Enhanced thermal stability by heterogeneous interface and columnar grain in nanoscale Cu/Ru multilayers

Abstract

The thermal stability of nanoscale Cu/Ru metallic multilayers with different modulation ratio (a fixed Ru layer thickness of 3 nm and varied Cu layer thickness (h) from 10 nm to 200 nm) was investigated by transmission electron microscope (TEM) analyses and nanoindentation tests. All of the Cu/Ru multilayers retained the layered structures up to 400 °C annealing. The Cu grain growth slowed with Cu layer thickness decreasing, illustrating an unusual size dependence, that is, “smaller is more stable.” Especially the average grain size was maintained as small as 25 nm for the Cu/Ru multilayers with h = 10 nm and the hardness only decreased by 6.5% after 400 °C annealing, which exhibits an excellent stability. The enhanced thermal stability of Cu/Ru multilayers originates from the textured columnar grains and semicoherent heterogeneous interfaces, which have a low interface energy. The amount of textured columnar grains and semicoherent heterogeneous interfaces increase with the Cu layer thickness decreasing is responsible for the enhanced thermal stability of Cu/Ru multilayers.