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H. U. JAGER Research Center Rossendorf Inc., Institute of Ion Beam Physics and Materials Research, P.O. Box 510119, D-01314 Dresden, Germany ABSTRACT A point defect-based model, which has been developed to describe diffusion and electrical activation of boron in crystalline silicon during post-implantation annealing, is used to simulate the anomalous diffusion of boron implanted into silicon along the random and [100] channeling directions. The model predictions are compared to data measured by Chu et al.

INTRODUCTION The formation of shallow junctions in silicon devices requires precise information about the distribution of dopant elements and their electrical activation. If low-energy boron ions are implanted into crystalline silicon to produce p-type layers, a transient enhanced diffusion (TED) occurs at the early stage of post-implantation annealing. Only the electrically active boron fraction is involved in TED. At low annealing temperatures of about 800 'C, the TED relaxes within a period of 30 min, whereas the time period for boron activation up to the solubility limit is of the order of 10 h. These processes have been extensively studied during the past decade. For the technologically relevant boron fluences and annealing parameters, many measured boron depth profiles can be found in refs.' and in the refs. cited therein. Chu et al.2 studied the dependence of TED on implantation direction. The amount of anomalous diffusion was found to be 10-50 % higher in the implants along the [100] channeling direction than along the random direction. It is clear that the TED of boron is controlled by the diffusion, annihilation, and clustering of implantation-induced silicon self-interstitials. But it has proved to be extremely difficult to explore the details of the process; for very recent, more refined experimental investigations advancing in this direction, see refs. 3 The point defect-based models which are used until now to provide simulation capabilities start from simplified ideas. We developed a model4' 5 where a system of diffusion-reaction equations for the dopant species and the silicon point defects is solved, but the initial conditions are detemined in a phenomenological way. The results of Chu et al.2 are thought to be a critical check for such an approach. It is the aim of this contribution to compare our model predictions to some data reported in 2 ref.

MODELING OF ANNEALING - THE INITIAL CONDITIONS Modeling the post-implantation annealing process, the boron concentration below the solubility limit c,,,, is assumed to consist of ionized substitutional atoms (Be) and of neutral 71 Mat. Res. Soc. Symp. Proc. Vol. 389 © 1995 Materials Research Society

immobile atoms (Bi) on interstitial sites. The boron concentration exceeding c5 ol is postulated to be perfectly clustered. The system of diffusion-reaction equations which is solved describes the diffusion of silicon point defects ( self-interstitials I, vacancies V ) and the diffusion of point defect boron pairs (B5]), (BV). The reactions Bi

k2 £ k 2r