In contrast to most wet cleans, cleaning reactions in the gas phase may be significantly affected by the surface hydration of the incoming wafer. Additionally, effective termination of the cleaning reaction, which in wet cleans is accomplished through a deionized water rinse, is not as straightforward in the case of dry cleaning processes. In this experiment, UV/ nitrogen exposure and slight oxide etching (30 Angstrom) were found to control the initiation of gas-phase oxide etching reactions by reducing the level of physisorbed and chemisorbed moisture on the oxide surface, respectively, while UV exposure, pressure, temperature, and ambient composition were used to control termination of surface reactions following silicon surface exposure to the UV/Cl-2 treatment.