Medium energy ion scattering for the characterisation of damage profiles of ultra shallow B implants in Si

High depth resolution medium energy ion scattering (MEIS) in the double alignment mode has been used to determine the pre- and post-annealing damage distributions following 0.1–2.5 keV B⁺ implantation into Si(1 0 0) at different substrate temperatures. Samples were irradiated to doses ranging from 1×10¹⁴ to 2×10¹⁶cm−2 at substrate temperatures of −150°C, 25°C and 300°C. Rapid thermal processing (RTP) was carried out to temperatures ranging from 400°C to 1000°C for 10 s, to monitor the annealing of damage caused by the B⁺ implant.

For the room temperature (RT) implants, two distinct damage distributions were observed. The first was a narrow, near-surface damage peak which grows out from the virgin Si surface peak to a maximum depth of 3 nm, much shallower than the TRIM predicted mean projected range of e.g. 1 keV B⁺ ions (R_p≈5.3 nm). The width of this damage layer showed only a weak dependence on the B⁺ ion energy and strong dependence on the dose. The number of displaced atoms in this layer for dilute damage conditions is in good agreement with modified Kinchin Pease predictions. For 1 keV B⁺, a second, deeper damage peak appeared only after a B dose of 1×10¹⁵cm−2, having a maximum at a depth of ≈7.5 nm, well beyond the R_p of 5.3 nm. MEIS showed that this post-implant damage structure which develops for irradiations performed at 25°C and 300°C, is the result of dynamic annealing processes that are highly effective in the region in between the two peaks, in which Frenkel defects have their maximum production rates. The observed growth of the surface damage layer with implant dose is ascribed to the migration of point defects, created along the bombardment cascade, to the Si/SiO₂ interface. For 500 eV B⁺ implants, due to proximity of this surface sink, the residual damage is greater even at 300°C. Implantations at −120°C resulted in a single, heavily damaged layer stretching from the surface to the position of the deep damage. These damage profiles show a direct correlation between the displaced Si and the implanted B distributions. MEIS yields approached random level, showing near or total amorphisation of the Si lattice; epitaxial regrowth, even after 30 s RTP at 600°, was however only partial, apparently arrested at B containing I clusters formed near R_p of the B distribution.

RTP at 400°C and 500°C of the samples implanted at room temperature leads to substantial reduction in the Si damage, especially in the width of the near-surface peak. It suggests a substantial rearrangement of Si atoms in the lattice that occurs without the release of Si interstitials, in view of the absence of TED at these temperatures and may involve a degree of realignment of the damage structure with the channelling direction. The annealing behaviour measured by MEIS at higher temperatures is consistent with XTEM observations, showing the formation and growth in size of extended interstitial defects and their ultimate dissolution at high temperature. As well as moving into the bulk where they cause TED, a fraction of the released interstitials migrate to the surface and increase the width of the surface damage region. MEIS studies also indicates the occurrence of reverse annealing for high temperature implant conditions.