Study On The Electromigration Of Copper Interconnects With Side Reservoir

The lifetime of a chip is determined by its reliability. With the advancement of process, the line width of the interconnect becomes smaller and smaller. While the current density that goes through the interconnect becomes larger and larger, electromigration has become a serious problem. The traditional method to solve the electromigration issue is increasing the line width, which will take large amount of routing resources. Adding a small reservoir to the via structure can remarkably improve the electromigration. But for traditional end-of-line reservoir, there exists critical reservoir length beyond which EM life time will not be improved any more. Some of current experiments show that side reservoir has better elecromigration resistance than end-of-line reservoir. So the effect of side reservoir on electromigration need to be further studied.In this paper, we compared the electromigration of the copper via structure with end-of-line reservoir and side reservoir respectively. We conduct a numerical simulation method to study the electromigration effect, which including a couple field FEM analysis based on ANSYS, a coordinate mapping method of data points based on interpolation, a gradient and divergence calculating method of temperature and stress based on Matlab. The final atomic flux divergence (Atomic Flux Divergence, AFD) distribution is used to study the effect of side reservoir on electromigration.By analyzing the electromigration effect of cathode via structure with side reservoir, we proposed the physical mechanism of side reservoir's influence on electromigration. It is demonstrated that side reservoir can move the maximum AFD sites from above via location to side reservoir location so that it can reduce the average AFD of above via location and improve EM lifetime further. The root cause for this improvement is that side reservoir can impede stress built-up at above via location.