Effect of fluorine on boron thermal diffusion in the presence of point defects

With the increased interest in the use of fluorine co-implantation with boron for boron diffusion suppression in MOSFET devices, it is important to understand the mechanisms by which fluorine reduces boron diffusion. Mechanisms, such as B–F chemical reaction, vacancy–fluorine clusters and fluorine–interstitials interactions have been proposed in the literature. In this paper, a point defect injection is done to investigate the mechanism responsible for boron TED and thermal diffusion suppression in F+ and B+ implanted silicon. A 5 keV, 7 × 1012 cm−2 B+ implant into silicon is used which is typical for halo implants in n-MOS. Three F+ energies, 5, 50 and 185 keV, are used. It is followed by rapid thermal annealing at 900–1000 °C for different times in N2 for an inert anneal and O2 for injection of interstitial point defects from the surface. Fluorine profiles for samples implanted with 185 keV F+ and annealed in N2 show two fluorine peaks at ∼Rp and ∼Rp/2. Under interstitial injection, the Rp/2 peak decreases in size and for long anneal times is completely eliminated, supporting an earlier claim that the Rp/2 peak is due to vacancy–fluorine clusters. The amount of suppression of both boron TED and thermal diffusion at 900 and 1000 °C anneal is correlated to the amount of fluorine remaining after anneal.