Anomalous depth distributions of bulk micro-defects were observed in wafers of Czochralski-type material which had been subjected to 2-step annealing (550 to 700C and 850 to 950C), where the time passed in each temperature range was between 1 and 100h. It was found that the number density of bulk micro-detects near to the surface was lower than that in the bulk when the time spent at the lower temperatures was short, and was higher when this time was long. The anomalous distribution extended to more than 100μ below the wafer surface, and could not be attributed to the behavior of interstitial O atoms. The distributions were examined as a function of various annealing parameters, such as annealing temperature, rate of temperature ramping, ambient atmosphere, and initial O concentration. The anomalous distributions were found to form in the early stages of the second step of annealing only when the annealing began with a rapid temperature rise. A formation model for the anomalous distributions was proposed which was based upon the assumptions that self-interstitials existed in the thermal equilibrium state, that wafer surfaces were permanent sources and sinks for self interstitials, that growing O precipitates produced self-interstitials, and that self-interstitial undersaturation enhanced the stable growth of precipitate nuclei while supersaturation suppressed stable growth. The non-equilibrium self-interstitial concentration which was introduced into the bulk after a rapid temperature rise was thought to be responsible for the anomalous distributions. All of the experimental observations were reasonably well explained by the model. The formation process of the anomalous distributions was detected by using 3- step annealing experiments. The basic properties of self-interstitials in Si were deduced from experimental results, combined with the model. The activation energy for migration was about 2.5eV. The diffusion coefficient was equal to about 10-6cm2/s at 900C. The thermal equilibrium concentration was estimated to be equal to about 1012/cm3 at 1000C. These data were similar to previous experimental results on impurity diffusion in float- zone material. Anomalous Depth Distributions of Bulk Microdefects in Heat-Treated Czochralski Silicon Wafers due to Nonequilibrium Self-Interstitials. Y.Satoh, H.Furuya, M.Kadoi, Y.Shimanuki: Journal of Applied Physics, 1995, 77[8], 3710-24