We investigated the effects of tungsten silicon nitride/tungsten silicide (WSiN/WSix) barrier layer thickness on its barrier capability. WSiN was obtained by nitridizing the WSix surface with electron cyclotron resonance nitrogen plasma. The total thickness of the WSiN/WSix barrier layer was reduced by thinning the initial WSix layer. When 5-nm-thick WSix was nitridized, the N and Si contents in the WSiN/WSix layer became smaller than when WSix initial thickness was 20 nm. This barrier layer diffused into the copper (Cu) layer when annealed, and did not act as a barrier layer. On the contrary, a WSiN/WSix barrier layer formed by nitridizing 10-nm-thick WSix showed good barrier capability against Cu diffusion. We evaluated the leakage current between Cu damascene interconnections with this barrier layer and found that this barrier layer formed on-the trench side wall prevents Cu diffusion when the thickness on the side wall is over 10 nm.