Diamond Nucleation Density Research Articles

Microwave Plasma Chemical Vapor Deposition of Diamond Films on Silicon From Ethanol and Hydrogen

Diamond films with very smooth surface and good optical quality have been deposited onto silicon substrate using microwave plasma chemical vapor deposition (MPCVD) from a gas mixture of ethanol and hydrogen at a low substrate temperature of 450°C. The effects of the substrate temperature on the diamond nucleation and the morphology of the diamond film have been investigated and observed with scanning electron microscopy (SEM). The microstructure and the phase of the film have been characterized using Raman spectroscopy and x-ray diffraction (XRD). The diamond nucleation density significantly decreases with the increasing of the substrate temperature. There are only sparse nuclei when the substrate temperature is higher than 800°C although the ethanol concentration in hydrogen is very high. That the characteristic diamond peak in the Raman spectrum of a diamond film prepared at a low substrate temperature of 450°C extends into broadband indicates that the film is of nanophase. No graphite peak appeared in the XRD pattern confirms that the film is mainly composed of SP3 carbon. The diamond peak in the XRD pattern also broadens due to the nanocrystalline of the film.

Effect of gas phase composition cycling on diamond nucleation density

The nucleation density of diamond films produced by microwave plasma enhanced chemical vapor deposition was enhanced up to four times using a cyclic methane flow modulation technique. In this technique, the plasma composition is alternated for short periods between a combination of methane and hydrogen and hydrogen only. This results in alternate periods of deposition and etching for the nucleation run. The highest nucleation density, 1.8×10 8 cm −2, was obtained for the highest methane:hydrogen ratio of 8:200 sccm at a growing/etching time ratio of 180/30 s for 10.5 min over 3 h total time on the glass substrates. The grain size likewise increased up to 0.85 μm with a higher growing/etching time ratio. In contrast, the diamond quality, as measured by the relative intensity of Raman peaks corresponding to diamond and amorphous carbon was inversely related with the growing/etching time ratio.

Diamond nucleation from the gas phase onto cold-worked Co-cemented tungsten carbide

Co-cemented tungsten carbide (WC–Co) substrates with fine (1 μm) and coarse (6 μm) grain size were sintered using 6 wt.% Co as a binder. The as-sintered samples were ground to the final geometry (10×10×3 mm 3). After the grinding treatment, the full width at half maximum (FWHM) of the WC X-ray diffraction (XRD) peaks indicated a high level of strain in a few micrometers thick surface layer, according to the penetration depth of Cu Kα radiation. The as-ground substrates were submitted to a two-step etching procedure with Murakami's solution, to roughen the surface, and 10 s acid wash to etch surface cobalt out. The Murakami's etching time was varied between 1 and 20 min. Fine- and coarse-grained substrates submitted to different chemical etching times were characterized by scanning electron microscopy and XRD, and then submitted to short diamond nucleation runs in a Hot Filament Chemical Vapour Deposition reactor. Both FWHM of WC peaks and diamond nucleation density decreased by increasing the Murakami's etching duration, providing that the etched layer did not exceed 2 μm thickness. When a layer thicker than a couple of micrometers was removed by etching, diamond nucleation density was very low and no more dependent on etching time. This occurrence suggested that diamond nucleation density correlates well with the amount of residual strain at the substrate surface and can be tailored by a suitable control of strain-related defects produced by mechanical treatments.

Raman analyses of residual stress in diamond thin films grown on Ti6Al4V alloy

The stress evolution in diamond films grown on Ti6Al4V was investigated in order to develop a comprehensive view of the residual stress formation. Residual stress is composed of intrinsic stress induced during diamond film growth and extrinsic stress caused by the different thermal expansion coefficients between the film and substrate. In the coalescence stage it has been observed that the residual stress is dominated by the microstructure, whereas on continuous films, the thermal stress is more important. In this work diamond thin films with small grain size and good size and good quality were obtained in a surface wave-guide microwave discharge, the Surfatron system, with a negative bias voltage applied between the plasma shell and substrate. For above of -100V applied bias, the ratio of carbon sp³/sp² bond may increase and the nucleation rate increase arising the high value at the -250V applied bias. Stress measurements and sp³ content in the film were studied by Raman scattering spectroscopy. The total residual stress is compressive and varied from -1.52 to -1.48 GPa between 0 and -200 V applied bias, respectively, and above the -200 V, the compressive residual stress increased drastically to -1.80 GPa. The diamond nucleation density was evaluated by top view SEM images.

