We have recently built a particle beam mass spectrometer (PBMS) for measuring ultrafine particle size distributions (0.005-0.25 mu m) at low pressures (>100 mTorr). The PBMS is being used to study nucleation and growth in low pressure chemical vapor deposition processes relevant to the production of semiconductor devices. In this article, the function and performance of the PBMS is summarized, and results of measurements made while depositing polysilicon and silicon dioxide films in tube furnaces are discussed. Measurements made during deposition of polysilicon films showed that there was a critical reactor pressure below which particles were not present; this critical pressure varied in proportion to the residence time in the reactor, and was insensitive to reactor temperature. Above the critical pressure, however, the concentration of particles produced was sensitive to reactor temperature. The average particle size was in the 0.003-0.03-mu m-diam range, with concentrations of similar to 10(4) cm(-3). In contrast, particles produced during the deposition of oxide films in a low-temperature oxide (LTO) furnace were considerably larger (0.1-0.3 mu m) and were present at lower concentrations (similar to 10(3) cm(-3)). These differences suggest that the chemical mechanisms of particle formation and growth in the two systems are quite different. Simultaneous measurements of particle accumulation on witness wafers during the LTO experiments showed a reasonable correlation with PBMS data, thereby illustrating the relevance of PBMS measurements to device fabrication.