Synthesis of diamond using ultra-nanocrystalline diamonds as seeding layer and their electron field emission properties

A modified nucleation and growth process was adopted so as to improve the electron field emission (EFE) properties of diamonds films. In this process, a thin layer of ultra-nanocrystalline diamonds (UNCD), instead of bias-enhanced-nuclei, were used as nucleation layer for growing diamond films in H₂-plasma. The morphology of the grains changes profoundly due to such a modified CVD process. The geometry of the grains transform from faceted to roundish and the surface of grains changes from clear to spotty. The Raman spectroscopies and SEM micrographs imply that such a modified diamond films consist of UNCD clusters (~ 10–20 nm in size) on top of sp³-bonded diamond grains (~ 100 nm in size). Increasing the total pressure in CVD chamber deteriorated the Raman structure and hence degraded the EFE properties of the films, whereas either increasing the methane content in the H₂-based plasma or prolonged the growth time improved markedly the Raman structure and thereafter enhanced the EFE properties of diamond films. The EFE properties for the modified diamond films can be turned on at E₀ = 11.1 V/μm, achieving EFE current density as large as (J_e) = 0.7 mA/cm² at 25 V/μm applied field.     

Deposition and characterization of nanocrystalline diamond films on Co-cemented tungsten carbide inserts

Nanocrystalline diamond films were deposited on Co-cemented tungsten carbides using bias-enhanced hot filament CVD system with a mixture of acetone, H₂ and Ar as the reactant gas. The effect of Ar concentration on the grain size of diamond films and diamond orientation was investigated. Nanocrystalline diamond films were characterized with field emission scan electron microscopy (FE-SEM), Atomic force microscopy (AFM), Raman spectroscopy and X-ray diffraction spectroscopy (XRD). Rockwell C indentation tests were conducted to evaluate the adhesion between diamond films and the substrates. The results demonstrated that when the Ar concentration was 90%, the diamond films exhibited rounded fine grains with an average grain size of approximately 60–80 nm. The Raman spectra showed broadened carbon peaks at 1350 cm^− 1 and 1580 cm^− 1 assigned to D and G bands and an intense broad Raman band near 1140 cm^− 1 attributed to trans-polyacetylene, which confirmed the presence of the nanocrystalline diamond phase. The full width at half maximum of the <111> diamond peak (0.8°) was far broader than that of conventional diamond film (0.28°–0.3°). The Ra and RMS surface roughness of the nanocrystalline diamond film were measured to be approximately 202 nm and 280 nm with 4 mm scanning length, respectively. The Ar concentration in the reactant gases played an important role in the control of grain size and surface roughness of the diamond films. Nanocrystalline diamond-coated cemented tungsten carbides with very smooth surface have excellent characteristics, which made them a promising material for the development of high performance cutting tools and wear resistance components.     

New insights into the mechanism of CVD diamond growth: Single crystal diamond in MW PECVD reactors

CVD Diamond can now be deposited either in the form of single crystal homoepitaxial layers, or as polycrystalline films with crystal sizes ranging from mm, μm or nm, and with a variety of growth rates up to 100s of μm h^− 1 depending upon deposition conditions. We previously developed a model which provides a coherent and unified picture that accounts for the observed growth rate, morphology, and crystal sizes, of all of these types of diamond. The model is based on competition between H atoms, CH₃ radicals and other C₁ radical species reacting with dangling bonds on the diamond surface. The approach leads to formulae for the diamond growth rate G and average crystallite size <d> that use as parameters the concentrations of H and CH_x (0 ≤ x ≤ 3) near the growing diamond surface. We now extend the model to show that the basic approach can help explain the growth conditions required for single crystal diamond films at pressures of 100–200 Torr and high power densities.     

Nucleation of nanocrystalline diamond on masked/unmasked Si3N4 ceramics with different mechanical pretreatments

