The origins of fluorine in dry ultrathin silicon oxides (< 1 nm) were investigated as the incorporation of fluorine in these layers may improve their reliability and interface quality. Oxides were grown at room temperature (RT) by UV/O-2 in an integrated tool combining vapor HF surface preparation and oxide growth in a single chamber. The chemical composition and thickness of the layers were characterized by x-ray photoelectron spectroscopy. It is shown that incorporation of fluorine in these oxides originated mainly from the contamination of the tool (dead spaces and surface adsorption) and from the fluorine left at the wafer surface by the in situ HF process. The levels of these sources of fluorine were about two orders of magnitude higher than from a wet HF dip. No evidence was found for the existence of subsurface fluorine [Kasi et al., Appl. Phys. Lett. 58, 2975 (199 1)] as oxides grown in a separate noncontaminated cell (UV/O-2, 1 atm O-2, RT to 200degreesC) did not show any significant increase in fluorine content as a function of oxide thickness. Hence the amount of fluorine incorporated into oxides grown in cluster tools under similar conditions will depend not only on the method selected for surface preparation but also on whether surface preparation and oxidation are performed in the same chamber or not. The possible benefits of determined fluorine levels are discussed.