Chemical vapour deposition (or CVD for the English chemical vapor deposition) is a vacuum deposition method of thin films, from gaseous precursors.
Many liquid or gaseous fluorides at room temperature are used as precursors of thin-film deposition, particularly in the field of semiconductors.
For example, tungsten, molybdenum, and rhenium fluorides are highly used products for the production of refractory coatings capable of withstanding high temperatures.
Germanium or phosphorus fluorides are advantageously used in ionic implantation in place of their hydrogenated equivalents (Phosphine, PH3, is a highly toxic gas).
Titanium fluorides, TiF4, and Tantalus, TaF5, yield thin layers of alkali metal and rare earth (CaF2, MgF2, LaF3, YF3…) fluorides and more rapidly than with commonly used hydrofluoric acid.
In General, high valency fluorides are lighter than their chlorinated correspondents and can be implemented more easily than the latter. However, they may have a larger dissociation energy value. For example, the melting point of tantalum chloride, TaCl5, is 216 °C, while tantalum fluoride, TaF5, is 96.8 °C. But the dissociation energy of the latter is 1.2 times higher than that of the chlorinated derivative.
Bibliography
■ “Chemical vapor deposition of refractory metals from fluoride gases”, Lakhotkin et al, inorganic materials, May 2000, Volume 36, issue 5, pp 454-457
■ “Plasma treated thermal CVD of TaN films from tantalum halide precursors”, Hautala et al, Brevet US6268288 (B1), 1999
■ “Atomic layer deposition for optical applications: metal fluoride thin films and novel devices”, Tero Pilvi, thesis, 2008, University of Helsinki, Finland
