Analysis of Peeling Mechanism in Annealed Tungsten Silicide Thin Films

Tungsten silicide (WSix) peeling is a noticeable issue from the manufacturing viewpoint, especially as WSix is widely applied in very large scale integrated circuit (VLSI) fabrication as a gate and interconnecting material, even on a 25-nm-node NAND flash device. In this study, we attempt to determine the margin of the Si/W atomic ratios and the mechanism of WSix film peeling after thermal annealing. The use of an as-deposited WSix > 2.0 film and 30 s rapid temperature annealing (RTP) at least 750 °C for a tungsten-rich WSi1.85 film are the minimal conditions for preventing peeling. Moreover, we found that silicon and phosphorus atoms diffuse upward on WSix films, driven out of the underlying doped poly-Si film, while they acquire sufficient thermal budgets based on energy dispersive X-ray (EDX) analysis and secondary-ion mass spectroscopy (SIMS). During this process, the strong mutual interaction and the rough interface formation between the tetragonal phase of WSix and polycrystalline silicon (poly-Si) enhance the adhesion of WSix films. A strong adhesion might reduce the risk of peeling in the subsequent processes.