Effects of anisotropic Beta-Sn alloys on Cu diffusion under a temperature gradient

• Main Author: 徐煒能
• Other Authors: Ouyang, Fan-Yi
• Format: Others
• Language: en_US
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/33735833478425329300

碩士 === 國立清華大學 === 工程與系統科學系 === 102 === As the packaging technology transitions toward the three-dimensional integrated circuit (3D IC), a larger temperature gradient is expected to be established when the heat is dissipated from the surface of the stacking module. Furthermore, one solder bump consists of only few grains when the size of the solder bump shrinks into few micrometers. The crystal structure of β-Sn is known to be body-centered tetragonal (BCT) structure and it is anisotropic. Therefore, the effect of anisotropy of Sn is an important issue in the reliability of next-generation packaging technology. We study the samples with a structure of Cu/Sn3.5Ag/Cu at a presence of temperature gradient. The experimental results and theoretical analysis indicated that thermomigration of Cu was strongly affected by anisotropic structure of Sn. When the c-axis of Sn grain was parallel to the direction of temperature gradient, a large thermomigration flux was induced and Cu atoms were driven to migrate from hot end to cold end. The migration resulted in a prominent asymmetrical feature in microstructure; serious dissolution of intermetallic compounds (IMC) and severe consumption of Cu substrate occurred at hot end whereas accumulation and growth of IMC were observed at cold end. On the other hand, when c-axis of Sn was perpendicular to temperature gradient, thermomigration was strongly mitigated due to a small induced flux. A near symmetrical feature in microstructure was found. Neither dissolution of hot-end IMC, nor abnormal growth of cold-end IMC was observed. The morphology of hot-end interface was discussed in more details. Different from the serrated dissolution often resulted from electromigration, a relative flat interface remained after thermomigration. The Gibbs-Thomson effect and thermomigration were taken into account to explain the morphological change and IMC dissolution at hot end. Furthermore, the immobile Ag3Sn at the presence of the temperature gradient was found to suppress the thermomigration of Cu.