Diamond Growth Rate Research Articles

Diamond growth from a mixture of fluorocarbon and hydrogen in a microwave plasma

The process of diamond deposition on Si(100) substrates is examined in a microwave plasma using tetrafluoromethane (CF 4) and trifluoromethane (CHF 3) as well as methane (CH 4) as the carbon source. Substrate pretreatment with diamond powder is effective for the enhancement of diamond nucleation for all the reactant systems tested. Well-faceted diamond of high quality is formed at a gaseous carbon concentration of about 1%. For CF 4-H 2 and CHF 3-H 2 systems the growth rate of diamond is nearly constant at temperatures above 850 °C, while in the temperature range from 640 to 850 °C it decreases not only with decreasing deposition temperature but also with increasing fluorine content of the reactant. The apparent activation energy for diamond growth is estimated as 119 kJ mol −1, 86 kJ mol −1 and 43 kJ mol −1 for the CF 4-H 2, CHF 3-H 2 and CH 4-H 2 systems respectively. The growth behaviour in the fluorocarbon gas systems differs from that in the CH 4-H 2 system. Fluorine compounds seem to terminate active sites of the growing diamond at temperatures lower than 850 °C and to remove non-diamond phases above 850 °C.

Characterization of tantalum impurities in hot-filament diamond layers

During hot-filament diamond deposition metal impurities ( e.g. tantalum) can evaporate from the filament. The incorporation of tantalum in diamond films was studied as a function of process conditions using secondary ion mass spectrometry, Rutherford backscattering spectrometry and transmission electron microscopy. It was found that the tantalum concentration depends on the initial carburization status of the filament, its temperature and the diamond growth rate. If diamond deposition is started with metallic tantalum filaments, TaC nanoprecipitates are formed at the interface.

Ultraviolet spectroscopy of gaseous species in a hot filament diamond deposition system when C2H2 and H2 are the input gases

The methyl radical density, acetylene mole fraction, filament properties, and diamond growth rate and film quality are measured in a hot filament chemical vapor deposition system when C2H2 and H2 are used as the input gases. The methyl radical density and acetylene mole fraction depend greatly on the degree of filament surface poisoning. This poisoning prevents diamond growth due to a lack of hydrogen atoms and/or methyl radicals. Understanding the large influence of the filament surface catalytic characteristics is important for developing a gas phase model of this system. The results obtained with C2H2 and H2 as the input gases are compared to those obtained with CH4 and H2 as the input gases. Under conditions when the filament surface is not poisoned, the methyl radical concentrations are similar when either C2H2 and H2 are the input gases or when CH4 and H2 are the input gases.

Temperature dependence of growth rate for diamonds grown using a hot filament assisted chemical vapor deposition method at low substrate temperatures

Diamond particles were deposited using a hot filament-assisted chemical vapor deposition (HFCVD) method in which the substrate temperature ranged from 210 to 700 °C. The size of the diamond particles measured as a function of time showed that a diamond grows via two periods of incubation and growth. Compared with an activation energy of 10–25 kcal/mol for substrate temperatures higher than 600 °C as reported in literature, the growth rate for a diamond grown using a HFCVD method was much less dependent on the substrate temperature for that temperature range investigated in our study. The apparent activation energy, determined from the Arrhenius plot of the substrate temperature versus diamond growth rate, decreased from 5 to 1 kcal/mol with decreasing temperature.

Diamond-coated carbon fiber

Diamond deposition was attempted on polyacrylonitrile (PAN) fiber and vapor grown carbon fiber (VGCF). PAN fibers were severely etched in a microwave plasma, whereas diamond was successfully deposited on VGCF. A diamond growth rate of 0.1 μm/h on VGCF was determined at a gas mixture of 99.9 sccm H2/0.1 sccm CH4 and a pressure of 30 Torr. It is proposed that diamond formation on VGCF occurs on not only the prism planes, but also the basal planes owing to the unique structure of VGCF. An explanation is proposed to explain diamond nucleation on the basal planes.

