Extremely anisotropic single-crystal growth in nanotwinned copper

By electroplating of nearly unidirectionally -oriented nanotwinned and fine-grained Cu on a Si wafer surface followed by annealing at 400–500 °C for up to 1 h, we grew many extremely large -oriented single crystals of Cu with sizes ranging from 200 to 400 μm. By patterning and annealing the nanotwinned Cu films, we grew an array of -oriented single crystals of Cu with sizes ranging from 25 to 100 μm on Si. In comparison, single-crystal nano-wire growth is a one-dimensional anisotropic growth process, in which the growth along the axial direction is much faster than in the radial direction. We report here a bulk-type two-dimensional crystal growth of an array of numerous -oriented single crystals of Cu on Si. This growth process has the potential for microbump applications in three-dimensional integrated circuit-packaging technology for hand-held consumer electronic products. Three-dimensional circuits: Better bonding with nanotwins Researchers in Taiwan and the United States have developed a high-precision method of growing copper crystals for use in three-dimensional integrated circuits. Chih Chen from National Chiao Tung University and colleagues lightly annealed a peculiar type of copper containing densely packed ‘nanotwins’ — boundaries within a crystal grain that separate regions with mirror image symmetry — to synthesize surprising quantities of microscopic copper grains containing a single crystallographic orientation. The researchers then demonstrated the potential of their technique by using lithographic patterning to generate single-crystal copper arrays, with on-demand grain sizes, on silicon surfaces. This material may aid in the development of ‘microbumps’ used to solder transistor elements in three-dimensional circuits. These tiny connections may have early failure when grain sizes are poorly aligned — a problem easily overcome with the team's crystal growth strategy. By electroplating the unidirectionally -oriented nanotwinned and fine-grained Cu on a Si wafer surface and followed by annealing at 400–500 °C up to an hour, we grow a number of extremely large -oriented single crystals of Cu of sizes from 200 to 400 μm, as illustrated by the left figure. By patterning the nanotwinned Cu film (middle figure), we grow an array of -oriented single crystals of Cu of sizes from 25 to 100 μm on Si after the annealing, as shown in the right figure.