Dc-arcjet Reactor Research Articles

Measurement and modeling of Ar∕H2∕CH4 arc jet discharge chemical vapor deposition reactors. I. Intercomparison of derived spatial variations of H atom, C2, and CH radical densities

Comparisons are drawn between spatially resolved absorption spectroscopy data obtained for a 6.4kW dc arc jet reactor, operating with Ar∕H2∕CH4 gas mixtures, used for deposition of thin, polycrystalline diamond films, and the results of a two-dimensional (r,z) computer model incorporating gas activation, expansion into the low pressure reactor, and the chemistry of the neutral and charged species. The experimental measurements, using either cavity ring-down spectroscopy or diode laser absorption spectroscopy, determined absolute number densities of H(n=2) atoms, and column densities of C2(aΠu3), C2(XΣg+1), and CH(XΠ2) radicals, with vibrational and rotational quantum state resolutions, and their variation with height through the horizontally propagating arc jet plume. Spectra were also analyzed to obtain temperatures and local electron densities [from Stark broadening of H(n=2) absorption lines]. The experimental data are directly compared with the output data of the model that returns spatially inhomogeneous temperature, flow velocities, and number densities of 25 neutral and 14 charged species. Under the base operating conditions of the reactor [11.4SLM (standard liters per minute) of Ar and 1.8SLM of H2 entering the primary torch, with addition of 80SCCM (SCCM denotes cubic centimeter per minute at STP) of CH4 downstream; 6.4kW input power; reactor pressure of 50Torr], the calculated and measured column and number densities agree to within factors of 2–3, the model reproduces the spatial dependence of column densities, and the mean temperatures of C2(a) and CH(X) radicals derived from spectra and model results are in good agreement. The model also captures the variation of these parameters with changes to operating conditions of the reactor such as flows of H2 and CH4, and input power. Further details of the model and the insights it provides are the subject of the accompanying paper [Mankelevich et al., J. Appl. Phys. 102, 063310 (2007) ].

Spatial profiling of H(n = 2) atom number densities in a dc arc jet reactor

Spatially resolved measurements of the absorption by H(n = 2) atoms on the Balmer-α transition in the plume of a dc arc jet reactor, operating at an input power of 6.4 kW and with an Ar/H2 feedstock gas mixture, are used to extract radially dependent H(n = 2) atom number densities at flows of H2 of 0.2, 0.5, 0.8 and 1.0 slm (standard litres per minute). The analysis to obtain number densities employs inverse Abel transformation of measured column densities and assumes cylindrical symmetry within the gas plume. The measured number density distributions are compared with the outcomes of a computer model of the arc jet and show quantitative agreement. Annular structure in the H(n = 2) distributions, evident at the lower added H2 flows, is a consequence of Ar+ reaction with H2 molecules diffusing radially into the plume from the cooler periphery of the reactor, followed by a dissociative electron attachment to ArH+ ions. At the higher H2 flows, the H(n ⩾ 2) distribution retracts towards the nozzle through which the plasma enters the reactor and peaks on the central axis, giving a conical structure to the luminous gas plume. These variations of plume structure with added H2 flow are well described by the sophisticated two-dimensional model, which includes heat, mass and radiation transfer and chemical kinetics of the expanding argon–hydrogen plasma, gas-surface processes at the substrate and reactor walls and carefully characterized initial conditions for the gas expansion from the nozzle orifice into the reactor chamber.

Measurement and modeling of a diamond deposition reactor: Hydrogen atom and electron number densities in an Ar∕H2 arc jet discharge

A combination of experiment [optical emission and cavity ring-down spectroscopy (CRDS) of electronically excited H atoms] and two-dimensional (2D) modeling has enabled a uniquely detailed characterization of the key properties of the Ar∕H2 plasma within a ⩽10-kW, twin-nozzle dc arc jet reactor. The modeling provides a detailed description of the initial conditions in the primary torch head and of the subsequent expansion of the plasma into the lower pressure reactor chamber, where it forms a cylindrical plume of activated gas comprising mainly of Ar, Ar+, H, ArH+, and free electrons. Subsequent reactions lead to the formation of H2 and electronically excited atoms, including H(n=2) and H(n=3) that radiate photons, giving the plume its characteristic intense emission. The modeling successfully reproduces the measured spatial distributions of H(n>1) atoms, and their variation with H2 flow rate, FH20. Computed H(n=2) number densities show near-quantitative agreement with CRDS measurements of H(n=2) absorption via the Balmer-β transition, successfully capturing the observed decrease in H(n=2) density with increased FH20. Stark broadening of the Balmer-β transition depends upon the local electron density in close proximity to the H(n=2) atoms. The modeling reveals that, at low FH20, the maxima in the electron and H(n=2) atom distributions occur in different spatial regions of the plume; direct analysis of the Stark broadening of the Balmer-β line would thus lead to an underestimate of the peak electron density. The present study highlights the necessity of careful intercomparisons between quantitative experimental data and model predictions in the development of a numerical treatment of the arc jet plasma. The kinetic scheme used here succeeds in describing many disparate observations—e.g., electron and H(n=2) number densities, spatial distributions of optical emission from the plume, the variation of these quantities with added flow of H2 and, when CH4 is added, absolute number densities and temperatures of radicals such as C2 and CH. The remaining limitations of the model are discussed.

