Diffusion of Fluorine at High Concentration in Silicon: Experiments and Models
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Diffusion of Fluorine at High Concentration in Silicon: Experiments and Models Robert R. Robison, Antonio F. Saavedra, Mark E. Law SWAMP Center, University of Florida, Gainesville, Florida 32611 USA ABSTRACT We have developed a model for high concentration fluorine diffusion and fluorine diffusion in amorphous silicon. In this context, we define high concentration fluorine to mean fluorine doses above the threshold of amorphization for implantation into silicon, which is approximately 1×1015/cm2 dose. We pre-amorphized with silicon to create a continuous amorphous region, and samples were subsequently implanted with the fluorine conditions of 16keV at 2×1015 cm-2 dose, 30keV at 2×1014 cm-2 dose, or 16keV at 8×1015 cm-2 dose. Samples were annealed by either conventional furnace or RTA with an N2 ambient for various times at temperatures of 550-750 °C . SIMS was used for depth profiling, and TEM images were also taken of the samples to check for defects and amorphous depth. We then created the model for the data by extending the fluorine model presented in our previous work, and it models the profile motion and the time dependence well. The model is also still capable of describing our previous work and fits it very well. INTRODUCTION Fluorine has long been implanted as a co-dopant during the implant of boron into silicon in the form of BF2. Fluorine can decrease the diffusion of the implanted boron and increase its activation which resulting in a shallower and less resistive junction, a requirement for continued transistor minimization. We have completed new experimental work examining high concentration fluorine and extended a model to explain the behavior. Previously, we developed a lower concentration fluorine model, showing the abnormal diffusion and its time-dependency [1]. However, other results have indicated that annealing behavior of fluorine in amorphized samples is distinct from its behavior in crystalline silicon. Robertson [2] showed experimental results with fluorine at amorphizing doses in which the majority of the dose was still present in the bulk after an anneal time on the order of hours. We have investigated the time dependent behavior through new experiments and have developed a model for the diffusion of fluorine in silicon at high concentrations. This model is a simple extension of our earlier work and continues to fit low concentration experimental work. This model will increase the understanding of fluorine diffusion and will allow for optimized coimplantation of dopants. EXPERIMENTAL SETUP P-type doped FZ silicon wafers of 80-200 Ω-cm were first pre-amorphized with dual Si implants of 150keV at 1×1015 cm-2 dose and 40keV at 1×1015 cm-2 dose. XTEM images taken showed that these implants created a uniform amorphous region that is 1800 Å deep. Samples were then implanted with the fluorine conditions in table 1. Samples were annealed by either traditional furnace or RTA with an N2 ambient, and SIMS was used for depth profiling. Both PTEM and XTEM images were taken to check for defects.
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