Boron Solubility Limits Following Low Temperature Solid Phase Epitaxial Regrowth
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Boron Solubility Limits Following Low Temperature Solid Phase Epitaxial Regrowth C. D. Lindfors and K. S. Jones Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611-6130 U. S. A. M. J. Rendon Semiconductor Products Sector, Motorola Inc. Austin, TX 44548 U. S. A. ABSTRACT The work described herein focuses on examining the effect of solid phase epitaxial regrowth (SPER) on boron implanted silicon. It is shown that boron levels within the silicon can greatly enhance or reduce the regrowth rate of the silicon. Electrical measurements show optimum sheet resistances for 5 keV, 2x1015 cm-2 implant conditions yielding sheet resistance values of ~140 Ω/sq at 500 oC annealing to ~120 Ω/sq at 650 oC. Results using Hall effect and four-point probe show lower doses of boron will become fully active but levels will drop significantly as dose is increased. Lastly, maximum active concentrations of boron appear to reach values of ~3-4x1020 cm-3 for a boron dose of 1x1015 cm-2 after SPER. Lower SPER anneal temperatures or higher doses tend to activate less boron. INTRODUCTION As the semiconductor industry continues to scale down device dimensions there is an increased need to activate higher levels of dopants to maintain low sheet resistance layers. The ITRS roadmap1 put forth by the Semiconductor Industry Association indicates the 35 nm technology node will need junctions as shallow as 13-17 nm with sheet resistances in the 100-400 Ω/sq range. However, conventional processing techniques, such as ion implantation and rapid thermal anneal, seem to be hitting a limit for the ability to generate shallow active layers. The major drawbacks arise from the phenomenon known as transient enhanced diffusion (TED) which has been shown to significantly increase junction depths2,3. To circumvent the effect of TED one needs to either anneal with a very high ramp rate for short periods or anneal at low temperatures which allows the underlying crystalline silicon to act as a seed to reorder the amorphous layer. The dopant atoms will rest on substitutional sites and become electrically active upon regrowth. There has been recent work4-6 and work done in the late 1970s to early 1980s7-10 that verifies high levels of active dopant can be achieved using SPER. The goal of this experiment was to study shallow junctions formed by SPER.
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EXPERIMENTAL N-type silicon wafers with orientation were amorphized using a 30 keV, 1x10 cm-2 Si+ implant and then implanted with B+ at 5 keV in the dose range of 5x1014–8x1015 cm-2. These wafers were then cut into 15x15 mm2 pieces so samples could be annealed at various times at temperatures. Anneals at 500 oC were carried out in a quartz tube furnace with a N2 purge at times of 20-45 minutes while the 550-650 oC anneals where performed using a rapid thermal anneal (RTA). Samples were annealed at 550 oC for 3-5.5 minutes, 600 oC for 25-50 seconds, and 650 oC for 4-15 seconds. To measure the success of the SPER process, several characterization techniques were employed. Vari
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