The Promotion of Silicide Formation using a Scanned Silicon Ion Beam
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THE PROMOTION OF SILICIDE FORMATION USING A SCANNED SILICON ION BEAM E.J. WILLIAMS, E.G. BITHELL, C.B. BOOTHROYD AND W.M. STOBBS University of Cambridge, Department of Materials Science and Metallurgy, Pembroke Street, Cambridge, CB2 3QZ, U.K. R.J. YOUNG AND J.R.A. CLEAVER University of Cambridge, Microelectronics Research Laboratory, Department of Physics, Milton Road, Cambridge, CB4 4FW, U.K. ABSTRACT The promotion of silicide reactions at the interface between silicon and a metal overlayer is described, the reactions being initiated by scanned ion beams. The relative effects of low and high energy Si÷ and Si2 + beams are discussed and the results of subsequent annealing are compared with those seen when using low energy (5keV) argon ion beams. The implications for the writing of metallisation lines are also noted. INTRODUCTION The uses of scanned ion beams for lithography, etching and even the electrical isolation of semiconductor device units are well known and have benefitted from improvements in the design and efficiency of liquid-metal ion-beam sources and appropriately filtered ion-optical columns [1,2]. We have become interested in the use of ion beams to promote silicide formation at the interface between deposited metal overlayers and a silicon substrate, with the ultimate aim of being able to write a fine metallisation interconnect. In principle it should even be possible to achieve an epitaxial orientation between the silicide and the substrate, in such a way that after removing the unreacted metal, further layers of single crystal silicon could be grown, with the potential for building up a three dimensional device structure. Here we report our current progress in determining whether or not such a method is feasible. A system which can react in an encouragingly simple manner on annealing consists of an overlayer of microcrystalline Pd 7oEr3o deposited on (001) Si. When annealed at 620K, discrete
islands of Pd 2 Si grow down into the silicon to dimensions of about 50nm by 25nm, retaining a flat upper surface. These islands are orientationally related to, and strongly constrained by the silicon, exhibiting two habits of a specific orientation relationship (henceforth designated A-type). This is such that (1210) Pd 2 Si is parallel to (001) Si (the substrate normal) and [1010] Pd 2Si and [0001] Pd2 Si are parallel to either [110] Si or [110] Si, the long dimension of the island being parallel to [0001] Pd 2Si [3]. It has been known for some time that a low energy (5keV) inert argon ion beam can promote continuous coverage by the silicide, but in a partially polycrystalline form [3,4], and it is appropriate to summarise our results on the annealing behaviour of silicide nucleated in this way, before going on to describe the effects we have found for higher energy Si ion beams. MATERIALS AND METHODS The Pd70 Er30 film examined was electron beam codeposited on an (001) Si substrate to a thickness of 140nm [5]. Work on the effects of a 5keV argon ion beam was carried out using a Gatan ion beam miller with a
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