Cryogenic BF2+ Implantation near Amorphization Threshold Dose for Halo Junctions in Sub-30 nm Device Technologies

Abstract

We report on cryogenic implantation of BF 2 + at doses near the amorphization threshold and its impact on junction characteristics. BF 2 + implant at a dose of 8×10 13 cm −2 does not amorphize silicon at room temperature. When implanted at −100 • C, it forms a 30 -35 nm thick amorphous layer, significantly reducing the depth of the boron distribution, both as-implanted and after anneals. The cryogenic BF 2 + implant also creates a shallower n + -p junction by steepening profiles of arsenic that is subsequently implanted in the surface region. It reduces sheet resistance after two sequential anneals including a 1025 • C spike anneal, with no residual defects after recrystallization.Cryogenic ion implantation has recently brought keen interest to the semiconductor device fabrication industry, particularly below the 30 nm technology node. Wafer temperature dependence of amorphization in silicon during ion implantation was previously characterized. [1][2][3][4] In conjunction with implant dose, implant temperature is recognized as one of the most important conditions that determine amorphization. For a given ion, there is an amorphization threshold dose above which a continuous amorphous layer is formed at room temperature. Cryogenic implant below room temperature lowers the amorphization threshold. The thermal effect on amorphization is explained by two competing processes of damage accumulation by the energetic collision cascades and by defect annihilation, 5 which is sensitive to silicon wafer temperature during implantation. To date, there have been experiments that show damage and junction reduction effects of cryogenic implants under amorphizing, high dose low energy conditions, using high current ion implanters. 6,7 In this letter, we present results that show significant junction advantages of cryogenic BF 2 + implant at −100 • C from experiments looking at doses that do not normally amorphize silicon at room temperature. We selected the sub-amorphizing doses and ion energies that are typically used for halo implants for NMOSFET device processing at the sub-30 nm technology node. By forming n + -p junctions with an additional high dose arsenic implant for n + extension, this work demonstrates the technological benefits of amorphization induced by the cryogenic implant using medium current implanters, and its application to halo confinement and ultra shallow junction formation in short channel devices.(100) n-type Si wafers were implanted with BF 2 + at 40 keV to a dose of 8×10 13 cm −2 , at a tilt angle of 25 • and at a twist angle of +90 • and −90 • in two steps (bi-mode). The implant was done on three groups of wafers at different platen temperatures: (i) room temperature; (ii) −40 • C; and (iii) −100 • C. All cryogenic implants were done using the Varian PTC II (Process Temperature Control) system. TEM (Transmission Electron Microscopy) and SIMS (Secondary Ion Mass Spectrometry) analyses were performed. The wafers were then annealed at 950 • C for 30 seconds in 100% N 2 ambient, and final boron profiles af...