Results are presented concerning different mechanical pretreatments performed on silicon nitride substrates and their influence on the nucleation and growth of nanocrystalline diamond (NCD). All substrates were equally sintered and finished, but differently pretreated. Then, they were diamond coated in a microwave chemical vapor deposition system (MPCVD) for relatively short periods, using Ar/H₂/CH₄ gas mixtures. The main objective was to identify the best pretreatment among those proposed, while verifying how it correlates with film uniformity and surface roughness after post-growth. The effect of a molybdenum mask during growth is investigated.The top surface analysis revealed major differences in the nucleation morphology of diamond nuclei on the pretreated samples, two different nucleation types having been identified. For all pretreatments, samples exhibited a very smooth and uniform underlayer of very fine grain particles before the formation of larger aggregates, suggesting a bi-phase nucleation mechanism. When no mask is used considerable changes in the nucleation concentration are found, the resulting films showing grain enlargement near the edges, where the morphology assumes microcrystalline nature. This effect is suppressed by the use of a mask that allowed obtaining very uniform smooth films (R_rms∼ 30 nm, thickness ∼ 1.3 μm, MUS pretreatment), indicating a strong edge effect for the unmasked case. This fact can be attributed both to increased local temperature, plasma density and gas turbulence.     

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.     

Investigation of the nucleation and growth mechanisms of nanocrystalline diamond/amorphous carbon nanocomposite films

Nucleation and growth, but especially the development of the morphology of nanocrystalline diamond/amorphous carbon (NCD/a-C) nanocomposite films have been investigated by systematic variation of three important parameters, namely the deposition time, the growth rate, and the substrate pre-treatment used to enhance the nucleation density. The films have been characterized, among others, by scanning electron microscopy, atomic force microscopy, and Fourier transform infrared spectroscopy. It is shown that, by successive addition of ultradispersive diamond powder to the suspension of nanocrystalline diamond powder in n-pentane used for the ultrasonic pre-treatment, the nucleation density can be enhanced by two orders of magnitude from 1 · 10⁸ cm^− 2 to > 1 · 10¹⁰ cm^− 2. This reduces the thickness required to achieve closed films from 1 µm to 100 nm. However, once coalescence of the individual nodules emerging from the nucleation sites has taken place the films loose “memory” of the nucleation step and start to develop the typical NCD morphology consisting of larger features with diameters of some hundreds of nm which are in turn composed of much smaller features. Irrespective of the feature size and of the parameters used, the films of this investigation possess AFM rms roughnesses of 9–13 nm, indicating that rms values are not sufficient to characterize NCD surfaces.     

Oxygen at the interface of CVD diamond films on silicon

The oxygen incorporation at the interface between the silicon substrate and chemical vapour deposited (CVD) diamond films nucleated by the bias-enhanced nucleation (BEN) procedure has been studied by heavy-ion elastic recoil detection (ERD). Using standard process conditions for the realisation of heteroepitaxial films, oxygen with a concentration equivalent to about 1 nm SiO₂ has been found, which was mainly incorporated during textured growth with a certain CO₂ admixture to the process gas. By completely omitting CO₂ during nucleation and growth, the oxygen at the interface can be reduced by nearly one order of magnitude to 6.3×10¹⁵ at cm⁻², corresponding to 0.14 nm SiO₂. Intentional addition of highly enriched C¹⁸O₂ to the gas phase shows that the oxygen incorporation is strongly enhanced during BEN with hydrocarbon in the gas phase. The results indicate that roughening of the surface, the deposition of Si_xO_yC_z phases and strong lateral inhomogeneities at the silicon interface may explain the coexistence of epitaxial crystallites and amorphous phases. It is suggested that a further reduction of the oxygen concentration at the interface may have consequences for an improved heteroepitaxy of diamond on silicon.     

Grain size dependent physical and chemical properties of thick CVD diamond films for high energy density physics experiments

We report on the grain size dependent morphological, physical and chemical properties of thick microwave-plasma assisted chemical vapor deposited (MPCVD) diamond films that are used as target materials for high energy density physics experiments at the Lawrence Livermore National Laboratory. Control over the grain size, ranging from several μm to a few nm, was achieved by adjusting the CH₄ content of the CH₄/H₂ feed gas. The effect of grain size on surface roughness, morphology, texture, density, hydrogen and graphitic carbon content was systematically studied by a variety of techniques. For depositions performed at 35 to 45 mbar and 3000 W microwave power (power density ~ 10 W cm^− 3), an abrupt transition from micro-crystalline diamond to nanocrystalline diamond was observed at 3% CH₄. This transition is accompanied by a dramatic decrease in surface roughness, a six percent drop in density and an increasing content in hydrogen and graphitic carbon impurities. Guided by these results, layered nano-microhybrid diamond samples were prepared by periodically changing the growth conditions from nano- to microcrystalline.     

