The study of the wafer atmosphere during processing has become a key practice in understanding interactions of the process gases with the wafer, internal chamber surfaces and other process gases. For today's ULSI fabrication, Low Pressure Chemical Vapor Deposition (LPCVD) of tungsten has become common for sub-micron contact and via filling and was the chosen process for this investigation. The wafer atmosphere during key process steps of the tungsten LPCVD process has been characterized by mass spectrometry. During the tungsten LPCVD process studied, the TiN surface was first conditioned for tungsten deposition by generating active nucleation sites. This is followed by the actual deposition. After the deposition, wafer backside etch with NF3 plasma, wafer removal and an in situ chamber clean with NF3 and H2 plasmas performed. This paper discusses the mass spectral analysis of the tungsten LPCVD process during the nucleation, backside etch and in situ chamber clean process steps. Additionally, this paper describes benefits of in situ gas analysis.