Microstructure evolution of Ru layer deposited in N-2 gas has been investigated and compared with that deposited in Ar gas for Cu diffusion barrier application. Deposition of Ru in N-2 results in high N dissolution, seemingly uniformly distributed in the Ru layer. The film microstructure is a mixture of nanocrystals and amorphous Ru instead of the columnar structure observed when deposited in argon. Annealing at T > 250 degrees C causes N out-diffusion and crystallization of the amorphous Ru. Voids were observed at Cu/Ru interface in Ru deposited in N-2 when annealed at T >= 275 degrees C as a consequence of N out-diffusion. (c) 2007 The Electrochemical Society.