Detecting buried voids in copper interconnect

In this paper we studied capturing buried void defects in copper (Cu) wires using an electron beam inspection (EBI) system. These are defects of interest (DOI) to integrated circuit (IC) manufacturers because typical defect inspection techniques cannot capture them: optical defect inspection, EBI voltage contrast (VC) mode, and EBI physical detection mode. We used an engineering system to study a copper CMP wafer with a silicon nitride (SiN) cap layer. EBI backscattered electron (BSE) mode captured many dark defects. Defect review using both secondary electron (SE) and BSE modes found that some dark defects were voids exposed to the Cu surface, while others were unique to BSE mode. We did physical failure analysis (PFA) for some of the BSE mode unique DOI. The cross-sectional scanning electron microscope (SEM) images confirmed that the unique DOI were buried voids in copper wires. Monte Carlo simulation results confirmed there is enough contrast for BSE to differentiate between the normal Cu wire and Cu wire with buried void.