Yi-Sa Huang

Unidirectional growth of microbumps on (111)-oriented and nanotwinned copper.

Tiny Tinny Bumps One challenge in moving to three-dimensional integrated circuit architectures is the need for aligned interconnects to join neighboring layers. Hsiao et al. (p. 1007) applied rapid stirring to the direct current electroplating of copper to produce films with oriented copper grains that have a high density of nanotwin defects. The resulting material was an excellent platform for the growth of copper-tin intermetallic compounds in the form of arrays of microbumps potentially suitable for the soldering of electronic components. Oriented copper grains grown using direct-current electroplating serve as a template for intermetallic microbumps. 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 Cu6Sn5 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.

Low-temperature direct copper-to-copper bonding enabled by creep on (111) surfaces of nanotwinned Cu

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.

Study of interaction between Cu-SnAg and Ni-SnAg interfacial reactions by low solder volume in 3D IC packaging

Metallurgical reactions in Ni/ SnAg2.3 solder/ Cu system are investigated by varying the solder thickness from 40µm to 10µm. We found that the growth rate of the interfacial intermetallic compounds (IMCs) strongly depend on the solder thickness. In the Ni/ 40-µm solder/ Cu samples, the (Cu, Ni)6Sn5 IMCs on the Ni side grew slightly faster than those on the Cu side. However, the trend reverses as the solder thickness decreases below 20µm. The (Cu, Ni)6Sn5 on the Ni side even stop growing after 4-min reflow at 260°C in the Ni/10µm solder/ Cu samples. Yet, the IMCs on the Cu side grew thicker than that in Ni/ 40µm solder/ Cu samples. Compositional analysis reveals that the Cu and Ni concentrations in the solder increases with the decreasing in solder thickness. The changes in the Ni and Cu concentration in the solder plays crucial role on the growth rates at the IMCs in the Ni/ SnAg solder/ Cu system.