MPCVD is a very important method that has been frequently used for diamond deposition. Here, microwave plasma is used to activate the hydrocarbon feed and dissociate molecular hydrogen. Typically, 2.45 GHz is used as an excitation source. Microwave plasma oscillates electrons, which in turn produces ions by colliding with gas atoms and molecules. A schematic representation of a microwave plasma reactor is shown below for understanding. However, the deposition area is restricted for a microwave CVD reactor. Typical 2–3 cm substrates are used for deposition of diamond in an MPCVD reactor. The size of the plasma ball increases with increase in microwave power. It has been observed that intimate contact of the plasma ball with the substrate is not essential for diamond deposition by MPCVD technique. A substrate of ∼10 cm diameter may be coated uniformly with diamond without having the plasma ball in direct contact with the substrate.
Diamond Synthesis by Microwave-Plasma Chemical Vapor Deposition Using CH3Cl and CH2Cl2 as Carbon Source
Polycrystalline diamond films have been synthesized from gas mixture systems of CH2Cl2-H2 and CH3Cl-H2 by the microwave-plasma chemical vapor deposition (CVD) technique. The gas reaction speed was promoted by using CH3Cl and CH2Cl2, although the deposition rate decreased, due probably to surface etching by the chlorine in the case of high-concentration CH2Cl2. A broad band centered at about 1500 cm-1 in the Raman spectra decreased considerably in the films compared with those synthesized from the CH4-H2 gas mixture. Very little residual chlorine was detected in the deposited films. The use of chlorine-permuted methane as a carbon source was found to be advantageous for high-purity diamond deposition at low temperature and high concentration.