Diffusion and gettering mechanisms of W and Fe in 300 mm silicon substrates

Abstract

Metallic contamination is a major reliability issue in CMOS image sensor processing, where recombination-active impurities can increase dark current and pixel defects. In this work, the behavior of tungsten and iron is studied in several 300 mm silicon-based stacks under the same annealing conditions characteristic for image sensor processing. A combined analysis is used to separate diffusion, segregation, gettering, and oxidation effects. Tungsten is found to be trapped only in the SOI substrate, where it accumulates near the Si/SiO 2 interface, indicating limited diffusion and interface-related trapping. Meanwhile in all the other stacks, tungsten forms surface oxidized species. In contrast, iron is trapped only in the poly-Si stack, near the poly-Si/Si interface where boron accumulates. A radial non-uniformity is also observed, with the highest gettering efficiency at half-radius of wafers corresponding to a non-homogeneity in crystalline defects. Iron and tungsten being characterized by different redistribution mechanisms, these results show that the successful gettering solution must be adapted to both the contaminant species and the stack architecture.