Investigation in the role of hydrogen on the properties of diamond films grown using Ar/H2/CH4 microwave plasma

The transition of diamond grain sizes from micron- to nano- and then to ultranano-size could be observed when hydrogen concentration is being decreased in the Ar/CH₄ plasma. When grown in H₂-rich plasma (H₂ = 99% or 50%), well faceted microcrystalline diamond (MCD) surface with grain sizes of less than 0.1 μm are observed. The surface structure of the diamond film changes to a cauliflower-like geometry with a grain size of around 20 nm for the films grown in 25% H₂-plasma. In the Ar/CH₄ plasma, ultrananocrystalline diamond (UNCD) films are produced with equi-axed geometry with a grain size of 5-10 nm. The H₂-content imposes a more striking effect on the granular structure of diamond films than the substrate temperature. The induction of the grain growth process, either by using H₂-rich plasma or a higher substrate temperature increases the turn-on field in the electron field emission process, which is ascribed to the reduction in the proportion of grain boundaries.     

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.     

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.     

Effect of N2 addition in Ar plasma on the development of microstructure of ultra-nanocrystalline diamond films

The effect of the N₂ and H₂ addition in Ar plasma on the characteristics of the UNCD films was systematically investigated. It is found that, while the N₂/Ar plasma results in UNCD films with ultra-small grains (~ 5 nm), incorporation of H₂ into the N₂/Ar plasma increased monotonously the size of the grains. Moreover, the diamond grains synthesized in H₂ free plasma are of equi-axed geometry and those grown in H₂-containing plasma are of plate-like one. The optical emission spectroscopic investigation indicated that the increase in electron temperature due to the addition of H₂ into Ar plasma is the main cause, altering the microstructure of the UNCD films. As the H₂ content increases, the spherical diamond grains first agglomerated to form elongated grains, which coalesce to form dendrite clusters. The proportion of grain boundaries is thus decreased that increased the turn-on field necessary for inducing the electron field emission process.     

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.     

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.     

Ar, O2, CHF3, and SF6 plasma treatments of screen-printed carbon nanotube films for electrode applications

This paper investigates the consequence of the material property and the plasma gas chemistry (herein referred to the plasma gas-feeding species and methods) on the electrode performance in plasma treatments of screen-printed carbon nanotube (CNT) films. Four plasma gases (Ar, O₂, SF₆, and CHF₃) and three gas-feeding methods were examined. The surface morphology, microstructure, and composition of 11 sample groups have been carefully characterized. Tests of the CNT film electrode subjected to gas discharge and field emission show that surface morphology modification is the most influential factor in respect of lowering the onset voltages. In detail, O₂/Ar (O₂ followed by Ar) and Ar + CHF₃ + SF₆ (mixed three gases) treatments are the best choices for ionization and field emission applications, respectively. The relevant results are even better than that of the samples of aligned CNT films prepared by chemical vapor deposition. The underlying mechanisms are modeled by two opposing processes (etching and coating), which phenomenally produce three competing effects, i.e., CNT protruding, bundle forming, and neo-nanostructure forming. The results and the correct behavior of our model suggest that the plasma gas chemistry is the most fundamental factor in the process of plasma treatments of CNT films.     

Effects of hydrogen on carbon nanotube formation in CH4/H2 plasmas

Carbon nanotubes (CNTs) were synthesized using CH₄/H₂ plasmas and plasmas simulated using a one-dimensional fluid model. The thinnest and longest CNTs with the highest number density were obtained using CH₄/H₂ = 27/3 sccm at 10 Torr. These conditions allowed CNTs to grow for 90 min without any meaningful loss of catalyst activity. However, an excess H₂ supply to the CH₄/H₂ mixture plasma made the diameter distribution of the CNTs wider and the yield lower. Hydrogen concentration is considered to affect catalyst particle size and activity during the time interval before starting CNT growth (=incubation period). With CH₄/H₂ = 27/3 sccm for a growth time of 10 min efficient CNT growth was achieved because the amount of carbon atoms in the CNTs and that calculated from simulation showed good agreement. The effect of hydrogen etching on CNTs was analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy by observing CNTs treated by H₂ plasma after CNT growth. It was confirmed that (a) multi-walled CNTs were not etched by the H₂ plasma, (b) the C 1s XPS spectra of the CNTs showed no chemical shift after the treatment, and (c) C-H bonds were produced in CNTs during their growth.     

