Optimization of BF2+ implanted and rapidly annealed junctions in silicon

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Abstract
The electrical properties and defect characteristics of shallow junctions fabricated by using BF2+‐ion implantation and followed by either furnace or rapid thermal annealing (RTA) have been investigated. The RTA temperature‐time cycle was optimized in terms of leakage current, junction depth, and sheet resistivity of junctions. Specifically, shallow p+ junctions (∼180 nm) with low leakage current (∼2 nA/cm2 at −5 V) were obtained with 49‐keV, 2×1015‐cm−2 BF2+ implantation and RTA at 1050 °C for 15 s. The effects of a Si+ preamorphizing implant and pre‐ or post‐RTA low‐temperature furnace anneal were also studied. p+/n diodes fabricated with a preamorphizing implantation exhibit about 3 orders of magnitude higher leakage current than the diodes without preimplantation. The excessive leakage current of the preamorphized junctions arises from a band of post‐annealing defects located in the depletion region of the n well (from ∼220 to 420 nm). Samples without the Si+ implant have very shallow and narrow defect bands (from ∼70 to 120 nm) located inside the heavily doped p+ region. The depth distribution of post‐annealing defects corresponds to the partially amorphized (heavily damaged) regions near the as‐implanted crystalline/amorphous interfaces. Furthermore, a low‐temperature furnace anneal (550 °C for 1 h) before or after RTA treatment reduces leakage current for diodes without preimplantation. This reduction of leakage current by the two‐step annealing coincides with the relaxation of the implant‐induced stress in the junction, as measured by substrate curvature. The post‐RTA anneal is found to be more effective than the pre‐RTA anneal, both in terms of leakage current and the amount of stress relief. Channeling tails were observed in secondary‐ion mass spectrometry boron profiles of BF2+‐implanted samples, and Si+ preamorphization eliminates them. However, identical carrier concentration profiles are obtained by spreading carrier‐resistance measurements for samples with or without preamorphization, which suggests that the channeled boron atoms remain in the interstitial sites after RTA and hence are inactive.