Optimization of BF2+ implanted and rapidly annealed junctions in silicon

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.