NEXAFS spectroscopy of CVD diamond films exposed to fusion relevant hydrogen plasma

A series of CVD diamond films have been exposed to hydrogen plasma in the linear magnetized plasma device, MAGPIE, with various applied sample stage biases between 0 V (no applied bias) to − 500 V. The plasma-induced damage to the surface structure of the diamond films has been investigated by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy in both the Auger electron yield (AEY) and total fluorescence yield (TFY) modes. The key diamond NEXAFS spectral features (diamond core exciton and second absolute band gap) are found to be diminished following plasma exposure as measured in the surface sensitive, AEY spectra, whilst these features remain unchanged relative to an unexposed diamond reference film as measured using the bulk sensitive, TFY spectra. These results, in conjunction with SRIM simulations, show definitively that the damage to the surface of the diamond films is restricted to the scale of the penetration depth of the H ions and no damage is induced at greater depths. The power and sensitivity of NEXAFS spectroscopy in assessing damage to the surface of diamond from fusion-relevant plasma-surface interactions are demonstrated.     

Friction force microscopy study of diamond films modified by a glow discharge treatment

A glow discharge treatment technique has been developed which enables control of the surface roughness and morphology of diamond films for applications in optical and electrical components. A conventional hot filament chemical vapour deposition (CVD) system was used to deposit the diamond films onto silicon substrates via a three-step sequential process: (i) deposition under normal conditions; (ii) exposure to either a pure hydrogen plasma or 3% methane in an excess of hydrogen using DC-bias; and (iii) diamond deposition for a further 2 h under standard conditions. The frictional characteristics and roughness of the film surfaces were investigated by atomic force microscopy (AFM) and the morphology and the growth rates determined from scanning electron microscope images. Lateral force microscopy (LFM) has revealed significant differences in frictional behaviour between the high quality diamond films and those modified by a glow discharge treatment. Friction forces on the diamond films were very low, with coefficients ∼0.01 against silicon nitride probe tips in air. However, friction forces and coefficients were significantly greater on the DC-biased films indicating the presence of a mechanically weaker material such as an amorphous carbon layer. A combination of growth rate and frictional data indicated that the exposure to the H₂ plasma etched the diamond surface whereas exposure to CH₄/H₂ plasma resulted in film growth. Re-Nucleation of diamond was possible (stage iii) after exposure to either plasma treatment. The resultant friction forces on these films were as low as on the standard diamond film.     

High quality,large surface area,homoepitaxial MPACVD diamond growth

The use of CVD diamond in electronics has very stringent requirements. For a CVD diamond industry to become viable it is mandatory to obtain very large growth rates (> 5 µm/h), all the while maintaining extremely high purity, a crystalline defect density as low as possible, and large usable surface areas. At the same time, one must keep the stress level within the growing crystal below acceptable limits to avoid crack formation and preserve the crystal structural integrity. These imperatives imply to work to improve both the plasma deposition process and the CVD diamond crystal growth. In this paper, we propose a three-pronged approach: (i) We use detailed plasma models to establish the influence of process parameters (in particular deposition pressure) on plasma chemistry in order to optimize film growth rate and diamond quality; (ii) We emphasize the need for careful substrate pre-treatment and selection (including choosing a single-sector face) to minimize defects in the growing films; (iii) We employ a 3D geometrical model to predict the crystal shape under given growth conditions, and exploit this knowledge to devise a growth strategy maximizing the usable film surface area while minimizing stresses inside the films.     

Pulsed laser surface modifications of diamond thin films

In view of practical applications requiring diamond films, plates and membranes with very smooth surfaces, ArF excimer laser polishing treatments were applied to thin (30 μm) diamond films grown by CVD on silicon substrates. The as-prepared diamond surfaces and the laser-treated parts of the samples were characterised by SEM analysis, Raman and micro-Raman spectroscopy. The presence on the laser-treated surface of a thin amorphous carbon layer responsible for the higher surface electrical conductivity and for the different optical reflectivity properties was evidenced. Using confocal micro-Raman spectroscopy a comparative depth profile analysis of the phase quality, below the surface in different regions of the films, was carried out. After short (10 min) treatment by H₂ plasma etching in the CVD chamber the graphitic top layer was completely removed from the samples.     

