Fluorine in thermal oxides from HF preoxidation surface treatments

The effect of fluorine added into thermal oxides via preoxidation HF/water and anhydrous HF/methanol surface treatments was investigated. Under typical conditions the resulting fluorine dose is in the range of 8×10 ¹² to 5×10 ¹³ cm ^-2, respectively. In ultra-thin oxides fluorine appears to be, within the limits of secondary ion mass spectroscopy depth resolution, uniformly distributed in a very limited volume of the oxide. In the course of an extended oxidation fluorine remains preferentially at the SiO ₂/Si interface which leads to nonuniform distribution of fluorine in the oxide. The higher concentration of fluorine at the SiO ₂/Si interface causes slight retardation of oxidation kinetics in the thick oxidation regime. No electrical characterization was carried out in this experiment, but it is postulated that because of this pronounced difference in distribution fluorine may have a somewhat different effect on the electrical characteristics of ultrathin and thick oxides.