Synthesis of CVD diamond at atmospheric pressure using the hot-filament CVD method

At low pressure, chemical vapor deposition (CVD) diamond growth by conventional techniques such as micro-wave plasma and hot-filament have been achieved by metastable precursor species. Moreover, bulk diamond at extremely high pressures and temperatures was consistently originated by the nature of diamond-graphite phase transition. CVD diamond growth has four problems with these conventional techniques. Excluding contaminated air from low pressure reactive systems has been problematic. It is very difficult to control the concentration of atomic hydrogen at high pressures. The growth rate is unsatisfactory and the running cost of gases are high.However, the hot-filament CVD technique at atmospheric pressure overcomes these problems. We have found that in order to control the concentration of atomic hydrogen, the residence time of the input gas and the methane-hydrogen concentration ratio needed to be varied at each pressure. The relationship between the quality of deposited diamond and the pressure have been also investigated by Raman spectroscopy and X-ray diffraction patterns (XRD).The growth rate at atmospheric pressure (106 000 Pa) was found to be greater than that at the conventional pressure (5000 Pa). At atmospheric pressure, the growth rate abruptly increases with the residence time. XRD analysis revealed that the quality of diamonds grown at atmospheric pressure was higher than that of diamonds produced at low pressures. Furthermore, high quality diamond growth was achieved with a long residence time of the input gas at atmospheric pressure.     

Homoepitaxial single crystal diamond growth at different gas pressures and MPACVD reactor configurations

Homoepitaxial growth of single crystal diamond by microwave plasma chemical vapor deposition in a 2.45 GHz reactor was investigated at high microwave power density varied from 80 W/cm³ to 200 W/cm³. Two methods of achieving high microwave power densities were used (1) working at relatively high gas pressures without local increase of electric field and (2) using local increase of electric field by changing the reactor geometry (substrate holder configuration) at moderate gas pressures. The CVD diamond layers with thickness of 100–300µm were deposited in H₂–CH₄ gas mixture varying methane concentration, gas pressure and substrate temperature. The (100) HPHT single crystal diamond seeds 2.5 × 2.5 × 0.3 mm (type Ib) were used as substrates. The high microwave power density conditions allowed the achievement of the growth rate of high quality single crystal diamond up to 20 µm/h. Differences in single crystal diamond growth at the same microwave power density 200 W/cm³ for two process conditions—gas pressure 210 Torr (flat holder) and 145 Torr (trapezoid holder)—were studied. For understanding of growth process measurements of the gas temperature and the concentration of atomic hydrogen in plasma were made.     

Characterization of the near-surface gas-phase chemical environment in atmospheric-pressure plasma chemical vapor deposition of diamond

A numerical model was developed and used to study the near-surface gas-phase chemistry during atmospheric-pressure radio-frequency (RF) plasma diamond chemical vapor deposition (CVD). Model predictions of the mole fractions of CH₄, C₂H₂, C₂H₄ and C₂H₆ agree well with gas chromatograph measurements of those species over a broad range of operating conditions. The numerical model includes a two-dimensional analysis of the sampling disturbance in the thin boundary layer above the substrate, accounts for chemistry in the gas chromatography sampling line, and utilizes a reaction mechanism that is significantly revised from a previously reported version. The model is used to predict the concentrations of H, CH₃, C₂H₂ and C at the diamond growth surface. It is suggested that methyl, acetylene and atomic carbon may all contribute significantly to film deposition during atmospheric-pressure RF plasma diamond CVD. The growth mechanism used in the model is shown to predict growth rates well at moderate substrate temperatures (∼1100 to 1230 K) but less well for lower (∼1000 K) and higher (∼1300 K) temperatures. The near-surface gas-phase chemical environment in atmospheric-pressure RF plasma diamond CVD is compared with several other diamond CVD environments. Compared with these other methods the thermal plasma is predicted to produce substantially higher concentration ratios at the surface of both H/CH₃ and C₂H₂/CH₃.     