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.     

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%.     

The effect of surface treatment on oxidation of oxalic acid at nanocrystalline diamond films

The surface investigation of undoped and boron doped nanocrystalline diamond (NCD/BDND) films associated to their electrochemical behavior of oxalic acid after four pre-treatments was studied. The films were produced using Hot Filament CVD technique on Si substrate with a gas mixture of CH₄/H₂/Ar. Surface pre-treatments were carried out to analyze the surface chemical changes induced by hydrogen and oxygen plasma and as well as cathodic and anodic treatments performed in 0.1 mol L^? 1 HClO₄. The films wetting analyzed by contact angle presented a strong dependence of their surface before and after each treatment was also confirmed by the electrochemical response from cyclic voltammograms. Independent of the surface pre-treatments, all the electrodes exhibited response for oxalic acid oxidation, but the electrode submitted to hydrogen plasma presented the lowest starting oxidation potential and the highest current density. Nonetheless, the BDND electrode presented higher oxidation current than that for NCD electrodes, after all pre-treatments studied. The use of square wave voltammetry with BDND electrode treated by hydrogen plasma for the analytical determination of oxalic acid is described. The detection limits of 0.75 μmol was obtained from the linear relationship between the peak currents of voltammograms as a function of the oxalic acid concentrations.     

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.     

High temperature thermal conductivity of free-standing diamond films prepared by DC arc plasma jet CVD

Free-standing diamond films with 1.68 mm in polished thickness have been prepared by DC arc plasma jet CVD. By means of simply changing the placing orientation of diamond films along the laser transmission direction while testing, the through-thickness thermal conductivity (κ_⊥) together with the in-plane (κ_//) thermal conductivity of free-standing diamond films were measured by laser flash technique over a wide temperature range. Results show that the thermal conductivity κ_⊥ and κ_// of free-standing diamond films are up to 1916 and 1739 Wm^− 1 K^− 1 at room temperature, respectively, showing small anisotropy (9%), and following the relationship κ ~ T^− n as temperature rises. The conductivity exhibits similar value compared to that of high-quality single crystal diamond above 500 K for both through-thickness and in-plane directions of CVD diamond films. The effects of impurities and grain boundaries on thermal conductivity of diamond films with increasing temperature were discussed.     

On the reduction of the non-diamond phase in nanocrystalline CVD diamond films

We present photocurrent spectra of nominally undoped nanocrystalline diamond (NCD) films grown on glass substrates by hot filament (HF) and microwave (MW) plasma enhanced chemical vapor deposition (CVD). The spectra were measured in a broad optical range (200–2000 nm) by dual-beam photocurrent spectroscopy (DBP) and Fourier-transform photocurrent spectroscopy (FTPS) in amplitude modulated step scan mode. The NCD films with carefully oxidized surface show photosensitivity and high dark resistivity. Unlike single crystal type IIa diamond with the photonization threshold at 5.5 eV, the photocurrent spectra of NCD films are dominated by the “non-diamond phase” with the photo-ionization threshold at about 0.8 eV. Some HF CVD samples have lower sub-band gap absorption (non-diamond phase contamination). The non-diamond phase content increases after annealing at elevated temperature. The non-diamond phase content can be reduced by exposing NCD to hydrogen plasma at temperature below 350 °C.     

