Diamond Growth Rate Research Articles

Supersonic DC-arcjet synthesis of diamond

A supersonic, direct-current arcjet, designed for expansion into low pressures (⪡ 100 Pa) and operating entirely on hydrogen, is employed to produce diamond films on molybdenum substrates. This technique features strongly reduced boundary layer effects due to the near rarefied flow conditions, and short residence times for post-plasma injected reactants, thereby providing better control of the gas-phase chemistry. The results from a parametric study of diamond growth for methane as a carbon source gas are presented. Despite the low pressures, relatively high diamond growth rates (up to 10 μm h −1) are obtained. A “saturation” is observed in the variation of the diamond film thickness with methane to hydrogen flow ratio. Non-diamond phases become more pronounced at higher methane flow ratios, where the film thickness appears to increase linearly with methane concentration. The results are interpreted within the framework of a simplified five-step heterogeneous mechanism that accounts for the competition between the growth of diamond and non-diamond carbon.

A model of diamond growth in low pressure premixed flames

Recognizing the potential importance of diamond thin film growth from combustion environments, a computational investigation of diamond synthesis in low pressure premixed flames has been conducted. The model employed solves the two-dimensional continuity, momentum, global energy, and species conservation equations in stagnation point flow geometry, and accounts for gas phase and surface reaction kinetics. The heterogeneous mechanism employed to describe diamond growth assumes that the methyl radical is the primary growth precursor. The gas phase mechanism includes elementary reaction pathways which generate methyl radicals from acetylene and in addition, includes a mechanism for cyclization (the formation of benzene) via acetylene and ethylene precursors. In this way, the pathway towards soot formation, which is believed to be a consequence of the formation of fused polycyclic aromatics, is shown to be a possible explanation for an eventual decrease in diamond growth rates at increasing fuel to oxygen flow ratios. A competition between oxidative pyrolysis of post flame hydrocarbons and cyclization establishes a criterion for optimum growth conditions.

Monte Carlo simulation of diamond growth by methyl and acetylene reactions

A dynamic Monte Carlo technique was applied to gas-surface reactions simulating diamond growth under chemical vapor deposition. A combined methyl-and-acetylene reaction mechanism was assumed, where the additions of methyl radicals and acetylene molecules are allowed to occur only when no steric interferences arise. The sterically resolved computations demonstrate nonlinear kinetic coupling: methyl and acetylene additions occur simultaneously and interdependently on each other−adsorption of CH3 creates sites for C2H2 addition, and addition of C2H2 creates sites for CH3 adsorption. It is also shown that the incorporation of acetylene by three-center additions only, irreversible on physical grounds, is capable of explaining the rate of diamond growth, thus dismissing the argument of reaction reversibility advanced against our proposed mechanism of acetylene addition.

In situ diamond growth rate measurement using emission interferometry

The growth rate of diamond synthesized in an oxygen-acetylene flame has been measured in situ with an infrared pyrometer. During the growth, the sample’s emitted radiation intensity is modulated in time by interference phenomena in the growing diamond film, causing a periodic variation of the pyrometer’s apparent temperature reading. Diamond film growth rates determined from the period of these oscillations agree well with rates derived from the film thickness. This technique is used to determine the variation of diamond growth rate with substrate temperature over the range of 444–1200 °C. An Arrhenius analysis of the data shows two distinct diamond growth regimes.

Diamond growth rates vs. acetylene concentrations

Abstract Previous measurements and analyses have pointed to either methyl CH 3 or acetylene C 2 H 2 as the primary growth species for diamond films grown from chemical vapor deposition. In this work we used a quartz microprobe and a mass spectrometer to measure how the concentration of C 2 H 2 varied with pressure and with the ratio of methane to hydrogen in the input gas. We compared these variation with variations in the growth rate, as measured with a microbalance. We found no consistent correlation between the growth rate and the C 2 H 2 concentration. This contrasts with a linear relationship that we found previously between the growth rate and the calculated concentration of the methyl radical CH 3 .

