A multilayer structure [TiN(5 nm)/interlayer metal(2 nm)/TiN(5 nm)] has been evaluated as a diffusion barrier between copper and silicon. Three metals (M=Ru, Cr, and Zr) were tested as interlayer materials for the multilayer diffusion barrier scheme, expecting that they will stuff the TiN grain boundaries by forming metal oxides therein. The diffusion barrier performance was found to be the best when the Zr was used as an interlayer material. X-ray diffractometry showed that the Zr interlayer diffusion barrier failed only after annealing at 800 °C for 30 min due to the formation of copper silicide while the reference sample without a metal interlayer failed when annealed at 600 °C. The insertion of Cr also was also found to enhance the diffusion barrier performance, whereas, Ru has little effect on the improvement of the barrier performance. High-resolution transmission electron microscopy and energy dispersive spectroscopy showed that Zr significantly diffused into TiN grain boundaries while Ru did not diffuse during annealing. An explanation of the variation of the diffusion barrier performance with the change of an interlayer metal is given in terms of Gibbs free energy change of the respective metal–oxide formation. © 2003 American Vacuum Society.