Improving the crack resistance and fracture toughness of Cu/Ru multilayer thin films via tailoring the individual layer thickness

The microstructure evolutions of Cu/Ru multilayered films with equal individual layer thickness h ranging from 1.5 nm to 200 nm were investigated. Based on systematic X-ray diffraction and transmission electron microscopy analyses, as h was varied, structure transition between face-centered cubic and hexagonal close-packed lattice was observed in Ru constituent layer. This structure transition was found to influence the length-scale dependent cracking behavior and fracture toughness of the Cu/Ru multilayers. In direct contrast to previous study that fracture toughness would decrease dramatically when h was reduced below 25 nm, the fracture toughness of Cu/Ru multilayer increased monotonically as h was as small as 1.5 nm. Along with the reduction of h, the coupling effects from the increased interface transparency as well as lattice structure transformation were considered as the main reasons for the enhanced toughness. In this way, the fracture toughness between fcc/hcp multilayers with structure transformation and fcc/bcc multilayers without structure transformation had been compared and discussed in detail.