Due to the gaps created in the surface by Ru and Co doping and the subsequent incorporation of Cu into this layer, these combined barrier/liner materials must be at minimum made up of two atomic layers (one layer of TaN and one layer of doped TaN) to retain their integrity as diffusion barriers. While this is not an issue for W doping, tungsten nitrides have been shown to have inferior barrier properties. For this reason, the "100% W-doped" layer of WN likely cannot be used on its own and will require an additional layer of TaN.