Growth kinetics of SiO₂ and the ellipsometric study of SiO₂ films on silicon

Abstract

In this study, silicon dioxide was grown at temperatures from 750°C to 11OO°C, and the thickness of the oxide film was measured by both ellipsometry and crosssectional transmission electron microscopy. For oxides thinner than about 250 Å, the ellipsometric thickness measurement gave higher thickness values than the TEM thickness measurement. In dry oxidation, the TEM oxide thickness-oxidation time data followed the linear-parabolic relationship throughout the oxide thickness range for all the oxidation temperatures. A model based onthe reduction of the diffusion coefficient of oxygen or water through the oxide layer byboth the compressive strain in the SiO₂ layer and a thin barrier SiOₓ layer at the oxide-silicon interface is proposed to explain the oxidation kinetics.

Routine ellipsometric measurements of oxide thickness use a simplified model based on a single, optically homogeneous oxide layer on silicon substrate with plane parallel boundaries between different phases and neglect oxide film stress, a non stoichiometric SiOₓ layer and the roughness at the Si-SiO₂ interface. In this study, the influence of oxide film stress, the SiOₓ layer and Si-SiO₂ interface roughness on the ellipsometric measurements are investigated. A modified ellipsometric model which includes the stress-birefringence effect on the oxide film and a intermediate layer at the Si-SiO₂ interface is also proposed to explain the discrepancy between TEM and ellipsometric thickness measurements. This model is tested and the model parameters are calculated by a liquid immersion technique. This technique replaces the surrounding medium of the sample (air) by liquids with different refractive indices, which provides more than one set of ellipsometric readings, Δ and Ψ, to solve the ellipsometric equations. The results show that a more complicated ellipsometric model than the single layer model is necessary to accurately calculate the oxide thickness for thin oxide films.

Silicon dioxide is a critical part of silicon semiconductor technology. The oxidation kinetics of silicon have been characterized by the linear-parabolic expression for both dry and wet oxidation over a wide range of process variables. For most of the kinetics studies, the oxide film thicknesses were measured by ellipsometry, and a departure from the linear-parabolic relationship for dry oxidation in the oxide thickness range from 0 to 300 Å was observed.