Formation mechanism and orientation of Cu3Sn grains in Cu–Sn intermetallic compound joints
Introduction
Three-dimensional (3D) package is generally considered to be the primary packaging technique in the coming period due to its low power consumption, low signal latency and high density integration [1], [2], [3], [4]. Through Silicon Via (TSV) is one of the most important technologies for 3D IC integration [5], [6].
As a kind of low temperature bonding method, solid–liquid-inter-diffusion (SLID) bonding provides low thermal stress, high melting point and highly reliable electrical interconnection for TSV technology [6]. Also the Cu/Sn/Cu system has been widely used in 3D integration. At present, the diameter of a TSV joint is about 10–20 μm [7] and will continue to decrease following the trend of higher-density integration. As a result, the joints are entirely composed of intermetallic compounds (IMCs) and the pads may be comprised of only one grain. Some findings recently have revealed that Cu6Sn5 grains had a preferred orientation with polycrystalline and single crystal Cu substrates [8], [9], [10], [11], [12]. Gong et al. [10] found that the scallop-type Cu6Sn5 grains forming on polycrystalline Cu substrate had a preferred orientation of the Cu6Sn5 (0001) plane being parallel to the Cu substrate. Results from Zou et al. [11] had shown that the prism-type Cu6Sn5 grains forming on (001) and (111) Cu substrates were elongated along two perpendicular directions and along three preferred directions with 60° angles. And this result was consistent with what Suh et al. [12] had obtained before. Shang et al. [13] found preferential growth of Cu3Sn at the interface of SnBi/(100) Cu substrate by TEM. However, the preferred orientation was quite different from that found in this paper. Since anisotropic properties of Cu6Sn5 and Cu3Sn grains [14], [15], [16] can largely influence the mechanical and electronic behaviors of Cu–Sn IMC joints, this paper investigated orientation of Cu3Sn in Cu–Sn IMC joints on polycrystalline and on single crystal Cu substrates separately by the electron backscattered diffraction (EBSD) method.
Section snippets
Experimental materials and method
In this study, polycrystalline oxygen-free Cu and (100), (111) single crystal Cu with the sizes of 3×3×1 mm3 were employed as Cu substrates and 99.9% pure Sn foils with the thickness of 30 μm were used as solders to fabricate the sandwich structure Cu/Sn/Cu joint. Before soldering, the surfaces (3×3 mm2) of the Cu substrates were grinded and polished to ensure the Cu/Sn bonding interfaces were clean and smooth. After being aligned with a specially designed clamp, the joints were bonded by SLID
Results and discussion
Fig. 1 shows the cross-sectional SEM images of (100) Cu/Sn/Cu samples bonded at 300 °C for different time. In the beginning (Fig. 1a), Cu6Sn5 and Cu3Sn layers near the residual Sn formed on the original Cu/Sn interfaces. The morphology of Cu6Sn5 grains was between prism-type and scallop-type. At the same time, Kirkendall voids were found in Cu6Sn5 layers adjacent to the Cu3Sn/Cu6Sn5 interfaces. With further reaction (Fig. 1b), the thicknesses of Cu6Sn5 and Cu3Sn layers increased until the
Conclusions
During the solid-state reaction of Cu–Sn IMC joints, columnar Cu3Sn grains grew in clusters along directions parallel to Cu6Sn5 grain boundaries. When the Cu3Sn grains from opposite sides came into contact, grain growth stopped. Cu3Sn did not show grain coarsening behavior over bonding time at 300 °C and some small equiaxed Cu3Sn grains were found in the joints. Cu3Sn grains had preferred orientation of Cu3Sn (100) being parallel to Cu substrate, whatever orientations of the Cu substrates were.
Acknowledgments
Authors are grateful to the financial supports from the National Science Foundation of China (Grant no. 51075103).
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