Synthesis and mechanical wear studies of ultra smooth nanostructured diamond (USND) coatings deposited by microwave plasma chemical vapor deposition with He/H2/CH4/N2 mixtures

Ultra smooth nanostructured diamond (USND) coatings were deposited by microwave plasma chemical vapor deposition (MPCVD) technique using He/H₂/CH₄/N₂ gas mixture. The RMS surface roughness as low as 4 nm (2 micron square area) and grain size of 5–6 nm diamond coatings were achieved on medical grade titanium alloy. Previously it was demonstrated that the C₂ species in the plasma is responsible for the production of nanocrystalline diamond coatings in the Ar/H₂/CH₄ gas mixture. In this work we have found that CN species is responsible for the production of USND coatings in He/H₂/CH₄/N₂ plasma. It was found that diamond coatings deposited with higher CN species concentration (normalized by Balmer H_α line) in the plasma produced smoother and highly nanostructured diamond coatings. The correlation between CN/H_α ratios with the coating roughness and grain size were also confirmed with different set of gas flows/ plasma parameters. It is suggested that the presence of CN species could be responsible for producing nanocrystallinity in the growth of USND coatings using He/H₂/CH₄/N₂ gas mixture. The RMS roughness of 4 nm and grain size of 5–6 nm were calculated from the deposited diamond coatings using the gas mixture which produced the highest CN/H_α species in the plasma. Wear tests were performed on the OrthoPOD®, a six station pin-on-disk apparatus with ultra-high molecular weight polyethylene (UHMWPE) pins articulating on USND disks and CoCrMo alloy disk. Wear of the UHMWPE was found to be lower for the polyethylene on USND than that of polyethylene on CoCrMo alloy.     

Modeling of nitrogen/diborane/methane/hydrogen plasma for nanocrystalline diamond growth: Comparison with experimental data

Gas phase thermodynamic equilibrium calculations involving N₂/B₂H₆/CH₄/H₂ mixtures were performed to investigate the chemical interactions leading to boron incorporation and microstructure variations in microwave plasma assisted chemical vapor deposition of diamond films. Molecular fractions of several BH_x (x = 1–3) species were calculated to study the incorporation mechanism of boron atom into diamond structure. A strong influence of the BH in causing the boron incorporation level in diamond lattice is confirmed by the correlation of its modeled equilibrium composition in the gas phase with boron content as determined experimentally. Nitrogen addition leads to nanocrystallinity and a reduction in boron incorporation due to a decrease in BH as additional B/N/H radicals are formed in the gas phase. We also obtained a good degree of agreement between the theoretically predicted CH₃/CN gas phase ratio and the experimental surface roughness trends as measured for all samples.     

The roles of argon addition in the hot filament chemical vapor deposition system

Nano-crystalline diamond films were successfully deposited using CH₄/H₂/Ar gas mixture by hot filament chemical vapor deposition (HFCVD) method. The characterizations of the as-grown films are carried out by using field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM). The results show that the film consists of nano-crystalline diamond grains with sizes ranging from 4 to 30 nm. High renucleation rates are found and attributed to the formation of these nano-diamond grains. The roles that Ar plays in HFCVD system are discussed.     

Mechanical properties of MWPECVD diamond coatings on Si substrate via nanoindentation

The mechanical properties of polycrystalline diamond coatings with thickness varying from 0.92 to 44.65 μm have been analysed. The tested samples have been grown on silicon substrates via microwave plasma enhanced chemical vapour deposition from highly diluted gas mixtures CH₄-H₂ (1% CH₄ in H₂). Reliable hardness and elastic modulus values have been assessed on lightly polished surface of polycrystalline diamond films.The effect of the coating thickness on mechanical, morphological and chemical-structural properties is presented and discussed. In particular, the hardness increases from a value of about 52 to 95 GPa and the elastic modulus from 438 to 768 GPa by varying the coating thickness from 0.92 to 4.85 μm, while the values closer to those of natural diamond (H = 103 GPa and E = 1200 GPa) are reached for thicker films (> 5 μm). Additionally, the different thickness of the diamond coatings permits to select the significance of results and to highlight when the soft silicon substrate may affect the measured mechanical data. Thus, the nanoindentation experiments were made within the range from 0.65% to 10% of the film thickness by varying the maximum load from 3 to 80 mN.     

