Enhanced nucleation of diamond on polycrystalline Ni by d.c. glow discharge in hot filament CVD

If a negative glow discharge is produced around a cathode wire above a polycrystalline Ni substrate, the diamond nucleation rate is greatly enhanced in the hot filament chemical deposition process. The diamond deposition depends on the surface orientation of the Ni grains, and occurs preferentially on faceted surfaces with hill-and-valley structures. The deposited diamond particles show a tendency to orient in one direction.     

Selective growth of diamond and carbon nanostructures by hot filament chemical vapor deposition

Diamond and carbon nanostructures have been synthesized selectively on differently pretreated silicon substrates by hot filament chemical vapor deposition in a CH₄/H₂ gas mixture. Under typical conditions for CVD diamond deposition, carbon nanotube and diamond films have been selectively grown on nickel coated and diamond powder scratched silicon surface, respectively. By initiating a DC glow discharge between the filament and the substrate holder (cathode), well aligned carbon nanotube and nanocone films have been selectively synthesized on nickel coated and uncoated silicon substrates, respectively. By patterning the nickel film on silicon substrate, pattern growth of diamond and nanotubes has been successfully achieved.     

Synthesis of diamond films and nanotips through graphite etching

A hot filament CVD process based on hydrogen etching of graphite has been developed to synthesize diamond films and nanotips. The graphite sheet was placed close to the substrate and only hydrogen was supplied during deposition. No hydrocarbon feed gases are required for this process. High quality diamond films were synthesized with high growth rate on P-type (1 0 0)-oriented silicon wafers without discharge or bias. The diamond growth rate is approximately five times higher than that through conventional hot filament chemical vapor deposition using a gas mixture of methane and hydrogen (1 vol.% methane) under similar deposition conditions. The diamond films synthesized in this process exhibit smaller crystallites and contain smaller amount of non-diamond carbon phases. Synthesis of well-aligned diamond nanotips with various orientation angles was achieved on the CVD diamond-coated Si substrate when the substrate holder was negatively biased in a DC glow discharge. The nanotips grown at locations far enough from the sample edges are aligned vertically, while those around the sample edges are tilted and point away from the sample center. The alignment orientation of the nanotips appears to be determined by the direction of the local electric field lines on the sample surfaces.     

A new polarised hot filament chemical vapor deposition process for homogeneous diamond nucleation on Si(100)

A new hot filament chemical vapor deposition with direct current plasma assistance (DC HFCVD) chamber has been designed for an intense nucleation and subsequent growth of diamond films on Si(100). Growth process as well as the I=f(V) characteristics of the DC discharge are reported. Gas phase constituents activation was obtained by a stable glow discharge between two grid electrodes coupled with two sets of parallel hot filaments settled in-between and polarised at the corresponding plasma potential. The sample is negatively biased with a small 10–15 V extraction potential with respect to the cathode grid. Such design allows to create a high density of both ions and radicals that are extracted and focussed onto the surface of the sample. The current density onto the sample can be finely tuned independently of the primary plasma. A homogeneous plasma fully covering the sample surface is visualized. Consequently, a high-density nucleation (⩾10¹⁰ cm⁻²) occurs.     

High rate of diamond deposition through graphite etching in a hot filament CVD reactor

Although a hot filament diamond CVD reactor has many advantages over other processes, the relatively low growth rate of the films has been a crucial drawback. We developed a new hot filament process that uses no hydrocarbon gas for diamond deposition. The graphite plate was placed below the silicon substrate and only hydrogen was supplied during the process. We could achieve the growth rate of 9 μm/h, which is approximately 9 times higher than that of conventional hot filament CVD using a gas mixture of methane and hydrogen. In spite of the high growth rate, the quality of diamond films was not degraded. Besides, the diamond films consisted of small crystallites with a smooth surface while the conventional diamond films of the same thickness tend to have a columnar structure with a rough surface.     

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.     

Influence of substrate nature on the d.c.-glow discharge induced nucleation of diamond

The nature of the nucleation centers, formed during the so called bias enhanced nucleation (BEN) of chemical vapor deposition (CVD) diamond is still an open question. We address this question by investigating the chemical composition and structure of the material deposited during the “nucleation” stage on various substrates by near edge X-ray absorption fine structure technique (NEXAFS) and Raman spectroscopy.The key step of the BEN of diamond in hot filament CVD systems is the generation of a stable d.c.-glow discharge between the grounded substrate and a positively biased electrode. This process results in the deposition of a carbon based film which contains the diamond nucleation and growth centers. Different materials, such as Si(100), CVD diamond films, and Si(100) onto which thin films of Ni were evaporated were used as substrates.It was found that the structure of the material deposited during the d.c.-glow discharge process is affected by the nature of the substrate. The d.c.-glow discharge process applied to the Si substrate resulted in the formation of a graphite-like film in the earlier stages (5 min), which after prolonged treatment time (30 min) was predominantly composed of nanosized diamond. The CVD diamond film, used as a substrate, promoted the formation of nanosized diamond particles even after 5 min of the d.c.-glow discharge process. However, C-13 labeling experiments have shown that microcrystalline diamond does not grow on the pre-existing CVD diamond substrate under the d.c.-glow discharge conditions. In the case of the Ni modified Si, the deposited film was graphitic in nature both after short and prolonged d.c.-glow discharge treatment times.     

