ANOMALOUS DIFFUSION OF FLUORINE IN SILICON

The diffusion of ion implanted F in Si has been studied by the use of secondary ion mass spectroscopy and thermal desorption spectroscopy. In the dose range studied (below amorphization threshold), F exhibits an anomalous out-diffusion behavior which is characterized by the depletion of F in Si substrate at temperatures greater-than-or-equal-to 550-degrees-C with complete suppression of diffusion deeper into the bulk of Si. F species which migrate to the surface react with native oxide and Si to form volatile Si oxyfluoride and Si fluoride, which then evaporate from the surface. There is clear evidence that the formation of Si oxyfluoride correlates strongly with the thermally activated anomalous migration of F. While the driving force for the anomalous F migration has not yet been identified, it appears that the electric field is not a dominant mechanism.