Manufacture of diamond-coated cutting tools for micromachining applications

Micro- and nanomachining cutting tools are currently being developed in order to extend the range of manufacturing technologies available to the micro and nano engineer. Conventional diamond tools used for grinding operations have a number of problems associated with heterogeneity of the crystallites, decreased cutting efficiency and short life. Micromachining cutting tools are manufactured by imbedding diamond particles into the tip of the microtool using a suitable binder matrix material. The use of a diamond coating may offer an improvement in microtool technology. Chemical vapour deposition (CVD) of diamond coatings onto a cemented tungsten carbide (WC-Co) substrate is difficult. Generally, the adhesion of the diamond coating to cemented carbide substrates is poor. It is reported that binder materials such as cobalt can suppress diamond growth and enhance graphitic deposits, which cause poor adhesion and low diamond nucleation density. The effects of process parameters such as filament position, filament and substrate temperature and pretreated substrate material on the coating properties have been investigated using a number of experimental techniques.

CVD diamond nucleation enhanced by ultrasonic pretreatment using diamond and mixture of diamond and TaC powders

Effects of ultrasonic pretreatment on chemical vapor deposition (CVD) diamond nucleation on Si substrates were systematically studied. Pure 1.5–40 μm-diamond powder and mixtures of 1.5–5 μm-diamond as well as 5–20 μm-Tantalum Carbide (TaC) powder were used in ultrasonic pretreatment. The root-mean-square (Rms) surface roughness of the pretreated substrates, residual diamond and TaC powders left on the substrates were examined using atomic force microscopy (AFM), Raman spectroscopy and X-ray diffraction (XRD), respectively. It was observed that the Rms surface roughness increases with increasing diamond or TaC powder size, and there is some diamond or TaC powder left on the substrates after ultrasonic pretreatment. Diamond films were deposited using microwave plasma chemical vapor deposition (MPCVD) technique and characterized by field emission scanning electron microscopy (FE-SEM). It was found that CVD diamond nucleation density strongly depends on particle size of diamond or TaC powder used, the nucleation density increases with increasing diamond or TaC powder size. A mixture of diamond and TaC powders enhances CVD diamond nucleation much more significantly than that of pure diamond powder. A mixture of 1.5 μm-diamond and 20 μm-TaC powders has an equivalent nucleation enhancement efficiency, which could be caused by pure 40 μm-diamond powder.

Precise patterning of diamond films for MEMS application

To apply diamond films in microelectromechanical systems (MEMS), it is necessary to develop suitable techniques to pattern diamond films in micrometer scale. In this paper, three different techniques capable of precise patterning diamond films will be demonstrated. One is to selectively grow diamond films in the desired region by pre-improving the diamond nucleation density in the said region using DC bias-enhanced microwave plasma chemical vapor deposition (MPCVD). Another technique is to selectively grow diamond films in the desired region by seeding the given region using diamond-powder-mixed photoresist. The third technique is to selectively etch large-area diamond film by an oxygen-ion beam under an Al mask. Several diamond-film patterns with a feature size of a few micrometers were successfully fabricated, and some technical problems encountered in the techniques are discussed.

Growth of diamond films with bias during microwave plasma chemical vapor deposition

Diamond films on 3-inch diameter (100) silicon wafers were synthesized with bias during microwave plasma vapor deposition (MPCVD). The deposition parameters were as follows: 0.13% CH4 in H2, pressure at 30 torr; deposition temperature at 768–869 °C, and a deposition time of 3–3.5 h. The bias voltage applied to the substrates during diamond growth varied from 0 to −450 V. The deposited films were characterized by Raman spectroscopy and electron microscopy. The results show that the film properties are uniform across the whole 3-inch diameter area. Raman spectra show that the ratio of sp3 to sp2 is increased with the bias voltage up to −350 V, while further increases in voltage resulted in a decreased ratio. It was found that column-shaped grains of diamond were directly grown from the substrate surface without grain coalescence in the lateral direction during growth. The nucleation density of diamond with biased growth was of the order of 109 cm−2.

Application of diamond coatings onto small dental tools

Small dental tools such as burs and drills are commonly used in dental practice and laboratory. Conventional diamond burs used for grinding operations have a number of problems associated with heterogeneity of the crystallites, decreased cutting efficiency, need for repeated sterilisation and short life. These burs are manufactured by imbedding diamond particles into the burs using a suitable binder matrix material. The use of a diamond coating may offer an improvement in dental bur technology. Chemical vapour deposition (CVD) of diamond coatings onto the cemented tungsten carbide WC–Co substrate is problematic. Generally the adhesion of diamond coating to cemented carbide substrate is poor. It is obvious that the binder materials such as cobalt can suppress diamond growth and enhance graphitic deposits, which cause poor adhesion and low diamond nucleation density. The effects of key process parameters such as filament position, filament and substrate temperature and pre-treated substrate material on the coating properties have been investigated using a variety of analytical techniques. Characterisations of the substrates and polycrystalline diamond film morphology were analysed by scanning electron microscopy (SEM). The chemical composition was evaluated by energy dispersive spectroscopy (EDS). Raman spectroscopy was used to assess the carbon-phase purity and give an indication of the stress levels in the as-grown polycrystalline diamond films.

