Dopant diffusion in amorphous silicon
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C10.2.1
Dopant diffusion in amorphous silicon R. Duffy1 , V.C. Venezia1 , A. Heringa 1 , M.J.P. Hopstaken2 , G.C.J. Maas2 , T. Dao2 , Y. Tamminga2 , F. Roozeboom2 1 2
Philips Research Leuven, Kapeldreef 75, 3001 Leuven, Belgium. Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands.
ABSTRACT In this work we investigate the diffusion of high-concentration ultrashallow boron, fluorine, phosphorus, and arsenic profiles in amorphous silicon. We demonstrate that boron diffuses at high concentrations in amorphous silicon during low-temperature thermal annealing. Isothermal and isochronal anneal sequences indicate that there is an initial transient enhancement of diffusion. We have observed this transient diffusion characteristic both in amorphous silicon preamorphized by germanium ion implantation and also in amorphous silicon preamorphized by silicon ion implantation. We also show that the boron diffusivity in the amorphous region is similar with and without fluorine, and that the lack of diffusion for low-concentration boron profiles indicates that boron diffusion in amorphous silicon is driven by high concentrations. Ultrashallow high-concentration fluorine profiles diffuse quite rapidly in amorphous silicon, and like boron, undergo a definite transient enhancement. In contrast, ultrashallow highconcentration phosphorus and arsenic profiles did not significantly diffuse in our experiments. EXPERIMENTAL Experiments were performed using 200 mm, Czochralski-grown, ‹100› oriented silicon wafers. Wafers were first implanted with germanium to a dose of 1×1015 cm-2 with an energy of 75 keV to preamorphize the silicon surface. Rutherford back-scattering (RBS) channeling measurements were used to determine the silicon amorphization depths, using a 2 MeV He+ beam. RBS channeling measurements indicated that the germanium implant produced a continuous amorphous layer to a depth of 103 nm. In some wafers a preamorphizing silicon implantation of dose 2×1015 cm-2 and energy 60 keV was used to produce a continuous amorphous layer to a depth of 113 nm. Following preamorphization, boron, fluorine, phosphorus, or arsenic was implanted to a dose of 1×1015 cm-2 with energies of 0.5 keV, 1 keV, 1.5 keV, and 2 keV, respectively. These energies were chosen so each profile would have approximately the same projected range. Some of the boron implanted wafers were first implanted with fluorine to a dose of either 1×1015 cm-2 or 2×1015 cm-2 with an energy of 6 keV. Finally, one wafer was implanted with boron to a dose of 1×1013 cm-2 with an energy of 0.5 keV to determine the low-concentration diffusion behavior. All implants were performed in single-quad mode at a tilt of 7° and a twist of 27°, with a native oxide covering the silicon substrate. Wafers were annealed in an inert N2 ambient, for various times, in the 400–600 °C temperature range. Dopant profiles were analyzed by secondary ion mass spectrometry (SIMS). RBS channeling measurements confirmed that all the dopant profiles presented here remained in amor
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