Thermal Diffusivity of Diamond Wafers Deposited with Multicathode dc Plasma-Assisted CVD

A thermal diffusivity map for diamond wafers of 10-cm diameter was obtained using a converging thermal wave technique in a nondestructive and noncontact manner. Diamond wafers were deposited by seven-cathode dc plasma-assisted chemical vapor deposition with different CH₄concentrations in pure hydrogen and applied powers of the plasma. Six cathodes were located at the apexes of a hexagon with an arm distance of 4.3 cm about a central cathode. The wafer deposited at a low-power plasma (13.47 kW) and a low concentration of CH₄(6%, by volume) shows three circular zones on the thermal diffusivity map. The thermal diffusivity shows the lowest value at the center. It increases to about 10% in a radius of 2 to 3 cm and then decreases with further increases in the radius. The optical photograph and the Raman lines of the wafer show patterns similar to those of the thermal diffusivity. These are affected by the locations of the cathodes in the deposition chamber when the plasma power is low. Diamond wafers deposited at a high-power plasma (20.58 kW) with high concentrations of methane (10%, by volume) show higher values of thermal diffusivity and better uniformity than wafers deposited at a low power and low methane concentration. A fine crack can be located on a wafer with the converging thermal wave technique.