Improved characterisation of C 2 and CH radical number density distributions in a DC arc jet used for diamond chemical vapour deposition

Modelling studies of the plasma chemistry prevailing in CH 4/H 2/Ar mixtures in a DC arc jet reactor used for diamond chemical vapour deposition are reported, together with complementary new experimental data. Gas temperatures, T gas, close to the substrate have been determined via analysis of the measured rotational state population distribution in C 2( a) radicals and found to be ∼3200 K—similar to the temperature established previously in the free plume region. These, and previous (J. Appl. Phys. 92 (2002) 4213), T gas and number density measurements are in good accord with the first results from a full 2D ( r, z) modelling of the plasma chemical transformations and heat and mass transfer processes within the evolving plume and the periphery of the reaction chamber. The modelling shows formation of a shock front in the supersonic expansion, a few millimeters downstream from the nozzle exit. The spatial distributions of the various species number densities are predicted to display localised maxima and minima within the reaction chamber, reflecting the complex balance between gas flow, diffusive transfer and chemical transformations in the widely varying range of local conditions (notably T gas and the H and H 2 concentrations). The calculations provide clear evidence of the importance of gas flow re-circulation in transporting the hydrocarbon feedstock gas (methane) from the injection ring to the hot plume. C 2H 2, C 2H, C, CH, C 2 and C 3 species are all predicted to be present at number densities >5×10 12 cm −3 in the plume incident on the substrate; it is suggested that all of the major C containing radical species (i.e. most notably C 2H, C, CH, C 2 and C 3) must make some contribution to material growth in order to satisfy the experimentally measured film deposition rate.

Cavity ring-down spectroscopy measurements of the concentrations of C 2(X 1Σ g+) radicals in a DC arc jet reactor used for chemical vapour deposition of diamond films

Absorption spectroscopy measurements are reported of the absolute number density (3.0 ± 0.9 × 10 12 cm −3) and vibrational temperature (3000 ± 500 K) of C 2(X 1Σ g +) radicals in a DC arc activated chemical vapour deposition reactor under the standard operating conditions (3.3% CH 4 in H 2 and 6.4 kW discharge) used to grow polycrystalline diamond films. The ratio of number densities of C 2 radicals in their ground (X 1Σ g +) and low-lying electronically excited (a 3Π u) states is close to that expected from a Boltzmann distribution at the measured temperature, despite the more rapid rates of reaction of C 2(X) with H 2 and hydrocarbon molecules.

Number density and temperature of acetylene in hot-filament and arc-jet activated CH 4/H 2 gas mixtures measured using diode laser cavity ring-down absorption spectroscopy

Continuous-wave cavity ring-down spectroscopy (CW-CRDS) with an external cavity diode laser has been used to make diagnostic measurements of acetylene absorption in hot-filament and DC-arc plasma jet reactors operating with 1% CH 4 in H 2 and ≤0.8% CH 4/H 2 in excess Ar, respectively, and used for diamond chemical vapour deposition. The acetylene was widely distributed throughout the reactor, extending well beyond the regions of highest thermal activation as a result of diffusion processes. The average acetylene concentration was determined as (2.9±0.6)×10 13 molecules cm −3 in the hot filament reactor and (1.2±0.2)×10 14 molecules cm −3 in the DC arc jet reactor, accounting for ∼1.7% and 40% of the total C budget in the respective cases. The average acetylene gas temperature along the viewing column was 550±150 K in both environments. Variations in the concentration and temperature of acetylene with the gas feed composition and input power processing conditions for the two reactors are presented. The experiments demonstrate the effectiveness of near-IR diode laser CRDS as a probe of highly luminous environments.

