Microstructure control of unidirectional growth of η-Cu6Sn5 in microbumps on 〈1 1 1〉 oriented and nanotwinned Cu

Anisotropic microstructure is becoming a critical issue in microbumps used in 3-D integrated circuit packaging. We report here an experimental approach for controlling the microstructure of η-Cu6Sn5 intermetallic compound in microbumps by using 〈1 1 1〉 oriented and nanotwinned Cu pads as the under-bump-metallization. By electroplating arrays of large numbers of 〈1 1 1〉 oriented and nanotwinned Cu pads and by electroplating the Sn2.3Ag solder on the pads, we form η-Cu6Sn5 in the reflow at 260 °C for 1 min. The η-Cu6Sn5 showed a highly preferential growth along the 〈0 0 0 1〉 direction. As reflow time is extended, the preferred texture of η-Cu6Sn5 changed to {21¯1¯3}. The results indicate that we can control the uniform microstructure of η-Cu6Sn5 intermetallic by controlling the microstructure of the Cu under-bump-metallization.