Nanometer rough, sub-micrometer-thick and continuous diamond chemical vapor deposition film promoted by a synergetic ultrasonic effect

In this paper we report on a surface treatment to seed substrates for the promotion of diamond nucleation. This surface treatment consists of an ultrasonic abrasion process using poly-disperse slurry composed of a mixture of small diamond particles (<0.25 μm) and larger particles (>3 μm) which may consist of diamond, alumina, titanium, etc. Whereas ultrasonic abrasion with a mono-disperse diamond slurry results in a diamond nucleation density of ∼2–3×108 particles/cm2, treatment with poly-disperse slurries results in diamond nucleation density of values up to ∼5×1010 particles/cm2. This effect was found to display a similar effectiveness on a variety of substrates such as silicon, sapphire, quartz, etc. The enhancement in diamond nucleation is interpreted by a ‘hammering’ effect whereby the larger particles insert very small diamond debris onto the treated surface, thus increasing the density of nuclei onto which diamond growth takes place during the chemical vapor deposition process. By increasing the nucleation density to values of ∼5×1010 particles/cm2, continuous diamond films of thickness of less than ∼100 nm were grown after only 5 min of deposition. The roughness of continuous diamond films grown on substrates treated at optimum conditions obtains values of 15–20 nm. The effect of ultrasonic treatment on silicon substrates and the deposited films was investigated by atomic force microscopy (AFM), high-resolution scanning electron microscopy (HR-SEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy.

Direct current bias effects in RF induction thermal plasma diamond CVD

Substrate biasing is an emerging technique extensively used in diamond and cBN chemical vapor deposition (CVD) processes. Direct current bias voltages between -400 V and +500 V are here applied to a diamond deposition surface in an RF induction thermal plasma CVD system. This is made possible by introducing a high-impedance filter network, eliminating the time-varying voltage drop across the plasma-substrate junction and suppressing the radio-frequency interference. A traditional limitation of thermal RF systems is thereby overcome. Negative. bias conditions enhance the initial diamond nucleation density. Positive bias improves the diamond quality and augments the film growth rate. A threefold increase in linear growth rate is attained at +500 V as compared to the unbiased case. In conjunction with optical emission spectroscopic diagnostics, the substrate is used as an electrical and thermal probe. Contrary to do arcjet CVD, there is no secondary discharge created in the RF system at positive bias voltage. Also, the role of ion bombardment at negative bias is shown to be of little importance. It is inferred that the predominant mechanism leading here to changes in the diamond deposit under bias conditions is the promotion and suppression of electron emission from the growing diamond.

Effect of substrate holder boundaries on reproducibility of bias-enhanced diamond nucleation

In this paper, we have reported how the substrate holder affects the generation of diamond nucleation by biasing in microwave plasma chemical vapor deposition (MWCVD). A high density of diamond nucleation was obtained on the Si substrate when a fresh holder was used, which decreases drastically to almost no nucleation on the substrate once there is a diamond coating on the boundaries of the holders. This was further confirmed when it became possible to reproduce the high nucleation density on the substrate while covering the diamond-coated holder boundaries with a fresh Mo-foil. This phenomenon can be understood when considering the fact that the boundaries of a fresh holder gets coated simultaneously with diamond during the deposition on substrates in the first few runs. Once there is a diamond layer on the boundaries of the holder, the field strength differs at the boundaries of the holder and at the substrate. The effect is so pronounced that even 5 mm wide boundaries of the Mo holder, unintentionally coated with diamond, can reduce the energy and density of positive ions on the 3×3 cm 2 substrate lying on it. This, in turn, results in no nucleation on the substrate.

Diamond nucleation enhancement on reconstructed Si(100)

Diamonds have been efficiently nucleated on the flash-annealed and reconstructed silicon (100) substrate by hot filament chemical vapor deposition. A relatively high diamond nucleation density of about 1010 cm−2 in the nucleation time of only 20 min was confirmed by a Raman peak at the wave number of 1332 cm−1 and a scanning electron microscope image of the oriented nuclei with square-like shapes. The core-level spectra of C1s and Si2p using x-ray photoelectron spectroscopy indicated that the diamond had been nucleated on the C-modified substrate believed to be a SiC layer. If so, this study demonstrates that the diamond can be efficiently nucleated on a high quality SiC surface.

