Study of diamond films grown at low temperatures and pressures by ECR-assisted CVD

We have studied a set of diamond films grown at low temperatures and pressures by electron cyclotron resonance (ECR)-assisted chemical vapor deposition (CVD). These films were grown on Si (100) substrates at temperatures ranging between 550 and 710 °C and pressures ranging between 1 and 2 Torr. Raman spectroscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) were employed to investigate the crystalline quality, diamond yield, and stresses developed in these films. Our Raman lineshape analysis indicates that most of the diamond films exhibit a net compressive stress. An estimate of the net stresses developed in these films was made by adding the thermal interfacial stress component to the calculated stress developed at the grain boundaries from the X-ray analysis. It was found that the residual stress is compressive in nature, but less compressive than that calculated from the Raman shift. The net stress exhibits a strong correlation with the relative amount of non-sp³ phase, thus implying that the non-sp³ phase is causing the measured excess compressive stress. However, the crystalline quality of the diamond phase improves as the overall non-sp³ component increases, thus indicating a process analogous to phase segregation within the films. These results indicate that the source of the excess compressive stress is non-sp³-bonded carbon accumulated at the grain boundaries.     

A study of the initial stages of diamond deposition on ferrous substrates coated by a nitrided chromium interlayer and on nitrided polycrystalline chromium substrates

The use of a nitrided chromium interlayer has been found to improve the interfacial properties of diamond films deposited on ferrous substrates. This is achieved by hindering diffusion process of carbon and iron, good adhesion of the interlayer to the steel substrate, and very stable mechanical and chemical bonding between the interlayer and the diamond film. In the present study the initial stages of diamond deposition on steel substrates coated by a nitrided chromium interlayer and on nitrided polycrystalline chromium substrates are reported. Nitridation of chromium films deposited by electrochemical methods and polycrystalline chromium substrates resulted in the formation of two chromium nitrides phases, CrN and Cr₂N, and a rough surface morphology. The initial stages of diamond deposition were found to be accompanied by carburization of the substrates surface resulting in chromium carbide formation. The incubation time, diamond particle density and growth rate at the very initial stages of the deposition process were found to differ for these two substrates. It is suggested that these differences originate from different carburization rates of the two substrates. Phase transformation, recrystallization and diffusion processes in the near surface regions of both substrates resulted in very stable chemical bonding and good adhesion of the diamond film to the substrates. Raman spectra of the deposited films, on both substrates, show shift of the diamond peak position to higher wave numbers and split of the peak. These effects are associated with compressive stresses in the diamond film. Residual stresses in the deposited films were calculated from the shift of the diamond Raman peak. The residual stresses, as calculated from the Raman spectra, were found to increase with deposition time reaching values of 8.4 and 6.9 GPa for continuous diamond films on steel substrate coated with the nitrided chromium film and on nitrided chromium substrates, respectively. Based on a simple model it was estimated that thermal stress, arising from mismatch between the thermal expansion coefficient of diamond and the underlying substrates, is the major component of the compressive stress in the diamond films.     

Comparative study of diamond films grown on silicon substrate using microwave plasma chemical vapor deposition and hot-filament chemical vapor deposition technique

Diamond films on the p-type Si(111) and p-type(100) substrates were prepared by microwave plasma chemical vapor deposition (MWCVD) and hot-filament chemical vapor deposition (HFCVD) by using a mixture of methane CH₄ and hydrogen H₂ as gas feed. The structure and composition of the films have been investigated by X-ray Diffraction, Raman Spectroscopy and Scanning Electron Microscopy methods. A high quality diamond crystalline structure of the obtained films by using HFCVD method was confirmed by clear XRD-pattern. SEM images show that the prepared films are poly crystalline diamond films consisting of diamond single crystallites (111)-orientation perpendicular to the substrate. Diamond films grown on silicon substrates by using HFCVD show good quality diamond and fewer non-diamond components.     

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.     

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.     

Selective deposition of diamond films on insulators by selective seeding with a double-layer mask

Polycrystalline diamond films have been patterned on Si₃N₄/Si and SiO₂/Si substrates by selective seeding with a double-layer mask via hot-filament chemical vapor deposition. High quality in the patterned diamond films and high selectivity were obtained by the process. The diamond films deposited on the insulators at different CH₄/H₂ concentrations were studied by scanning electron microscopy and Raman spectroscopy. The process proved to be far less damaging to the substrates, and yet effective in developing patterns of diamond films on a large and different substrate.     

