It was recalled that, upon annealing a B-implanted Si sample at about 800C, the B in the tail of the implanted profile diffused very rapidly. That is, faster than the normal thermal diffusion rate by a factor 100 or more. After annealing for a sufficiently long time, this enhanced diffusion saturated. The enhanced diffusion was temporary. Upon annealing the sample again after saturation, no enhanced diffusion occurred. The high-concentration peak of the implanted B profile, which was electrically inactive, did not diffuse. The mechanism of transient enhanced diffusion was believed to be that a Si interstitial kicked-out the substitutional B atom into an interstitial position, where it could diffuse easily. An alternative explanation was that the interstitials and B atoms formed highly mobile pairs. In both cases, Si interstitials were required for the diffusion of B. The enhanced B diffusivity was therefore proportional to the concentration of excess Si interstitials. The interstitials were injected during implantation with Si or dopant ions. They were also injected during oxidation of the Si surface. The diffusivity therefore increased temporarily in both cases. Even at annealing temperatures of about 800C, the mobility of the interstitials was high. The transient enhanced diffusion at this temperature lasted for more than 1h. This lengthy transient enhanced diffusion time could be explained by the presence of interstitial clusters and interstitial-B clusters. The interstitial clusters were {311} extended defects and dislocation loops. The precise structure of the interstitial-B clusters was not yet known. The clusters were reservoirs of interstitials and, when the supersaturation of interstitials became low, the clusters dissolved and emitted interstitials. The interstitials emitted from clusters sustained the transient enhanced diffusion. It had been suggested that the rate of emission of interstitials was governed by Ostwald ripening of the clusters. However, transient enhanced diffusion changes could also be explained without invoking the Ostwald ripening of {311} defects. The evidence for Ostwald ripening of dislocation loops was more clear. In this case, Ostwald ripening was confirmed by measurements of the size distributions of dislocation loops at various annealing times and temperatures. At higher temperatures, the extended clusters were unstable and coupling between the interstitials and B atoms was reduced. At high temperatures, transient enhanced diffusion therefore lasted for only a short time. At high temperatures, the displacement during transient enhanced diffusion was also small. This suggested that, if rapid thermal annealing at high ramping rates was used, transient enhanced diffusion should be suppressed. So far, ramping rates of 300 to 400C/s had been used to try to suppress transient enhanced diffusion.

Transient Enhanced Diffusion of Boron in Si. S.C.Jain, W.Schoenmaker, R.Lindsay, P.A.Stolk, S.Decoutere, M.Willander, H.E.Maes: Journal of Applied Physics, 2002, 91[11], 8919-41