Growth of diamond films with high surface smoothness

Diamond films with highly smooth backside surface have been deposited by positively biasing the substrate during diamond growth in a hot-filament chemical vapor deposition (HFCVD) system. By bonding the diamond film on the glass and wet etching to remove silicon, the highly smooth diamond surface can be exposed and used directly for the fabrication of diamond devices.Silicon substrate was first treated by diamond powder of 625 nm in an ultrasonic bath. By positively biasing the substrate, electron bombardment during diamond growth increases the nucleation density from 10⁸ ∼ 10⁹ cm^− 2 to 4 × 10¹¹ cm^− 2. The surface smoothness on the backside of diamond film has thus been improved significantly, inducing root-mean-square roughness of 5 nm. Owing to the extremely high surface smoothness and the high crystalline quality on the backside of diamond film and the high diamond growth rate, the backside surface of the diamond film grown under electron bombardment is particularly suitable for device fabrication.     

Diamond–carbon nanocomposites: applications for diamond film deposition and field electron emission

Diamond–carbon nanocomposites (DCN) containing diamond and graphitic particles, both a few nanometers in size, were studied as the material for field electron emission. Diamond–carbon mass ratio and grain size were varied to optimize field emission properties. The stable and uniform electron emission was observed at fields E=10 V μm⁻¹ with a negligible hysteresis of I–V curves. Treatment in microwave hydrogen plasma was found to reduce the threshold field for emission owing to preferential etching of carbon component and surface relief sharpening. Ultrathin chemical vapour deposition diamond films can be easily grown on DCN due to the very high nucleation density inherent to this composite.     

Influence of SiC particle addition on the nucleation density and adhesion strength of MPCVD diamond coatings on Si3N4 substrates

It is generally accepted that SiC layers are often involved in the adhesion efficiency of chemical vapour deposition (CVD) diamond films on Si-containing substrates. Si₃N₄–SiC composite substrates with different amounts of SiC particles (0–50 wt%) were then used for diamond deposition. Samples were produced by pressureless sintering (1750°C, N₂ atmosphere, 2–4 h). The diamond films were grown on a commercial MPCVD reactor using H₂/CH₄ mixtures. Despite there being no special substrate pre-treatment, the films were densely nucleated when SiC was added (N_d≈1×10¹⁰ cm⁻²) with primary nanosized (∼100 nm) particles, followed by a less dense (N_d≈1×10⁶ cm⁻²) secondary nucleation. Indentation experiments with a Brale tip of up to 588 N applied load corroborated the benefit of SiC inclusion for a strong adhesion. The low thermal expansion coefficient mismatch between Si₃N₄ and diamond resulted in very low compressive stresses in the film, as proved by micro-Raman spectroscopy.     

Some novel aspects of nanocrystalline diamond nucleation and growth by direct current plasma assisted chemical vapor deposition

Some novel aspects of nanocrystalline diamond (NCD) film nucleation and growth by DC-PACVD were investigated, which focused on the effect of methane injection timing at ramp stage (see discussion in the text) and cathode temperature as well. NCD films were deposited for 4 h on a 4 in. Si wafer which was ultrasonically seeded in a methanol slurry of diamond powder with a 5 nm average diameter. The H₂/CH₄/N₂ gas mixture with a composition of 96.7%/3%/0.3% was used as precursor gas. The total gas flow rate and chamber pressure were 150 sccm and 150 Torr, respectively. Discharge voltage and current of 500 V and 45 A were used respectively at a substrate temperature of 800 °C. The nucleation density, microstructure, growth rate and crystallinity of the obtained NCD films were characterized by SEM, XRD, NEXAFS and Raman spectroscopy. The nucleation density was found to be sensitive to methane injection timing in the ramp stage. In addition, the cathode temperature greatly affected the nucleation density, grain size and growth rate.     

Low temperature and large area deposition of nanocrystalline diamond films with distributed antenna array microwave-plasma reactor

Diamond films grown at low temperature (< 400 °C) on large area of different substrates can open new applications based on the thermal, electrical and mechanical properties of diamond. In this paper, we present a new distributed antenna array PECVD system, with 16 microwave plasma sources arranged in a 2D matrix, which enables the growth of 4-inch nanocrystalline diamond films (NCD) at substrate temperature in the range of 300–500 °C. The effect of substrate temperature, gas pressure and CH₄ concentration in the total gas mixture of H₂/CH₄/CO₂ on the morphology and growth rate of the NCD films is reported. The total gas pressure is found to be a critical deposition parameter for which growth rates and crystalline quality both increasing with decreasing the pressure. Under optimized conditions, the process enables deposition of uniform (~ 10%) and high purity NCD films with very low surface roughness (5–10 nm), grain size of 10 to 20 nm at growth rates close to 40 nm/h. Nanotribology tests result in the friction coefficient of the NCD films close to that obtained for the standard tetrahedral amorphous carbon coatings (ta-C) indicating the suitability of this low-temperature diamond coating for mechanical applications such as bearing or micro-tools.     

