Effect of Fluorine on the Diffusion of Boron in Amorphous Silicon
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Effect of Fluorine on the Diffusion of Boron in Amorphous Silicon J. M. Jacques, L. S. Robertson, and K. S. Jones Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 Joe Bennett International SEMATECH, Austin, TX 78741 Mike Rendon Motorola, Austin, TX 78741 Abstract Fluorine and boron co-implantation within amorphous silicon has been studied in order to meet the process challenges regarding p+ ultra-shallow junction formation. Previous experiments have shown that fluorine can reduce boron TED (Transient Enhanced Diffusion), enhance boron solubility and reduce sheet resistance. In this study, boron diffusion characteristics prior to solid phase epitaxial regrowth (SPER) of the amorphous layer in the presence of fluorine are addressed. Samples were pre-amorphized with Si+ at a dose of 1x1015 ions/cm2 and energy of 70 keV, leading to a deep continuous amorphous surface of approximately 1500 Å. After preamorphization, B+ was implanted at a dose of 1x1015 ions/cm2 and energy of 500 eV, while F+ was implanted at a dose of 2x1015 ions/cm2 and energies ranging from 3 keV to 9 keV. Subsequent furnace anneals for the F+ implant energy of 6 keV were conducted at 550oC, for times ranging from 5 minutes to 260 minutes. During annealing, the boron in samples coimplanted with fluorine exhibited significant enhanced diffusion within amorphous silicon. After recrystallization, the boron diffusivity was dramatically reduced. Boron in samples with no fluorine did not diffuse during SPER. Prior to annealing, SIMS profiles demonstrated that boron concentration tails broadened with increasing fluorine implant energy. Enhanced dopant motion in as-implanted samples is presumably attributed to implant knock-on or recoil effects. Introduction As the semiconductor industry continues to scale silicon based microelectronic devices smaller and smaller, the pressure to achieve shallower junction depths becomes more significant. There has been increasing interest in the co-implantation of boron and fluorine for the formation of ultra-shallow junctions. The Transient Enhanced Diffusion (TED) of boron and other dopant species has been attributed as one of the primary limiting factors surrounding the formation of ultra-shallow, low resistivity junctions within the source and drain extension regions of transistors [1]. Several previous studies have described the unexpected ability of fluorine to significantly inhibit boron TED, while continuously increasing boron solubility limits during high temperature furnace annealing and rapid thermal annealing (RTA) [2-4]. These particular effects have been associated with the ability of fluorine to bind with excess silicon interstitials, thereby reducing the prevalence of boron cluster formation. However, these observations have only been studied within crystalline silicon. During conventional processing, implants are conducted within amorphous silicon, rather than crystalline silicon. Thus, in order to develop a succinct understanding of dopant diffusion pro
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