Microwave ͑MW͒ plasma-enhanced chemical vapor deposition ͑PECVD͒ reactors are widely used for growing diamond films with grain sizes spanning the range from nanometers through microns to millimeters. This paper presents a detailed description of a two-dimensional model of the plasma-chemical activation, transport, and deposition processes occurring in MW activated H/C/Ar mixtures, focusing particularly on the following base conditions: 4.4%CH 4 / 7%Ar/balance H 2 , pressure p = 150 Torr, and input power P = 1.5 kW. The model results are verified and compared with a range of complementary experimental data in the companion papers. These comparators include measured ͑by cavity ring down spectroscopy͒ C 2 ͑a͒, CH͑X͒, and H͑n =2͒ column densities and C 2 ͑a͒ rotational temperatures, and infrared ͑quantum cascade laser͒ measurements of C 2 H 2 and CH 4 column densities under a wide range of process conditions. The model allows identification of spatially distinct regions within the reactor that support net CH 4 → C 2 H 2 and C 2 H 2 → CH 4 conversions, and provide a detailed mechanistic picture of the plasma-chemical transformations occurring both in the hot plasma and in the outer regions. Semianalytical expressions for estimating relative concentrations of the various C 1 H x species under typical MW PECVD conditions are presented, which support the consensus view regarding the dominant role of CH 3 radicals in diamond growth under such conditions.

Spatially resolved optical emission spectroscopy ͑OES͒ has been used to investigate the gas phase chemistry and composition in a microwave activated CH 4 / Ar/ H 2 plasma operating at moderate power densities ͑ϳ30 W cm −3 ͒ and pressures ͑Յ175 Torr͒ during chemical vapor deposition of polycrystalline diamond. Several tracer species are monitored in order to gain information about the plasma. Relative concentrations of ground state H ͑n =1͒ atoms have been determined by actinometry, and the validity of this method have been demonstrated for the present experimental conditions. Electronically excited H ͑n = 3 and 4͒ atoms, Ar ͑4p͒ atoms, and C 2 and CH radicals have been studied also, by monitoring their emissions as functions of process parameters ͑Ar and CH 4 flow rates, input power, and pressure͒ and of distance above the substrate. These various species exhibit distinctive behaviors, reflecting their different formation mechanisms. Relative trends identified by OES are found to be in very good agreement with those revealed by complementary absolute absorption measurements ͑using cavity ring down spectroscopy͒ and with the results of complementary two-dimensional modeling of the plasma chemistry prevailing within this reactor.

Absolute column densities of C 2 ͑a͒ and CH radicals and H͑n =2͒ atoms have been measured in a diamond growing microwave reactor operating with hydrocarbon/Ar/ H 2 gas mixtures as functions of height ͑z͒ above the substrate surface and process conditions. The monitored species are each localized in the hot plasma region, where T gas ϳ 3000 K, and their respective column densities are each reproduced, quantitatively, by two-dimensional ͑r , z͒ modeling of the plasma chemistry. The H͑n =2͒ distribution is seen to peak nearer the substrate, reflecting its sensitivity both to thermal chemistry ͑which drives the formation of ground state H atoms͒ and the distributions of electron density ͑n e ͒ and temperature ͑T e ͒. All three column densities are found to be sensitively dependent on the C/H ratio in the process gas mixture but insensitive to the particular choice of hydrocarbon ͑CH 4 and C 2 H 2 ͒. The excellent agreement between measured and predicted column densities for all three probed species, under all process conditions investigated, encourages confidence in the predicted number densities of other of the more abundant radical species adjacent to the growing diamond surface which, in turn, reinforces the view that CH 3 radicals are the dominant growth species in microwave activated hydrocarbon/Ar/ H 2 gas mixtures used in the chemical vapor deposition of microcrystalline and single crystal diamond samples.

A microwave (MW) activated hydrogen plasma operating under conditions relevant to contemporary diamond chemical vapor deposition reactors has been investigated using a combination of experiment and self-consistent 2-D modeling. The experimental study returns spatially and wavelength resolved optical emission spectra of the d → a (Fulcher), G → B, and e → a emissions of molecular hydrogen and of the Balmer-α emission of atomic hydrogen as functions of pressure, applied MW power, and substrate diameter. The modeling contains specific blocks devoted to calculating (i) the MW electromagnetic fields (using Maxwell's equations) self-consistently with (ii) the plasma chemistry and electron kinetics, (iii) heat and species transfer, and (iv) gas−surface interactions. Comparing the experimental and model outputs allows characterization of the dominant plasma (and plasma emission) generation mechanisms, identifies important coupling reactions between hydrogen atoms and molecules (e.g., the quenching of H(n > 2) atoms and electronically excited H 2 molecules (H 2 *) by the alternate ground-state species and H 3 + ion formation by the associative ionization reaction of H(n = 2) atoms with H 2 ), and illustrates how spatially resolved H 2 * (and H α ) emission measurements offer a detailed and sensitive probe of the hyperthermal component of the electron energy distribution function.

CH4 and C2H2 molecules (and their interconversion) in hydrocarbon/rare gas/H2 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/H2 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 CH4 and C2H2 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 CH4 and C2H2. 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. CH4→C2H2 conversion occurs most efficiently in an annular shell around the central plasma (characterized by 1400