Discrete control of individual dopant or impurity atoms is critical to the electrical characteristics and fabrication of silicon nanodevices. The unavoidable introduction of defects into silicon during the implantation process may prevent the uniform distribution of dopant atoms. Cottrell atmospheres are one such nonuniformity and occur when interstitial atoms interact with dislocations, pinning the dislocation and trapping the interstitial. Atom probe tomography has been used to quantify the location and elemental identity of the atoms proximate to defects in silicon. We found that Cottrell atmospheres of arsenic atoms form around defects after ion implantation and annealing. Furthermore, these atmospheres persist in surrounding dislocation loops even after considerable thermal treatment. If not properly accommodated, these atmospheres create dopant fluctuations that ultimately limit the scalability of silicon devices.
The anomalous diffusion of ion implanted boron into silicon is shown to be a transient effect with a decay time that decreases rapidly with increasing anneal temperature. The decay time is approximately 45 min at 800 °C and decreases to the order of a second at 1000 °C. The anomalous displacement in the low concentration region is greater at low temperatures but a larger fraction of the boron is redistributed at high temperature. Sheet resistance measurements agree with the idea that the moving fraction of the boron atoms is electrically active and limited to the intrinsic carrier concentration at the anneal temperature. The activation energy for the decay of the transient is greater than that for the diffusion coefficient, which makes an appropriate rapid thermal anneal cycle an important practical process in the fabrication of shallow p-n junctions.
Quantitative two-dimensional maps of electrostatic potential in device structures are obtained using off-axis electron holography with a spatial resolution of 6 nm and a sensitivity of 0.17 V. Estimates of junction depth and variation in electrostatic potential obtained by electron holography, process simulation, and secondary ion mass spectroscopy show close agreement. Measurement artifacts due to sample charging and surface "dead layers" do not need to be considered provided that proper care is taken with sample preparation. The results demonstrate that electron holography could become an effective method for quantitative 2D analysis of dopant diffusion in deep-submicron devices.
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