Active sampling mass spectrometry has been used for process sensing in gate oxide growth by rapid thermal chemical vapor deposition from SiH4 and N2O. Equipment and process behavior throughout the short process cycle were revealed in the detailed time-dependent changes of downstream mass spectroscopic signals. A H2 reaction product was clearly identified during SiO2 deposition for SiH4/N2O ratios of 0.5%–2.0% at 5 Torr total pressure and in the temperature range 750–850 °C. No H2O product was observed, suggesting that the process is dominated by a two-step reaction involving SiH4 pyrolysis and subsequent N2O oxidation of the deposited Si to form SiO2. The evolution of the H2 product signal during a process was then used as a process indicator. The integrated H2 signal was found linearly proportional to the deposited oxide thickness, providing the basis for real-time, noninvasive thickness metrology applications. This work demonstrates that properly configured real-time mass spectrometry is capable of providing not only time-dependent chemical information about system behavior, but also quantitative metrology for the film deposition process. © 1999 American Vacuum Society.