Transient enhanced diffusion of boron in Si
Authors: Jain S. C. , Schoenmaker W. , Lindsay R. , Stolk P. A. , Decoutere S. , Willander M. , Maes H. E. 
Published: July 2002

On annealing a boron implanted Si sample at ∼800 °C, boron in the tail of the implanted profile diffuses very fast, faster than the normal thermal diffusion by a factor 100 or more. After annealing for a sufficiently long time, the enhanced diffusion saturates. The enhanced diffusion is temporary, on annealing the sample a second time after saturation, enhanced diffusion does not occur. It is therefore designated as transient enhanced diffusion (TED). The high concentration peak of the implanted boron profile, which is electrically inactive, does not diffuse. TED makes it difficult to fabricate modern Si based devices, in particular TED produces the parasitic barriers which degrade the performance of the SiGe heterostructure bipolar transistors and TED can limit the fabrication of shallow junctions required for sub-100 nm complementary metal–oxide–semiconductor technology. The mechanisms of TED have been elucidated recently. A Si interstitial “kicks out” the substitutional boron atom to an interstitial position where it can diffuse easily. Alternatively the interstitials and boron atoms form highly mobile pairs. In both cases Si interstitials are required for the diffusion of boron. Therefore the enhanced boron diffusivity is proportional to the concentration of the excess Si interstitials. The interstitials are injected during implantation with Si or dopant ions. The interstitials are also injected during oxidation of the Si surface. Therefore the diffusivity increases temporarily in both cases. Even at relatively low annealing temperatures (∼800 °C) the mobility of the interstitials is high. The TED at this temperature lasts for more than 1 h. This large TED time can be explained by the presence of interstitial clusters and interstitial–boron clusters. The interstitial clusters are the {311} extended defects and dislocation loops. The precise structure of interstitial–boron clusters is not yet known though several models have been proposed. The clusters are the reservoirs of the interstitials. When the supersaturation of interstitials becomes low, the clusters dissolve and emit interstitials. The interstitials emitted from the clusters sustain the TED. Many groups have suggested that the rate of emission of interstitials is determined by Ostwald ripening of the clusters. However, recently TED evolution has also been explained without invoking Ostwald ripening of the {311} defects. The evidence of Ostwald ripening of dislocation loops is more direct. In this case the Ostwald ripening has been confirmed by the measurements of the size distributions of the dislocation loops at different times and temperatures of annealing. At higher temperatures the extended clusters are not stable and coupling between the interstitials and boron atoms is reduced. Therefore at high temperatures TED lasts only for a short time. At high temperatures the displacement during TED is also small. This suggests that if rapid thermal annealing with high ramp rates is used, TED should be suppressed. Currently high ramp rates, 300–400 °C/s are being tried to suppress TED.