NEXAFS characterization of nanocrystalline diamond thin films synthesized with high methane concentrations

Diamond thin films were deposited on silicon in gas mixtures of methane and hydrogen with different methane concentrations ranging from 1% to 100% using microwave plasma assisted chemical vapor deposition. Both Raman spectroscopy and synchrotron near edge extended X-ray absorption fine structure spectroscopy (NEXAFS) were used to characterize the electronic structure and chemical bonding of the synthesized films. The NEXAFS spectra of the nanocrystalline diamond (NCD) films exhibit clear spectral characteristics of diamond. Close observation reveals that the films (10% CH₄ or above) exhibit a slightly broadened exciton transition with a 0.25 eV blue shift. With the increase in methane concentration, the growth rate, the surface smoothness, and the sp² carbon concentration of the films increase while the grain size decreases. Well-faceted microcrystalline diamond films were synthesized with a methane concentration of 5% or lower, while NCD films were formed with a methane concentration of 10% or higher. Diamond thin films with low surface roughness and fine nanocrystalline structure have been synthesized with high methane concentrations (50% or above). It has been observed that the diamond growth rate increases with methane concentration. The growth rate at 100% methane concentration is approximately 10 times higher than at 1%.     

Synthesis of nanocrystalline diamond films by DC plasma-assisted argon-rich hot filament chemical vapor deposition

Continuous nanocrystalline diamond (NCD) films were grown in an argon-rich gas atmosphere with relatively high growth rates by sustaining a low power (5 W) DC plasma in a hot filament chemical vapor deposition system (HFCVD). The parameter window for the synthesis of NCD films was studied as a function of argon, methane and hydrogen concentrations, as well as substrate temperature and DC bias. The results are consistent with reports indicating that the DC plasma induces re-nucleation by ion bombardment during the initial growth step and helps to maintain the atomic H and hydrocarbon species near the growing surface. It was found that DC plasma-assisted HFCVD enables high NCD growth rates and expands the parameter window, rendering it unnecessary to heat the filament above 2800 K.     

Structural and optical properties of diamond and nano-diamond films grown by microwave plasma chemical vapor deposition

We compare structural and optical properties of microcrystalline and nanocrystalline diamond (MCD and NCD, respectively) films grown on mirror polished Si(100) substrates by microwave plasma chemical vapor deposition. The films were characterized by SEM, Raman spectroscopy, XRD, and AFM. Optical properties were obtained from transmittance and reflectance measurements of the samples in the wavelength range of 200–2000 nm. Raman spectrum of the MCD film exhibits a strong and sharp peak near 1335 cm⁻¹, an unambiguous signature of cubic crystalline diamond with weak non-diamond carbon bands. Along with broad non-diamond carbon bands, Raman spectra of NCD films show features near 1140 cm⁻¹, the intensity of which is significantly higher in the film grown at 600°C compared to the NCD film grown at higher temperature. The Raman feature near 1140 cm⁻¹ is related to the calculated phonon density of states of diamond and has been assigned to nanocrystalline or amorphous phase of diamond. XRD patterns of the MCD film show sharp peaks and NCD films show broad features, corresponding to cubic diamond. The rms surface roughness of the films was observed to be approximately 60 nm for MCD film that reduced substantially to 17 and 34 nm in the NCD films grown at 600 and 700°C, respectively. Tauc's optical gap for the diamond film is found to be approximately 5.5 eV. NCD grown at 700°C has a high optical absorption coefficient in the whole spectral region and the NCD film grown at 600°C shows very high transmittance (∼78%) in the near IR region, which is close to that of diamond. This indicates that the NCD film grown at 600°C has the potential for applications as optical windows since its surface roughness is significantly low as compared to the MCD film.     

