A chemical vapor deposition furnace for depositing silicon nitride films is disclosed. The furnace having a process chamber elongated in a substantially vertical direction and a wafer boat for supporting a plurality of wafers in the process chamber. A process gas injector inside the process chamber is provided with a plurality of vertically spaced gas injection holes to provide gas introduced at a feed end in an interior of the process gas injector to the process chamber. A valve system connected to the feed end of the process gas injector is being constructed and arranged to connect a source of a silicon precursor and a nitrogen precursor to the feed end for depositing silicon nitride layers. The valve system may connect the feed end of the process gas injector to a cleaning gas system to provide a cleaning gas to remove silicon nitride from the process gas injector and/or the processing chamber.