Quantitative In Situ Growth Measurements of Chlorine-Activated Homoepitaxial Diamond Cvd

ABSTRACTIn situ quantitative studies of the effects of substrate temperature, methane and chlorine flow rates on homoepitaxial diamond growth rates on (110) surfaces in a chlorine-activated diamond CVD reactor have been carried out using a Fizeau interferometer. The temperature dependence of diamond growth rates was found to display three distinct growth activation energies, ranging from 9±2 kcal/mol in the substrate temperature of 750-950°C, to 3.2±0.2 kcal/mol in the temperature range of 300-650°C, followed by 1.2±0.2 kcal/mol in the temperature range of 102-250°C. Atomic hydrogen is believed to be the dominant activating species in the highest temperature range, and atomic chlorine is believed to be the dominant species in the lower temperature regions. Studies of the methane flow effect on diamond growth rates revealed a linearity, indicating that the diamond growth rate was of the first order in methane flows. Diamond growth rates were also found to increase linearly with the chlorine flow. At high chlorine flow rates, however, an accelerated diamond growth rate was observed. Discussion is given to explain the observed results.

Gas flow effects in synthesis of diamond by hot-filament chemical vapor deposition

Abstract Hot-filament chemical vapor deposition was employed to study the effect of gas flow dynamics on diamond growth. The density and growth rate of diamond within the patterned region of copper and silicon substrates was found to be twice as large as compared with that on a flat (unpatterned) surface. The density of diamond nucleation increased on patterned silicon and copper substrates with an increase in the methane concentration from 1.0% to 1.5% in the gas mixture of methane and hydrogen (CH4 + H2). Selective growth of diamond was achieved without using diamond seed or paste, or scratching the patterned silicon substate. About 90% diamond coverage and a continuous diamond film (75 μm) were achieved on the square-patterned silicon and copper substrates, respectively, at a methane concentration of 1.5%. The growth of diamond within the wells was discussed on the basis of gas flow dynamics. The present analysis is consistent with available theoretical models.

Diamond Growth Rates and Quality: Dependence on Gas Phase Composition

ABSTRACTDiamond growth rates and quality were studied as a function of source gas composition and correlated with position on the ternary C-H-O diagram. The chemical potentials of carbon and oxygen change dramatically on either side of the H2-CO tie line, leading to large differences in the equilibrium distribution of species. These differences are reflected in the species flux reaching the diamond surface, and hence in the quality and growth rate of the diamond. In situ microbalance measurements in a hot-filament reactor show that the reaction rate is independent of the CO concentration, but decreases with increasing O2. Quality, as measured by Raman spectroscopy, increases as the C/C+O ratio in the source gases is reduced to approach the critical value of 0.5. The stability of the filaments to decarburizing and oxidation are correlated with the carbon and oxygen chemical potentials and hence to the position on the C-H-O diagram. A preliminary ternary diagram for the C-H-F system is presented.

Fundamental limits to growth rates in a methane-hydrogen microwave plasma

The fluxes of the major stable products, acetylene and ethylene, emanating from a 2.45 GHz methane-hydrogen plasma (0.2–5 vol.% methane; 800 W; 33 Torr) have been measured using quadrupole mass spectrometry. The diminution in the methane flux was also measured to allow the overall carbon balance to be determined. The ball plasma was sustained within a spherical ultrahigh vacuum chamber, out of contact with the substrate and chamber walls. Experiments were performed either in the presence of a heated (720°C) silicon substrate of 4 in diameter or with the substrate heater completely removed from the chamber. Within experimental error, the product yields and methane losses were identical for the two experimental arrangements. The carbon mass imbalance was compared with the growth rates of polycrystalline diamond using in-situ laser interferometry. The measured growth rates were of the order of 15% of the maximum permissible rates, for a 0.5 vol.% methane mixture, based on the carbon imbalance. Thermodynamic calculations, based on minimizing the Gibbs free energy, have been used to determine the relative amounts of the species involved in the decomposition of dilute mixtures of methane in hydrogen. Both diamond and graphite phases have been included in the heterogeneous reactions. Isotherms based on quasi-equilibrium theory are used to define the conditions of temperature and pressure under which both phases coexist. Such calculations extending the phase diagrams to include oxygen enable the ternary diagrams developed by Bachmann to be rationalized. The conditions for the exclusive growth of diamond have been calculated, employing values for the surface enthalpy of diamond, taking into account the enhanced surface stability of hydrogenated diamond surfaces.