From micro to nanocrystalline transition in the diamond formation on porous pure titanium

A novel composite material of nanocrystalline (NCD) and/or microcrystalline (MCD) diamond films grown on porous titanium (Ti) substrate was obtained by hot filament chemical vapor deposition technique. Diamond films were grown using 1.5 vol.% CH₄ in a balanced mixture of Ar/H₂. The grain size control was obtained by varying the argon concentration from 0 up to 90 vol.% at substrate temperature of 870 K. Porous Ti substrates were obtained by powder metallurgy and presented an inter-connected open porosity. Scanning electron microscopy images of diamond/Ti exposed the substrate covered by a continuous textured coating which changed from MCD to NCD morphology; depending on the amount of Ar concentration in the feed gas. Micro-Raman spectra showed the characteristic t-polyacethylene peaks around 1150 cm^− 1 and 1470 cm^− 1, associated to NCD formation for samples grown with Ar concentration higher than 40 vol.%. X-ray diffraction patterns identified the diamond and TiC peaks, where the crystallinity of (111) TiC phase decreased as the Ar amount increased. This behavior was associated to diamond (220) peak increase for films grown with Ar concentration higher than 70 vol.%. Diamond crystallite size was also evaluated from Sherrer's formula in the range of 11 up to 20 nm.     

Structural and electrical properties of nanocrystalline diamond (NCD) heavily doped by nitrogen

Gas mixtures containing up to 40% nitrogen by volume and 1% CH₄ with the balance being argon have been used for the deposition of nitrogen doped nanocrystalline diamond (NCD) films by means of microwave plasma enhanced chemical vapour deposition (MPECVD). The CVD plasma was monitored by optical emission spectroscopy to reveal the plasma species, e.g., CN molecules, as a function of the nitrogen additive. Structural properties of the deposited NCD films were studied by FESEM and Raman spectroscopy. Effects of nitrogen doping on the electrical resistivity and electron field emission characteristics of the NCD films were measured. In this work, correlation between the structural and electrical properties of NCD films and the nitrogen additive to the CVD plasma will be presented and discussed.     

A new regime for high rate growth of nanocrystalline diamond films using high power and CH4/H2/N2/O2 plasma

In this work, we report high growth rate of nanocrystalline diamond (NCD) films on silicon wafers of 2 inches in diameter using a new growth regime, which employs high power and CH₄/H₂/N₂/O₂ plasma using a 5 kW MPCVD system. This is distinct from the commonly used hydrogen-poor Ar/CH₄ chemistries for NCD growth. Upon rising microwave power from 2000 W to 3200 W, the growth rate of the NCD films increases from 0.3 to 3.4 μm/h, namely one order of magnitude enhancement on the growth rate was achieved at high microwave power. The morphology, grain size, microstructure, orientation or texture, and crystalline quality of the NCD samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction, and micro-Raman spectroscopy. The combined effect of nitrogen addition, microwave power, and temperature on NCD growth is discussed from the point view of gas phase chemistry and surface reactions.     

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.     

Adhesion strength of diamond films on heat-treated WC–Co cutting tools

The adhesion strength and deposition behavior of diamond films with different grain size onto heat-treated WC–Co cutting tool inserts were investigated. The diamond film was deposited on WC–6%Co cutting tool inserts by the hot-filament chemical vapor deposition method, with H₂/3% CH₄ mixed gas. The N₂ gas was incorporated in the mixed gas to refine the grain size of the deposited diamond film (nanocrystalline diamond: NCD).Pores were observed in the interface region between the micrometer-size diamond film (MCD) and the WC–Co cutting tool insert. This suggested that the growth of diamond grains on top of elongated WC grains, which was induced by heat treatment to improve the adhesion strength of the deposited film, hindered the deposition of diamond in the valley area between the elongated WC grains. By contrast, in the case of the NCD film with a grain size of less than 50 nm obtained by addition of N₂ gas, no pores were observed, due to the fact that the refined diamond grains filled the interface region regardless of the existence of the elongated WC grains. The adhesion strength of the NCD film was likely to be greater than that of the MCD film on the heat-treated WC–Co cutting tool insert, which was explained by the full coverage with small diamond grains at the rough interface region.     

