Oxygen precipitation retardation and recovery phenomena in Czochralski silicon: Experimental observations, nuclei dissolution model, and relevancy with nucleation issues
We report experimental results of an oxygen precipitation study carried out using Czochralski silicon wafers. A two-step anneal scheme was employed: a lower-temperature step (at 650 or 750 °C for 0-128 h) for SiO2 precipitate nucleation and a higher temperature step (at 1050 °C for 0-64 h) for growth. The oxygen precipitation rate is monitored by measuring the interstitial oxygen (Oi) concentration in the silicon lattice. We have found that (i) a precipitation retardation phenomenon exists for wafers that received prolonged nucleation annealing treatment (from 2 to 16 h), and (ii) this retardation phenomenon gives way to a precipitation recovery phenomenon for wafers that received still longer nucleation annealing treatment (for 8-128 h). We also describe a nuclei dissolution model to explain the retardation/recovery phenomena. The dissolution proceeds against an O i supersaturation which would normally drive the nuclei into growth. The most important aspects of the model are (i) the dissolution occurs at the onset of the high-temperature growth anneal step, (ii) the factor directly opposing the Oi supersaturation and leading to the nuclei dissolution process is attributed to a large supersaturation of silicon self-interstitials (I supersaturation), essentially also generated at the onset of the growth anneal step, (iii) the generation of the I supersaturation is associated with a nuclei/precipitate polymorph change again occurring at the onset of the growth anneal step. The fundamental physical cause leading to these phenomena is the exigent-accommodation volume (or exigent volume) associated with precipitate growth. This model can explain the present results and is consistent with many other experimental results. We then examine the relevance of the exigent-volume factor with nucleation issues such as its effect on ramping, the effect of carbon, the existence of multiple polymorphs of SiO2, multiple nucleation paths, and a nucleation incubation phenomenon. We believe contributions of the exigent-volume factor to the nucleation phenomena generally exist.
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