Effects of electrode arrangement and pressure on synthesis of diamond films by arc discharge plasma jet chemical vapor deposition

The setup and deposition conditions of electrode arrangement and pressure have been studied to synthesize diamond films at high growth rate on wide area efficiently by arc discharge plasma jet chemical vapor deposition. An apparatus has been used in which four plasma torches, one is used for cathode and the others for divided anodes, are arranged and the positions of these torches are changeable. Growth rate, deposition area and thickness of diamond films have increased with changing the electrode arrangements without improvement of thickness variation. Maximum growth rate of our apparatus has occurred at the pressure of 6.7 kPa and diamond films that have less variations of quality and surface roughness have been synthesized at lower pressure during deposition. Moreover, a high conversion rate, which is the ratio of carbon atoms that form diamond in supplied methane gas, of 16% has been obtained at the pressure of 6.7 kPa and methane concentration of 2%.     

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.     

Selective synthesis of diamond and CNT nanostructures directly on stainless steel substrates

Without surface pretreatment or applying additional interlayer, diamond films have been directly synthesized on an Fe-25Cr-5Al steel substrate by a hot filament chemical vapor deposition method from an H₂-1vol.% CH₄ gas mixture. Due to an effective removal of intermediate graphite phase from the diamond-steel interface, the coated diamond films were continuous and adherent well to the steel substrate. Aligned conical diamond structures were also achieved on this steel substrate by negatively biasing the substrate holder and inducing a glow discharge. The deposition behavior of carbon on Fe-Cr-Ni steel substrate was different. A graphite-rich carbon film incorporated with diamond particles grew in the absence of biasing, then aligned carbon nanotube bundles were formed in the presence of negative biasing and glow discharge. The different deposition behavior of carbon on the two kinds of steel substrates was addressed in terms of the effect of their chemical compositions.     

MPCVD diamond tool cutting-edge coverage: dependence on the side wedge angle

Microwave plasma CVD usually produces uniform diamond coatings and high-quality diamond films. However, abnormal deposits appear near the sample edges — the so-called ‘edge effect’. Wedge-shaped silicon nitride inserts with 30°-, 60°-, 75°- and 90°-edge angles were vertically and horizontally exposed to MPCVD diamond coating to systematically study this effect. Finite element method (FEM) analysis was used to simulate the temperature distribution on such geometries. Diamond morphology and quality were assessed by SEM and micro-Raman techniques. The edge effect, a consequence of plasma concentration and thermal phenomena in this experimental set-up (activation by electromagnetic gas discharge), is more accentuated on samples that are vertically wedge-oriented towards the plasma. A grain-size gradient is established along the exposed surface, steeply increasing at the hot edge. An extreme effect occurs in the sharpest wedge samples, avoiding diamond growth at the edge.     

High quality heteroepitaxial diamond films on silicon: recent progresses

This paper reports the progresses made recently on the nucleation and growth of high-quality, [001]-oriented diamond films and discusses the problems to be resolved. The interface structure of diamond on silicon has further been investigated by transmission electron microscopy (TEM). Heteroepitaxial diamond films with increased lateral grain size and reduced grain boundary density were prepared in both microwave plasma chemical vapour deposition (MW-CVD) and hot filament chemical vapour deposition (HF-CVD) processes. Using a growth process combining a bias-assisted H⁺ etching and a [001]-textured growth smooth diamond films with large lateral grain size up to 10 μm can be obtained at a film thickness of approximately 10 μm. By controlling the [001]-textured growth process thick diamond films with a lateral grain size up to 30 μm has been achieved in HF-CVD.     

Hot filament CVD diamond coating of TiC sliders

Hot filament chemical vapour deposition (HF CVD) process with low filament temperature (∼ 1650 °C) was utilized for the diamond coating of TiC samples. Porous substrates were fabricated by pulse discharge sintering (PDS) to create more nucleation sites. Nucleation density and morphology of deposited diamond films were studied using scanning electron microscopy (SEM). It was found that the highest growth rate occurs at substrate temperature of 980 °C. Evaluation of the residual stress in deposited films was carried out by Raman spectroscopy. Ball on disk tests were performed with steel as a counterface material. After polishing diamond films demonstrated good sliding performance: friction coefficient of 0.08 and wear rate of 10^− 17 m³/N m.     

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.     

