Iridium coatings grown by metal–organic chemical vapor deposition in a hot-wall CVD reactor

Deposition of uniform coatings on relatively large size and/or complex shaped pieces require generally isothermal rather than cold-wall chemical vapor deposition (CVD) reactor. After a review of the state-of-the-art of Ir CVD processes aiming the selection of the starting materials, Ir thin films were deposited on W substrates by thermal decomposition of Ir(COD)(MeCp) either in presence of H2 or O2. The growth was carried out in a horizontal hot-wall metal–organic chemical vapor deposition reactor under reduced pressure and low temperature (573–673 K). Using this CVD reactor the process is more difficult to control using H2 rather than O2 as co-reagent. The purity, the microstructure, the growth rate and the thickness uniformity depend on the deposition conditions. Oxygen avoids carbon incorporation in the layers and enhances significantly the growth rate. However, co-deposition of Ir and IrO2 was observed using a high excess of O2. Polycrystalline, compact, untextured and pure Ir coatings were deposited with a satisfactory thickness uniformity over a length of approximately 15 cm and with a typical thickness of 1–2 μm. These coatings have attractive properties to be used as oxidation barriers at high temperature. Optimal deposition conditions were found using the trends predicted by a kinetic model simulating the growth rate along the CVD reactor. A good thickness uniformity along the reactor requires a very short residence time of the reactive species. As a result, the conversion rate is low leading to a poor efficiency of the process.