Quantum cascade laser investigations of CH_4 and C_2H_2 interconversion in hydrocarbon/H_2 gas mixtures during microwave plasma enhanced chemical vapor deposition of diamond

摘要

CH_4 and C_2H_2 molecules (and their interconversion) in hydrocarbon/rare gas/H_2 gas mixtures in a microwave reactor used for plasma enhanced diamond chemical vapor deposition (CVD) have been investigated by line-of-sight infrared absorption spectroscopy in the wavenumber range of 1276.5-1273.1 cm~(-1) using a quantum cascade laser spectrometer. Parameters explored include process conditions [pressure, input power, source hydrocarbon, rare gas (Ar or Ne), input gas mixing ratio], height (z) above the substrate, and time (t) after addition of hydrocarbon to a pre-existing Ar/H_2 plasma. The line integrated absorptions so obtained have been converted to species number densities by reference to the companion two-dimensional (r,z) modeling of the CVD reactor described in Mankelevich et al. [J. Appl. Phys. 104, 113304 (2008)]. The gas temperature distribution within the reactor ensures that the measured absorptions are dominated by CH_4 and C_2H_2 molecules in the cool periphery of the reactor. Nonetheless, the measurements prove to be of enormous value in testing, tensioning, and confirming the model predictions. Under standard process conditions, the study confirms that all hydrocarbon source gases investigated (methane, acetylene, ethane, propyne, propane, and butane) are converted into a mixture dominated by CH_4 and C_2H_2. The interconversion between these two species is highly dependent on the local gas temperature and the H atom number density, and thus on position within the reactor. CH_4 →C_2H_2 conversion occurs most efficiently in an annular shell around the central plasma (characterized by 1400