Investigation of nanocrystalline diamond films grown on silicon and glass at substrate temperature below 400 °C

We present investigation of nanocrystalline diamond films deposited in a wide temperature range. The nanocrystalline diamond films were grown on silicon and glass substrates from hydrogen based gas mixture (methane and hydrogen) by microwave plasma CVD process. Film composition, nano-grain size and surface morphology were investigated by Raman spectroscopy and scanning electron microscopy. All samples showed diamond characteristic line centred at 1332 cm^− 1 in the Raman spectrum. Nanocrystalline diamond layers revealed high surface flatness (under 10 nm) with crystal size below 60 nm. Surface morphology of grown films was well homogeneous over glass substrates due to used mechanical seeding procedure. Very thin films (40 nm) were successfully grown on glass slides (i.e. standard size 1 × 3″). An increase in delay time was observed when the substrate temperature was decreased. A possible origin for this behaviour was discussed.     

Improvements of crystallinity of single crystal diamond plates produced by lift-off process using ion implantation

A single crystal diamond substrate cut from a 9 mm thick ingot which was grown by chemical vapor deposition (CVD) was used to produce freestanding single crystal CVD diamond plates with improved crystallinity by the lift-off process using ion implantation. To reduce dislocations on the substrate surface, the ingot was sliced along the {100} plane parallel to the growth direction. In addition, the repeated lift-off processes reduced the surface damage on the substrate. These treatments were shown to improve the crystallinity of the CVD diamond plates produced by polarized light microscopy (PLM) and high-resolution X-ray.     

Diamond CVD film formation onto WC–Co substrates using a thermally nitrided Cr diffusion-barrier

Diamond film deposition onto WC-Co substrates exhibits several limitations regarding the final diamond quality in the film and its adhesion due to the chemical interaction between the Co in the substrate and the diamond CVD environment. In the present study, the use of a ~ 1.5 μm thermally nitrided Cr interlayer was examined as an effective diffusion barrier throughout the CVD process. Nitridation of the Cr PVD layer in NH₃ environment resulted in the formation of a graded CrN/Cr₂N layer comprised mainly of the CrN phase, accompanied with the formation of a porous ‘net-like’ microstructure at the surface. During both thermal nitridation and exposure to the CVD environment up to 360 min, the diffusion of C and Co from the substrate into the interlayer was limited to the region adjacent to the Cr–N interlayer/WC–Co substrate interface, which contained the Cr₂N phase. In this region, the Co interacted with the Cr lattice to form a CoCr phase, which was suggested to enhance the chemical binding between the interlayer and the substrate. The region containing the CrN phase was suggested to act as an effective diffusion barrier due to its fully occupied interstitial sites and relatively high crystalline density compared to the underlying Cr₂N phase. It was evident that the deleterious effects of Co during the CVD process were successfully suppressed using the Cr–N interlayer and the deposited diamond film exhibited improved adhesion and higher diamond quality.The formation of phases within the interlayer during nitridation and the diamond CVD process, and diamond quality evaluation in the deposited films were investigated by complementary techniques: SEM, XRD, XPS, SIMS and Raman spectroscopy.     

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.     

Interface features of the HPHT Ib substrate and homoepitaxial CVD diamond layer

This is a detailed study of the interface features of the HPHT Ib substrate and the homoepitaxial CVD diamond layer. Homoepitaxial diamond layers were prepared by a commercial type 30 kW dc arc plasma Jet CVD on (100) substrates with gas mixture of Ar/H₂/CH₄. The internal stress and the fluorescence properties of the cross-section of the single crystal diamond bulk were characterized with polarizer optical microscopy, micro-Raman spectroscopy, and photoluminescence, as well as DiamondView luminescence imaging. A higher stress region in the homoepitaxial diamond layer near the substrate with width about 15 μm was found exhibiting higher nitrogen content and presented different fluorescence properties. Studies on the surface morphology of the initial growth diamond indicated that the transformation of the surface morphology from growth hillocks to macrosteps, may play a crucial role in the interface feature.     

