Boron diffusion in ion‐implanted and annealed single‐crystal and amorphized Si is compared to determine the effect of amorphization on the initial transient boron motion reported for single crystal. The boron was implanted at 20 keV and at doses of 1×1015 and 3×1015cm−2. The Si was either preamorphized or postamorphized to a depth of 320 nm by implantation of Si ions at three different energies. In the amorphized samples the entire boron profile was always contained within this distance. The samples were annealed by furnace or rapid thermal annealing to 900–1100 °C with or without a preanneal at 600 °C. The initial rapid diffusion transient in the tail region of the boron profile was observed in all the crystal samples. This transient was totally absent in the amorphized samples. This is manifest by careful comparison of boron concentration profiles determined by secondary ion mass spectrometry of single‐crystal and amorphized samples after annealing. For anneals where significant motion occurs, the profi...

Transient boron diffusion in ion-implanted crystalline and amorphous silicon

The fabrication of ultra large-scale integrated (ULSI) circuits with ever shrinking feature size requires a continued reduction of diffusion lengths of dopants in Si. However, boron implants undergo an “anomalous” diffusion upon annealing, which is detrimental to the ultra-shallow junction formation. Over the last decade, boron transient diffusion has been much better understood. This implantation-induced transient-enhanced diffusion is known to be driven by the large supersaturation of self-interstitial silicon atoms emitted from the extended defects. The form and evolution of implantation-induced defects with respect to implant species, energy, dose, and annealing temperature have been systematically studied. The improved understanding has led to the development of new doping techniques, such as carbon co-implant, multiple step annealing and point defect engineering (PDE) using co-implantation of high-energy ions. In the PDE approach, high-energy ion bombardments inject vacancies near the surface region and create excessive interstitials near the end of projected range deep inside the substrate. Such manipulation of point defects can retard boron diffusion and enhance the activation of boron. The control of boron diffusion could be used to form ultra-shallow junctions for next generation Si device. In this article, we review the history and recent progress in PDE.

Boron diffusion in silicon: the anomalies and control by point defect engineering

The residual extended defects due to end‐of‐range ion implantation damage can be totally eliminated by Ti silicidation. Shallow p+ junctions were formed by amorphizing the silicon with Ge implantation (85 keV, 1×1015 cm−2) prior to implanting boron (85 keV, 1×1015 cm−2), recrystallizing the amorphous region at 550 °C, and then rapid thermal annealing at 1050 °C for 10 s. A buried sheet of interstitial dislocation loops lying below the surface remained. However, following a self‐aligned Ti silicide process, the end‐of‐range defects due to Ge ion implantation damage were no longer observed in cross‐sectional transmission electron micrographs. The annihilation of these end‐of‐range interstitial dislocation loops is attributed to the injection of vacancies during Ti silicidation.

Defect annihilation in shallow p+ junctions using titanium silicide

The tradeoffs involved in alternative processes for the formation of ultra shallow junctions are described. Low energy implantation, preamorphization to eliminate channeling and low thermal budget processing are adequate to form junctions that are 0.1 to 0.3μm deep. For junctions less than about 100 nm, however, the enhanced diffusion resulting from the amorphization implant reduces its benefits. Athermal diffusion can result in considerable junction motion even when low thermal budget processing is used. Junctions this shallow typically require silicide or metal cladding to reduce the sheet resistance; however, the dopant redistribution associated with siliciding pre-existing junctions increases the contact resistance which diminishes the potential benefit of silicidation. In addition, high leakage can result from excessive silicon consumption. While the use of silicide as a diffusion source can overcome some of the limitations of silicided junctions, this technique can be especially hindered by slow dopant diffusion or compound formation in the silicide and by the limited thermal stability of the silicide. One outstanding issue associated with silicide diffusion sources is understanding the seemingly enhanced diffusivity of dopant in the silicon.

Formation of silicided, ultra-shallow junctions using low thermal budget processing

Growth- and processing-induced defects in semiconductors

Amorphizing n‐type 〈100〉 silicon by tin implantation prior to implanting 10 keV boron and then annealing for 30 min at 800 °C results in a 0.22‐μm‐deep p+/n junction. The implanted tin prevents boron channeling, enhances the quality of the solid phase epitaxial regrowth of the silicon, and shows no measurable diffusion. A discontinuous band of dislocation loops, 20–30 nm in diameter, with a density below 1010 cm−2 remains at the original amorphous‐crystalline interface after annealing. Junctions are nearly ideal and are characterized at −5 V reverse bias by an areal leakage of −5 nA cm−2 and a peripheral leakage less than −0.1 fA μm−1.

M. Delfino

Papers

Shallow junction formation by preamorphization with tin implantation