Iron Oxide Nanoparticles Employed as Seeds for the Induction of Microcrystalline Diamond Synthesis

Iron nanoparticles were employed to induce the synthesis of diamond on molybdenum, silicon, and quartz substrates. Diamond films were grown using conventional conditions for diamond synthesis by hot filament chemical vapor deposition, except that dispersed iron oxide nanoparticles replaced the seeding. X-ray diffraction, visible, and ultraviolet Raman Spectroscopy, energy-filtered transmission electron microscopy , electron energy-loss spectroscopy, and X-ray photoelectron spectroscopy (XPS) were employed to study the carbon bonding nature of the films and to analyze the carbon clustering around the seed nanoparticles leading to diamond synthesis. The results indicate that iron oxide nanoparticles lose the O atoms, becoming thus active C traps that induce the formation of a dense region of trigonally and tetrahedrally bonded carbon around them with the ensuing precipitation of diamond-type bonds that develop into microcrystalline diamond films under chemical vapor deposition conditions. This approach to diamond induction can be combined with dip pen nanolithography for the selective deposition of diamond and diamond patterning while avoiding surface damage associated to diamond-seeding methods.     

Fracture toughness of the interface between CVD diamond film and silicon substrate in the relation with methane concentration in the source gas mixture

Diamond films produced by chemical vapor deposition (CVD) have been reported to show various excellent properties. However, low toughness of diamond films, especially the interface between the films and substrates, has been a severe problem. In order to find the dominant factors to control the adhesive strength of CVD diamond films, we obtained diamond films with various crystalline structures deposited on silicon (100) substrates under various methane concentrations in the source gas mixture. The toughness of the interface between the diamond film and silicon substrate was evaluated for the first time by a recently developed method. The toughness showed an interesting behavior with respect to the variation of methane concentration. The obtained results were quantitatively compared to the data already obtained for the case of CVD diamond particles deposited on silicon substrates.     

Early stage of diamond growth at low temperature

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

Determination of bulk residual stresses in superhard diamond-SiC materials

Superhard silicon carbide bonded diamond materials can be produced by liquid silicon infiltration of diamond containing preforms. These materials can be produced as bulk materials and as layered materials with the SiC bonded diamond only in areas where it is required. In order to understand the behaviour of the materials it is necessary to know the internal stresses in the different phases and at the interface. These stresses were determined by Raman spectroscopy and in the bulk by neutron diffraction using the SALSA instrument in the ILL. In the SiC bonded diamond material the diamond and the remaining Si are under compressive stresses. The SiC-phase is under tensile stresses up to 500 MPa. The Raman investigations and the neutron diffraction resulted in similar results. At the interface between the SiC-bonded diamond and the SiSiC no significant additional stresses could be observed.     

Chemically vapor deposited diamond-tipped one-dimensional nanostructures and nanodiamond–silica–nanotube composites

Composite thin films of nanodiamond and silica nanotubes were synthesized by means of microwave plasma assisted chemical vapor deposition (MPCVD) on silica nanotube matrix that was seeded with nanodiamond particles. SEM, Raman spectroscopy, and EDX were used to analyze the composite. Wet chemical etching was applied to selectively remove exposed silica from the composites for further revealing the nanostructure of the composites. Nanodiamond grew around silica nanotubes and filled the space left between silica nanotubes to form a continuous film. When appropriately selected sizes of nanodiamond particles were used as diamond seeds, silica nanotubes capped with CVD-grown diamond crystals were also obtained. Potential applications and implication of composites of nanodiamond and 1-D nanostructures will be discussed.     

Characterization of diamond fluorinated by glow discharge plasma treatment

The surface fluorination of diamond by treatment in glow discharge plasmas of CF₄ for different times has been investigated. High quality diamond films were deposited onto silicon substrates using hot filament chemical vapor deposition (HFCVD). Subsequently, the films were exposed to a radiofrequency glow discharge plasma of CF₄ for times ranging from 5 min to 1 h. The effects of the plasma treatment on the surface morphology, diamond quality and elemental composition were investigated using atomic force microscopy (AFM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. Differences in film roughness caused by the plasma treatment were detected by AFM and confirmed by scanning electron microscopy (SEM). Raman spectroscopic analyses showed that the original diamond was of high quality and that the bulk of each film was unchanged by the plasma treatment. Analyses using XPS revealed increased surface fluorination of the films at longer treatment times. In addition, the density of free radicals in the films was probed using electron paramagnetic resonance spectroscopy (EPRS), revealing that untreated diamond possesses an appreciable density of free radicals (6×10¹² g⁻¹) which initially falls with treatment time in the CF₄ plasma but increases for long treatment times.     

