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The possibility of ~8nm-thick ruthenium (Ru) grown by ALD as a diffusion barrier to Cu was tested and evaluated in the present study. After additional deposition of Copper (Cu) layer by sputtering method, Cu/~8nm-Ru/Si stack was vacuum-annealed for 30 min at various temperature ranges to appreciate thermal stability of the structure. XRD, XRR, TEM and resistivity measurements were conducted and the failure temperature was revealed to be larger than 500 oC. Ru was proved to be a promising barrier against Cu in future.