower dissolution in solders than Cu and Ni, thus potentially being a more effective solder diffusion barrier with improved electro- and thermo-migration resistance in interconnect applications (Ishida et al., 1991; Okamoto et al., 2006; Wang et al., 2013; Kanatzidis et al., 2005; Liu et al., 2004; Labie, 2007) [5–7]. In this work, we extend prior data (Vakanas et al., 2013; M.O. et al., 2012; Vakanas et al., 2014) on formation, processing, characterization and quantification of Co–Sn IMC phases generated upon deposition, reflow and aging of electroplated and solder paste screen printed samples. Cobalt oxides growth (vs. Cu) was characterized by X-ray Photoelectron Spectroscopy (XPS) as a function of multiple reflows. Cobalt oxides cleaning by microwave Ar/H2 plasma was also demonstrated. Co–Sn IMC growth rate (vs. Cu–Sn and Ni–Sn) was quantified after one reflow cycle and aging in solid-state temperature range of 180–220°C. Relevant kinetic parameters were calculated assuming a diffusion-driven, parabolic growth model. IMC phases were identified by Energy-Dispersive X-ray Spectroscopy (EDS) verifying Sn-rich Co IMC’s. Young’s modulus (E) and hardness (H) were measured by nanoindentation indicating that Co–Sn IMC’s are more compliant and softer than Cu–Sn IMC’s. Implications of the findings for material selection in 3D micro-bumping and stacking integration are discussed with a future outlook and recommendations to improve interconnect reliability.