Improved microwave plasma cavity reactor for diamond synthesis at high-pressure and high power density

Microwave plasma assisted synthesis of diamond is experimentally investigated using high purity, 2–5% CH₄/H₂ input gas chemistries and operating at high pressures of 180–240 Torr. A microwave cavity plasma reactor (MCPR) was specifically modified to be experimentally adjustable and to enable operation with high input microwave plasma absorbed power densities within the high-pressure regime. The modified reactor produced intense microwave discharges with variable absorbed power densities of 150–475 W/cm³ and allowed the control of the discharge position, size, and shape thereby enabling process optimization. Uniform polycrystalline diamond films were synthesized on 2.54 cm diameter silicon substrates at substrate temperatures of 950–1150 °C. Thick, freestanding diamond films were synthesized and optical measurements indicated that high, optical-quality diamond films were produced. The deposition rates varied between 3 and 21 μm/h and increased as the operating pressure and the methane concentrations increased and were two to three times higher than deposition rates achieved with the MCPR operating with equivalent input methane concentrations and at lower pressures (≤ 140 Torr) and power densities.     

Recent advances in high-growth rate single-crystal CVD diamond

There have been important advances in microwave plasma chemical vapor deposition (MPCVD) of large single-crystal CVD diamond at high growth rates and applications of this diamond. The types of gas chemistry and growth conditions, including microwave power, pressure, and substrate surface temperatures, have been varied to optimize diamond quality and growth rates. The diamond has been characterized by a variety of spectroscopic and diffraction techniques. We have grown single-crystal CVD diamond over ten carats and above 1 cm in thickness at growth rates of 50–100 μm/h. Colorless and near colorless single crystals up to two carats have been produced by further optimizing the process. The nominal Vickers fracture toughness of this high-growth rate diamond can be tuned to exceed 20 MPa m^1/2 in comparison to 5–10 MPa m^1/2 for conventional natural and CVD diamond. Post-growth high-pressure/high-temperature (HPHT) and low-pressure/high-temperature (LPHT) annealing have been carried out to alter the optical, mechanical, and electronic properties. Most recently, single-crystal CVD diamond has been successfully annealed by LPHT methods without graphitization up to 2200 °C and < 300 Torr for periods of time ranging from a fraction of minute to a few hours. Significant changes observed in UV, visible, infrared, and photoluminescence spectra are attributed to changes in various vacancy centers and extended defects.     

Early stage of diamond growth at low temperature

We investigate the first stages of nanocrystalline diamond (NCD) thin film growth at low substrate temperature. NCD films were grown on silicon substrates by microwave plasma enhanced chemical vapor deposition (CVD) for 0–300 min at a temperature of 410 °C. Si substrates were ultrasonically seeded in suspension of detonation nanocrystalline diamond powder. The seeding density approached values up to 1 ⁎ 10¹² cm^− 2, which allows growth of ultra-thin fully closed layers. Stagnation of the AFM roughness indicates that the low temperature NCD growth is a) delayed due to the surface contamination of the used nanodiamond powder and b) possibly dominated by the growth in the lateral direction. XPS measurements showed that the measured surface exhibits changes from a multi-phase composite (seeding layer) to single-phase one (NCD layer).     

Investigation of nitrogen addition on hydrogen incorporation in CVD diamond films from polycrystalline to nanocrystalline

The effect of nitrogen addition in the gas phase on hydrogen impurity incorporation into CVD diamond films was investigated. A series of thick diamond films of different morphology and quality ranging from large-grained polycrystalline to fine-grained nanocrystalline were deposited on silicon wafers using a 5 kW microwave plasma assisted CVD system. They were obtained only by changing the small amount of oxygen and nitrogen addition while keeping all other input parameters the same. Bonded hydrogen impurity in these diamond films was studied by using Fourier-transform infrared spectroscopy. It was found that with increasing the amount of nitrogen addition in the gas phase, the produced diamond films from large-grained polycrystalline gradually shift to fine-grained nanocrystalline and their crystalline quality is drastically degraded, while the amount of incorporated hydrogen impurity in the diamond films increases sharply. The role of nitrogen additive on diamond growth and hydrogen incorporation is discussed. These results shed light into the growth mechanism of CVD diamond films ranging from polycrystalline to nanocrystalline, and the incorporation mechanism of hydrogen impurity in CVD diamonds.     

Development of a plate-to-plate MPCVD reactor configuration for diamond synthesis

The design and performance of a microwave plasma chemical vapor deposition (MPCVD) reactor based on compressed microwave waveguides and plate-to-plate substrate holders are described. This reactor can be operated at pressures from 10 to 40 kPa with microwave power of 0.4–1.2 kW, and a high plasma power density up to 500 W/cm³ can be obtained. The single-crystal diamond (lower substrate holder) and polycrystalline diamond (upper substrate holder) have been grown by the plate-to-plate MPCVD reactor using high pressure CH₄-H₂ mixture gases. Experimental results show that high quality single-crystal diamond and polycrystalline diamond were simultaneously synthesized at a growth rate of 25 μm/h and 12 μm/h, respectively. The results indicate that our MPCVD reactor is unique for the synthesis of diamond with high efficiency.     

