Study of fluorine behaviour in silicon by selective point defect injection

This letter reports a point defect injection study of 185 keV 2.3x1015cm?2 fluorine implanted silicon. After an inert anneal at 1000°C, fluorine peaks are seen at depths of 0.3Rp and Rp and a shoulder between 0.5–0.7Rp. The shallow peak (at 0.3Rp) is significantly smaller under interstitial injection than under both inert and vacancy injection conditions. For a longer anneal under interstitial injection, both the shallow peak and the shoulder are eliminated. These results support earlier work suggesting that the shallow fluorine peak is due to vacancy-fluorine clusters which are responsible for suppression of boron thermal diffusion in silicon. The elimination of the shallow fluorine peak and the shoulder is explained by the annihilation of vacancies in the clusters with injected interstitials

110GHz fT Silicon Bipolar Transistors Implemented using Fluorine Implantation for Boron Diffusion Suppression

This paper investigates how fluorine implantation can be used to suppress boron diffusion in the base of a double polysilicon silicon bipolar transistor and hence deliver a record fT of 110 GHz. Secondary Ion Mass Spectroscopy (SIMS) and transmission electron microscopy are used to characterize the effect of the fluorine implantation energy and dose, the anneal temperature and ambient and the germanium pre-amorphisation implant on the fluorine profiles. These results show that retention of fluorine in the silicon is maximised when a high-energy fluorine implant is combined with a low thermal budget inert anneal. TEM images show that a high-energy fluorine implant into germanium pre-amorphised silicon eliminates the end of range defects from the germanium implant and produces a band of dislocation loops deeper in the silicon at the range of the fluorine implant. Boron SIMS profiles show a suppression of boron diffusion for fluorine doses at and above 5?1014cm-2, but no suppression at lower fluorine doses. This suppression of boron diffusion correlates with the appearance on the SIMS profiles of a fluorine peak at a depth of approximately Rp/2, which is attributed to fluorine trapped in vacancy-fluorine clusters. The introduction of a fluorine implant at this critical fluorine dose into a bipolar transistor process flow leads to an increase in cut-off frequency from 46 to 60GHz. Further optimisation of the base-width and the collector profile leads to a further increase in cut-off frequency to 110GHz. Two factors are postulated to contribute to the suppression of boron diffusion by the fluorine implant. First, the elimination of the germanium end of range defects, and the associated interstitial population, by the fluorine implant, removes a source of transient enhanced diffusion. Second, any interstitials released by the dislocation loops at the range of the fluorine implant would be expected to recombine at the vacancy-fluorine clusters before reaching the boron profile

Reduction of boron diffusion in silicon-germanium by fluorine implantation

This paper investigates the effect of a 185 keV, 2.3×1015cm-2 F+ implant on boron thermal diffusion and boron transient enhanced diffusion (TED) in SiGe by characterising the diffusion of a boron marker layer in samples with and without a 288 keV, 6×1013cm-2 P+ implant. In samples implanted with F+ only, the fluorine suppresses boron thermal diffusion by 58%. In samples given both P+ and F+ implants, the fluorine completely eliminates boron transient enhanced diffusion caused by the P+ implant and also significantly reduces boron thermal diffusion. SIMS profiles after anneal show a fluorine concentration in the SiGe layer that is approximately 8× higher than after implant, indicating that fluorine accumulates in the SiGe layer during anneal. A comparison with fluorine profiles in comparable silicon samples also shows that the fluorine concentration after anneal is dramatically higher in SiGe samples than in Si samples. This accumulation of fluorine in the SiGe layer during anneal will have major benefits for boron diffusion suppression in devices like SiGe HBTs, where boron must be kept within the SiGe layer

Fluorine complexes in Si-SiGe-Si structures

Fluorine-vacancy complexes have been directly observed in the Si0.94Ge0.06 layer in a Si-SiGe-Si structure, using variable-energy positron annihilation spectroscopy. These complexes are linked to the significant reduction of boron diffusion in the SiGe layer via interstitial trapping. Vacancies were introduced into the samples by ion implantation with 185 keV F+ at doses in the range 9x1014 to 1x1016 cm?2; the samples were subsequently subjected to rapid annealing in nitrogen ambient at 950 °C for 30 s. The VEPAS results, in combination with F profiles obtained by secondary ion mass spectrometry, are consistent with F4nVn complexes being associated with the SiGe layer and that they preferentially accumulate at the Si/SiGe interfaces. Their concentration is critically dependent on annealing temperature, decreasing significantly after annealing at 1000 °C

Properties and benefits of fluorine in Si and SiGe devices

This paper reviews the behaviour of fluorine in silicon and silicon-germanium devices. Fluorine is shown to have many beneficial effects in polysilicon emitter bipolar transistors, including higher values of gain, lower emitter resistance, lower 1/f noise and more ideal base characteristics. These results are explained by passivation of trapping states at the polysilicon/silicon interface and accelerated break-up of the interfacial oxide layer. Fluorine is also shown to be extremely effective at suppressing the diffusion of boron, completely suppressing boron transient enhanced diffusion and significantly reducing boron thermal diffusion. The boron thermal diffusion suppression correlates with the appearance of a fluorine peak on the SIMS profile at approximately half the projected range of the fluorine implant, which is attributed to vacancy-fluorine clusters. When applied to bipolar technology, fluorine implantation leads to a record fT of 110GHz in a silicon bipolar transistor

Properties and benefits of fluorine in silicon and silicon-germanium devices

This paper reviews the behaviour of fluorine in silicon and silicon-germanium devices. Fluorine is shown to have many beneficial effects in polysilicon emitter bipolar transistors, including higher values of gain, lower emitter resistance, lower 1/f noise and more ideal base characteristics. These results are explained by passivation of trapping states at the polysilicon/silicon interface and accelerated break-up of the interfacial oxide layer. Fluorine is also shown to be extremely effective at suppressing the diffusion of boron, completely suppressing boron transient enhanced diffusion and significantly reducing boron thermal diffusion. The boron thermal diffusion suppression correlates with the appearance of a fluorine peak on the SIMS profile at approximately half the projected range of the fluorine implant, which is attributed to vacancy-fluorine clusters. When applied to bipolar technology, fluorine implantation leads to a record fT of 110 GHz in a silicon bipolar transistor