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

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

Measurement of the adhesion of diamond films on tungsten and correlations with processing parameters

Adhesion between diamond films and tungsten substrates is reported as a function of the deposition processing parameters. Diamond films were grown by a hot filament method as a function of seven different processing parameters: substrate scratching prior to diamond deposition, substrate temperature, methane content of the input gas mixture, filament temperature, filament-substrate distance, system pressure, and total gas flow rate. Adhesion was measured by using a Sebastian Five A tensile pull tester. Testing was complicated by the non-uniformity of the film thickness, diamond quality, film cohesion, and surface preparation across the full substrate surface area. Various types of film failure mode were observed, which did not correlate with the film processing parameters. The measured adhesion values showed larger variations from point to point across the sample surface and from identically prepared samples than variations as a function of the film processing parameters. Weak correlations of adhesion with the processing parameters were found using statistical analysis of the results from multiple pulls on a large number of samples. The statistical results suggest that substrate preparation, gas flow rate, and gas pressure are the most important processing parameters affecting the film adhesion, while the temperature of the hot filament has little or no effect on the adhesion of the film. However, improvements in film processing and adhesion testing need to be made before true quantitative adhesion testing of high-quality diamond films can be accomplished.     

Deposition of diamond/β-SiC composite gradient films by HFCVD: A competitive growth process

Surfaces featuring gradients of chemical composition and/or morphology allow high-throughput investigations and systematic studies in disciplines such as physics, chemistry, materials science, and biology. In this work, novel diamond/β-SiC composite films exhibiting a gradient composition were synthesized by a hot filament chemical vapor deposition (HFCVD) technique utilizing H₂, CH₄, and tetramethylsilane (TMS) as reaction gases. A specific filament-sample arrangement in the HFCVD chamber induced a gradation in chemical composition of the gas phase above the substrate surface, which, in turn, leads to a gradual change in the composition of the deposited films potentially ranging from pure diamond to pure β-SiC. It was possible to control the actual details of the diamond/β-SiC ratio in the gradient films by adjusting deposition pressure and TMS concentration. Aside from film characterization by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy were employed to determine the presence and quality of both diamond and β-SiC phases, respectively. The wealth of information provided by such diamond/β-SiC composite films allowed for a systematic investigation of the mechanism governing their growth. It turned out, that the growth process features nonequilibrium characteristics. It is dominated by a competition between a kinetic product (diamond) and kind of a thermodynamic product (β-SiC) to occupy any available positions on the substrate and the growing surface, respectively. With higher hydrogen radical concentration [H] and substrate temperature, the deposition is kinetically controlled, leading to diamond dominated films. On the other hand, a lower [H] and substrate temperature, consequently resulted in a predominantly thermodynamically controlled deposition, featuring a higher β-SiC content in the film.     

Interplay between adhesion and interfacial properties of diamond films deposited on WC-10%Co substrates using a CrN interlayer

In this study, the effect of deposition temperature on the adhesion of diamond films deposited on WC-10%Co substrates with a Cr-N interlayer is investigated. Diamond films were deposited at different temperatures (550, 650 and 750 °C), using a hot filament chemical vapor deposition reactor. It was found that the optimal adhesion is obtained for the film deposited at 650 °C. The interplay between carbon interfacial diffusion and the adhesion of diamond films deposited at different deposition temperatures were investigated. The combined use of different characterization techniques (Indentation tests, SIMS, XPS, XRD and SEM) shows that the adhesion strength depends on the thickness of Cr-C layer formed at the interface during diamond deposition, which is strongly influenced by the deposition temperature. It is suggested that at the optimum deposition temperature, thickness of the Cr-C layer is too low to introduce a large thermal stress at the interface and sufficiently thick enough to withstand the propagation of indentation induced cracks.     

Simultaneous growth of well-aligned diamond and graphitic carbon nanostructures through graphite etching

A hot filament chemical vapor deposition process based on hydrogen etching of graphite has been developed to synthesize diamond and graphitic carbon nanostructures. Well-aligned diamond and graphitic carbon nanostructured thin films have been synthesized simultaneously on differently pretreated silicon substrates in a pure hydrogen plasma. Graphitic nanocones, diamond nanocones and carbon nanotubes were selectively grown on uncoated, diamond and nickel pre-coated silicon substrates, respectively, in a single deposition process. The nanocones are solid cones with submicron scale roots and nanometer-size sharp tips. The nanotubes are hollow tubes with outer diameter of approximately 50 nm. The orientation of the well-aligned carbon nanostructures depends on the direction of the electric field at the samples surface. Nucleation and growth of diamond on the nanocones were further investigated under similar conditions without plasma. Diamond nanocomposite films have been obtained by depositing a nanocrystalline diamond film on the layer of diamond nanocones.     

Identification of carbon phases and analysis of diamond/substrate interfaces by Raman spectroscopy

Raman spectroscopy is employed to characterize thin diamond and diamond-like carbon films deposited by hot filament chemical vapour deposition (HFCVD). A method is proposed and experimentally verified for a contact-less measurement of the actual substrate temperature by Raman spectroscopy.