Grain-scale adhesion strength mapping of copper wiring structures in integrated circuits

The adhesion strength distribution at the interface between damascene copper lines and the cap layers covering the lines was explored. A novel system composed of a dual-beam SEM/FIB equipped with a nanoindenter and an EBSD camera enabled successful evaluation with a resolution of the order of 300 nm, which is the scale of crystal grains. The adhesion strength was found to scatter in a range spanning roughly half to double the average value. Although no clear correlation was observed with the geometry of the copper line, the impact of the copper grain structure beneath the specimens was rather distinct, where the adhesion strength of specimens with grain boundary junctions observed on their footprints was significantly lower than that of specimens without junctions. This result suggests that the microscopic structure of deposited materials may strongly influence the strength of adhesion to neighboring layers. Therefore, the evaluation of local adhesion is important from an engineering point of view as a means for avoiding unexpected fractures at possible weak points and assessing the mechanical reliability of the layered systems.