Diamond coating on hard metals by traveling methods in a plasma-assisted chemical vapor deposition method above the liquid

In this work, two approaches were developed to extend the coating area of diamond by continuous deposition in a plasma-assisted chemical vapor deposition (CVD) method above the liquid. The techniques were based on the methods previously developed by our research group and the characteristic was to use dc (direct current) plasma generated between the liquid surface and the metal electrode. In the first approach, a tungsten rod was rotated in a chamber at reduced pressure so that a diamond film was formed as a ‘belt’ in 6 mm width around the side of the rod. The deposited diamond was polycrystalline with a grain size of 1–3 μm. The film thickness increased almost linearly with deposition time, whereas the grain size was almost constant against the deposition time. The second approach was for a plate substrate. A tungsten plate was hung with an iron wire and the plasma was horizontally generated between the liquid and plate surfaces. When the W plate was vertically slid down slowly, a diamond film was continuously deposited on the surface. The deposited film was covered with a soot-like carbon layer on the top and the post-treatment with H₂O/N₂ gas at 600 °C was effective in removing it. The continuous deposition successfully demonstrated the expansion of the deposition area with the novel plasma CVD method above the liquid.     

In situ doping of diamond coatings with silicon,aluminum and titanium through a modified laser-based CVD process

The aim of this study was to demonstrate the feasibility of in situ doping of chemical vapor deposition (CVD) fabricated diamond coatings through simultaneous evaporation of solids in a CVD plasma-based process. In order to achieve maximum flexibility and energy density, a laser-based plasma-jet CVD process was chosen, and expanded with the introduction of dopant rods. The rods, with diameters varying from 0.5 mm to 3.0 mm, were fed at rates from 0.25 mm/min to > 100 mm/min, and positioned 3 mm below the optical breakthrough which generates the plasma. Gas flows of 20.0 slm (standard liters per min) argon, 2.0 slm hydrogen and 0.02 slm methane were used for diamond coating deposition. At a surface temperature of about 1100 °C, an average linear diamond growth rate of 20 μm/h was achieved. The materials selected as solid precursors for the rods were SiO₂, Al₂O₃, and Ti due to their differing electrical characteristics, as they are an insulator, semiconductor, and conductor, respectively. The evaporation rate of these rods varied by more than six orders of magnitude, from < 1 × 10^− 8 g/min (Ti) to > 7 × 10^− 2 g/min (SiO₂). The doped diamond coatings were produced by simultaneous evaporation and CVD. To prove that the precursors were vaporized and the atomic bonds were broken by the plasma, the optical emission spectra are compared with published and calculated spectral lines. Analyses of the layers were performed using EDX (energy-dispersive X-ray) spectroscopy and WDS (wavelength dispersive X-ray spectroscopy). As a result, the maximum doping densities in the diamond coating were determined, and were 3.460 wt.% for silicon, 0.957 wt.% for aluminum, and 0.03 wt.% for titanium. To prove the diamond-like characteristics of these coatings, Raman measurements were performed.     

A novel microwave plasma reactor with a unique structure for chemical vapor deposition of diamond films

With the aid of numerical simulation, a novel microwave plasma reactor for diamond films deposition has been designed. The new reactor possesses a unique structure, neither purely cylindrical nor purely ellipsoidal, but a combination of the both. In this paper, the design strategy of the new reactor together with a simple but reliable phenomenological simulation method will be described. Preliminary experiments show that uniform diamond films of high quality could be deposited using the new reactor, and the deposition rate of diamond films is typically about 3 μm/h at 6 kW input power level on a 2 inch diameter silicon substrate.     

Optical-quality diamond growth from CO2-containing gas chemistries

From interpretation of the Bachmann diagram, it is conceivable that there may be some advantage to be gained by moving up the H–CO tie line for optical-quality diamond deposition. A convenient system for achieving this are gas chemistries containing CO₂, which, when combined with gases such as CH₄ and C₂H₄ (ethylene), enables the diamond deposition region to be traversed and, with the addition of hydrogen, to move along the H–CO tie line. The fabrication of free-standing diamond wafers using combinations of these feed-stock gases with a high-pressure, 2.45-GHz microwave source reactor (HPMS) able to operate at up to 140 Torr and 6 kW has been investigated. The FWHM line width of the 1332 cm⁻¹ Raman peak is found to be predominately a function of the gas composition. The growth rate is also dependent on the input power and the deposition pressure. The deposition plasmas are bright green in colour, and optical emission spectroscopy (OES) of the plasma reveals distinctive C₂ and Hα peaks. In some cases, it is possible to correlate characteristics of the deposited diamond layer to features in the OES spectra.     

