The effectiveness of using a tungsten silicon nitride (WSixN) layer as a dopant diffusion barrier is investigated. The WSixN layer can be formed by simple WSix surface nitridation using electron cyclotron resonance (ECR) plasma of nitrogen gas without substrate heating. Structural analysis reveals that the WSixN layer is very thin (5.0 to 6.0 nm) and that it remains intact and has an amorphous structure free from grain boundaries even after annealing at 850 degrees C. The WSixN layer formed with ECR nitrogen plasma is found to act as an excellent barrier to dopant diffusion.