Current Crowding For Cu Interconnects

Together with the development of silicon-based integrated circuit technology and the continuous shrinkage of critical dimensions, the metallization system for the interconnect structures has shifted from Al(Cu) to Cu in order to obtain reduced resistance–capacitance delay, enhanced Electromigration (EM) resistance, and reduced cost of production.Today, the cost of setting up a modern 12-inch (300 mm) foundry is over billions of dollars. In processing a 300 mm Si wafer, over half of the production cost comes from interconnects. At the same time, over 80% of the defects and yield loss is also attributed to interconnects. Therefore, reliability of Cu interconnects has become one of the most critical problems. As the minimum feature size of copper interconnections shrinks down, the current density is getting higher. Thus, the EM of Cu interconnects is becoming so serious that it must be monitored and controlled carefully during fabrication. The standard accelerated EM test, like SWEAT, is typically done at high temperature and high current density. The data are then extrapolated to obtain the expected lifetime under normal operation conditions. However, typically, the failure mechanisms are not the same under accelerated experimental conditions and under normal operation conditions for Cu interconnects. Furthermore, the traditional Blech test structure will lose its effectiveness, not being able to separating the effect of current crowding from that of back-stress. Therefore, a new test structure and a new monitoring means needs to be developed to effectively measure the reliability decrease of Cu interconnects caused by current crowding.A slit-test structure is designed in this thesis to structurally induce and accelerate current crowding in Cu damascene lines. EM experiments, theoretical calculation and quantitative finite element analysis is applied to study the impact of current crowding on EM-induced mass transport along damascene Cu wires on the interface between Cu/TaN barrier layer.