Plasma-assisted chemical vapor deposition process for depositing smooth diamond coatings on titanium alloys at moderate temperature

A simple process has been perfected to deposit smooth fine-grained diamond coatings at 600°C on titanium alloys or titanium-coated surfaces. It consists of a two-step microwave plasma-assisted chemical vapor deposition (PACVD) procedure including first the deposition of a sacrificial sp²-carbon containing layer from a methane-rich CH₄–H₂ mixture and then the diamond growth from a CO₂–CH₄ inlet mixture. Scanning electron microscopy, X-ray diffraction, visible and UV Raman spectroscopy show that the coatings are smooth and mainly composed of crystalline diamond with a fine-grained morphology. The results are compared with the results obtained with classical rough polycrystalline coatings deposited from 8% CO–H₂. Optical emission spectroscopy reveals important differences between the plasma species produced for the deposition of these smooth coatings and the plasma species produced for the deposition of both polycrystalline coatings from 1% CH₄–H₂ or 8% CO–H₂ mixture and nanocrystalline films from Ar–CH₄(–H₂).     

Effects of nitrogen addition on morphology and mechanical property of DC arc plasma jet CVD diamond films

Nitrogen-doped diamond films have been synthesized by 100 KW DC arc plasma jet chemical vapor deposition using a CH₄/Ar/H₂ gas mixture. The effect of nitrogen addition into the feed gases on the growth and surface morphology and mechanical property of diamond film was investigated. The reactant gas composition was determined by the gas flow rates. At a constant flow rate of hydrogen (5000 sccm) and methane (100 sccm), the nitrogen to carbon ratio (N/C) were varied from 0.06 to 0.68. The films were grown under a constant pressure (4 KPa) and a constant substrate temperature (1073 K). The deposited films were characterized by scanning electron microscopy, Raman spectroscopy and X-ray diffraction. The fracture strength of diamond films was tested by three point bending method. The results have shown that nitrogen addition to CH₄/H₂/Ar mixtures had led to a significant change of film morphology, growth rate, crystalline orientation, nucleation density and fracture strength for free-standing diamond films prepared by DC arc plasma jet.     

The role of inert gas in MW-enhanced plasmas for the deposition of nanocrystalline diamond thin films

Nanocrystalline diamond thin films have been deposited using microwave plasma enhanced deposition with gas mixtures of composition H₂/CH₄/X, where X was one of the inert gases He, Ne, Ar and Kr and typically constituted > 90% of the total gas flow. The diamond films obtained with each gas mixture deposited at approximately the same rate (0.15–0.5 µm h^? 1), and all showed similar morphologies and average grain sizes, despite very obvious differences in the appearance and gas temperatures of the respective plasmas. These plasmas were probed by optical emission and cavity ring-down spectroscopy, and results from companion 2D chemical kinetic modelling of the Ar/H₂/CH₄ and He/H₂/CH₄ plasma were used to guide interpretation of the experimental observations. We conclude that the inert gas, though acting primarily as a buffer, also has significant effects on the thermal conduction of the gas mixtures, the electron temperature and electron energy distribution, and thereby changes the main channels of ionization and input power absorption. As a result, inert gas dilution elevates the electron and gas temperatures, enhances the hydrogen dissociation degree and affects the H/C mixture composition and deposition mechanisms.     

Preparation of diamond whiskers using Ar/O2 plasma etching

Radio frequency (RF) plasma etching of chemical vapor deposition (CVD) diamond film has been investigated in Ar/O₂ plasmas, with an emphasis to elucidate the effects of reacting gas on the fabrication of diamond whiskers. Diamond whiskers were formed on diamond films pre-coated with Al. It was found that diamond whiskers preferentially formed at the diamond grain boundaries. The densities of diamond whiskers increased with O₂ / Ar ratio. Whiskers obtained in pure O₂ plasma etching were 50 nm in diameter and 1 μm in height. The etching rate was increased by mixing Ar with appropriate volume of O₂. Al coated on the diamond surface reacted with O₂ to form Al₂O₃, serving as mask to restrain the etching underneath. Raman spectroscopy measurement confirmed that the whiskers kept sp³ diamond bonding structure after RF plasma etching. The field emission characteristics of the whiskers were also inspected.     

