Spectroscopic ellipsometry studies of very thin thermally grown SiO2 films: Influence of oxidation procedure on oxide quality and stress

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Abstract
The qualitative and structural modifications of very thin SiO2 films with thicknesses between 80 and 500 Å, caused by the oxidation procedure, were studied with spectroscopic ellipsometry (SE). Analysis of the experimental dielectric function obtained by SE provides the structural characteristics and the thickness of the oxide in fairly good agreement with electron microscopy results in cross‐sectional geometry. The calculated voids volume fraction was found to drop below 3% (2%) for oxides thicker than 250 (400) Å grown at 900 (1000) °C. The densification of thermal oxides grown at low oxidation temperature predicted by Fourier transform infrared is discussed and compared to the SE results, whereas a relation between the final oxidation time and the viscoelastic relaxation time was found. Furthermore, it is shown that in situ SE can be used to monitor the process of oxide removal with very low‐energy Ar+ ions and to control the conditions in order to avoid oxide and Si substrate damage. It is found that Ar+ ions with energy of about 10 eV are required to avoid oxide damage and Si substrate amorphization at a depth below 6 Å. The latter finding is also corroborated by atomic force microscopy images obtained from Si substrates after oxide etching. Finally, the influence of oxide thickness and the oxidation procedure on the E1 structure of c‐Si was studied and from the results the stress applied by the oxide on the Si substrate was determined. From this study it was found that the stress depends strongly on the oxidation duration as well as on the oxidation procedure and the type of Si substrate and is minimized at an oxidation temperature 900 °C for oxidation duration longer than 70 min.