Oxidation, defects and vacancy diffusion in silicon

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Abstract
The mechanism causing extrinsic faults to grow into a silicon wafer during thermal oxidation has been investigated by annealing experiments on thin electron microscope foils. The defects grow on annealing in air at 1100°C and shrink on annealing in vacuo at the same temperature; this behaviour is explained in terms of the diffusion of vacancies between defect and surface. The sense of this flow is dependent on the vacancy concentration in equilibrium with the surface which is reduced to approximately 0·8 of the bulk equilibrium value because the vacancies are annihilated by the inward-growing oxide. During oxidation the faults emit vacancies to the surface, causing fault growth, whereas the vacancy flow is reversed on annealing in vacuo. The activation energy for fault shrinkage has been determined to be 2·1 ev, which is consistent with pipe diffusion along the core of the bonding Frank dislocation. The shrinkage rate of these faults at constant temperature is proportional to the equilibrium vacancy concentration and thus provides a comparative method of determining the equilibrium vacancy concentration as a function of dopant concentration. From such measurements it has been shown that the equilibrium vacancy concentration increases with increasing phosphorus (n-type) concentration and decreases with increasing boron (p-type) concentration. These results have been interpreted in terms of the vacancy accepting electrons from the Fermi level and are consistent with a vacancy acceptor level lying about midway between the valence and conduction bands.

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