Mechanism of Oxidation of Si Surfaces Exposed to O2/Ar Microwave-Excited Plasma

Plasma oxidation of 200-mm-diameter Si wafer surface at low temperature (400 °C) with use of high-density microwave plasma in O2/Ar gas was investigated. Dependence of the time-averaged oxidation rate on the percentage O2 showed a maximum value of 0.95 nm/min at a few % O2 in Ar. However, the measured O radical density was considerably low in this condition, and hence the O radical is not considered to be a key species in the plasma oxidation. A good correlation was found between the oxidation rate and the electron density; the oxidation rate averaged for 10 min discharge is proportional to the square root of the electron density measured near the Si wafer. The experimental results are basically explained by the Jorgensen-Mott model: in a presence of electron flux from plasma, the O2 molecule adsorbed on SiO2 surface is dissociated into negative ions which are transported to the SiO2–Si interface.