In this paper, a novel inspection mode of electron beam inspection (EBI) that can effectively detect buried voids in tungsten (W) plugs is reported for the first time. Buried voids in metal are a defect of interest (DOI) that cannot be captured by either optical inspection or traditional EBI modes. The detection of buried voids is achieved by using energetic electron beam (e-beam) with energy high enough to penetrate into metal and reach the buried void. By selecting desired secondary electrons to form the inspection images, strong contrast between the defective tungsten plugs and normal ones can be achieved. Failure analysis was performed on the DOI that is unique to this new EBI mode. After optical microscope locating and laser marking, we successfully recaptured DOI with scanning electron microscope (SEM) and capped the DOI with e-beam assisted platinum (Pt) deposition. Later a dual-beam focused ion beam (FIB) system was used to re-locate the Pt-capped DOI and prepare samples for transmission electron microscope (TEM). TEM images confirmed the unique DOI were buried voids in the metal plugs, which could affect resistance of interconnect in integrated circuit (IC) chip and impact the IC yield.

The Voltage Contrast (VC) [1-3] and Dopant Contrast [4-7] in Scanning Electron Microscopy (SEM) [8] have been widely used in the Silicon (Si) semiconductor manufacturing field to localize the failure site from plane-view and inspect the doping profile along cross-section with spatial resolution in the nanometer (nm) range. In this article, we demonstrate how the surface effect, such as topography or material variation, impacts the conventional prediction for the voltage and dopant contrast in the SEM images. The mechanisms and applications for the SRAM and real products are described.

In-line e-beam inspection is performed to detect dark voltage contrast (DVC) defects on normally bright W-plugs. Cross-sectional SEM and TEM in an FA lab verified that the different gray level values (GLV) of DVC defects are caused by different resistances of the W-plugs. We found that DVC defects with lower GLV (GLV1) are W-plugs that are open and almost open. DVC defects with GLV2 are caused by partially open W-plugs and in-plug voids.