In-situ monitoring of chemical vapor deposition process by mass spectrometry

A method and apparatus are provided for controlling a CVD process used to deposit films on semiconductor substrates wherein the by-products of the reaction are measured and monitored during the reaction preferably using mass spectrometry and the results used to calculate the concentrations of the by-products and to control the CVD reaction process based on the by-product concentrations. An exemplary CVD process is the deposition of tungsten metal on a semiconductor wafer. A preferred method and apparatus uses a capillary gas sampling device for removing the by-product gases of the reaction as a feed for the mass spectrometer. The capillary gas sampling device is preferably connected to a differential pump.