Large-area polycrystalline diamond (PCD) coatings are important for fields such as thermal management, optical windows, tribological moving mechanical assemblies, harsh chemical environments, biological sensors, etc. Microwave plasma chemical vapor deposition (MPCVD) is a standard technique to grow high-quality PCD films over large area due to the absence of contact between the reactive species and the filament or the chamber wall. However, the existence of temperature gradients during growth may compromise the desired uniformity of the final diamond coatings. In the present work, a thick PCD coating was deposited on a 100-mm silicon substrate inside a 915-MHz reactor; the temperature gradient resulted in a non-uniform diamond coating. An attempt was made to relate the local temperature variation during deposition and the different properties of the final coating. It was found that there was large instability inside the system, in terms of substrate temperature (as high as ΔT = 212 ℃), that resulted in a large dispersion of the diamond coating's final properties: residual stress (-15.8 GPa to +6.2 GPa), surface morphology (octahedral pyramids with (111) planes to cubo-octahedrals with (100) flat top surfaces), thickness (190 µm to 245 µm), columnar growth of diamond (with appearance of variety of nanostructures), nucleation side hardness (17 GPa to 48 GPa), quality (Raman peak FWHM varying from 5.1 cm-1 to 12.4 cm-1 with occasional splitting). This random variation in properties over large-area PCD coating may hamper reproducible diamond growth for any meaningful technological application.
CSIR, India provided financial support for the network project "Very High Power MW Tubes: Design and Development Capabilities (MTDDC)" (CSIR Grant Nos. NWP 0024 and PSC0101) under the 11th and 12th five-year plan periods. Dr. J. C. Mendes is thankful to the Instituto de Telecomunicações for providing the necessary working conditions. Lambda Technologies Inc. USA was helpful in carrying out the CVD experimental runs.
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