Annealing effects of tantalum thin films sputtered on [001] silicon substrate

Tantalum is an important barrier material for copper metalization in integrated-circuit fabrication. A nano-structured tantalum film of 550 nm thickness was grown on [001] Si substrate. This Ta/Si system was then annealed from 500 °C to 750 °C under various vacuum conditions. The phases and microstructures of the as-deposited and annealed films were analyzed by grazing incident angle (3°) X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The interface of tantalum and substrate was analyzed by secondary-ion mass spectrometry (SIMS). The inter-diffusion behavior happened at the Ta/Si interface when annealed at a temperature lower than 600 °C, and the tetragonal Ta5Si3 was formed after annealing at 750 °C. In addition, Ta surface oxidation has been detected after annealing in a vacuum as low as 2×10−4 Torr. The increase of oxygen content in Ta films caused higher compressive stress and resulted in film peeling from the substrate. The residual oxygen in vacuum may build up stress in Ta thin films during thermal processes, which can cause major reliability problems in electronic and X-ray optics applications.