The effect of rapid thermal annealing on the precipitation of oxygen in silicon

Abstract

We report observations of the effect of heat pulses on the precipitation of oxygen in single-crystal silicon. In our experiments, heat pulses are applied for various durations at a fixed temperature of 1200 °C prior to a two-step precipitation sequence. When no pulse is applied, precipitation is at a maximum and is close to that expected from considerations of solid solubility, provided account is taken of the loss of oxygen due to evaporation. For very short pulses (e.g., 2 s), precipitation is suppressed by several fold. As the length of the pulse is increased to roughly 100 s, the precipitation recovers to its initial value. For very long pulses (greater than 10 000 s), precipitation decreases to nearly zero. There are no further changes in the precipitation characteristics for pulse times up to 50 000 s. We interpret these results in terms of a model in which the thermal pulses modify an initial distribution of heterogeneous nucleation sites. The fraction of heterogeneous sites that survive the two-step precipitation cycle is assumed to decrease continuously during the pulse due to thermal dissolution of sites. For short pulses, the decrease in sites accounts for the suppressed precipitation. For longer pulse times (about 100 s), we believe the recovery of precipitation is due to a reduction by out-diffusion of silicon self-interstitials which otherwise limit the rate of precipitation. For very long pulses (10 000 s), all nuclei have dissolved, so that negligible precipitation occurs. This interpretation is supported by experiments in which the interstitial population is altered by changes in ambient and by experiments in which the effects of more complex pulse sequences are investigated. It is suggested that studies of this type can be used to characterize the defect population of silicon for device applications as well as to investigate precipitation processes.