We use kinetic nonlattice Monte Carlo atomistic simulations to investigate the physical mechanisms for boron cluster formation and dissolution at very high B concentrations, and the role of Si interstitials in these processes. For this purpose, high-dose, low-energy B implants and theoretical structures with fully active box shaped B profiles were analyzed. Along with the theoretical B profile, different Si interstitial profiles were included. These structures could be simplifications of the situation resulting from the regrowth of preamorphized or laser annealed B implants. While for B concentrations lower than 1020 cm−3, B clusters are not formed unless a high Si interstitial concentration overlaps the B profile, our simulation results show that for higher B concentrations, B clusters can be formed even in the presence of only the equilibrium Si interstitial concentration. The existence of a residual concentration of Si interstitials along with the B boxes makes the deactivation faster and more severe.
The role of fluorine in suppressing boron diffusion was investigated by utilizing a buried dopant marker to monitor the interaction of fluorine with interstitials. A boron spike with a peak concentration of 1.2×1018 cm−3 followed by 500 nm of undoped silicon was grown in a low pressure chemical vapor deposition furnace. The wafers were then preamorphized and implanted with either B, B and F, BF2, As, As and F, or F, respectively. Following the implants, the samples were rapid thermal annealed (RTA) at 1050 °C for very short times (spike). The use of preamorphization allows the chemical effect of fluorine to be analyzed independently of implant damage, and the buried layer functions as an indicator of point defect (in this case Si self-interstitial) perturbation. As expected, secondary ion mass spectroscopy shows that the presence of fluorine retards the diffusion of boron. In addition, the retained fluorine dose after the RTA is highest in the boron-implanted samples. In all samples the buried layer has diffused by the same amount, indicating that there is no change to the silicon self-interstitial population due to fluorine. These results suggest that fluorine has a chemical effect, and retards boron diffusion by mainly bonding with boron.
The kinetics of boron electrical activation is studied for both pre-amorphized (PAI) and non-amorphized (non-PAI) samples. It is found that the electrical activation mechanism in both cases is similar and is dominated by a 5eV native point defect driven activation energy barrier, substantially greater than the 3.5eV diffusion activation energy. The physical origins of this mechanism are explained through atomistic simulations and the physical basis of the activation energy difference was used to design a flash anneal capable of achieving highly active and ultra shallow p-type junctions meeting the 32nm node lTRS specifications.
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