Negative Ion Transfer Model of Low-Temperature Oxidation of Silicon Surface by High-Density Microwave Plasma

High-quality thin SiO2 layer is rapidly formed at low temperature (~400 °C) by oxidation of silicon wafer using high-density microwave plasma in argon containing a few % O2. The oxidation rate is proportional to the square root of electron density but hardly correlates with the measured O radical density. Secondary ion mass spectrometry (SIMS) depth analysis of SiO2 layer with 18O as an isotope tracer suggests that predominant mobile species in the oxide layer is not neutral oxygen atom but oxygen negative ion, possibly O2-, as proposed by Jorgensen and Mott. The time evolution of oxidation process calculated in a negative ion transfer model well explains the experimental observation and accounts for the square root dependence on the electron density.