Metal Silicides Formed by Direct Ion Beam Deposition
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METAL SILICIDES FORMED BY DIRECT ION BEAM DEPOSITION* R. A. ZUHR, S. J. PENNYCOOK, T. E. HAYNES, AND 0. W. HOLLAND, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 ABSTRACT Thin films of transition metal silicides have been produced at low temperatures on Si substrates by direct ion beam deposition (IBD) of the metal ion. Using a mass-analyzed beam of metallic ions rastered over the target at energies on the order of 100 eV and at substrate temperatures near 500*C, stoichiometric silicide films of varying thicknesses up to 300 nm have been formed on both n- and p-type Si. The advantages of this technique over other methods for silicide formation include good control of thickness by current integration, high purity due to the mass analysis, and control of incident ion energy which permits formation of the disilicide phase at low temperatures, thereby minimizing the thermal budget and the associated dopant diffusion in the underlying substrate. Films were characterized by Rutherford backscattering, transmission electron microscopy, and electrical measurements. Co, Fe, Ni, Ti, and W silicides have been formed by this direct deposition process. The effectiveness of the technique has been found to be dependent upon the diffusion characteristics of the particular metal/Si couple involved, with systems in which Si is the dominant diffuser, such as Ti/Si, giving the best results. Stoichiometric TiSi 2 films produced at 550'C by this process show low bulk-like resistivity (15 piQ-cm) without subsequent high-temperature annealing. All of these characteristics make silicide formation by IBD attractive for integrated circuit fabrication and shallow junction technology. I. INTRODUCTION Metal silicides are important as conducting layers or interconnects in the formation of integrated circuits in silicon because of their low resistivity, resistance to electromigration, and thermal stability. Specifically, they have been used to lower the sheet resistance of active areas and gate electrodes in complimentary metaloxide-semiconductor (CMOS) transistors.1 ,2 Physical vapor deposition by either cosputtering using separate metal and Si targets, or sputtering from a single silicide target, has been used to form stoichiometric silicide films. Silicide films have also been deposited by chemical vapor deposition. Another technique consists of depositing a thin film of the pure metal on a Si substrate and thermally reacting the metal/Si couple. While each of these techniques has its own advantages and disadvantages, they share two common drawbacks. First, the film purity is only as good as the starting material. Any impurities in the sputter-targets or gases will be incorporated directly into the films resulting in a higher intrinsic resistivity. Secondly, as-deposited films must be annealed at high temperatures to lower their resistivity, which is usually much higher than the bulk value. For example, temperatures in excess of 800 0C are needed to reduce the resistivity of TiSi 2 films to their lowest value. 3 Signi
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