Semiconducting to metallic-like boron doping of nanocrystalline diamond films and its effect on osteoblastic cells

The impact of boron doping level of nanocrystalline diamond (NCD) films on the character of cell growth (i.e., adhesion, proliferation and differentiation) is presented. Intrinsic and boron-doped NCD films were grown on Si/SiO₂ substrates by microwave plasma CVD process. The boron-doped samples were grown by adding trimethylboron (TMB) to the gas mixture of methane and hydrogen. Highly resistive (0 ppm), semiconducting (133 or 1000 ppm), and metallic-like (6700 ppm) NCD films were tested as the artificial substrates for the cultivation of osteoblast-like MG 63 cells. The conductivity and surface charge increased monotonically with the increasing boron content. All NCD substrates showed good biocompatibility and stimulated the adhesion and growth of MG 63 cells. Higher osteocalcin concentration (by more than 30%) for the cells growing on 1000 and 6700 ppm boron-doped NCD films was found which indicates an enhancement in the cell growth biochemistry.     

Optical and mass spectroscopy measurements of Ar/CH4/H2 microwave plasma for nano-crystalline diamond film deposition

Nano-crystalline diamond (NCD) thin film with grains of about 5–100 nm in size attracts much attention as new functional materials in various industrial fields, due to its unique properties that are different from the conventional microcrystalline diamond (MCD) thin film. Most commonly, NCD film can be synthesized using CH₄/Ar plasma with/without a small amount of hydrogen gas added. In this study, we carried out the measurements of quadrupole mass spectroscopy (QMS) and optical emission spectroscopy (OES) to investigate CH₄/H₂/Ar mixture plasma in detail. Combined with our experimental results of NCD film synthesis, the mechanism of CH₄ dissociation and the precursors of NCD were further explored.     

Bioproperties of nanocrystalline diamond/amorphous carbon composite films

Nanocrystalline diamond/amorphous carbon (NCD/a-C) nanocomposite films have been deposited by microwave plasma chemical vapour deposition (MWCVD) from CH₄/N₂ mixtures. The films have been thoroughly characterized by a variety of methods with respect to their composition, morphology, structure and bonding environment. Thereafter, the bioproperties of these films have been investigated. Tests with osteoblast-like cells and pneumocytes proved that the NCD/a-C films are not cytotoxic. In addition, exposure of the films to a simulated body fluid revealed that they are bioinert. Further experiments addressed the question whether biomolecules such as RNA or proteins bind unspecifically on the surfaces of NCD/a-C films. By means of atomic force microscopy (AFM) and scanning force spectroscopy measurements it was established that, in contrast to control experiments with mica and glass, no interaction between the nanocrystalline diamond and either RNA or protein molecules took place. The results of these experiments concerning the biologically relevant properties of NCD/a-C films are discussed in view of possible future applications, e.g. as a material for the immobilization of biomolecules and their characterization by AFM measurements and related techniques.     

Boron doped diamond deposited by microwave plasma-assisted CVD at low and high pressures

Boron doped diamond is deposited over a range of pressures and chemistries including pressures from 35–120 Torr and gas chemistries including hydrogen–methane–diborane and argon–methane–hydrogen–diborane mixtures. The diamond deposition system is a 2.45 GHz microwave resonant cavity system. Diborane (B₂H₆) gas chemistry has been utilized with flow rates of 2.5–100 ppm. At low pressures of 35 Torr polycrystalline films are deposited using a feed gas mixture of hydrogen and 0.5% methane. At moderate pressures of 95 Torr, diamond films are grown using 60% Ar, 39% H₂ and 1% CH₄. For the high pressure experiments of 120 Torr, polycrystalline films are deposited using 98% H₂ and 2% CH₄. The deposition rate ranges from 0.3 to 1.6 μm/h. This investigation describes the relationship of the diborane flow rate and pressure versus the resulting film morphology, electrical properties, and morphology of the deposited films. The deposition of boron-doped polycrystalline diamond is done on 5 cm diameter silicon and silicon dioxide coated substrates. The resistivity spatial variation across the wafer was ± 5% indicating a good uniformity.     