Characterization of boron-doped micro- and nanocrystalline diamond films deposited by wafer-scale hot filament chemical vapor deposition for MEMS applications

Micro- and nanocrystalline diamond (MCD and NCD) films are deposited on 4-inch silicon substrates by a large-area multi-wafer-scale hot filament chemical vapor deposition (HFCVD) system. The films are in-situ doped by boron. The chemical and crystalline structures are studied by electron probe microanalysis (EPMA), Raman spectroscopy and X-ray diffraction (XRD). The microcrystalline films have a preferred (111) texture, while the nanocrystalline films exhibit (220) texture. Strain gauges and cantilever beam arrays are micro-fabricated by surface micro-machining techniques to characterize the residual strain and strain gradient of the diamond films. Both micro- and nanocrystalline films have small compressive strains of − 0.052% and − 0.040% respectively, with the strain gradient of about 10^− 5 μm^− 1. These values are low enough to enable the realization of many MEMS devices.     

Graphitization effects of CH4 addition on NCD growth by first and second Raman spectra and by X-ray diffraction measurements

Nanocrystalline diamond (NCD) films were grown on silicon substrates by hot filament chemical vapour deposition in Ar/H₂/CH₄ gas mixtures. In the current study, the methane volume concentration varied from 0.5 to 3.5 vol.% in order to estimate its effect on nanodiamond morphology and structure. Film micrograph obtained from scanning electron microscopy showed diamond grain agglomerate, also called ballas diamond, which presented the grain size variation as a function of methane concentration increase. The transition from diamond agglomerate to graphite structure was also observed when CH₄ concentration is higher than 2.0 vol.%, confirmed from second order Raman measurements. The film local stress was estimated from the G-peak shift analyses and showed critical values necessary for the graphite/diamond phase formation. Structural investigations carried out by X-ray diffraction measurements for films deposited from 0.5 up to 2.0 vol.% CH₄ presented characteristic diamond diffraction peaks corresponding to 〈111〉, 〈220〉 and 〈311〉. A preferential orientation, changed from 〈110〉 to 〈111〉, was observed during NCD film deposition as a function of the methane concentration.     

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.     

Diamond growth by hollow cathode arc chemical vapor deposition at low pressure range of 0.02–2 mbar

An attempt was made to synthesize diamond films on (001) silicon substrates by means of a graphite or tungsten hollow cathode arc chemical vapor deposition at a lower pressure range of 0.02–2 mbar. The hollow cathode arc provides the advantage of the generation of a large area, high-flux electron beam, a very high-density plasma, and the high kinetic reaction species due to relatively low pressure operation. Diamond films have been characterized by scanning electron microscopy and Raman spectroscopy. The quality of diamond films deposited using the graphite hollow cathode was better than that using the tungsten hollow cathode at 2 mbar pressure. With further decreasing the deposition pressure, the evaporation and sputtering of the graphite hollow cathode are increased and the film quality was deteriorated. The growth rate of diamond films decreased and the nucleation density increased with decreasing deposition pressure.     

Evolution and properties of adherent diamond films with ultra high nucleation density deposited onto alumina

