XRD and ToF-SIMS study of intermetallic void formation in Cu-Sn micro-connects

An identified reliability challenge of significant importance to Cu-Sn bonding for 3D integration is Cu-Sn intermetallic void formation. Voids, often referred to as Kirkendall voids, form within the inter-diffusional zone between Cu and Sn, more specifically within the intermetallic compound Cu₃Sn. The root-cause(s) of void formation is not well understood, therefore this study is designed to understand under what conditions voids form. The two main hypotheses for the root-causes of void formation are (i) the imbalance of diffusion rates between Cu and Sn during the formation of Cu-Sn intermetallic compounds and the resulting residual stresses and (ii) the co-deposition of impurities during Cu electroplating to void formation. Therefore, an ex- and in-situ x-ray diffraction (XRD) study is used to probe the material state as a function of thermal annealing, and a time-of-flight mass spectroscopy (ToF-SIMS) study is used to detect impurities co-deposited during Cu electroplating and to understand the effects of thermal annealing on the impurities' kinetic behaviour.