Mechanisms and Kinetics of the Hydrogen-Free CVD of Protective Tantalum Coatings

The kinetics and mechanisms of the hydrogen-free chemical-vapor-deposition (CVD) method for protective tantalum coatings in the TaBr5–Cd system on St3, tungsten, and copper substrates in the temperature range of 700–950°C are investigated by X-ray diffraction, scanning electron microscopy, glow-discharge optical emission spectroscopy, and polarization curves. The thickness of the coatings obtained on St3, tungsten, and copper is 2.8–15.7, 2.2–5.3, and 2.0 µm respectively. The calculated activation energy of the CVD process during deposition onto St3 and tungsten (68 and 28 kJ/mol, respectively) indicates a diffusion-limiting stage. In the series of copper-tungsten-St3 substrates, the deposition rate of the tantalum coating increases and agrees with the negative enthalpy of formation of intermetallic compounds ΔHMeTa (Me is the substrate metal), which is associated with an increase in the adsorption interaction between the substrate and TaBr5 in it. It is shown that dense α-Ta-based coatings are deposited onto St3 at T = 700–750°C, and loose coatings based on face-centered cubic (fcc) tantalum with an admixture of lamellar hexagonal close-packed (hcp) tantalum crystals are deposited at T = 800°C and higher. Accordingly, dense body-centered tetragonal (bct) β-Ta-based coatings are deposited onto tungsten at T = 700–750°С; at 800–900°С, loose α-Ta-based coatings are deposited. A coating consisting of a mixture of α and β phases is obtained on copper at 800°C. The sums of the probabilities of deformation (α) and twinning (β) stacking faults (SFs) (1.5α + β) in deposited bcc ({112} planes) and fcc ({111} planes) tantalum are calculated by the harmonic analysis of diffraction lines according to Warren, and they range from 0.04 up to 1.2 and from 0.03 to 2%, respectively. The discovered SFs are likely to be closely related to the formation mechanisms of nonequilibrium bct, fcc, and hcp tantalum phases. Additional annealing (1000°С) of the β-Ta-based coating (tungsten substrate) leads to the formation of α-Ta, while annealing of the α-Ta-based coating (St3 substrate) leads to the formation of fcc tantalum. The formation of fcc tantalum crystals on an St3 substrate at T ≥ 725°C during CVD or as a result of annealing is assumed to be due to the α → γ phase transition in St3. The obtained α-Ta-based coatings demonstrate high corrosion properties.