Diamond growth on silicon nitride by microwave plasma chemical vapor deposition

Diamond films were grown on hot-pressed silicon nitride (HPSN) substrates by microwave plasma chemical vapor deposition. The pre-growth surface preparation was varied and its effect on diamond quality and growth rate was studied. Scratching the HPSN with diamond powder is necessary for growth of continuous and uniform films.

Growth of diamond film by a plug-flow flat flame method

Abstract The conventional acetylene-oxygen mixture torch flame can grow diamond film at fast rates, but it suffers from problems such as small area coverage, non-uniform growth, and poor reproducibility due to the large concentration and temperature gradients. Considering the effects of momentum, heat and mass transfer in the flame, we proposed to study diamond growth by the plug-flow flat flame method. Our results indicated that, owing to the large burning velocity of the acetylene-oxygen mixture, dilution was required for resonable gas consumption. The growth of uniform diamond films by a flat flame method was successful. Dilution, however, reduced the growth rate of diamond significantly and the crystal qualities deteriorated. The effects of dilution gases were studied by comparing nitrogen and argon. Argon was more favorable for diamond growth than nitrogen. The possible reasons will be discussed.

Diamond growth on a (100)-type step

Abstract Recent measurements with scanning tunneling microscopes and atomic force microscopes show that diamonds grown with the chemical vapor deposition process are covered with steps, kinks, defects, and reconstructions. In the present work we model growth at a step site with (100) character. The model is a modified version of a previous mechanism which has been highly successful in predicting growth rates under a wide range of experimental conditions. Thus, it could serve as a valuable practical tool for predicting diamond growth rates in systems which have not previously been studied experimentally.

Diamond film preparation by arc-discharge plasma-jet-CVD and thermodynamic calculation of the equilibrium gas composition

In order to obtain a high growth rate of diamond, the DC plasma-jet chemical vapor deposition technique has been used. The plasma consists of argon (99.998%) and hydrogen (99.9%). CH 4 (99.5%) is used as a carbon source and is mixed into the plasma jet. The plasma jet is sprayed onto a substrate fixed on a water-cooled substrate holder. The mean growth rate of diamond obtained on Si (111) and Mo substrates with this technique is approximately 80 μm/h. The influence of the synthesis parametersCH 4/H 2 gas ratio and the temperature of the substrate on the morphology and the properties of the diamond film have been examined by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and Vickers hardness. It was found that the quality of the deposits came close to that of natural diamond. Additionally, the equilibrium concentrations of the various hydrocarbon species in the gas phase for the CH 4 Ar H 2 gas system, temperatures from 500 to 6000°C and a total pressure of 0.25 atm (25 kPa) has been computed using the thermodynamic computer program “microtherm and sage”. It was found, that at temperatures, typical for the gas film near the surface of the substrate, C 2H 2 and C 2H are the most important species.

Low-temperature deposition of diamond in a temperature range from 70 °C to 700 °C

Diamond films were deposited on silicon wafer at low temperatures ranging from 70 °C to 700 °C by filament-assisted chemical vapor deposition. A silicon wafer substrate scratched with diamond powder was cooled by a stream of water flowing around a substrate holder. The films were analyzed as diamond by Raman spectroscopy and scanning electron microscopy. Even at a temperature as low as 135 °C formation of diamond was confirmed. Clearly faceted crystals were shown in scanning electron micrographs. The maximum concentration to realize the growth of diamond seems to decrease with decreasing temperature. The diamond growth rate, delined as the diameter of diamond particle divided by the deposition time, was found to be constant during deposition.

In situ growth rate measurement and nucleation enhancement for microwave plasma CVD of diamond

Laser reflection interferometry (LRI) has been shown to be a useful in situ technique for measuring growth rate of diamond during microwave plasma chemical vapor deposition (MPCVD). Current alternatives to LRI usually involve ex situ analysis such as cross-sectional SEM or profilometry. The ability to measure the growth rate in ‘real-time’ has allowed the variation of processing parameters during a single deposition and thus the extraction of much more information in a fraction of the time. In situ monitoring of growth processes also makes it possible to perform closed loop process control with better reproducibility and quality control. Unfortunately, LRI requires a relatively smooth surface to avoid surface scattering and the commensurate drop in reflected intensity. This problem was remedied by greatly enhancing the diamond particle nucleation via the deposition of an intermediate carbon layer using substrate biasing. When an unscratched silicon wafer is pretreated by biasing negatively relative to ground while in a methane-hydrogen plasma, nucleation densities much higher than those achieved on scratched silicon wafers are obtained. The enhanced nucleation allows a complete film composed of small grains to form in a relatively short time, resulting in a much smoother surface than is obtained from a film grown at lower nucleation densities.

