Study on Effect of Microstructure Evolution on Wafer Warpage for High-Temperature Annealed and Self-Annealed Copper Thin Films

Copper is widely used as an interconnect material in the back-end-of-line (BEoL) because it has high electrical conductivity and good electromigration failure resistance. Different applications, however, require a large number of ultrathick copper metal interconnects with varied line widths. A high wafer warpage is induced in the wafer due to the coefficient of thermal expansion (CTE) mismatch between the copper and the silicon during the BEoL process steps. High-temperature annealing and self-annealing of copper after deposition also result in high wafer warpage during fabrication. In this work, blanket copper thin films, and high-temperature annealed copper damascene interconnects with varied cross sections were investigated for their textural and microstructural evolution. The investigations show the directional changes of texture and grain size of copper cross sections at different annealing conditions. In addition, measurements also show variation in roughness and hardness magnitude for copper cross sections at different annealing conditions. In the case of blanket copper films, measurements confirmed the self-annealing behavior of copper within 48 h after deposition. In order to analyze a relationship between the stress state of the wafer and the microstructural evolution, the wafer warpage was measured after each BEoL process step. Through this investigation, it was found that the nonhomogenous stress concentrations with different cross sections are an important parameter for understanding the warpage change after high-temperature annealing processes. This study, moreover, gives insights into the structural change of the material during different annealing processes.