Hybrid diamond/graphite films: Morphological evolution,microstructure and tribological properties

Diamond films were deposited on silicon substrate by microwave plasma enhanced chemical vapor deposition (MPCVD) using H₂ and CH₄ gas mixture. The morphological evolution process was characterized systematically by means of field-emission scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. Special attention has been paid to the influence of the methane concentrations on the microstructures of diamond films, which shows a gradual transition from nanocrystalline to microcrystalline films, and finally displays a hybrid diamond-graphite nanostructure with the length of a few micrometers. Finally, the friction behaviour of the hybrid films was studied. The value of the friction coefficient of the hybrid films is 0.10 and the corresponding wear resistance is below 1.9 × 10^− 7 mm³/N·m in diamond composites/Al₂O₃ sliding system in ambient atmosphere under dry sliding conditions. It is a convenient path to synthesize hybrid diamond/graphite nanostructures by MPCVD depending on higher methane concentrations in the absence of nitrogen or argon. The structure is appropriate for the potential applications as high efficient mechanical tools.     

Effects of CCl4 concentration on nanocrystalline diamond film deposition in a hot-filament chemical vapor deposition reactor

The effect of CCl₄ concentration on the nanocrystalline diamond (NCD) films deposition has been investigated in a hot-filament chemical vapor deposition (HFCVD) reactor. NCD films with a thickness of few-hundred nanometers have been synthesized on Si substrates from 2.0% and 2.5% CCl₄/H₂ at a substrate temperature of 610 °C. Polycrystalline diamond films and nanowall-like films with higher formation rates than those of the NCD films were deposited from lower and higher CCl₄ concentrations, respectively. The grain sizes of the diamond film grown using 2.0% CCl₄ increased with film thickness while a diamond film with uniform nanocrystalline structure all over a thickness of 1 μm can be deposited in the case of 2.5% CCl₄. We suggest that both the primary nucleation and the secondary nucleation processes are crucial for the growth of the NCD films on Si substrates.     

Microcrystalline diamond growth in presence of argon in millimeter-wave plasma-assisted CVD reactor

The additions of argon and oxygen to H₂–CH₄ feed gas and high-electron-density plasma generated by the millimeter-wave power were used to deposit microcrystalline diamond films having high quality and high growth rate simultaneously. Microcrystalline diamond films were grown on silicon substrates with 60–90 mm diameter in the millimeter-wave plasma-assisted CVD reactor based on 10 kW gyrotron operating at a frequency of 30 GHz. The growth process and morphology of diamond films at wide variation of parameters (gas pressure, substrate temperature, microwave power, argon and oxygen concentrations in gas mixtures Ar–H₂–CH₄ and Ar–H₂–CH₄–O₂) are investigated. For understanding of growth conditions the investigations of the plasma parameters (electron density and gas temperature) in novel CVD reactor are presented.     

Bilayered coatings of BN/diamond grown on Si3N4 ceramic substrates

Cubic boron nitride (c-BN) is a well known material to be used in machining of ferrous metallic alloys, namely as a coating. However, in most practical cases, there is a lack of adhesion to the substrate material. In this work, BN coatings were deposited by magnetron sputtering on silicon nitride (Si₃N₄) ceramic substrates using an intermediate layer of CVD microcrystalline (MCD) or nanocrystalline diamond (NCD). The goal was to improve the c-BN content by using diamond interlayers, and to optimize film adhesion to the substrate by employing such ceramic, which is known to provide very high adhesion strength to CVD diamond. The BN/NCD/Si₃N₄ combination demonstrated to be unique regarding the absence of delamination at both the BN/diamond and diamond/substrate interfaces, also providing the highest c-BN phase content.     

High compressive stress in nanocrystalline diamond films grown by microwave plasma chemical vapor deposition

Hard and smooth nanocrystalline diamond (NCD) thin films were deposited on mirror polished silicon substrates by biased enhanced growth in a microwave plasma chemical vapor deposition system. The films were characterized by Raman spectroscopy, X-ray diffraction and atomic force microscopy. Stress in the films was calculated by measuring the radius of curvature of the films on substrates and hardness was measured using a Nanoindenter. Stress in the films increases, first, with decreasing methane concentration in the gas phase while keeping biasing voltage constant, and second, with increasing biasing voltage while keeping the methane concentration constant. Observation of enormous stress (∼30 GPa) was possible in the films, which is due to strong adhesion between the films and substrates. To the best of our knowledge, this is the maximum value of stress reported so far in any kind of carbon thin films. It was hypothesized that it is mostly hydrogen content of the films in the methane series and graphitic content of the films in voltage series that are responsible in generating compressive stress in the respective films. The hardness follows almost a reverse trend than stress with the two growth parameters and can be well-defined from the relative concentration of NCD to graphitic content of the films, as estimated from Raman spectroscopy.     