A roll-to-roll microwave plasma chemical vapor deposition process for the production of 294 mm width graphene films at low temperature

Roll-to-roll microwave plasma chemical vapor deposition (CVD) has been used for the continuous deposition of graphene films for industrial mass production. Using surface wave plasma, a pair of roll-to-roll winder and unwinder system has been built into a CVD apparatus, which has a deposition area of 294 mm × 480 mm. A graphene film was deposited onto the Cu film with 294 mm width under CH₄/Ar/H₂ plasma below 400 °C. It was found from cross-sectional transmission electron microscopy that few layer graphene, had been produced which consists of flakes with a nanometer size. After transferring the film onto a polyethylene terephthalate film, a uniform graphene film with high optical transmittance was confirmed.     

Investigation of diamond deposition uniformity and quality for freestanding film and substrate applications

In this paper, we report on microwave CVD deposition of high quality polycrystalline diamond and on related post-processing steps to produce smooth, flat and uniformly thick films or diamond substrates. The deposition reactor is a 2.45 GHz microwave cavity applicator with the plasma confined inside a 12 cm diameter fused silica bell jar. The deposition substrates utilized are up to 75 mm diameter silicon wafers. The substrate holder is actively cooled with a water-cooled substrate holder to achieve a substrate surface temperature of 600–1150 C. The pressure utilized is 60–180 Torr and the microwave incident power is 2–4.5 kW. Important parameters for the deposition of thick films with uniform quality and thickness include substrate temperature uniformity as well as plasma discharge size and shape. As deposited thickness uniformities of ± 5% across 75 mm diameters are achieved with simultaneous growth rates of 1.9 μm/h. The addition of argon to the deposition gases improves film deposition uniformity without decreasing growth rate or film quality, over the range of parameters investigated. Post-processing includes laser cutting of the diamond to a desired shape, etching, lapping and polishing steps.     

Distribution in angular mismatch between crystallites in diamond films grown in microwave plasma

High resolution electron backscattered diffraction (EBSD) has been used for analysis of grain size, texture and stress distribution on growth side of free-standing polycrystalline diamond films of different grade. The undoped and moderate boron-doped films of 0.3–0.5 mm thickness were grown by microwave plasma CVD. The highest number of stressed domains, mostly located at grain boundaries, and the largest average grain misorientation angle (θ ≈ 6°) have been found for B-doped film. Highly defected and highly [001] oriented “black” diamond exhibited much more rear stress domains, this being ascribed to angular mismatch as small as θ = 0.5° in that film. The samples of “white” diamond showed somewhat intermediate pictures, with stress observed both in bulk and on grain boundaries. Evolution of texture (columnar growth) and stress distribution with film thickness has been observed with EBSD study of film cross-sections.     

Experimentally defining the safe and efficient,high pressure microwave plasma assisted CVD operating regime for single crystal diamond synthesis

The detailed experimental behavior of a microwave plasma assisted chemical vapor deposition (MPACVD) reactor operating within the high, 180–300 torr, pressure regime is presented. An experimental methodology is described that first defines the reactor operating field map and then enables, while operating at these high pressures, the determination of the efficient, safe and discharge stable diamond synthesis process window. Within this operating window discharge absorbed power densities of 300–1000 W/cm³ are achieved and high quality, single crystal diamond (SCD) synthesis rates of 20–75 μm/h are demonstrated. The influence of several input experimental variables including pressure, N₂ concentration, CH₄ percentage and substrate temperature on SCD deposition is explored. At a constant pressure of 240 torr, a high quality, high growth rate SCD synthesis window versus substrate temperature is experimentally identified between 1030 and 1250 °C. When the input nitrogen impurity level is reduced below 10 ppm in the gas phase the quality of the synthesized diamond is of type IIa or better.     

