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The stability of selectively formed TiSi2 films on single crystal and polycrystalline silicon layers at elevated process temperatures is reported. Extensive electrical and analytical studies were performed to understand the high-temperature stability of TiSi2 films as a function of (i) substrate dopant concentration, (ii) titanium silicide thickness, (iii) silicide formation sequence, and (iv) silicide post-processing steps. It is shown that all four process variations have a profound influence on the thermal stability of TiSi2 films. It is observed that titanium silicide films formed on single crystal silicon are stable at higher processing temperatures compared to those formed on polysilicon substrates under similar conditions. The degradation of high-temperature stability of TiSi2 films on polycrystalline silicon can be related to increased number of defects and grain boundaries. It is shown that TiSi2 films can be successfully used in silicon integrated circuit applications where the post-silicide processing temperatures do not exceed 1000 °C.

I. INTRODUCTION Polycrystalline silicon is extensively applied as lowresistivity gates in MOS devices, as emitter junctions in scaled bipolar transistors, as conducting substrates in display technologies, and in developing a variety of devices for advanced optoelectronic applications.1 Heavily doped polycrystalline silicon is also used to fabricate low-resistivity electrical and optical interconnections. The phosphorus solid-solubility limit in polysilicon, polysilicon thickness limitations resulting from lithographic and wafer processing constraints, and postgate deposition temperatures determine the nature of phosphorus confinement in polysilicon. In MOS-based technologies, it is important to ensure that phosphorus in polysilicon does not diffuse into the gate oxide because it leads to MOS gate threshold voltage nonuniformities and results in degraded MOS channel transport. The diffusion of phosphorus from polysilicon may lead to increased cross-talk and degraded noise immunity when used as interconnections. Generally, a polysilicon sheet resistance below 15 O / D is difficult to achieve at the completion of device processing. Refractory silicides of transition metals have been extensively used in silicon device technologies for fabricating low-resistivity MOS gates, interconnections, and ohmic contacts.2 The desirable silicide thin film properties for these applications include high conductivity and selectivity, improved thermal stability and adhesion, good process compatibility and step coverage, and mina

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1502

http://journals.cambridge.org

J. Mater. Res., Vol. 6, No. 7, Jul 1991

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imal perturbation in silicon junction parameters. Low thermal budgets required in scaled device technologies to maintain the integrity of shallow junctions place an upper bound on the silicide formation and subsequent post-processing temperatures.3 Previous stud