Optimisation of BF2 implanted pnp polysilicon emitter bipolar transistors using rapid thermal annealing

BF implantation into polysilicon and its subsequent rapid thermal diffusion into single crystal silicon is investigated for the fabrication of shallow pnp polysilicon emitter bipolar transistors. The use of RTA, instead of furnace annealing, is shown to give shallower junctions, with a higher doping concentration at the polysilicon/silicon interface. The effect of fluorine, which is introduced into polysilicon during the BF₂ implant, is also investigated. Electrical results show that, in the presence of fluorine, the base current drops by a factor of ≈1.6. This is explained by the action of fluorine in accelerating the break up of the interfacial layer.