Diamond Deposition Process Research Articles

Diamond —A Novel Sensing Material

The excellent electrical and optical properties of di amond are the driving force behind the current research in produc ing thin films of diamond. Based upon the unique properties of dia mond, enormous state-of-the-art novel electron devices and sensor configurations are possible. The synthesis of crystalline diamond in the laboratory provides tremendous opportunities to benefit from its unique properties. A critical review of the diamond depo sition process is presented. Recent NRL studies suggest deposition of good quality diamond films in a turbulent flow regime where CVD processes normally operate in local thermal equilibrium (LTE). Modifications are suggested to further enhance the deposi tion rates of diamond films. A brief account is provided of hybrid microelectronics, opto-electronic, and other novel devices realizable with the diamond material. Thermal management of electronic devices is extremely important in the design of sensors and high-speed, high-density electronic circuits with power de vices fabricated monolithically. A preliminary theoretical compu tation for comparison of the power dissipation vs. gate propagation delay for diamond, silicon, and gallium arsenide-based devices is presented. Thermal interaction of semiconductor devices on the substrates as it relates to their close proximity and spacing is ad dressed. Computations of simulated analytical expressions indicate that diamond devices can withstand higher temperatures of opera tion without any significant deterioration in electrical perfor mance.

Diamond Deposition with Ar ‐ CO ‐ H 2 Plasma Jet at Reduced Pressure

The diamond deposition on molybdenum and niobium substrates with plasma jets was carried out at 26 kPa by varying hydrogen concentration and exposure time. At 2 and 3 v/o hydrogen added to the plasma jet , the diamond deposition was obtained. When the hydrogen concentration was reached to 5 v/o, the substrate was only carburized. The diamond deposition process is considered from both the kinetic study and characterizations of the deposits. Diamond was deposited on the metal carbides which were formed prior to the deposition of diamond. molecules would contribute to the diamond deposition with radicals which were formed in the plasma jet by the reaction between and .

Application of thermodynamics to the examination of the diamond CVD process II. A model of diamond deposition process from hydrocarbon-hydrogen mixtures

A model of the diamond deposition process from gas phases activated by thermal methods, consisting of the four following stages, is proposed: (1) activation of a hydrocarbon-hydrogen mixture; (2) transport of the activated gas phase to the substrate surface; (3) simultaneous deposition of carbon in sp 3 and sp 2 hybridization at the growth surface; (4) etching of co-deposited sp 2 hybridized carbon. Carbon deposition with maintenance of sp 3 hybridization can proceed only on the condition that the gas phase is in a non-equilibrium state, generally called the activated state, in which a non-equilibrium partial pressure of atomic hydrogen is present. A definition of the activated state is proposed. Depending on the degree of activation carbon is deposited in stage (3) by potentially many different reactions. When the gas phase is activated to a “high”, “very high” or “extremely high” degree, the most important reactions, according to which carbon is deposited, are the reactions C 2H 2(g) + 2 H(g) = C(diamond & graphite) + CH 4(g) , C 2H(g) + 3 H(g) = C(diamond & graphite) + CH 4(g) , or 2 CH(g) + 2 H(g) = C(diamond & graphite) + CH 4(g) , respectively. The most important reaction, according to which graphite is etched or gasified in stage (4), is reaction C( graphite) + 4 H( g) = CH 4( g). Atomic hydrogen in a super-equilibrium concentration is necessary not only to etch graphite in stage (4) but also to precipitate carbon in stage (3). Conclusions following from the model are in good agreement with the results of experimental investigations reported in the current literature.

The nucleation and morphology of diamond crystals and films synthesized by the combustion flame technique

