Real-time in situ mass spectrometry has been applied to poly-Si rapid thermal chemical vapor deposition (RTCVD) (from SiH4) on thermally grown SiO2 as a way to determine film thickness at the end of the process and to infer dynamic deposition rate during the process for run-to-run and real-time control applications. Monitoring process ambient at 5 Torr is achieved using two-stage differential pumping of a sampling aperture in the exhaust stream, and a rapid response time (similar to 1 s for a similar to 30 s process cycle) allows for real time sensing of reactant input, product generation, and reactant depletion. Active mass spectrometric sampling of the reaction by-product (H-2 generated by SiH4 decomposition) provides a monitor of the total reaction/deposition rate during poly-Si RTCVD in the range 550-850 degrees C. Product generation as a function of temperature is readily distinguished from reactant cracking fragments by spectral analysis. A well-defined monotonic correlation between the time-integrated H-2(+) product signal and the poly-Si film thickness, determined ex situ by single-point interferometry (Nanometrics), demonstrates that the integrated mass spectrometric signal can provide real-time thickness metrology. In addition, the time-dependence of product and reactant signals provides a real-time indication of detailed equipment behavior during the process.