Development of Time-Lag Method on Barrier Properties Evaluation of Ultrathin PVD-Co(W) Films against Cu Diffusion

To realize highly reliable ULSI-Cu interconnects, it is essential to employ liner and diffusion barrier materials that exhibit strong adhesion to Cu and excellent Cu diffusion barrier properties. We previously demonstrated that Co(W) alloy thin films fulfill these requirements; however, a quantitative evaluation of their barrier properties at ultrathin thicknesses (∼1 nm), as used in actual ULSI devices, has not yet been conducted. In this study, we applied the time-lag method, which was originally developed to measure the diffusivity ( D ) of H 2 in porous rubber membranes, to quantitatively evaluate the barrier performance of a 1-nm-thick Co(W) film fabricated by magnetron sputtering (physical vapor deposition, PVD). The time-lag method enabled the measurement of Cu diffusion coefficients at various temperatures. The results revealed that the 1-nm-thick PVD-Co(W) films exhibited excellent barrier performance, characterized by a high diffusion activation energy of 2.1 eV, indicative of lattice diffusion. These results are comparable to those obtained for a 20-nm-thick PVD-Co(W) film, suggesting that reducing the film thickness does not compromise its barrier properties. This study demonstrates the applicability of the time-lag method for quantitatively assessing the barrier properties of ultrathin diffusion barrier layers.