Abstract: We have developed a new experimental vacuum beam system to study radical-surface interactions. The beam system is novel because of the ability to make three in situ measurements. First, we can quantitatively characterize the flux of radicals to the surface using a threshold ionization mass spectrometer. Second, we can detect etch/deposition rates, with a quartz crystal microbalance, and surface bond modifications, with attenuated total internal reflectance Fourier transform infrared spectrometer. The measurements are made in situ during the surface exposure to radicals and ions. Third, we can characterize the flux of species coming off the surface using the threshold ionization mass spectrometer. Our experimental studies with the system indicate that the new system is capable of obtaining fundamental insights into radical-surface interactions. The effects of F atoms and Ar+ on porous hydrogen silsesquioxane (HSQ were studied and compared to the effects on blanket SiO₂ films. The parameters studied included substrate temperature, Ar+ incident angle, Ar+ energy, and transitional effect of pulsing Ar+ flux. The observations indicate significant F atom uptake in HSQ pores after F atom exposure and no significant F atom uptake for blanket SiO₂. These observations implied that pores play a significant role in surface chemistries. Si and SiO₂ surfaces were exposed first to un-dissociated c-C₄F₈ with and without Ar+ at the surface, then to a mixture of characterized C_xF_y radical and stable species with and without Ar+. The mixture of reactive neutral species impacting the surfaces included CF, CF₂, CF ₃ and various heavy C_xF_y species. The neutral fluorocarbon flux to Ar+ flux and the energy of the Ar + beam impacting the surfaces were varied. Large C_xF _y species were shown to play an unexpectedly large role in the plasma-surface chemistry. Two types C_xF_y films were exposed to NH₃ and O atoms. It was observed that O atoms spontaneously etched both C _xF_y films with less than 1% reaction probability at room temperature. For NH₃ exposure, our observations indicated a large deposition rate of NH₃ on the surface and a large flux of reaction products leaving the surface. This implied that NH₃ is highly reactive on C_xF_y films.