Nanodiamond formation by hot-filament chemical vapor deposition on carbon ions bombarded Si

Nanocrystalline diamond films were grown by a two-step process on Si(1 0 0) substrate, which was first pretreated by pure carbon ions bombardment. The bombarded Si substrate was then transformed into a hot-filament chemical vapor deposition (HFCVD) system for further growth. Using the usual CH 4/H 2 feed gas ratio for microcrystalline diamond growth, nanodiamond crystallites were obtained. The diamond nucleation density is comparable to that obtained by biasing the substrate. The uniformly distributed lattice damage is proposed to be responsible for the formation of the nanodiamond.

Micro-defects produced on a substrate by a glow discharge and the role of such defects on diamond nucleation

The process of diamond nucleation by hot filament chemical vapor deposition was investigated by atomic force microscopy. It was observed that a large number of micro-defects (pits) were produced on the surface of the silicon substrate due to ion bombardment under the negative potential and diamond nucleated on the pits. The formation of the pits and their role in diamond nucleation were theoretically approached. The results indicate that the number of the pits increased with increasing potential, the diamond nucleation density and the nucleation rate were functions of the pits, and they were in agreement with the experiment results.

Selective Deposition of Diamond Film on Glass Substrate via the Enhancement of the Diamond Nucleation Density by the Cyclic Process

Diamond films were deposited on the pretreated silicon or on the pretreated glass substrate in a microwave plasma enhanced chemical vapor deposition (MPECVD) system. We can increase the diamond nucleation density by the cyclic process, irrespective of the substrate kinds and deposition conditions. Using the cyclic process, we can certainly enhance the selective deposition of diamond film on glass substrate. The cyclic process is the in situ method carried out by the cyclic modulation of the source gas flow rate during the initial deposition stage. Surface morphologies and diamond qualities of the films have been investigated. Based on these results, we discuss the cause for the enhancement of the selectivity of diamond film deposition on glass substrate by the cyclic process. © 2001 The Electrochemical Society. All rights reserved.

Early stage of diamond formation on carbon/carbon composites

Diamond formation on two types of carbon composites was investigated using a plasma-enhanced chemical vapor deposition system. The effects of oxygen and the addition of copper into the carbon composites at the early stage of diamond formation were examined. Linear growth rates of diamond crystals under various deposition conditions were determined. It was found that the nucleation density of diamond was not affected by the addition of O 2 in the region of 0–0.3%. However, the growth rate decreased with increasing oxygen content. Also, the presence of copper enhanced the growth rate. This effect could be, however, hindered due to the addition of oxygen.

Ion bombardment as the initial stage of diamond film growth

It is believed that during the initial stage of diamond film growth by chemical-vapor deposition (CVD), ion bombardment is the main mechanism in the bias-enhanced-nucleation (BEN) process. To verify such a statement, experiments by using mass-separated ion-beam deposition were carried out, in which a pure carbon ion beam, with precisely defined low energy, was selected for investigating the ion-bombardment effect on a Si substrate. The results are similar to those of the BEN process, which supports the ion-bombardment-enhanced-nucleation mechanism. The formation of sp3 bonding is based on the presumption that the time of stress generation is much shorter than the duration of the relaxation process. The ion-bombarded Si is expected to enhance the CVD diamond nucleation density because the film contains amorphous carbon embedded with nanocrystalline diamond and defective graphite.

Bias Enhanced Nucleation of Diamond Films on Palladium Substrate.

A three-step process consisting of formation of a carbon supersaturated layer, density improvement of diamond nucleation and film growth was used to synthesize diamond films on polycrystalline Pd substrates. The results were compared with those of a two-step process in which diamond was deposited by particle growth, and the nucleation density was 107cm-2. A uniform diamond film could be obtained on the entire substrate surface by a three-step process the nucleation density was 109cm-2. The results indicate that formation of a carbon supersaturated layer using highly concentrated methane (40%) play an important role in improving the density of diamond nucleation.

Nucleation Control of Diamonds by Reducing Flame with High-Voltage Pulse and Synthesis of Smoothed Surface Diamond Film.

A control method of crystal growth for diamond films has been established with improving the surface quality by use of highly reducing name with high-voltage pulse discharge. The nucleation density, the crystal diameter and the surface roughness can be controlled actively by applying the high-voltage pulse on the substrate. Especially, it is also possible that pulse current over 40 mA changes the crystal surface direction. When ion flux flows to substrate, the nucleation density of diamond increases with the pulse voltage, and then smoothed surface films can be obtained. On the other hand, when electron flux flows to substrate the surface morphology hardly changes. This is due to the CH3+ ion flux, which is considered to be diamond precursor, increases with the electric field and become higher than CH3 thermal flux multiplied the sticking probability. This phenomena results in the increase of supersaturation of the radicals on the substrate and in the enhancement of nucleation. Simultaneously, CH3+ sticking coefficient itself increases, because of both high incidence speed of CH3+ flux to the substrate and high directionality.