Mesotaxy: Formation of Buried Single-Crystal CoSi 2 Layers by Implantation
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MESOTAXY: FORMATION OF BURIED SINGLE-CRYSTAL CoSi 2 LAYERS BY IMPLANTATION
Alice E. White, K. T. Short, R. C. Dynes, J. P. Garno, and J. M. Gibson AT&T Bell Laboratories, Murray Hill, NJ 07974
ABSTRACT Using high dose implantation of 200 keV Co ions followed by high temperature annealing, we have created buried layers of CoSi 2 in crystalline Si of both (100) and (111) orientations. For a dose of 3X 1017 Co/cm 2 , the layer that forms is -1100A thick and the overlying Si is -600A thick. A lower dose of 2X1017 Co/cm2 yields a thinner layer, 700A thick, under 1200A of crystalline Si. Rutherford Backscattering and channeling analysis of the layers shows that they are aligned with the substrate (Xmin of the Co as low as 6.4%.) and TEM inspection of the (100) CoSi 2 /Si interfaces shows that they are abrupt and epitaxial (wit0 occasional small facets). Moreover, electrical characterization of these layers yields resistance ratios that are better than epitaxial CoSi2 films grown by more conventional UH\T methods.
INTRODUCTION Metal sUicides are of fundamental as well as practical interest because of their low resistivities, their resistance to high temperature processing, and their compatibility with silicon. Recently, much progress has been made in growing surface epitaxial cobalt disilicide (CoSi 2 ) films by ultra-high vacuum (UHV) deposition of Co onto Si wafers followed by solid-state reaction at elevated temperature (-650°C) [1]. Such films typically have very smooth. abru~pt interfaces when grown on (111) Si, but they tend to form large facets on (10 S1) i. We have successfully fabricated buried single-crystal CoSi.2 layers iising a completely different technique: high dose implantation and annealing 12]. These layers are in registry with the Si surface and substrate lattices of both (100) and (111) orientations. Since the alignment and compound formation are occuring inside the crystal, we call this process "mesotaxy" in analogy with epitaxy which refers to alignment on a surface.
EXPERIMENTAL The Co implants were carried out using an Eaton Nova NV-4T ion implanter at 200 keV with a - 20pA/cm 2 59 Co+ beam. The Si substrates, which were of both (100) and (111) orientations, were clamped to a stainless steel block which provided implant temperature variation from -196"C to 450 *C. Due to beam heating and thermal mismatch between the wafer and the holder, the temperature of the wafer Mat. Res. Soc. Symp.Proc. Vol 74. ' 1987 Materials Research Society
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may have risen up to -- 100C above the quoted temperature of the holder. 17 2 Implant doses were varied from 1X1016 to 4X10 /cm . The post-implant annealing was performed in two steps: a low temperature anneal (-- 6000 C) for 100 min. to partially regrow the Si and a higher temperature anneal (ranging from 900aC to 1200aC) to complete the reaction. Anneals done below 1000'C were performed in a conventional tube furnace under vacuum (3X101a/cm2 always yielded Co-rich surface layers, implying that the implant was sputter-limited. We also observed that such imp
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