Epitaxial lateral overgrowth (ELO) of homoepitaxial diamond through an iridium mesh

In the present work iridium layers forming a mesh on diamond have been studied as potential candidates for buried electrodes or stopping layers in an ELO process for heteroepitaxial diamond. Thin iridium layers (∼ 15 nm) were deposited by e-beam evaporation at ∼ 700 °C on the facets of individual (001)-oriented CVD diamond crystallites and macroscopic Ib HPHT substrates with off-axis angles of several degrees. The heteroepitaxial iridium films formed a mesh with 10–200 nm large holes. These were penetrated by homoepitaxial diamond in a microwave plasma chemical vapour deposition process (MWPCVD) burying the iridium layer completely after 15 min of diamond growth. High resolution X-ray diffraction including reciprocal space mapping and Raman spectroscopy was used to characterize the structural properties of the diamond overlayer on the Ib HPHT substrate. It was monocrystalline with an FWHM of 0.03–0.05° of the X-ray rocking curve. Its lattice planes were tilted by ∼ 0.01° with respect to the substrate and showed a macroscopic strain of − 10^− 4 perpendicular to the surface. Besides the smaller lattice constant due to the lack of nitrogen the strain is mostly attributed to a tensile in-plane stress state. Strain and tilt can be attributed to the lateral overgrowth and the off-axis angle of the substrate.     

Studies of heteroepitaxial growth of diamond

Large-scale heteroepitaxial growth of diamond depends critically on the development of a suitable lattice-matched substrate system. Oxide substrates, notably MgO and SrTiO₃, on which thin epitaxial films of iridium serve as a nucleation layer for diamond have already shown considerable promise. We describe here improvements in the growth of single crystal diamond by low-pressure microwave plasma-enhanced CVD. Oxide substrates with flat, low-index surfaces form the initial basis for the process. Iridium was deposited on heated substrates in a UHV electron-beam evaporation system resulting in epitaxial films, typically 150–300 nm thick, with Ir (1 0 0) parallel to the surface of all substrates as confirmed by X-ray and electron backscattering diffraction. Following Ir deposition, the samples were transferred to a CVD reactor where a bias-enhanced nucleation step induced a dense condensate that completely covered the Ir surface. Uniform nucleation densities of order 10¹² cm⁻² were observed. Interrupted growth studies, carried out at intervals from seconds to minutes subsequent to terminating the nucleation step, revealed a rapid coalescence of grains. One hour of growth resulted in a smooth, nearly featureless, (0 0 1) diamond film. For extended growth runs, slabs of diamond were grown with thickness as great as 38 μm and lateral dimensions near 4 mm. The crystals were transparent in visible light and cleaved on (1 1 1) planes along 〈1 1 0〉 directions, similar to natural diamond. Of particular significance is the successful use of sapphire as an underlying substrate. Its high crystalline perfection results in epitaxial Ir films with X-ray linewidths comparable to those grown on SrTiO₃. However, Al₂O₃ possesses superior interfacial stability at high temperatures in vacuum or in a hydrogen plasma with a better thermal expansivity match to diamond. Since sapphire is available as relatively inexpensive large diameter substrates, these results suggest that wafer-scale growth of heteroepitaxial diamond should be feasible in the near future.     

Nanocrystalline diamond growth in a surface-wave plasma

A surface-wave excited plasma is exploited in a diamond growth process by microwave plasma chemical vapor deposition method. Nanocrystalline diamond films with smooth surfaces are obtained from the plasma. As well as characterizing the deposited diamond films, the electron density and the electron temperature of the plasma are determined by using double-probe measurements. The plasma diagnosis reveals low electron temperatures of 2–3 eV in the process region, which is a distinctive characteristic of the surface-wave plasma. The low electron temperature is essential for the continuous re-nucleation of diamond in a hydrogen-rich plasma during the nanocrystalline diamond growth for a wide range of substrate temperature from under 100 to over 700 ^°C.     

Hydrogen plasma etching of diamond films deposited on graphite

Poly- and nanocrystalline diamond films have been deposited using microwave plasma enhanced CVD with gas mixtures of x%CH₄/15%H₂/Ar (x = 0.5, 1, 3, and 5). After deposition the resulting films were exposed to a hydrogen plasma etching for 30 min. The hydrogen plasma produced preferential etching of non-diamond carbon on the surface of the samples and the development of steps and pits. Raman spectroscopy and X-ray photoelectron spectroscopy analyses on the etched films showed increased sp³/sp² ratio and decreased surface oxygen. The etch mechanism proposed is regression of pre-existing steps and step flow.