Interactions of Low Energy Oxygen Ions with Silicon Surfaces.

The interactions between accelerated ions with energies in the 100-eV range and semiconductor surfaces have been investigated by studying the bombardment of (100) silicon by broad ion beams containing variable mixtures of argon and oxygen. The process is applied to the growth of ultra-thin films of silicon dioxide on unheated silicon substrates. The properties of the obtained oxides are studied by electrical characterization methods, composition analysis and optical measurements. Extremely uniform film thickness is observed over large substrate areas. The high quality of these films is demonstrated through the fabrication of n-channel MOS transistors with gate dielectrics grown at room temperature using oxygen-containing ion beams. The ion beam oxides quickly reach a thickness of 40-60 A after which they are largely insensitive to additional ion doses. The dependence of the film properties on ion energy, ion dose, ion flux, substrate temperature and angle of incidence has been investigated. The oxide growth is governed by the penetration of the ions into the target. The limited thicknesses are explained by the enhancement of the reaction rates only at depths which are currently being reached by the beam. The volume expansion which accompanies the incorporation of the oxygen ions into the target and the conversion of silicon to SiO_2 decreases the effective penetration range of the ion beam. As the oxide-semiconductor interface moves beyond the ion penetration range, the oxidation effectively stops. At higher ion energies the sputtering of the surface by the beam further limits the maximum oxide thickness. A model for the observed processes is developed based on calculating the ion range distribution, the target swelling, the enhanced reaction rates associated with the slowed-down oxygen atoms and the enhanced outdiffusion of excess oxygen through the silicon dioxide film. The range distribution is calculated using the transport theory of linear collision cascades. Very good agreement with the experimental data is observed, indicating that true ion penetration into the target occurs at these energies. The accompanying sputtering of the target surface is determined experimentally. A simple model is proposed for obtaining the sputtering rates of oxides near threshold. (Abstract shortened with permission of author.).