In situ mass spectrometric sensing has been implemented in a 10 Torr H-2/WF6 W chemical vapor deposition process as a real-time process and wafer state metrology tool. Dynamic sensing through the process cycle reveals HF byproduct generation as well as H-2 and WF6 reactant depletion as real-time quantitative indicators of deposition on the wafer. Thickness metrology is achieved by integrating the HF byproduct signal through the process cycle and comparing it to post-process measurements of film weight. To evaluate the quantitative precision of this metrology, multiwafer runs have been performed under different sets of conditions: (1) fixed process conditions, (2) intentionally introduced run-to-run process temperature drift, and (3) run-to-run deposition time variation. These results demonstrate that real-time thickness metrology is achievable at a level of 1% or better in two application settings: (1) when an essentially fixed process recipe is employed, as in high-volume manufacturing; and (2) when more substantial changes in process recipe are explored, as in a development environment. In situ mass spectrometry presents an attractive option for real-time advanced process control with the prognosis for real-time course correction demonstrated here and its already established benefit to fault detection and classification. (C) 2004 American Vacuum Society.