In this work, we report on adherent diamond films with thickness of up to 4.5 μm grown on polycrystalline alumina substrates. Prior to deposition, alumina substrates were ultrasonically abraded with mixed poly-disperse slurry that allows high nucleation density of values up to ∼5×10¹⁰ particles/cm². It was estimated that the minimal film thickness achieved for continuous films was ∼320 nm, obtained after a deposition time of 15 min with diamond particles density (DPD) of ∼4×10⁹ particles/cm². Continuous adherent diamond films with high DPD (∼10⁹ particles/cm²) were obtained also on sapphire surface after abrasion with mixed slurry and 15 min of deposition. However, after longer deposition time, diamond films peeled off from the substrates during cooling.The poor adhesion between the diamond and sapphire is attributed to the weak interface interaction between the film and the substrate and to difference in coefficient of thermal expansion. On the other hand, it is suggested that the reason for good adhesion between diamond film and alumina substrate is that high carbon diffusivity onto alumina grain boundaries allows strong touch-points at the grooves of alumina grains, and this prevents the delamination of diamond film. This adhesion mechanism, promoted by sub-micron diamond grain-size, is allowed by initial high nucleation density.The surface properties, phase composition and microstructure of the diamond films deposited onto alumina were examined by electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and high-resolution scanning electron microscopy (HR-SEM). The residual stress in the diamond films was evaluated by diamond Raman peak position and compared to a theoretical model with good agreement. Due to the sub-micron grain-size, the intrinsic tensile stress is high enough to partially compensate the thermal compressive stress, especially in diamond films with thickness lower than 1 μm.     

Surface composition,bonding, and morphology in the nucleation and growth of ultra-thin,high quality nanocrystalline diamond films

The morphology, composition, and bonding character (carbon hybridization state) of continuous, ultra-thin (thickness ∼ 60 nm) nanocrystalline diamond (NCD) membranes are reported. NCD films were deposited on a silicon substrate that was pretreated using an optimized, two-step seeding process. The surface after each of the two steps, the as-grown NCD topside and the NCD underside (revealed by etching away the silicon substrate) is examined by X-ray PhotoElectron Emission spectroMicroscopy (X-PEEM) combined with X-ray absorption near edge structure (XANES) spectroscopy, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The first step in the seeding process, a short exposure to a hydrocarbon plasma, induces the formation of SiC at the diamond/Si interface along with a thin, uniform layer of hydrogenated, amorphous carbon on top. This amorphous carbon layer allows for a uniform, dense layer of nanodiamond seed particles to be spread over the substrate in the second step. This facilitates the growth of a homogeneous, continuous, smooth, and highly sp³-bonded NCD film. We show for the first time that the underside of this film possesses atomic-scale smoothness (RMS roughness: 0.3 nm) and > 98% diamond content, demonstrating the effectiveness of the two-step seeding method for diamond film nucleation.     

Optimization of the mechanical properties of magnetron sputtered diamond-like carbon coatings

Diamond-like carbon coatings containing hydrogen, a-C:H, were deposited by use of reactive DC magnetron sputtering with an industrial deposition system. The reactive gas C₂H₂ was used in combination with carbon targets. Using Raman spectroscopy, nanoindentation and Rockwell C indentation, the mechanical properties of the coatings were optimized. Excessively high compressive stresses, which were measured with Raman spectroscopy, were found in the coatings with high hardness, resulting in poor adhesion to the substrates. By thermal annealing, these compressive stresses were reduced without altering the hardness, resulting in diamond-like carbon coatings with good adhesion.     

Real time investigation of diamond nucleation by laser scattering

Diamond nucleation onto diamond-free substrates remains a major challenge for most diamond films applications. In order to quickly form a continuous film across a given surface, several pre-treatments of the substrate have been developed to increase the nucleation density. Amongst those, Bias Enhanced Nucleation (BEN) has been used intensively for many applications, including for instance the synthesis of ultra-thin diamond films, heteroepitaxial diamond films, or nanodiamond films. The determination of the nucleation kinetics during the BEN pretreatment is particularly relevant in order to obtain fundamental informations about plasma/surface interactions and associated nucleation mechanisms. Besides, it is a key challenge to optimise the BEN step for specific applications, such as epitaxy or high nucleation density. The sequential approach which consists of interrupting the process at different time intervals for nucleation density measurement is time consuming and not accurate enough. We propose a real time investigation of diamond nucleation by laser scattering applied to the Bias Enhanced Nucleation (BEN) pre-treatment on silicon carbide. The Microwave Plasma Chemical Vapour Deposition (MPCVD) reactor was equipped with a laser reflectometry system associated with a lock-in laser intensity measurement. In parallel, a kinetics model of nucleation was drawn based on light diffusion of diamond nanoparticles according to their size and density. The modelling results were compared to the experimental data, and characteristic kinetic parameters were worked out for diamond nucleation on silicon carbide. In this study we demonstrated that using a model based on nanoparticles laser scattering it is possible to determine in real time the kinetics of diamond nucleation.