Fabrication of arrays of (100) Cu under-bump-metallization for 3D IC packaging

Due to the thousands of microbumps on a chip for 3D ICs, the precise control of the microstructure of all the material is required. The nearly <111>-oriented nanotwinned and fine-grained Cu was electroplated on a Si wafer surface and annealed at 400-500 °C for 1 h, many extremely large <100>-oriented Cu crystals with grain sizes ranging from 200 to 400 μm were obtained. The <111>-oriented Cu grains were transformed into super-large <100>-oriented grains after the annealing. In addition, we patterned the <111>-oriented Cu films into pad arrays of 25 to 100 μm in diameter and annealed the nanotwinned Cu pads with same conditions. An array of <100>-oriented single crystals Cu pads can be obtained. Otherwise, single-crystal nano-wire growth displays a process by one-dimensional anisotropic growth, in which the growth along the axial direction is much faster than in the radial direction. This study reported here a bulk-type two-dimensional crystal growth of an array of numerous <100>-oriented single crystals of Cu on Si. The growth process in 3D IC has the potential for microbump applications packaging technology.