Effects of metal-cap coverage on electro-migration (EM) tolerance for scaled-down Cu interconnects

Ultra-high electro-migration tolerant Cu interconnect was achieved by full-coverage metal-cap combined with a porous low-k film having a closed-pore structure such as a molecular-pore-stack (MPS) SiOCH film (k~2.5). We found that the key feature for the high reliability is “full coverage” of the Cu surface with Co-based metal-cap without Co-penetration into the porous film. The full-coverage metal cap on the pure Cu interconnect improved EM lifetime drastically by “6000 times”, while a partial-coverage metal cap limits the improvement only by 10 times. The interconnect resistance was kept low within +3.2% increment from the pure Cu one, which is far less than that of a Cu-alloy interconnect. Perfect block of the Cu surface-diffusion by the metal cap takes us into another dimension regarding the Cu interconnect reliability, desirable for deeply scaled-down SoCs below 20nm-nodes and/or MCUs under very high temperature environments.