Excited state density distributions of H, C, C2, and CH by spatially resolved optical emission in a diamond depositing dc-arcjet reactor

Spatially resolved optical emission spectroscopy is used to investigate excited species in a dc-arcjet diamond depositing reactor. Temperature measurements indicate a cold plasma with electrons, excited states, and gas in nonthermal equilibrium. The H, C, C2, and CH excited state number densities decrease exponentially with the distance from the nozzle and have a pronounced increase in the shock structure above the substrate. The H emission increases throughout the boundary layer to the substrate surface, whereas emission from other species has a maximum in the boundary layer and then decreases again towards the substrate. The reconstructed radial distribution of excited state concentrations are Gaussian, with the C and C2 distributions broader than the H and CH ones. The optical emission is calibrated with either Rayleigh scattering or laser-induced fluorescence to furnish absolute number densities. We find all the excited species to be present in concentrations two or more orders of magnitude smaller than the corresponding ground states measured in the same reactor and conditions. We find that C2(d-a) emission intensity correlates well with laser-induced fluorescence measurements of C2(a) concentration in the arcjet plume. Ground state concentrations of the other species do not vary as their emission intensity except near the substrate, where the variations of CH(A-X), CH(B-X), and C2(d-a) emission intensities are good monitors of the corresponding concentration changes.

Absolute concentration, temperature, and velocity measurements in a diamond depositing dc-arcjet reactor

Spatial distributions of the absolute concentration of H, CH, C 2 , and C 3 reactive intermediate species have been measured by laser-induced fluorescence (LIF) in the freestream plume of a dc-arcjet. Diamond film grows from this reactive mixture on a water-cooled molybdenum substrate in a 25 Torr reactor. The temperature of the gas jet is determined from LIF measurements of the rotational distribution of CH radicals in the plume. The directed velocity of the gas plume is derived from the Doppler shift of LIF. The fraction of the feedstock hydrogen dissociated is obtained from calorimetry. The atomic hydrogen concentration and gas temperature are used to constrain a model of the gas phase chemistry in the reactive plume. Concentration measurements of CH, C 2 , and C 3 provide a stringent test for the model of the diamond precursor chemistry.

Optical diagnostics for temperature measurement in a DC arcjet reactor used for diamond deposition

a-state in this plume. LIF measurements of CH are complicated by the presence of C3 and we discuss strategies to deal with this interference. The gas temperature describing the rotational distributions obtained for NO, C2 and CH agree within experimental error. Optical emission measurements indicate that the rotational and vibrational distributions of the excited A-state of CH and a-state of C2 are characterized by vibrational and rotational populations which are at least 1000 K above the ground state distributions. The excited states are collisionally quenched before their population distributions equilibrate with the gas temperature. We also determine relative populations of the ground and excited states along the axis of the plume between the arcjet nozzle and the substrate and relative populations for a cross section of the jet, midway between the nozzle orifice and the substrate. The measured relative ground and excited state populations for both CH and C2 show different trends along the plume axis, indicating that the ground and excited states of these molecules are products of different chemical mechanisms; such mechanisms are discussed.

Absolute concentration measurements of CH radicals in a diamond-depositing dc-arcjet reactor

Laser-induced fluorescence in the CH (B - X) and CH (A - X) electronic transitions is used to measure absolute number density versus position for CH radicals in the plume of a 25-Torr hydrogen/argon/methane (0.8:1:0.005) dc arcjet during the chemical vapor deposition of diamond film. The laser-induced-fluorescence signal is calibrated with argon Rayleigh scattering, and the resultant concentration of the CH radical in the center of the arcjet plume is found to be (3.5 +/- 0.8) x 10(12) molecules/cm(3). The characterization of the plasma plume shows three different regions in the reacting gas: nozzle, plume, and boundary layer. We observe substantial differences in spatial distribution of gas temperature, collisional quenching, and CH number density among these regions.

Electron densities and temperatures in a diamond-depositing direct-current arcjet plasma

Langmuir probe measurements of electron density and temperature are made in the plume of a dc arcjet reactor. A dc arc is struck in an argon or argon/hydrogen mixture at 6 atm pressure and expands through a converging/diverging nozzle into the reactor with a pressure of 25 Torr. Methane and methane/nitric oxide are added in the diverging nozzle, and diamond film grows on a substrate in the gas plume. Electron temperatures of 1–2 eV are significantly hotter than the neutral gas temperature in the plume. Electron densities range from 1010 to 1013 cm−3, well above the Saha equilibrium for the gas temperature and pressure and well below the equilibrium for the electron temperature.