Strength of nanoscale copper under shear

Abstract

Properties and processing of copper have received much interest among the electronics community mainly because of its low electrical resistance. One of its important application fields is in electrical interconnections. Mechanical properties of such nanoscale copper connections are, however, not well known and understood. In this article we study the effects of shear strain and stress in several nanoscale copper systems consisting of about 200 000 atoms with the effective-medium theory and molecular dynamics method. The role of polycrystalline microstructure on the shear strength is analyzed. It seems that the strength decreases as the grain size decreases, in contrast to macroscopic systems. In single crystals we study dislocation initiation and propagation. The results indicate that both sub- and supersonic dislocations may be present in copper. The minimum stress and strain, at which a dislocation initiated in a single crystal was 1.2 GPa and 4.6%. These values can be much larger than the values for a large scale copper sample, since polycrystalline microstructure was seen to decrease the system strength.