Diffusion behavior of Cu in Cu through-silicon-vias (TSVs) fabricated using low-temperature plasma enhanced chemical vapor deposition (LT-PECVD) has been evaluated. Silicon oxynitride (SiON) barrier films were formed by LT-PECVD at 150 °C. Cu diffusion rate was found to increase with decreasing film density. The critical density and thickness for prevention of Cu diffusion into Si substrate have been estimated. In case of a film with density >60% of the bulk value and/or thickness >100 nm, no change of electrical resistance for stacked wafers containing TSVs was observed after 1000 cycles of thermal stress. According to above results, SiON film formed at 150 °C can be used for the TSV process without any degradation of electrical characteristics and reliability, enabling a reduction in total process temperature in the wafer-on-wafer technology.