Oxygen Precipitation in Silicon
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OXYGEN PRECIPITATION IN SILICON
S. M. HU* IBM General Technology Division East Fishkill Facility, Hopewell Junction, NY 12533 ABSTRACT The precipitation of oxygen in silicon exhibits many interesting phenomena: the precipitates are often distributed in striated zones; the morphology of precipitates changes with annealing temperatures; the kinetics of precipitation in the bulk is affected by annealing ambients and other surface treatments. These phenomena are briefly reviewed. A model consistent with all these phenomena is presented. Then, the energetics and the kinetics of nucleation and precipitation are discussed. Finally, a growth law is derived in general for disc precipitates, and applied in particular to oxygen precipitates in silicon. INTRODUCTION The interest in the subject of oxygen in silicon first appeared in the mid 1950's, but it had almost disappeared by the end of the 60's. Then, around 1970, it was discovered that making integrated circuits on silicon wafers from the seed-end portion of a Czochralski crystal tended to have higher yields than on those from the tail-end portion. This brought forth two urgent problems: (a) should we wastefully discard the tail portion of a silicon crystal, and (b) what caused this to happen. Without an understanding of what was happening, one could not hope to be in control of production yield. Among many clues sought to explain this finding was the known fact that Czochralski silicon crystals contain oxygen, and that the oxygen concentration is highest in the seed-end portion. The interest in oxygen in silicon was thus rekindled and it rapidly intensified. A number of possible effects due to oxygen were soon postulated, and were experimentally demonstrated in the subsequent several years. The most important are the improvement of the high temperature mechanical strength of the silicon substrates [1-5] and the internal gettering of metallic impurities by oxygen precipitates [6-10]. In fact, the tendency of metallic impurities such as copper to gather in regions rich in oxygen or its precipitates had been observed in the early 1960's; [11, 12] but its technological implications were not recognized at that time. Unfortunately, not all effects of oxygen in silicon are beneficial. It has been shown that oxygen precipitates can act as dislocation nucleation agents, and thus promote slips and warpage in silicon wafers which have incurred thermal stresses [5, 6, 13-16]. There even have been efforts to reduce oxygen concentrations, by such means as growing silicon crystals in magnetic fields [16], in order to prevent precipitation during the course of device processings. These effects impose conflicting demands on the specification of desirable oxygen concentrations as well as the thermal processing conditions. While such effects have been established, the mechanism and kinetics of oxygen precipitation are still not completely understood. This understanding is important for the control of oxygen precipitation in the right amount, at the right time and in the right place withi
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