The nucleation and morphology of diamond crystals and films synthesized by the use of a combustion flame have been investigated. By operating an oxy-acetylene torch in a fuel-rich mode, diamond crystals and films have been deposited on mechanically abraded molybdenum, on in situ created molybdenum carbide, and on thin diamond-like carbon (DLC) layers synthesized on molybdenum. Scanning electron microscopy, Auger and Raman spectroscopy have been used to characterize the films and crystals. Diamond is found to be uniformly deposited in the region of the substrate that intersects the inner, acetylene-rich region of the flame. The nucleation density, the growth rate, and the morphology of the diamond crystals and films are found to be strongly influenced by the surface condition of the substrate. On mechanically abraded molybdenum, abraded with 600 mesh silicon carbide, and on molybdenum carbide, well-formed cubo-octahedrons of diamond, up to 45 μm in diameter, are formed for deposition times of 90 min. Film formation is seldom observed under these conditions. To enhance nucleation, thin layers of DLC were formed on molybdenum substrates by reducing the O2/C2H2 ratio in the gas mix to ∼ 0.75 for short periods of time under 30 s. This was followed by increasing the O2/C2H2 ratio to conditions that produce diamond (an O2/C2H2 ratio of ∼ 0.9). Under these conditions the nucleation density of diamond was increased by an order of magnitude and the growth rates by about 60%, as compared to diamond deposited on abraded molybdenum and molybdenum carbide. In addition, the morphology of the diamond crystals and films was substantially affected with indications of dendritic growth. The DLC layer is effective in promoting diamond nucleation due to the high surface defect density and the high hydrogen concentration of these films. The combination of surface defects, in the form of dangling bonds, and the evolution of hydrogen from the DLC layer during the diamond deposition process, which is characterized by higher temperatures, result in a high concentration of active surface sites for diamond nucleation. The nucleation density, the distribution on the substrate, and the morphology of diamond crystals and films are not driven by the transport of reactive specie in the flame to the substrate, but rather by nucleation processes, temperature distribution across the surface, and attendant surface phenomena.

CVD diamond deposition processes investigation: CARS diagnostics/modeling

We have applied Coherent Anti-Stokes Raman Spectroscopy (CARS), using a narrowband, scanned colinear configuration, to measure temperatures, relative concentrations, and detect species in low pressure CVD of polycrystalline diamond. CARS measurements were obtained for methane, hydrogen, and acetylene in either or both a rf plasma reactor and a hot filament reactor. In the rf PACVD experiments a mixture of 1% CH4 in H2 was used at a total pressure of 5 torr. The rf power input to the plasma was 300 watts and the H2 and CH4 flow rates were 99 and 1 seem, respectively. As acetylene (C2H2) has been proposed as an intermediate in diamond growth, it was selected for the initial series of measurements. In the absence of rf power, a sensitivity of 5 mtorr was observed; in the plasma downstream of the rf coils, no observable signal attributable to C2H2 was evident. This places an upper limit to conversion of methane to acetylene at 20%, a figure representing the observed sensitivity to C2H2. In the hot filament reactor, the gas flow was 200 sccm of 1% CH4 in H2 at a total pressure of 150 torr. Under these conditions, C2H2 was detectable. Absolute concentrations were not calculated, but the observed spectra are within an order of magnitude of our sensitivity limit. This allows estimation of the C2H2 partial pressure near the substrate as 5–50 mtorr or from 0.66 to 6.6% conversion from methane. In view of this low conversion percentage, the absence of a signal in the rf experiments must be taken as inconclusive. CARS spectra of methane were also obtained in both reactors. In the rf reactor, under similar conditions to those described previously, the methane relative concentration decreased to 25% as the rf power was increased from zero to 400 watts. In the hot filament reactor, CH4 CARS signal profiles were obtained as a function of axial distance from the hot filament, and parametrically as a function of filament temperature. Comparison of these profiles, in which the observed signal decayed monotonically as the filament was approached and increased monotonically downstream of the filament, was made with theoretical calculations. This comparison showed that the fluctuations were attributable to temperature/pressure effects and not to chemistry. To determine if the observed depletion in the rf plasma was similarly attributable, the CARS signal of hydrogen was observed as a function of axial distance downstream of the rf coil centerline and parametrically as a function of rf power. In contrast to expected behavior in the thermal hot filament reactor, little rotational excitation was observed in the plasma. Rotational temperatures were assigned to hydrogen based upon comparison with theoretically derived spectra. At 450 watts of rf power, rotational temperatures of 340 K were observed 4 to 6 cm downstream of the coil, the region where the 25% decrease in CH4 was observed. Little or no density fluctuations accrue due to these temperatures, indicating that the observed depletion in methane signal is attributable to decomposition or chemical reaction in the plasma. In summary, CARS is applicable to reactant species (CH4) axial profiling in both reactors, but can be limited by sensitivity in the detection of intermediate or product species (C2H2). In addition, CARS thermometry can be utilized to profile the rotational temperatures of selected species.

