SEM-based X-ray tomography of sub-micrometer defects in 3D integration

Abstract

3D integration is a promising technology to overcome miniaturization challenges and chips densification. It consists in increasing the number of components by realizing vertical pileups of interconnected chips. Solder balls, through silicon vias (TSVs) and copper pillars are widely used for that purpose. It is essential to characterize these metallic structures in order to validate the fabrication process and to ensure an optimal connection between the different chips. Here, X-ray tomography is proposed as a non-destructive technique to investigate metallic interconnections. In particular, the potential of an innovative computerized tomography (CT) system, the X-ray tomography hosted in a scanning electron microscope (SEM), is presented. Since sample preparation is an important step of the X-ray characterization process, the use of a Plasma Focused Ion Beam (P-FIB) to extract large chunks out of the bulk silicon is reported. We expose an original work towards systematic studies: in order to validate a new copper filling chemistry, five samples containing TSVs at various filling rates have been scanned and the 3D results are shown. Chunks containing completely filled TSVs have then been scanned and their resulting 3D reconstructions demonstrate that the instrument is able to detect 500 nm diameter voids. The advantages and limitations of this characterization method are finally pointed out.