Oxygen-related defects in the top silicon layer of SIMOX; the effect of thermal treatments

Commercial wafers of silicon implanted with oxygen (SIMOX), with a 190 nm silicon top layer and 360 nm buried oxide layer, were analysed using positron beam spectroscopy. Depth profiles of defects have been obtained in the depth range from 0 to 2 μm. The S parameter, related to annihilations with low-momentum electrons, has a value 10% lower in the top layer than in the silicon substrate. This low value can uniquely be ascribed to annihilations in oxygen-related traps. Positron beam analysis indicates that these defects are probably negatively charged and present at parts per million levels or lower. These defects escape detection by transmission electron microscopy, which suggests that the size of the defects must be smaller than 1 nm. By exposing the samples to deuterium plasma, these defects are passivated. In addition, the effects of annealing the samples from 600 to 800°C in deuterium ambients with pressures between 75 and 1000 kPa were studied. After these treatments, the annihilation parameters for defects in the top layer are very close to those of bulk silicon, indicating passivation of the oxygen-related defects. Further annealing in vacuum leads to a reversible process independent of the previous treatments applied to the samples. Thus, positron analysis has provided information about the existence and evolution of oxygen-related defects in the top layer of SIMOX.