The nucleation and growth characteristics of CVD-W on Ti/TiN barrier layers with SiH4 and H2 reduction chemistries are presented. In particular, the reaction between WF6 (precursor used for depositing W) and the underlying Ti of the barrier stack was studied to better understand the chemistry of ‘volcano’ formation - a phenomena that causes severe defects in the deposited W film. Ti/TiN processing parameters and stack thicknesses were varied, along with the CVD-W deposition chemistry (gas flows, pressures, temperatures and times), to evaluate film properties and characteristics after SiH4 passivation, nucleation and full W plug deposition. The analysis was augmented with cross-sectional Scanning Electron Microscopy (SEM) on short-loop testers and films were characterized using Rutherford Backscattering Spectroscopy (RBS), Secondary Ion Mass Spectroscopy (SIMS), X-ray Diffraction (XRD) and Auger Electron Spectroscopy (AES) techniques. Several processing conditions are recommended for ‘volcano-free’ and ‘defect-free’ CVD-W films.