Measuring Vacancy Diffusivity and Vacancy Assisted Clustering by Nitridation Enhanced Diffusion of Sb IN Si(100) Doping
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ABSTRACT An investigation of the properties of Si native point defects was undertaken using low temperature annealing of molecular beam epitaxially grown Sb and B doping superlattice structures. The superlattice structures, consisting of 10 nm B or Sb doping spikes separated by 100 nm, were annealed in NH 3 at 810°C, 860°C, and 910'C. During thermal nitridation under these annealing conditions, we observed enhanced Sb diffusivity caused by the injection of vacancies, and retarded B diffusivity, resulting from the depletion of interstitials. Since the diffusivities of Sb and B are proportional to the vacancy and interstitial concentrations respectivity, spatially resolved diffusivity measurements permit effective point defect diffusivities to be inferred. From the spatial decay length of the Sb diffusion enhancement, lower limits for the effective vacancy diffusivity were obtained (limited by possible vacancy trapping). Evidence of vacancy assisted Sb clustering during NH 3 anneals is also presented.
INTRODUCTION As the trend toward smaller IC feature sizes continues, the need for increasingly accurate IC process simulation tools becomes more critical. These tools depend greatly on detailed modeling of point defect and dopant interactions, including point defect coupled diffusion and defect assisted dopant clustering. In addition, quantitative experimental values for fundamental point defect parameters are required for accurate predictive modeling of dopant and point defect profiles. Unfortunately, there is no direct method for observing Si native point defects in the bulk. Monitoring their interactions with dopant atoms, however, can be used as an indirect means to trace the diffusive behavior of Si self-interstitials and vacancies since it is well established that dopant diffusion in Si is mediated by point defects [1]. For example Sb diffuses predominantly through a vacancy mechanism, and B by an interstitialcy mechanism and thus the intrinsic diffusivity of Sb or B is directly proportional to the vacancy or Si self-interstitial concentration, respectively [2,3]. In this study, Si(100) doping superlattice (DSL) structures were used to examine the depth dependent diffusion of point defects. These thin film test structures, consisting of six narrow Sb or B doping spikes periodically spaced in a Si epilayer, were used to monitor dopant diffusivity at various depths. Since the diffusivity of each Sb or B doping spike measures the local time averaged concentration of vacancies or Si self-interstitials, respectively, the DSL structure as a whole enables measurements of the depth profile of the time averaged point defect concentration. Point 157 Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society
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Fig. ib: Intrinsic Sb diffusivity Dsb versus depth, obtained from analyzing the DSL peaks in Fig. la. Since DSb cx Cv, this plot can also be interpreted as the time averaged vac
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