Stability of As and B doped Si with respect to overlaying CoSi 2 and TiSi 2 thin films
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G. Ghosh and L. Delaey Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, de Croylaan 2, 3030 Leuven, Belgium
V. Probst Siemens AG, Corporate Research and Development, Otto-Hahn-Ring 6, D-8000 Munchen 83, Federal Republic of Germany
P. Lippens, L. Van den hove, and R. F. De Keersmaecker Interuniversity Microelectronics Center (IMEC v.z.w.), Kapeldreef 75, 3030 Leuven, Belgium (Received 31 October 1988; accepted 17 April 1989)
The thermodynamic equilibrium of structures consisting of a thin film silicide (TiSi2 or CoSi2) on doped Si (with As or B) is investigated. Isothermal sections of the ternary phase diagrams for Ti-Si-B, Co-Si-B, Ti-Si-As, and Co-Si-As have been evaluated, indicating the stability of high B concentrations in Si underneath a CoSi2 layer, the instability of high As concentrations in Si underneath a CoSi2 layer, and of B and As concentrations underneath a TiSi2 layer. The obtained thermodynamic predictions agree very well with experimental results (i) on the redistribution of dopants during silicide formation, (ii) on the diffusion of dopants from an ion implanted silicide, and (iii) on the stability of highly doped regions underneath the silicide, both for the case of TiSi2 and CoSi2. It is shown that even though the inaccuracy of reported thermodynamic data is substantial, thermodynamic calculations provide a useful guidance and are consistent with the experimental results.
I. INTRODUCTION Silicides have found several applications in VLSI integrated circuits, both for low-resistivity interconnection lines and as part of the contact of these interconnection lines to the silicon substrate.1 The dopant concentration at the silicide/Si interface directly determines the contact resistance. Hence, the redistribution of dopants during and after silicide formation becomes an extremely important issue if shallow profiles with a high interface concentration level are required. In a widespread endeavor to select the silicide with optimal properties for implementation in MOS or bipolar processes, comparative studies of transition-metal silicides have been undertaken. Various silicides and their formation technologies have been compared to obtain a high quality thin film silicide on a highly doped Si substrate.2"10 The various silicide formation and doping technologies aim at a final structure consisting of a low resistivity silicide on top of a highly doped Si exhibiting a low contact resistivity, independent of the technological path that has been followed. Moreover, it is desirable that this structure can withstand high temperatures without degeneration of the silicide or of the doping level at the silicide/Si interface. Up to now the behavior of dopants with respect to silicides has been interpreted mainly in terms of kinetic arguments such as the main moving species during silicide formation,11 point defect generation during silicide formaJ. Mater. Res., Vol. 4, No. 5, Sep/Oct 1989
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