Nucleation and Growth Processes During the Chemical Vapor Deposition of Diamond

AbstractIn situmicrobalance measurements of diamond growth rates are described. These results can be used to test proposed mechanisms for diamond growth and suggest mechanisms for sp2impurity incorporation. The Thiele modulus is a simple criterion for growth uniformity and is used to compare hot-filament and combustion-assisted growth.

Nucleation and Growth Processes During The Chemical Vapor Deposition of Diamond

ABSTRACTThe nucleation of independent diamond crystals can occur through graphitic, sp2, intermediates. Evidence from several sources indicates that diamond growth may take place through sp2 surface layers. Steady state analysis of simple reaction networks leads to expressions for the diamond growth rates, the concentration of sp2 impurities in the diamond, and for the nucleation rate of new crystals.

Sequential deposition of diamond from sputtered carbon and atomic hydrogen

The growth of diamond thin films on a scratched silicon crystal surface by a chemical-vapor deposition technique is reported. The substrate was bombarded by sputtered carbon from a graphite target in a helium dc glow discharge, and subsequently exposed to atomic hydrogen generated by a hot tungsten filament. The resulting diamond films were characterized by Raman spectroscopy and scanning electron microscopy. Deposited film quality and growth rate were studied as functions of carbon and atomic hydrogen exposure. An increase in growth rate of diamond was observed with atomic hydrogen exposure. We also observe that only the first monolayer of carbon deposited with each exposure appears to be utilized. These observations suggest that the diamond growth is a surface reaction. Further, calculations based upon the carbon utilization in traditional hot filament reactors indicate that a gas-phase reaction process can account for neither the growth rate nor the saturation behavior observed. Based on this work it is proposed that the growth of diamond films is governed by surface reactions, and that the necessity of gas-phase precursors can be precluded.

Diamond growth by hot-filament chemical vapor deposition: state of the art

Low-pressure diamond deposition using hot-filament gas activation was the first method to achieve nucleation and continuous diamond growth on various substrates. The method itself is simple but the strongly interdependent parameters involved must be strictly controlled in order to obtain reproducible diamond quality. The material used for the hot filament and its reactions during diamond deposition are important for gas activation. Together with this the chemical stability of the substrate and the deposition parameters determine the quality of the diamond layers produced. The growth conditions must be optimized for each specific application. When scaling up the hot-filament method, a uniform temperature distribution and the gas flow become additional factors of major importance for obtaining good quality diamond coating. The C:O:H ratio in the reaction gas also influences diamond growth rates and the resulting quality of the diamond films.

Effects of Oxygen Addition on Growth of Diamond Film by Arc Discharge Plasma Jet Chemical Vapor Deposition

Effects of oxygen addition on growth and crystallinity of diamond films are discussed. The etch rate of graphite by Ar-O2 plasma jet is 130 times larger than that by Ar-H2 plasma jet. Nucleation density of diamonds decreases with oxygen addition. The growth rate of diamond decreases with increase in the amount of added oxygen. Surface unevenness and porosity also decrease with increase in the amount of added oxygen, and it was found that the optimum oxygen concentration is about 33%. A diamond film of 1.2 mm thickness was fabricated under these conditions in 20 h. Surface unevenness of the diamond film was reduced to about 100 µm. The growth rates of the {111} planes and {100} planes become isotropic with 33% oxygen content. Therefore the diamond film surface is only slightly uneven and the film shows good crystallinity.

