Dynamical X-ray Microscopy Study of Stress-Induced Voiding in Cu Interconnects

High-resolution x-ray imaging with a spatial resolution in the 20 nm range offers unique capabilities for process development in semiconductor industry. Buried copper on-chip interconnect structures can be studied with excellent element specific contrast. In addition, it is possible to obtain 3-D views of fully embedded copper interconnect lines and vias. Moreover, transmission x-ray microscopy (TXM) is a very suitable technique to study dynamical, reliability-limiting processes like electromigration (EM) or stress-induced voiding (SIV). Stress-induced void formation is investigated in dual damascene Cu/SiOx and Cu/low-k interconnect stacks. The initial pre-stressing of the about 1.5 mu m thick focused ion beam(FIB)-prepared cross-sections was performed by heating at a temperature of 175 degrees C for 50 hours. Subsequently, a series of x-ray images was recorded using images after every five hours of heating at the same temperature. The dynamical SIV experiments show for the first time directly that voids are formed and grown further underneath the via.