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Mechanical Characterization of CuSn Intermetallics for Advanced Flip-Chip Bonding
Keywords: Mechanical characterization, CuSn intermetallic, fracture strength
Continuous shrinking in device dimensions in the integrated circuits requires fine pitch interconnects. In case of a solder bump connection, the height of solder medium has to decrease with the bump size. However, the intermetallic joint that is formed between the solder bump and the under bump metallization (UBM) is not scaled. Therefore, intermetallic compounds comprise a large volume fraction in the small bumps, which strongly influences the overall mechanical behavior of interconnect. An extremely case is the Transient Liquid Phase (TLP) flip-chip bonding, during which all solder is transformed into intermetallic compounds that have much higher melting points than the solder itself. TLP bumping recently drew much attention in 3D chip stacking technology because it enables repeated stacking of additional layers without remelting the bumps at lower levels of the stack. [1-3] Therefore, detailed study of intermetallic compounds is of great importance for the most advanced interconnect technology. In this paper, we investigate the mechanical properties of CuSn intermetallics in the Cu/Sn system. This is because first the Sn-based solder is the most leading Pb-free solder candidate and Cu is one of the common UBM materials, and second there are no consensus on the Young’s modulus and hardness of CuSn intermetallics. [4-7] In addition, the fracture strength of CuSn intermetallics has not been reported yet to the authors’ best knowledge. In order to accurately evaluate these parameters, we apply three different approaches as follows: First, we characterize CuSn intermetallics films by nano-indentation. Here Cu and Sn are deposited sequentially by electroplating, and then are annealed at 200C for more than 100 hours to form Cu3Sn dominated films. These films are then investigated by nano-indentation to achieve the Young’s modulus and hardness. Second, we carry out Cu/Sn flip-chip bonding to form CuSn intermetallic joints and then characterize them also by nano-indentation. Before we characterize the intermetallics that are formed in each bump, we create a series planar surface with the size of Ǿ60 um by shear test. After that, we can precisely load the nano-indenter on the intermetallic surface. Thirdly, unlike the afro-mentioned two approaches, we fabricate a free-standing CuSn intermetallic beam for the micro-tensile test, during which we measure the stress-strain curve. The fracture strength can be obtained directly from this curve. The Young’s modulus can be extracted from the slope of the stress-strain curve, which is quite linear at the early stage. Finally, combining all these experiments, we find the Young’s modulus of CuSn intermetallic is between 75 to 150 MPa, the hardness is about 5 GPa, and the fracture strength is ~980 MPa. [1]J. W. Roman, and T. W. Eagar, Proceedings of the International Society for Hybrid Microelectronics (ISHM) 1992, p. 1. [2]S. Wakiyama, H. Ozaki, Y. Nabe, et al., Proc. of ECTC 2007, p. 610. [3]K. Sakuma, P.S. Andry, B. Dang, et al., Proc. of ECTC 2007, p. 627. [4]R.J. Fields, S.R. Low III, and G.K. Lucey, Met. Sci. Joining, p. 165 (1992). [5]B. Subrahmanyan, Trans. JIM 13, 93 (1972). [6]R.R. Chromic, R.P. Vinci, S.L. Allen, and M.R. Notis, J. Mater. Res. 18, 2251 (2003).
Wenqi Zhang, Senior Researcher
IMEC
Leuven B-3001,
Belgium


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