Si Ultra Shallow Junctions Dopant Profiling with ADF-STEM
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Si Ultra Shallow Junctions Dopant Profiling with ADF-STEM A. Parisini1, D. Giubertoni2, M. Bersani2, V. Morandi1, P. G. Merli1, and J. A. van den Berg3 1 CNR-IMM, Sezione di Bologna, via P. Gobetti 101, Bologna, I-40129, Italy 2 Fondazione Bruno Kessler-irst, via Sommarive 14, Povo (Tn), I-38050, Italy 3 Joule Physics Laboratory, Institute of Materials Research, University of Salford, Salford, M5 4WT, United Kingdom ABSTRACT In this work, we show how the Z-contrast annular dark field scanning transmission electron microscopy technique can provide reliable dopant profiles in ultra shallow junctions in Si. Dopant profiles obtained with this technique are compared with those obtained by spectroscopic techniques like secondary ion mass spectroscopy and medium energy ion scattering. The comparison demonstrates how this non-spectroscopic technique represents a complementary tool to supply a reliable dopant localization at a sub-nm scale.
INTRODUCTION The huge scaling down of the electronic devices of the last 20 years has reduced the depth of the Shallow Junctions, SJ, by about one order of magnitude. These new Ultra Shallow Junctions, USJ, are localized at a depth of a few nanometers from the sample surface that consequently assumes a critical role in the determination of the dopant distribution during postimplantation annealing. Unexpected phenomena as dopant diffusion towards the surface, against the concentration gradient, have already been reported [1]. Thus the determination of the dopant profiles in USJ challenges the state-of-the-art of ion and electron beam characterization techniques. For example, Secondary Ion Mass Spectroscopy, SIMS, requires complementary techniques if accurate characterization of the top few nanometers of the dopant distribution is required [2]. In the same field of ion beam techniques, Medium Energy Ion Scattering, MEIS, offers the advantage of accurate quantification and high depth resolution, < 1nm, but suffers from a limited visibility of dopant atoms in substitutional sites in the double alignment mode [3]. As concerns electron beam characterization techniques, dopant localization with atomic resolution has recently been achieved by high resolution Annular Dark Field Scanning Transmission Electron Microscopy, ADF-STEM [4]. However, the rather extreme sample preparation requirements (thickness t < 2.5 nm, surface roughness < 0.1 nm rms [4]) as well as the possibilities of surface relaxation effects in these very thin samples suggest the development of a more routine characterization technique able to define, in samples of standard thickness, an in-depth dopant distribution at a sub-nm level. The Z-contrast method has been recently reconsidered by Merli et al. [5] and applied to the investigation of shallow implants in Si using low-energy electrons in a scanning electron microscope, SEM. In this work, it is shown that the resolution is given by the probe size whereas the signal contrast depends on the beam broadening. In the following, we will extend this approach to the hig
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