Junction Depth Reduction of ion Implanted Boron in Silicon Through Fluorine ion Implantation
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Junction depth reduction of ion implanted boron in silicon through fluorine ion implantation L. S. Robertson, P. N. Warnes, and K. S. Jones Dept. of Materials Science and Engineering, University of Florida S. K. Earles and M. E. Law Dept. of Electrical and Computer Engineering, University of Florida D. F. Downey, S. Falk, and J. Liu Varian Ion Implant Systems ABSTRACT The interaction between boron and excess silicon interstitials caused by ion implantation hinders the formation of ultra-shallow, low resistivity junctions. Previous studies have shown that fluorine reduces boron transient enhanced diffusion, however it is unclear whether this observed phenomenon is due to the fluorine interacting with the boron atoms or silicon self-interstitials. Amorphization of a n-type Czochralski wafer was achieved with a 70 keV Si+ implantation at a dose of 1x1015/cm2. The Si+ implant produced a 1500Å deep amorphous layer, which was then implanted with 1.12 keV 1x1015/cm2 B+. The samples were then implanted with a dose of 2x1015/cm2F+ at various energies ranging from 2 keV to 36 keV. Ellipsometry measurements showed no increase in the amorphous layer thickness from either the boron or fluorine implants. The experimental conditions allowed the chemical species effect to be studied independent of the implant damage caused by the fluorine implant. Post-implantation anneals were performed in a tube furnace at 750° C. Secondary ion mass spectrometry was used to monitor the dopant diffusion after annealing. Transmission electron microscopy (TEM) was used to study the end-of-range defect evolution. The addition of fluorine reduces the boron transient enhanced diffusion for all fluorine energies. It was observed that both the magnitude of the boron diffusivity and the concentration gradient of the boron profile vary as a function of fluorine energy. Introduction The inherent damage produced by ion implantation results in a large supersaturation of silicon self-interstitials during post-implantation annealing. This interstitial supersaturation leads to an increase in the diffusivity of dopants1-4 such as B, P, and As during the initial stages of annealing, a phenomenon commonly known as transient enhanced diffusion (TED). The formation of ultra-shallow, low resistivity junctions in the source and drain extension regions of transistors is hindered by TED. Unless resolved, this issue will prevent the current scaling trend known as Moore's Law from continuing through the end of the decade. In previous studies5-10, fluorine has been co-implanted with boron, mainly in the + form of a BF2 molecular implant, to determine its effect on the characteristics of implanted boron. Many important observations have been brought forth by these
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previous investigations. One of the most noteworthy conclusions of these studies was + + that a BF2 molecular implant produced a shallower junction than a B implant with equivalent energy for the boron ion. These results were promising, however some ambiguity remained. This is due to the fact that crossin
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