High toughness of nanotwinned copper lines after annealing

Abstract

High toughness and tensile strength are favorable factors to advance the applications of Cu lines. In this study, 60-μm-wide nanotwinned Cu (NT-Cu) and regular Cu lines were electroplated and further heat-treated. The annealing-induced effect on the mechanical properties was then investigated. Results showed that the tensile strength of the NT-Cu lines was apparently greater than that of the regular Cu counterparts. Under annealing, the tensile strength of the NT-Cu lines dropped slightly due to thermal-induced detwinning effect. However, such a heat treatment at 150 °C for 3 h greatly enhanced their elongation (7.0%–13.4%) and toughness (32.0 MJ/m3 to 64.9 MJ/m3). No obvious grain growth was observed in the NT-Cu after annealing at 250 °C for 3 h. Meanwhile, grain coarsening occurred in the regular Cu lines leading to significant decrease in the tensile strength, and their toughness was only 6.6 MJ/m3 after the annealing at 150 °C for 3 h. Additionally, the microstructures of the NT-Cu were analyzed by transmission electron microscopy and electron backscattered diffraction. Hall-Petch equation and confined layer slip (CLS) model were also employed to estimate and correlate the yield strength of the NT-Cu lines with twin spacing and average grain size.