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Quadrupole mass spectrometry has been used to monitor reactant and product partial pressures in a selective W chemical vapor deposition process. A 4/1H₂/WF₆ molar reactant ratio was used to produce W films on Si wafers, at 67 Pa (0.5 Torr) total pressure, and for wafer temperatures around 400 °C. A relatively fast response time (~4 s) sensor system sampled gas directly from a commercial Ulvac ERA-1000 reactor in order to minimize the effect of wall reactions. The signal from the volatile HF product, integrated over the deposition cycle, and corrected for contributions from reactions in the ion-source region of the quadrupole and for sensor drifts, was found to vary linearly with the weight of the W film deposited, to within an uncertainty of ~7%. This provides the basis for real-time, noninvasive thickness metrology to drive process control. Depletion of both H₂ and WF₆ reactants was observed. The time integral of the H₂ reactant depletion was also linearly related to the film weight, though the data exhibited a somewhat larger scatter due to the low conversion efficiency of the process. In addition, volatile SiF₄ and SiHF₃ products of the initial rapid W nucleation reaction on the Si surface were clearly observed, indicating that initial surface conditions may be monitored in real time under selective growth conditions. ©2000 American Vacuum Society.