Focused ion beam time-of-flight secondary ion mass spectroscopy tomography of through-silicon vias for 3D integration

Abstract

Two important issues for the integration of through-silicon via (TSV) interconnects are whether the TSVs are completely filled with Cu and whether unwanted Cu diffusion occurs. To address these issues, specific analysis protocols were developed using custom TSV geometry, sample preparation, and time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis. To obtain three-dimensional (3D) information on the filling of the Cu TSV, an in situ Ga focused ion beam (FIB) was used to cut the TSVs, after which a TOF-SIMS image was acquired and a 3D volume was thereby acquired using a slice-and-view approach. In this way, voids in the TSVs were observed, and defects in the barrier layer were identified. To analyze Cu diffusion, a special square cross-section TSV geometry was used. A protocol was developed to allow depth profiling of TSVs using precise wafer cleaving followed by an ex situ plasma FIB cut that left the sidewalls of the TSVs covered with only a thin SiO2 layer. Dual-beam depth profiling was then used on the cleaved and FIB-polished side of the sample which was placed flat in the sample holder. Several TSVs were simultaneously depth-profiled, and the diffusion profiles of copper were analyzed at different TSV depths.