Growth of microcrystalline and nanocrystalline diamond films by microwave plasmas in a gas mixture of 1% methane/5% hydrogen/94% argon

Well-faceted microcrystalline diamond (MCD) films were deposited along with nanocrystalline diamond (NCD) films on the same substrate by a microwave plasma in the gas mixture of 1% CH₄+5% H₂+94% Ar. This was achieved by forcing a microwave plasma ball generated at 170 torr gas pressure to touch a silicon substrate that was pre-seeded by nanocrystalline diamond powder resulting in a high concentration of atomic hydrogen on the surface of growing diamond. Previously reported compositional mapping of the argon–methane–hydrogen system for MCD and NCD growth was not valid in this process parameter space. The non-uniform concentrations of atomic hydrogen and carbon containing radicals such as C₂ as well as varied local substrate temperature resulted in the simultaneous deposition of well-faceted MCD films in some areas with nanograined NCD films in others. Dilution of methane/hydrogen microwave plasmas by as much as 94% of argon alone could not suppress the growth of MCD.     

Oxygen functionalization of multiwall carbon nanotubes by Ar/H2O plasma treatment

To increase the applicability of multiwall carbon nanotubes (MWCNTs), oxygen-containing functional groups were introduced on the surfaces of MWCNTs by using microwave-excited Ar/H₂O surface-wave plasma. X-ray photoelectron spectroscopy and Raman spectroscopy were used to determine dependencies of Ar/H₂O gas partial pressure, treatment time and microwave power. The oxygen functionalization of MWCNTs by plasma can be achieved very rapidly, about 10 min. The C-O and O-C═O fractions firstly increase and then decrease with increasing Ar partial pressure. The C-O and O-C═O fractions increase with increasing microwave power from 400 W to 700 W. A slight increase of the R (I_D/I_G ratio) value for the treated MWCNTs indicated disordering in the surface microstructure of MWCNTs coincident with the introduction of surface oxygen. The oxygen-containing groups introduced on the surfaces of MWCNTs by plasma treatment are hydrophilic. The dispersion of plasma treated MWCNTs is therefore improved.     

Study the effect of O2 addition on hydrogen incorporation in CVD diamond

The effect of a small amount of O₂ addition on film quality and hydrogen incorporation in chemical vapour deposition (CVD) diamond films was investigated and the films were grown using a 5-kW microwave plasma CVD reactor. Film quality and bonded hydrogen were characterized using micro-Raman and Fourier transform infrared (FTIR) spectroscopy, respectively. It was found that in general for films grown using CH₄/H₂ plasma both without and with O₂ addition, the hydrogen incorporation increases with increasing substrate temperature, while a small amount of O₂ addition (O₂/CH₄=0.1) into CH₄/H₂ (4%) plasma strongly suppresses the incorporation of hydrogen into the film. Raman spectra show that the added oxygen improved film quality by etching and suppressing the amorphous carbon component formed in the film. The above effect of oxygen addition on hydrogen incorporation and film quality is discussed according to the growth mechanism of CVD diamond. The CVD diamond specific hydrogen related IR vibration at 2828 cm⁻¹ appears as a sharp and strong peak only in the FTIR spectra of poor quality films grown at high temperature both without and with O₂ addition, but it appears much stronger in the film grown without O₂ addition. This result experimentally excludes the assignment of the 2828 cm⁻¹ peak arises from hydrogen bonded to oxygen related defect in the literature.     

Growth of conformal single-walled carbon nanotube films from Mo/Fe/Al2O3 deposited by electron beam evaporation

We discuss growth of high-quality carbon nanotube (CNT) films on bare and microstructured silicon substrates by atmospheric pressure thermal chemical vapor deposition (CVD), from a Mo/Fe/Al₂O₃ catalyst film deposited by entirely electron beam evaporation. High-density films having a tangled morphology and a Raman G/D ratio of at least 20 are grown over a temperature range of 750-900 °C. H₂ is necessary for CNT growth from this catalyst in a CH₄ environment, and at 875 °C the highest yield is obtained from a mixture of 10%/90% H₂/CH₄. We demonstrate for the first time that physical deposition of the catalyst film enables growth of uniform and conformal CNT films on a variety of silicon microstructures, including vertical sidewalls fabricated by reactive ion etching and angled surfaces fabricated by anisotropic wet etching. Our results confirm that adding Mo to Fe promotes high-yield SWNT growth in H₂/CH₄; however, Mo/Fe/Al₂O₃ gives poor-quality multi-walled CNTs (MWNTs) in H₂/C₂H₄. An exceptional yield of vertically-aligned MWNTs grows from only Fe/Al₂O₃ in H₂/C₂H₄. These results emphasize the synergy between the catalyst and gas activity in determining the morphology, yield, and quality of CNTs grown by CVD, and enable direct growth of CNT films in micromachined systems for a variety of applications.