Morphological instabilities in the low pressure synthesis of diamond

Morphological instabilities attending the high growth rate of diamond films are examined. Pertinent literature on morphological instabilities and microstructure evolution in vapor deposited films is reviewed and theoretical treatments related to the case of diamond growth are discussed. Diamond films of various thicknesses have been synthesized utilizing the combustion flame synthesis technique involving diamond growth rates of ∼1 μm/min. Films of thicknesses under 20 μm are found to be dense and the surface smoothness of such films is governed by facets on the individual crystallites that make up the film. Increasing film thicknesses, at high growth rates, results in extremely rough surfaces, the trapping of voids and discontinuities, and the incorporation of non-diamond phases in the growing film. These characteristics are typical of morphological instabilities when surface diffusion and re-evaporation processes are absent and instability is promoted by the high rate arrival of the appropriate species from the flame ambient to the surface. Factors contributing to morphological instabilities include competitive shadowing and nutrient starvation and growth anisotropy of the different crystallographic faces on individual diamond crystals. It is shown that surface temperature and the presence of oxidizing species in the flame ambient contribute to anisotropic growth of diamond crystals and hence to morphological instabilities in diamond films. An approach to avoiding these instabilities is briefly discussed.

The Role of Heavy Hydrocarbons in Cvd Diamond Growth

ABSTRACTA mass spectrometric analysis of heavy hydrocarbons (HHCs) during hot-filament CVD diamond growth was performed together with in situ monitoring of the growth rate. Many HHCs were detected and tentatively identified. Of all HHCs studied, only diacetylene shows good correlation with the diamond growth rate under various deposition conditions. Its possible role is discussed.

Effects of microwave plasma deposition parameters on diamond coating formation on SiAlON substrates

Diamond films were grown on SiAlON substrates from different CH 4/H 2 gas mixtures by microwave-activated chemical vapor deposition (MW-CVD), and the effects of gas pressure, methane concentration, microwave power and substrate temperature on growth rate and morphology were examined. Scanning electron microscope pictures showed that at low methane concentrations the synthesized diamond crystals were characterized mainly by triangular (111) facets, whereas at higher methane concentrations (100) facets were predominant. Correlations between diamond morphology, diamond growth rates, homogeneity of the deposition and the size of the plasma ball are given. The effects on diamond growth and diamond morphology with respect to deposition area, gas pressure and the variations in plasma intensity will be discussed.

プラズマジェットＣＶＤ法によるダイヤモンド成膜面積の拡大

A direct current arc discharge plasma jet Chemical Vapor Depositon (CVD) method has achieved very high growth rates of diamond by spraying a plasma jet onto a water-cooled substrate. The initial, experimental scale diamond films thus synthesized on a substrate were small in area, less than 10 mm in diameter. A large area of diamond film formed with a high growth rate is presupposed to be required, but this only has partially been obtained. By rotating a tungsten, substrate during spraying, the area of diamond deposited on the substrate was observed to be about 4 times as large as that on a fixed tungsten substrate. Raman spectroscopy determined the diamond obtained by this method included a small quantity of amorphous carbon. However, diamond Raman spectra obtained from within the diamond indicated the diamond had a diameter of about 20 mm. More recently, a diamond film that has a diameter of about 30 mm has been obtained by rotating the substrate while moving it horizontally.

