Intrinsic gettering of iron in silicon has been studied with a novel approach that involved the combination of two complementary techniques. Electron Paramagnetic Resonance (EPR) was used to determine the concentration of interstitial iron atoms which are electrically active in the silicon matrix. A systematic comparison of the effect of various oxygen precipitates of well defined morphologies on the solubility of iron was performed. A surprising result was found for bulk samples containing oxygen precipitates obtained after low temperature heat treatments (700ºC): the solubility of iron was reduced byalmost two orders of magnitude from the values obtained in untreated Cz silicon samples or in samples containing oxygen precipitates obtained at high temperature (1150ºC). The practical importance of this new result was demonstrated by a series of iron concentration measurements after various gettering and high temperature re-heating treatments.
Transmission Electron Microscopy (TEM) was used in order to confirm the proposed mechanism of formation of a new phase, different from FeSi2 in the silicon matrix. This new phase is assumed to limit the possible electrically active metallic content by acting as a boundary phase in quasi-equilibrium with interstitial iron atoms. Evidence ofthe formation of Fe2SiO4 was found in wafers that had been oxygenimplanted and subsequently subjected to similar precipitation and diffusion treatments than those performed with the bulk samples.
These findings suggest the possibility of dramatic improvements in the efficiency and the stability of the gettering process.