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We have used mass spectroscopy to observe and analyze, in real-time, gas phase reactants and product species in plasma enhanced chemical vapor deposition (PECVD) of silicon. We describe a doubly differentially pumped mass spectrometry system to sample the exhaust stream of a large area plasma CVD reactor operating at 0.4–1.5 Torr. We show real-time quantitative analysis of silane consumption and hydrogen production for deposition of hydrogenated amorphous silicon and for pulsed-gas selective area silicon deposition. The ability of mass spectrometry to observe process faults in real time is also demonstrated. Mass spectroscopy is a useful nonintrusive process-state sensor for real-time metrology of plasma deposition, for example, to quantify gas phase species, and to characterize reactions occurring on the substrate surface. Based on our results, we discuss potential advanced manufacturing applications of real-time mass spectrometry in amorphous silicon and selective area silicon plasma deposition, including indirect wafer-state sensing, fault analysis and classification, and run-to-run and real-time process control. © 1997 American Vacuum Society.