Mixed-mode crack initiation at the edge of Cu/Si interface due to nanoscale stress concentration

To investigate the crack initiation behavior under the mixed-mode at the edge of a Cu/Si interface in nanoscale components, fracture experiments are conducted by a novel torsion method using a double nano-cantilever specimen consisting of a 20-nm-thick Cu layer and a Si substrate. This nano-cantilever torsion method can apply the shear stress to the Cu/Si interface with the nanoscale stress concentration through the transverse arm. The mode mixity, which is the ratio of the shear stress to the normal stress, can be precisely controlled by varying the loading position. The interface crack is successfully initiated at the Cu/Si edge for various mode mixities by the developed method. The detailed stress fields along the Cu/Si interface at the critical loads for crack initiation are analyzed by the finite element method, and the stress concentration region near the interface edge in all specimens is within the scale of 100 nm. The critical normal stress and maximum shear stress at crack initiation have a circular relation.