Diamond synthesis in supersonic direct-current arcjet plasma at subtorr pressures

A new approach to diamond thin film synthesis is presented. A supersonic, direct-current (d.c.) arcjet designed for expansion into low pressures (approximately 1 Torr), operating on mixtures of methane (0.5–25%) in hydrogen and argon, was employed to produce diamond films on silicon and molybdenum substrates. At subtorr pressures, free molecular flow conditions are approached, resulting in strongly reduced boundary layer effects. Large area coatings (up to 20 cm 2) due to the wide expansion of the plasma plume were obtained. The high molecular velocity, on the order of 10 km s −1 at the arcjet exit, and the high activation energies by the thermal plasma provide a high reactive species flux, both of the above giving rise to relatively high diamond growth rates (1–20 μm h −1, despite the lower pressures. Results of parametric studies of diamond growth over ranges of substrate temperatures and reactant mixture ratios show good agreement with commonly observed trends in other vapor deposition systems.

Diamond film growth in a flowtube: A temperature dependence study

Data are presented on the rate of diamond film growth in a flowtube at uniform temperature and pressure, and on the rate of H-atom decay in the tube during growth. Diamonds are grown by adding methane to a flow of atomic hydrogen in a carrier gas. The study covers temperatures from 600 to 900 °C, and a range of methane and H-atom concentrations. The profile of diamond growth rate in the tube is a measure of the gas-phase rise and decay of chemical species required for growth. A rudimentary computer model, fitted to the measured H-atom loss rates, is used to compare the gas-phase chemistry with the profiles. Using an assumed rate law for diamond growth, reasonable agreement with the data can only be obtained by including rapid, nonproductive wall loss of more than 95% of the methyl radicals. The data also suggest that methane (or methyl) increases the loss rate of H atoms, and that the converse is true: H atoms increase the loss rate of methyl radicals. Both processes are too fast to be accounted for by gas-phase chemistry. The model comparison yields an estimate for an activation energy of 19±10 kcal for the heterogeneous growth process. This number is meaningful only if the assumed rate law is correct.

Simulations of high-rate diamond synthesis: Methyl as growth species

The results of numerical simulations of two high-rate diamond growth environments (oxygen-acetylene torch and dc arcjet) are reported. The calculations account in detail for boundary-layer transport, gas-phase chemistry, and gas-surface chemistry. Diamond growth rates are calculated self-consistently with the gas-phase concentrations, using a recently proposed methyl growth mechanism. The calculated growth rates agree well with the measured values, indicating that this growth mechanism can account for both high- and low-rate diamond growth.

Selective and low temperature synthesis of polycrystalline diamond

Polycrystalline diamond thin films have been deposited on single crystal silicon substrates at low temperatures (⋚ 600 °C) using a mixture of hydrogen and methane gases by high pressure microwave plasma-assisted chemical vapor deposition. Low temperature deposition has been achieved by cooling the substrate holder with nitrogen gas. For deposition at reduced substrate temperature, it has been found that nucleation of diamond will not occur unless the methane/hydrogen ratio is increased significantly from its value at higher substrate temperature. Selective deposition of polycrystalline diamond thin films has been achieved at 600 °C. Decrease in the diamond particle size and growth rate and an increase in surface smoothness have been observed with decreasing substrate temperature during the growth of thin films. As-deposited films are identified by Raman spectroscopy, and the morphology is analyzed by scanning electron microscopy.

Growth of diamond films on si(100) with and without boron nitride buffer layer

Diamond films were grown on Si(100) and boron nitride deposited Si(100) substrates using hot filament chemical vapor deposition (HFCVD) technique. Microstructure and morphology of diamond films have been investigated systematically as a function of CH4 and H2 ratio and the ambient pressure. The deposited films were characterized by employing techniques such as scanning electron microscopy (SEM) and laser Raman spectroscopy. The average size and growth rate of diamond particles were found to increase with the CH4 to H2 ratio and decrease with the ambient pressure. Maximum growth rate of synthetic diamond deposited on Si(100) was found to be ∼3.5 µm/hr for the film deposited at 20 Torr with CH4:H2 ∼ 1.5:100 (substrate temperature ∼850°C). In most of these depositions, the morphology of the diamond crystals was cubic with significant secondary nucleation at higher methane concentrations and ambient pressure. The diamond film deposited on Si(100) with BN buffer layer shows an improvement in growth rate and the coverage, and the secondary nucleation was found to be substantially reduced, resulting in relatively smooth morphology. MicroRaman investigations show less amorphous graphite formation and better structural quality of diamond film than the one deposited without the BN buffer layer.