Growth of atomically step-free surface on diamond {111} mesas

Diamond homoepitaxy by microwave plasma-enhanced chemical vapor deposition was investigated on {111} substrate. Growth at a low CH₄/H₂ ratio of 0.025% in a gas phase results in the formation of an atomically step-free surface over 10 × 10-µm² mesas of diamond {111} substrate, when there are no screw dislocations in the mesas. This was achieved through ideal lateral growth, in which two-dimensional terrace nucleation was completely suppressed. The application of the selective formation of the step-free surface and the lateral growth of diamond films will open the way for the realization of high-quality electronic devices using diamond.     

Grain size dependent mechanical properties of nanocrystalline diamond films grown by hot-filament CVD

Nanocrystalline diamond (NCD) films with a thickness of ~ 6 µm and average grain sizes ranging from 60 to 9 nm were deposited on silicon wafers using a hot-filament chemical vapor deposition (HFCVD) process. These samples were then characterized in order to identify correlations between grain size, chemical composition and mechanical properties. The characterization reveals that our films are phase pure and exhibit a relatively smooth surface morphology. The levels of sp²-bonded carbon and hydrogen impurities are low, showing a systematic variation with the grain size. The hydrogen content increases with decreasing grain size, whereas the sp² carbon content decreases with decreasing grain size. The material is weaker than single crystalline diamond, since both stiffness and hardness decrease with the reduction in crystal size. These trends suggest gradual changes in the nature of the grain boundaries, from graphitic in case of 60 nm grain size material to hydrogen terminated sp³ carbon in 9 nm grain size material. The films exhibit low levels of internal stress and free-standing structures with a length of several centimeters could be fabricated without noticeable bending     

High-quality diamond film deposition on a titanium substrate using the hot-filament chemical vapor deposition method

Diamond film on titanium substrate has become extremely attractive because of the combined properties of these two unique materials. Diamond film can effectively improve the properties of Ti for applications as aerospace and biomedical materials, as well as electrodes. This study focuses on the effects of process parameters, including gas composition, substrate temperature, gas flow rate and reactor pressure on diamond growth on Ti substrates using the hot-filament chemical vapor deposition (HFCVD) method. The nucleation density, nuclei size as well as the diamond purity and growth tendency indices were used to quantify these effects. The crystal morphology of the material was examined with scanning electron microscopy (SEM). Micro-Raman spectroscopy provided information on the quality of the diamond films. The growth tendency of TiC and diamond film was determined by X-ray diffraction analysis. The optimal conditions were found to be: CH₄:H₂ = 1%, gas flow rate = 300 sccm, substrate temperature T_sub = 750 °C, reaction pressure = 40 mbar. Under these conditions, high-quality diamond film was deposited on Ti with a growth rate of 0.4 μm/h and sp² carbon impurity content of 1.6%.     

Extreme insulating ultrathin diamond films for SOD applications: From coalescence modelling to synthesis

The synthesis of diamond films with extreme insulating properties is of great interest for most diamond film applications in nanoelectronics. SOD (Silicon-On-Diamond) is a promising alternative to standard SOI (Silicon-On-Insulator) because of the high heat-spreading capability of diamond material. Current Fully Depleted MOS processing technologies require a thickness of the dielectric buried layer of 150 nm. Synthesis of polycrystalline diamond films is already well documented. Nonetheless, the difficulties here are to keep their high thermal conductivity and their high electrical resistivity in spite of the reduction of the diamond layer thickness. This study aims at the fine control of both the nucleation density and the growth process to enable the fabrication of optimized fully covered diamond films as thin as possible.A mathematical model describing the coalescence was used to determine the surface coverage of the diamond film according to the linear growth of the diamond nanocrystals for different nucleation densities. The model gives information on the nucleation density needed to obtain a covering diamond film within ultrathin diamond layer thickness. To corroborate the coalescence model, diamond layers with different surface coverages were characterized. Our work led to ultrathin diamond layers (thickness below 140 nm) exhibiting electrical resistivities above 2 × 10¹³ Ω cm.     

