Evaporation of Ag Nanotwinned Films on Si Substrates With Ion Beam Assistance

High-density Ag nanotwinned films with a very thin twin spacing of about 6.05 nm have been deposited by electron beam evaporation for the first time. With ion beam assistance, Ag thin films with a highly <111> textured structure can be deposited on Si substrates pre-coated with Ti/Ag (111) seed layers. In contrast, the Ag films deposited without ion beam assistance exhibit mostly fine grains with about $0.8~\mu \text{m}$ grain size and random annealing twins. The nanotwinned Ag films have a nanoindentation hardness of 1.23 GPa, about five times higher than that of Ag films deposited without ion beam assistance. For the mechanism of enhancing nanotwin formation in Ag films deposited with ion beam assistance, the energy and stress perspectives are both discussed. The formation of Ag nanotwins is ascribed to sufficient stress induced by ion bombardment under suitable parameters, which leads to the generation of twinning partial dislocations, formation of twin boundaries, and release of strain energy.