Nanovoid formation mechanism in nanotwinned Cu.

Nanotwinned metals have been intensely investigated due to their unique microstructures and superior properties. This work aims to investigate the nanovoid formation mechanism in sputter-deposited nanotwinned Cu. Three different types of epitaxial or polycrystalline Cu films are fabricated by magnetron sputtering deposition technique. In the epitaxial Cu (111) films deposited on Si (110) substrates, high fractions of nanovoids and nanotwins are formed. The void size and density can be tailored by varying deposition parameters, including argon pressure, deposition rate, and film thickness. Interestingly, nanovoids become absent in the polycrystalline Cu film deposited on Si (111) substrate, but they can be regained in the epitaxial nanotwinned Cu (111) when deposited on Si (111) substrate with an Ag seed layer. The nanovoid formation seems to be closely associated with twin nucleation and film texture. Based on the comparative studies between void-free polycrystalline Cu films and epitaxial nanotwinned Cu films with nanovoids, the underlying mechanisms for the formation of nanovoids are discussed within the framework of island coalescence model.