Low-temperature deposition of diamond using chloromethane in a hot-filament chemical vapor deposition reactor

Chloromethanes (CH 2Cl 2, CHCl 3 and CCl 4) were used as carbon sources to grow diamond at low temperature (from 380°C to 700°C). In comparison with methane, which is inefficient at growing diamond below 600°C, chloromethane was quite suitable for the growth of diamond films at low temperature. Diamond growth was possible even at 380°C, which was the lowest temperature possible in the system utilized here, using CCl 4 reactant. Scanning electron micrographs, X-ray diffraction patterns and Raman spectra confirmed the presence of diamond crystallites. However, the growth rate at 380°C was only 0.05 μm h −1. Improved growth rates were achieved with hydrogen passing through the hot-filament and carbon source gas bypassing the filament (bypass configuration). The growth rate of diamond was indeed enhanced very significantly using chloromethane in the bypass configuration. However, problems such as non-uniform growth and a narrow range of possible growth parameters arose using the bypass configuration. In contrast, the growth rate was not enhanced by using methane in the bypass configuration. The advantages of chloromethane are demonstrated. Possible reasons for such advantages are also discussed.

Influence of atomic hydrogen gradients on the growth rate and nucleation of diamond produced by microwave plasma assisted deposition

Experiments to show the influence of the relative atomic hydrogen content on diamond deposition in hydrogen plasmas with small admixtures of alcohols are reported. By using a planar microwave plasma source and placing the substrates outside the active plasma excitation region, we could ensure that the substrate heating was dominated by the surface recombination of atomic hydrogen. Therefore, the concentration of atomic hydrogen could be assumed to be proportional to the substrate temperature. By varying the position of the substrate, deposition experiments with a variation in concentration of 60% were possible. Growth conditions yielding well isolated crystals were used. A dependence of the medium crystal diameter on position, similar to the spatial dependence of the atomic hydrogen concentration was observed. This indicated a power law dependence of the linear growth rate on the concentration of atomic hydrogen. An analysis of the crystallite size revealed an asymmetric size distribution. In contrast to the crystal diameters, the nucleation densities were independent of the atomic hydrogen concentration.

Synthesis and morphology of CVD diamond on Ta and TaC film

Synthetic diamond films have been deposited on the Si(111) surface, polycrystalline Ta plate, TaC/Si, and TaC/Ta substrates using an electron assisted chemical vapor deposition (EACVD) method. The effects of substrate pretreatment and existence of carbide layer on the diamond nucleation, subsequent growth and morphology have been studied. The substrate pretreatment, scratching by diamond powder, affects nucleation behavior, subsequent growth and morphology of diamond. Existence of carbide layer and formation of carbide on the substrate affects nucleation density, growth rate and morphology of diamond.

Plasma-assisted CVD of diamond films by hollow cathode arc discharge

The preparation of polycrystalline diamond particles and thin diamond films by deposition in the plasma discharge of a hollow cathode arc has been investigated. Although this discharge is known as a very simple and effective system for plasma generation, there has been little information about the growth of diamond by means of this method up to now. This type of discharge combines thermal and plasma dissociation of the reactant gases for diamond deposition. The temperature of the deposition area can be easily controlled by the electron current to the substrate and thus no auxiliary heating system is required, in contrast to many other diamond deposition methods. Furthermore, we assume additional decomposition of the reactant gases at the surface of the substrate due to the strong electron bombardment. As-deposited diamond particles and films were investigated by various electron microscopy methods, Raman spectroscopy and optical methods. They clearly show a polycrystalline structure with good crystallinity and quality. The diamond growth rate reaches about 2 μm h −1 on a deposition area of about 4 cm 2.

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.