The Formation of SiO₂ in an RF Generated Oxygen Plasma: I. The Pressure Range Below 10 mTorr

Abstract

This paper deals with the formation of SiO2 on silicon in an rf generated oxygen plasma in the pressure range below 10 mTorr. A companion paper describes the behavior of the oxide growth process in the pressure range above 10 mTorr. The oxide was formed in an open-tube pumped fused silica system without substrate biasing. Oxide formation rate was studied as a function of substrate temperature, conductivity type, resistivity, distance from the end of the plasma generating coil, oxygen pressure, and generator power output. Based on the results obtained on masked substrates, using the silicon-mask interface as a marker, it was determined that within the limits of experimental error, all the oxide formed in the pressure range studied was deposited on the surface and not a result of oxidation. The rate of deposition increased with increasing power, decreasing pressure, and decreasing wafer to plasma generation zone distance. This rate was temperature independent and was unaffected by substrate parameters. Excellent uniformity was obtained over 56 mm diam wafers, ±3-5%. The properties of the oxide deposited at 600°C and above are only slightly poorer than thermal oxides grown at 1000°-1100°C. They exhibit somewhat higher fixed charge and interface state density while the breakdown strength, etch rate, and refractive index are essentially the same as thermal oxides. A key observation of the plasma process conducted at pressures below 10 mTorr is that the oxide always forms on the silicon surface facing the plasma. This is opposite to the results obtained at higher pressures described in Part II of this study. © 1981, The Electrochemical Society, Inc. All rights reserved.