Synthesis of Buried Silicon Compounds Using Ion Implantation
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SYNTHESIS OF BURIED SILICON COMPOUNDS USING ION IMPLANTATION
Alice E. White, K. T. Short, R. C. Dynes, J. M. Gibson, and R. Hull AT&T Bell Laboratories Murray Hill, NJ 07974
ABSTRACT Ion implantation is widely used for doping semiconductors at low concentration, but, with the advent of a new generation of high current implanters, synthesizing new materials rather that simply doping them has become feasible. This technique has been successfully applied to fabricating silicon-on-insulator (SOI) structures with oxygen and nitrogen for several years. Since we are interested in understanding the mechanisms of formation of these layers, we have concentrated on sub-stoichiometric implantation doses of oxygen where it is easier to observe the coalescing layer. In order to determine whether this process of compound formation is more general, our studies were expanded to include implantation of the transition metals. Here, elevated substrate temperatures are necessary to minimize Si surface damage. The resulting disilicide layers are of remarkably high quality: they are single crystals in registry with the silicon wafer and they have better residual resistivities than comparable UIV-reacted silicides.
INTRODUCTION Since ion implantation was invented almost 40 years ago, implanters have become permanent fixtures on integrated circuit processing lines. In fact, manufacture of today's chips can involve as many as 10 implantation steps. Implantation is used primarily at doses of 1012 - 1013 ions/cm 2 for doping the semiconductor to tailor the electrical properties. Other conventional uses of implantation include amorphization for isolation or fundamental solid state studies, mixing for phase formation, and heavy doping for contacts. In the past, applications of implantation were dominated by the small beam currents that were available, but recently a new generation of high current implanters has entered the market. It is this high current capability that allows us to implant the large concentrations required for compound formation--about five orders of magnitude higher than those required for doping. In this paper, we will briefly discuss our results on the formation of substoichiometric buried oxide layers as the first example of compound synthesis in silicon. In order to determine whether this process was limited to Si0 2 and Si 3 N4 , we decided to explore the silicides. Several of the silicides are very good conductors and, in general, they have high mechanical and thermal stability and are compatible with silicon processing. We started with CoSi 2 because of the demonstration of epitaxial growth of such films on silicon using Co deposition and reaction [1]. It has the CaF 2 structure and a small lattice mismatch with silicon, -1.2% at room temperature. Using this technique of
Mat. Ras. Soc. Symp. Proc. Vol. 100. c1918 Materials Research Society
implantation and annealing, we have grown buried single-crystal layers of CoSi 2 which are oriented with the silicon substrate. We call this technique mesotaxy, for arrangement
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