An investigation of the surface reactivity of diamond photocathodes with molecular and atomic oxygen species

The surface reactivity of CVD diamond films, which function as UV photocathodes, with thermal and electronically excited O₂, as well as atomic oxygen, has been explored. The CVD films show a low, but measurable reactivity with O₂, which increases markedly with electronic excitation, to form dilute oxygen adlayers which are stable thermally to 800 K. In contrast, reaction with atomic O produces significantly higher surface concentrations of oxygen, although these adlayers decompose thermally below 500 K, evolving CO₂. Greater surface reaction probabilities are observed if the diamond surface is pre-hydrogenated before interaction with oxygen. The consequences of these observations for the stable operation of H-terminated diamond photocathodes are discussed.     

Investigation of heterostructure between diamond and iridium on sapphire

Diamond thin film has outstanding physical and chemical properties. Diamond-on-iridium configurations have been prepared by several methods, such as microwave enhanced plasma CVD, direct currency plasma CVD, and hot filament CVD. In this study, an Ir interlayer was deposited on single crystal sapphires (Al₂O₃) with A-planes {1120} by an RF magnetron sputtering method after annealing samples. In addition, a diamond thin film was deposited by a microwave enhanced plasma chemical vapor deposition (MPCVD) method using a mixture of hydrogen and methane gases after a bias enhanced nucleation (BEN) procedure.Ir (001) was grown on the A-plane of sapphire by X-ray pole figure measurement. Diamond thin films were synthesized on each Ir/sapphire substrate and characterized by SEM, Raman spectroscopy. D {100} faces were exhibited in substantial areas of diamond films, and a flat D {100} plane was partially obtained. It is considered that diamond thin films on Ir {100} were mainly grown towards the <100> direction and were epitaxially grown in part.     

Etching and micro-optics fabrication in diamond using chlorine-based inductively-coupled plasma

The effect of Inductively-Coupled Plasma (ICP) etching on diamond using chlorine-based plasma has been investigated. The diamond materials studied include type IIa natural diamond, High Pressure and High Temperature (HPHT) diamond and Chemical Vapour Deposition (CVD) diamond. It was found that argon and chlorine (Ar/Cl₂) ICP plasma etching can improve the smoothness of the diamond surface. By using this method, a minimum root-mean-squared (rms) surface roughness of 0.19 nm has been achieved. To demonstrate optimized Ar/Cl₂ plasma etching, diamond spherical micro-lenses and micro-trenches were fabricated. Compared to argon and oxygen (Ar/O₂) plasma etching, Ar/Cl₂ plasma etching has a low selectivity with respect to the photo-resist mask, which enables an accurate control over the dimensions of the microstructures fabricated. The surface quality and profiles of these micro-lenses and micro-trenches were characterized by atomic force microscopy (AFM) and were shown to be better than those fabricated by Ar/O₂ ICP plasma.     

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.     

The annealing effect on the electrical property of the Al/undoped diamond film

Polycrystalline diamond films were deposited using a methane–hydrogen gas mixture in a microwave plasma assisted chemical vapor deposition system. Prior to deposition the silicon substrate was seeded, by photoresist, with 0.1 μm diamond powder. Then, the polycrystalline diamond films were annealed at 800°C under N₂ gas flow for 1 h. From the ESCA analyses, it was observed that the oxygen signal increases substantially, the carbon signal became very weak and the silicon (Si) signal showed up appreciably after the 800°C annealing process. In this study, a conduction mechanism was successfully established for the Al/annealed undoped diamond structure in the temperature ranges of 30–300°C. It was considered that a Schottky contact was also formed in the diamond grain boundaries. The modified equivalent circuit for the Al/annealed undoped diamond structure is an ideal Schottky diode in series with the bulk resistance for the bulk diamond crystallites, which is also in parallel with an opposite pole of the ideal Schottky diode in series with the grain boundary resistance for the diamond grain boundaries. It was observed that the electrical characteristics of the Al/annealed undoped diamond structure showed more ohmic behavior at high temperatures. It was suggested that the oxidation layer in the Al/annealed diamond interface was degraded after the high temperature measuring process. It was found that the field-activated transport mechanism, in which the series resistance can be represented by the modified Frenkel–Poole equation, the effective oxidation layer thickness and the electromigration effects both being included, gave a better fit to the experimental data.