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UBC Theses and Dissertations

Thin oxide films: mechanisms of growth, dielectric and optical properties and applications in semiconductor devices Smith, David J.


Dielectric films are important in the fabrication of semiconductor devices. Thermally grown and CVD SiO₂, and CVD silicon nitride films are perhaps the most widely used dielectric materials at present. Future developments in both Si and GaAs technology, however, would seem to require the introduction of both new materials and processes. Since it is a room temperature process, anodic oxidation is attractive for some device applications. The mechanism by which the oxide film grows, however, is not well understood. Low temperature thermal oxidation of certain deposited metals is also of interest. For this thesis, anodic and thermally grown tantalum oxide films were investigated in connection with their possible engineering applications. Part of the work described in this thesis involved developing instrumentation for computer-controlled experiments and data acquisition. This included development of a new balancing algorithm and control program for a computer-controlled ellipsometer. Careful attention was also given to calibration of the ellipsometer and other equipment used in this work. The optical properties of tantalum oxide films grown in dilute H₂SO₄ were measured by in situ ellipsometry. The ellipsometer data are consistent with a model in which a nonabsorbing single-layer film overlays a thin inter-facial film of variable thickness and refractive index. The interfacial film is due to surface roughness and a fluorine-rich film remaining from the surface preparation. The oxide film thicknesses were compared with thicknesses determined from the wavelengths of minimum reflectivity. Anodic oxides are grown in a thermally activated process in which ions are transported across the film under the influence of a large electric field. It is argued that the observed history dependence of the current-field relations due to changes in the structure of the oxide film. It is then shown that equations can be developed which are equivalent to those of Dignam's current-driven dielectric polarization model but which are based on a more acceptable physical model involving these structural changes. The process of high field ionic conduction in the oxide film was investigated by stepped current and open circuit transient methods. The data were analyzed numerically and compared with data predicted by Dignam's equations. The experimental and computed data are shown to be in close agreement. However, small but possibly significant anomalies were observed regarding the linearity of the stepped current data and parameter values used in fitting relations to the experimental data. For possible device applications, tantalum films were sputtered onto silicon and thermally oxidized. The optical properties of the oxide film were investigated by ellipsometry. Current-voltage measurements of MIS capacitors indicate the present oxide films would be suitable as a second but not a first dielectric layer in a silicon MIS device. The capacitance of the MIS capacitors depended strongly on the oxidation time and post-oxidation heat treatment.

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