Unusual Morphological Evolution of the Cu Pillar/Solder Micro-joints During High-Temperature Annealing
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-chip technology provides a joining method to efficiently connect the Si integrated-circuit (IC) chips and substrates.[1–3] In flip-chip technology, solder balls are formed on the Si chips in an area array pattern and joined to the substrates with a high input/output (I/O) density. In view of the development trends of miniaturization, portability, and multi-functionalities of consumer electronic products, a more efficient interconnection method using smaller, finer-pitched, and more reliable solder joints is needed. The Cu pillar capped with a solder bump is one of the solutions for next-generation IC interconnection techniques.[4–6] Owing to the Cu pillar, the solder joint possesses many advantages such as adjustable standoff height, high thermal and electrical conductivities, and good anti-electromigration performance.[7,8] Traditionally, a Ni diffusion layer is inserted in between the Cu substrate and Sn-based solders to avoid excessive HSI-KUEI CHENG, Ph.D. Student, and TZENG-FENG LIU, Professor, are with the Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, ROC. Contact e-mail: tfl[email protected] YU-JIE LIN, Ph.D. Student, HOU-CHIEN CHANG, Associate Professor, and CHIHMING CHEN, Professor, are with the Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC. Contact e-mail: [email protected] KUO-CHIO LIU, Director, is with the Backend Operation Division, Taiwan Semiconductor Manufacturing Company, Ltd., Hsinchu, Taiwan, ROC. YING-LANG WANG, Professor, is with the Institute of Lighting and Energy Photonics, National Chiao Tung University, Hsinchu 300, Taiwan, ROC. Contact e-mail: [email protected]. Manuscript submitted December 3, 2014. Article published online March 10, 2015 1834—VOLUME 46A, MAY 2015
growth of brittle Cu-Sn intermetallic compounds (IMCs).[3,9] However, the recent cost-cutting measures allow the Cu pillar interconnection technique to join the Cu pillar directly to a solder cap without a Ni diffusion layer. Inevitably, the Cu pillar solder joints must reconfront the ‘‘old’’ reliability concern due to excessive IMC growth. In this study, Cu pillars capped with Sn-1.8 wt pct Ag solder were subjected to thermal annealing at elevated temperatures to investigate the growth of the Cu-Sn IMCs. Typical Cu6Sn5 and Cu3Sn IMCs formed at the interface between the Cu pillar and solder reacted at 423 K (150 C). When the annealing temperature was raised to 473 K (200 C), surface diffusion was greatly enhanced and the Cu-Sn IMCs grew rapidly around the brim of the Cu pillar. The Cu pillar was consumed at a faster rate around the pillar brim than at the interior IMC/Cu interface. An unusual morphological evolution of the Cu pillar was observed due to such irregular Cu pillar consumption. The pillar top transformed from an initially planar surface into a truncated cone, and finally into a hemisphere. The test samples used in this study were fabricated using a typical Cu pillar solder bumping process. A bilayer of Ti/Cu thin films were
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