Status review of the science and technology of ultrananocrystalline diamond (UNCD™) films and application to multifunctional devices

This review focuses on a status report on the science and technology of ultrananocrystalline diamond (UNCD) films developed and patented at Argonne National Laboratory. The UNCD material has been developed in thin film form and exhibit multifunctionalities applicable to a broad range of macro to nanoscale multifunctional devices. UNCD thin films are grown by microwave plasma chemical vapor deposition (MPCVD) or hot filament chemical vapor deposition (HFCVD) using new patented Ar-rich/CH₄ or H₂/CH₄ plasma chemistries. UNCD films exhibit a unique nanostructure with 2–5 nm grain size (thus the trade name UNCD) and grain boundaries of 0.4–0.6 nm for plain films, and grain sizes of 7–10 nm and grain boundaries of 2–4 nm when grown with nitrogen introduced in the Ar-rich/CH₄ chemistry, to produce UNCD films incorporated with nitrogen, which exhibit electrical conductivity up to semi-metallic level. This review provides a status report on the synthesis of UNCD films via MPCVD and integration with dissimilar materials like oxides for piezoactuated MEMS/NEMS, metal films for contacts, and biological matter for a new generation of biomedical devices and biosensors. A broad range of applications from macro to nanoscale multifunctional devices is reviewed, such as coatings for mechanical pumps seals, field-emission cold cathodes, RF MEMS/NEMS resonators and switches for wireless communications and radar systems, NEMS devices, biomedical devices, biosensors, and UNCD as a platform for developmental biology, involving biological cells growth on the surface. Comparisons with nanocrystalline diamond films and technology are made when appropriate.     

Effect of H2/Ar plasma on growth behavior of ultra-nanocrystalline diamond films: The TEM study

Incorporation of H₂ species into Ar plasma was observed to markedly alter the microstructure of diamond films. TEM examinations indicate that, while the Ar/CH₄ plasma produced the ultrananocrystalline diamond films with equi-axed grains (~ 5 nm), the addition of 20% H₂ in Ar resulted in grains with dendrite geometry and the incorporation of 80% H₂ in Ar led to micro-crystalline diamond with faceted grains (~ 800 nm). Optical emission spectroscopy suggests that small percentage of H₂-species (< 20%) in the plasma leads to partially etching of hydrocarbons adhered onto the diamond clusters, such that the C₂-species attach to diamond surface anisotropically, forming diamond flakes, which evolve into dendrite geometry. In contrast, high percentage of H₂-species in the plasma (80%) can efficiently etch away the hydrocarbons adhered onto the diamond clusters, such that the C₂-species can attach to diamond surface isotropically, resulting in large diamond grains with faceted geometry. The field needed to turn on the electron field emission for diamond films increases from E₀ = 22.1 V/μm (J_e = 0.48 mA/cm² at 50 V/μm applied field) for 0% H₂ samples to E₀ = 78.2 V/μm (J_e < 0.01 mA/cm² at 210 V/μm applied field) for 80% H₂ samples, as the grains grow, decreasing the proportion of grain boundaries.     

Fabrication and application of nano–microcrystalline composite diamond films on the interior hole surfaces of Co cemented tungsten carbide substrates

Nano–microcrystalline composite diamond films are deposited on the interior hole surface of Co cemented tungsten (WC–6%Co) drawing using a squirrel-cage hot filament passing through the interior hole with large aperture by the bias-enhanced hot filament CVD. A new process is used to deposit nano–microcrystalline composite diamond coatings by a two-step hot filament chemical vapor deposition (HFCVD) procedure. Research results show that the as-deposited composite diamond films exhibit nanocrystalline diamond crystallites with grain sizes ranging from 60 to 90 nm and their surface roughness is measured as approximately Ra 220 nm with 4 mm scanning length. The Raman spectrum mainly exhibits three features near 1332, 1560 cm^? 1 (G peak), and a weak peak at approximately 1150 cm^? 1, which is attributed to the transpolyacetylene. XRD pattern indicates good crystallite quality of the composite films. The as-fabricated diamond coated dies show obvious performance enhancement in the practical application. Comparing with the WC–Co drawing die, the working lifetime of the diamond coated drawing die increases by a factor of above 15. Furthermore, the surface quality of the drawn copper pipes is greatly improved.