In spite of much work, the formation of epitaxial CoSi2 from Ti/Co on (100) Si remains something of a mystery. It has been proposed that epitaxy occurs via the formation of an intermediate phase of CoSi with a (311) preferred orientation. In the absence of sufficient information it is impossible to validate or to invalidate the specific original claim. However, one shows that the formation of preferably oriented CoSi is not a necessary condition for the subsequent growth of epitaxial CoSi2. Careful measurements of diffraction intensities reveal the probable, temporary formation of a metastable form of CoSi2, based on a diamond cubic rather than the usual CaF2 structure.
Tungsten disilicide (WSi2) was formed by annealing tungsten films deposited by low-pressure chemical vapor deposition on 〈100〉-silicon substrates. The influence of oxygen on the silicidation rate was studied. Si wafers with different oxygen content in the form of Czochralski, float-zone, and epitaxial wafers were used. Oxygen was also ion implanted into either the silicon substrate or the as-deposited tungsten film. The Rutherford backscattering technique was used to follow the progress of the silicidation. The silicidation rate was found to be dependent on the oxygen content of the Si substrates. The rate was lowest for Czochralski substrates and highest for float-zone substrates. Secondary ion mass spectroscopy was used to study the oxygen and fluorine profiles in the films prior to and after silicidation. Growth of WSi2 was found to be retarded concurrently with a pile-up of fluorine at the tungsten side of the W/WSi2 interface and a gettering of oxygen from the annealing atmosphere at the interface. Growth of WSi2 was then transferred to the tungsten surface. Oxygen implantation into silicon and tungsten, respectively, reduced the rate of silicide formation. Oxygen implantation into tungsten altered the distribution of fluorine and suppressed WSi2 growth at the tungsten surface. The observations led to a conceptual model, which ascribes the retardation in the growth of the inner WSi2 to a‘‘poisoning’’ effect caused by the increase of oxygen and fluorine levels at the interface.
Contactless and accurate measurement of the temperature is an important issue in rapid thermal processing. In this letter we report on the variation of the emissivity of the surface during chemical vapor deposition of polycrystalline silicon onto oxidized silicon wafers. The observed periodic change in the emissivity of the polycrystalline silicon/silicon dioxide/silicon structure is due to interference phenomena in the growing layer. In these experiments the silicon wafers were heated by direct absorption of microwaves in a single wafer reactor. It is shown that with this method of heating, the change of emissivity does not influence the actual temperature of the substrate.
The redistribution of titanium during the formation of epitaxial CoSi 2 , grown from the reaction of Co͑20 nm͒/Ti͑10 nm͒ bilayers with Si ͗100͘, has been investigated. Annealing of Co/Ti/Si structures, at temperatures between 850 and 1050°C, is shown to be associated with the growth of an inhomogeneous CoSi 2 layer having Ti-rich surface layer͑s͒ on top. The formation of inhomogeneities in the CoSi 2 layer is conclusively attributed to the presence of Ti-rich surface layer͑s͒. It is shown that smooth and morphologically stable CoSi 2 layers can be grown by removing these surface layers followed by a high-temperature treatment in nitrogen atmosphere. We propose that the underlying mechanism for the inhomogeneity formation within the CoSi 2 layer is a nucleation-controlled process, induced by an anticipated reaction between the CoSi 2 layer and Ti-rich phases near the surface.
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