Diamond deposition by chemical vapor transport with hydrogen in a closed system

The carbon-hydrogen chemical vapor transport system was examined in accordance with a four-stage transport model. A result of this examination is that graphite co-deposition could be avoided when diamond is deposited from gas solutions undersaturated with regard to diamond. Actual deposition experiments showed that this unusual requirement can be fulfilled but only for the condition that the transport distance between the carbon source and the substrate surface is short. In such a case diamond can be deposited equally from supersaturated as well as from undersaturated gas solutions. On the basis of thermodynamic considerations, a possible explanation of this unusual phenomenon is given. It is shown that there is a possibility of deposition of diamond from both supersaturated and undersaturated gas solutions but only on the condition that they are in a non-equilibrium state generally called the activated state. A model of the diamond deposition process consisting of two steps is proposed. In the first step diamond and graphite are deposited simultaneously. The most important carbon deposition reaction is C 2H 2(g)+2H(g) = C(diamond+graphite) +CH 4(g). The amount of co-deposited graphite is not a direct function of the saturation state of the gas phase. In the second step graphite is etched according to the most probable reaction C(graphite)+4H(g) = CH 4(g). Atomic hydrogen in a concentration exceeding equilibrium is necessary not only to etch graphite, but also to precipitate diamond and graphite.

Factors influencing the adhesion of diamond coatings on cutting tools

CVD diamond coatings can greatly extend the tool life of cemented carbide cutting tools when machining abrasive non-ferrous materials. However, reliable tool performance is difficult to obtain due to the problem of controlling coating adhesion. In this work, diamond coatings have been deposited on cemented carbide tool inserts with variation of substrate chemical composition and surface pre-treatment. The coatings have been characterized with respect to nucleation density, crystallinity, non-diamond material content and residual stresses. The adhesion and wear resistance of the coatings were investigated by machining tests in Al-18%Si alloy. The diamond deposition process and the resulting coating adhesion are both strongly influenced by the surface characteristics of the substrate surface. Formation of voids and non-diamond material at the coating-substrate interface in combination with high compressive stresses in the coating are believed to be the main reasons for poor adhesion.

Electron microscopic characterization of diamond thin films and substrates for diamond heteroepitaxial growth

Because of diamond’s wide band gap, high thermal conductivity, high breakdown voltage and high radiation resistance, there is a growing interest in developing diamond-based devices for several new and demanding electronic applications. In developing this technology, there are several new challenges to be overcome. Much of our effort has been directed at developing a diamond deposition process that will permit controlled, epitaxial growth. Also, because of cost and size considerations, it is mandatory that a non-native substrate be developed for heteroepitaxial nucleation and growth of diamond thin films. To this end, we are currently investigating the use of Ni single crystals on which different types of epitaxial metals are grown by molecular beam epitaxy (MBE) for lattice matching to diamond as well as surface chemistry modification. This contribution reports briefly on our microscopic observations that are integral to these endeavors.

The role of hydrogen in vapor deposition of diamond

The effect of hydrogen addition on growth of diamond films under the conditions of chemical vapor deposition was investigated computationally. A detailed chemical kinetic mechanism was composed to describe the evolution of reaction species in pyrolysis of hydrogen- and argon-diluted methane mixtures with imposed temperature profiles, simulating the gas-phase conditions of diamond film growth in an idealized hot-filament reactor. The reaction mechanism was comprised of two basic parts: decomposition of methane, and formation and growth of polycyclic aromatic hydrocarbons; it contained a total of 120 elementary reactions and 45 chemical species. The reaction rate coefficients included temperature and pressure dependencies. The computations were performed for a variety of initial conditions, elucidating the effects of critical parameters on the product composition in the regime of diamond deposition. Analysis of the computational results indicated that the key role of the hydrogen addition in the diamond deposition process is to suppress the formation of aromatic species by H2 in the gas phase and thereby to prevent the formation and growth of nondiamond, graphitic phases on the deposition surface.