Surface modification of nanocrystalline diamond/amorphous carbon composite films

The surfaces of nanocrystalline diamond/amorphous carbon (NCD/a-C) nanocomposite films deposited from a 17% CH₄/N₂ mixture have been subjected to a variety of plasma and chemical treatments, namely H₂ and O₂ microwave plasmas, a CHF₃ 13.56 MHz plasma, and a chemical treatment with aqua regia (HCl:HNO₃ 3:1). The resulting surfaces have been studied with respect to their chemical nature by X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (TOF-SIMS), concerning their morphology with atomic force microscopy, and by contact angle measurements to study their hydrophobicity and their stability. As-grown surfaces are hydrogen terminated, but the number of C–H bonds can slightly be increased by a H₂ microwave plasma, while treatment with aqua regia considerably lowers the number of C–H bonds at the surface. O₂ and CHF₃ plasmas, on the other hand, lead to a replacement of the terminating C–H bonds by C–O or C–OH and C–F_x groups, respectively. Finally, by contact angle measurements over a period of 150 days it could be shown that the H-terminated surface is very stable whereas the contact angle of the O-treated surface changed considerably with time, probably due to the adsorption of contaminants.     

Properties of nanocrystalline diamond thin films grown by MPCVD for biomedical implant purposes

Nanocrystalline diamond (NCD) thin films have been grown by microwave plasma chemical vapor deposition (MPCVD) and investigated to determine their suitability for biomedical applications. Growth conditions were chosen to produce very uniform films over the surface of curved temporomandibular joint implants. These parameters include methane flow rates exceeding 20% of the hydrogen gas flow rate, and chamber pressure and microwave power were maintained at 30 Torr and 0.73 kW, respectively, in a Wavemat 6 kW MPCVD device. Films (3 μm thick) that completely coated 2.54-cm-diameter Ti–6Al–4V disks under these conditions exhibit mean grain size of 30.4 nm as determined by XRD peak broadening, hardness of 80 GPa as determined by nanoindentation, RMS mean roughness of 15.3±5.3 nm as determined by stylus profilometry, and film adhesion toughness (Γ_C) of 158 J/m² as determined by a Rockwell indentation method. Similar deposition performed on small Ti–6Al–4V hemispheres produce films with smaller mean grain size of 21.1 nm and correspondingly lower hardness and roughness. Overall, these films exhibit properties well suited for use in biomedical applications.     

Investigation of the combined effect of argon addition and substrate bias on the growth of ultrananocrystalline diamond layers

In our recent project the combined effect of argon addition and substrate bias was investigated in the microwave plasma assisted chemical vapor deposition of diamond, focused on the ultrananocrystalline phase. Over the conventional qualifying techniques, i.e., Raman and SEM studies, we have led a special in-situ mass spectrometry investigation to explore the growth mechanism of UNCD, analysing the gas composition close to the surface. To achieve this aim, ion beam mass spectrometry (IBMS) was used for in-situ, real time, mass-selective analysis of the incoming species playing an important role in the MWPECVD (Microwave Plasma Enhanced Chemical Vapor Deposition) of the ultrananocrystalline diamond. In our experiments Ar, CH₄, and H₂ gases were used as source gases in a wide range of concentrations applying different values of substrate bias to deposit different phases of diamond. By the IBMS technique we can measure the fluxes of different species: C_xH_y (x = 1–2, y = 0–2) during the phases of deposition, either under the conditions of microcrystalline diamond (MCD), nanocrystalline diamond (NCD) and ultrananocrystalline diamond (UNCD) synthesis. As a result of it, we can compare the different mechanisms of layer formation: i.e.: whether C₁ species or C₂ mediated growth method takes place, or probably both C₁ and C₂ species propagate the diamond lattice. Based on the given tendency by comparing the IBMS data (i.e.: fluxes of surface species) with the growth rate, morphology, and Raman spectra of the layers we propose, that in the case of UNCD a similar (but not exactly the same) growth mechanism can be found as in the case of MCD i.e.: C₁ species are the most likely precursors.