Due to attractive surface properties and to intrinsic brittleness of Complex Metallic Alloys
(CMAs), most of their potential applications involve materials with high surface-to-volume ratios, including
thin films and coatings. While physical vapor deposition techniques are efficient for the processing of CMA films
on line-of-sight surfaces, chemical vapor deposition (CVD) is well positioned for their application on complex surfaces.
However, for CVD process to be implemented efficiently in the processing of CMA films a number of challenges must be
addressed. Because numerous CVD reagents, commonly called precursors, are low vapor pressure liquids or solids, one of
these challenges is the production of vapors of such precursors, which are decomposed in the deposition chamber to provide
the desired film. Such a production has to be ensured at high rate and must be reproducible and stable during the
whole process. Actual solutions to this question involve (i) bubbling inert gas through thermally regulated liquid precursors,
(ii) leaching the surface of fixed precursor powder beds, and (iii) in situ generating the precursor flow by passing a
reactive gas through a thermally regulated bed of the metallic element to be transported. Such solutions neither may be
satisfactory for actual R&D needs nor may be transferable to industrial environments. These reasons are in part responsible
for the limited implementation of advanced materials (including CMA-based ones) in numerous industrial and hence
societal needs. More recently, innovative solutions have been proposed to feed deposition systems based on vapor phase
chemical techniques (CVD and Atomic Layer Deposition, ALD). Such solutions are Direct Liquid Injection (DLI) of dissolved
solid precursors and also sublimation of the latter in fluidized beds or in elaborated fixed beds. Such technological
responses show promise for industrial applications of CVD, especially for the deposition of metals and ceramic compounds
for which the available molecular and inorganic precursors present low vapor pressures. This review provides an
overview of the methods by which precursor vapors are transported to the deposition chamber. Early and recent patents dedicated
to such technologies will be revisited and considered in the light of the deposition of multimetallic alloy coatings.
Keywords: CVD, chemical vapor deposition, CMA, coatings, complex metallic alloys, delivery system, fluidization, liquid
precursor, solid precursor, sublimation, thin films, vaporization.
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