Highly oriented [111] Cu grains with densely packed nanotwins have been fabricated by direct-current electroplating with a high stirring rate. The [111]-oriented and nanotwinned Cu (nt-Cu) allow for the unidirectional growth of Cu(6)Sn(5) intermetallics in the microbumps of three-dimensional integrated-circuit packaging; a uniform microstructure in a large number of microbumps of controlled orientation can be obtained. The high-density twin boundaries in the nt-Cu serve as vacancy sinks during the solid-state reaction between Pb-free solder and Cu and greatly reduce the formation of Kirkendall (or Frenkel) voids.

Direct Cu-to-Cu bonding was achieved at temperatures of 150–250 °C using a compressive stress of 100 psi (0.69 MPa) held for 10–60 min at 10−3 torr. The key controlling parameter for direct bonding is rapid surface diffusion on (111) surface of Cu. Instead of using (111) oriented single crystal of Cu, oriented (111) texture of extremely high degree, exceeding 90%, was fabricated using the oriented nano-twin Cu. The bonded interface between two (111) surfaces forms a twist-type grain boundary. If the grain boundary has a low angle, it has a hexagonal network of screw dislocations. Such network image was obtained by plan-view transmission electron microscopy. A simple kinetic model of surface creep is presented; and the calculated and measured time of bonding is in reasonable agreement.

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We investigate the growth of Cu films on two different Cu seed layers: one with regular <111>-oriented grains and the other with very strong <111>-preferred orientation. It is found that densely-packed nanotwinned Cu (nt-Cu) can be grown by pulsed electroplating on the strong <111>-oriented Cu seed layer without a randomly-oriented transition layer between the nt-Cu and the Cu seed layer. The electroplated nt-Cu grow almost epitaxially on the seed layer and formed <111>-oriented columnar structures. However, with the regular <111>-oriented Cu seed, there is a randomly-oriented transition layer between the nt-Cu and the regular <111>-oriented Cu seed. The results indicate that the seed layer plays a crucial role on the regularity of <111>-oriented nanotwinned Cu.