Enhancement Of Diamond Nucleation Research Articles

Optical emission spectroscopy study of positive direct current bias enhanced diamond nucleation

High nucleation density and crystalline diamond films were deposited on a mirror-polished Si(100) substrate by horizontal microwave plasma chemical vapor deposition using a two step process consisting of positive direct current (dc) bias enhanced nucleation and growth. Optical emission spectroscopy was employed to investigate in situ the plasma emission characterization during positive biasing process. Emission lines from the Balmer series of atomic hydrogen, molecular hydrogen, CH, C 2, and Ar were observed in the visible and ultraviolet ranges when CH 4, H 2, and Ar were used as the reactant gases. The dependence of plasma emission spectra on the deposition parameters, such as biasing voltage, methane concentration and working pressure was investigated. The relative concentrations of neutral atomic hydrogen were estimated by using the Ar emission at 750.4 nm as an actinometer. A significant variation in the emission intensity of the radicals was measured with a change in the biasing voltage. The correlation between the spectra of some species and the quality of diamond films was studied. The results show that CH and C 2 both were important precursor in the diamond deposition, while C 2 was associated with the presence of amorphous phase in the films during positive dc biasing process.

Bias enhanced diamond nucleation on Mo and CrN coated stainless steel substrates in a HFCVD reactor

Bias enhanced nucleation was explored in order to obtain a high diamond nucleation density on Mo and CrN coated stainless steel substrates in a hot-filament chemical vapor deposition (HFCVD) reactor. Several bias geometries were tested on their nucleation enhancement for diamond on molybdenum. It was found that the nucleation properties observed for Mo were not much different for CrN coated stainless steel. The application of a bias between the filament and a cathode above the substrate at floating potential resulted in locally high diamond nucleation densities of up to 3∙10 9 cm − 2 .

Enhanced diamond nucleation on copper substrates by graphite seeding and CO 2 laser irradiation

Diamond nucleation on copper (Cu) substrates was investigated by graphite seeding and CO 2 laser irradiation at initial stages of the combustion-flame deposition. A graphite aerosol spray was used to generate a thin layer of graphite powders (less than 1 μm) on Cu substrates. The graphite-seeded Cu substrates were then heated by a continuous CO 2 laser to about 750 °C within 1 min. It was found that diamond nucleation density after this treatment was more than three times as much as that on the virgin Cu substrates. As a consequence, diamond films up to 4 μm were obtained in 5 min. The enhancement of diamond nucleation on the graphite-seeded Cu substrates was attributed to the formation of defects and edges during the etching of the seeding graphite layers by the OH radicals in the flame. The defects and edges served as nucleation sites for diamond formation. The function of the CO 2 laser was to rapidly heat the deposition areas to create a favorable temperature for diamond nucleation and growth.

Effect of 3C-SiC(100) initial surface stoichiometry on bias enhanced diamond nucleation

Two 3C-SiC(100) reconstructed surfaces have been exposed to bias enhanced nucleation (BEN) treatments performed in a microwave plasma chemical vapor deposition reactor. For both BEN steps, a significant enhancement of the diamond nucleation density has been observed on the initial C-terminated surface compared to the Si-rich surface. Samples have been characterized by in situ surface analysis, namely, x-ray photoelectron spectroscopy and x-ray Auger electron spectroscopy, and by field emission gun scanning electron microscopy.

Diamond chemical vapour deposition on seeded cemented tungsten carbide substrates

Diamond particles were deposited onto seeded cemented tungsten carbide (WC–Co) substrates using conventional hot-filament chemical vapour deposition (HFCVD) and time-modulated CVD (TMCVD) processes. The substrates were pre-seeded ultrasonically with diamond particles of different grit sizes. In this investigation, we employ timed methane (CH 4) gas modulations, which are an integral part of our TMCVD process in order to enhance diamond nucleation density. During diamond deposition using the conventional HFCVD process, methane gas flow was maintained constant. The total hydrogen flow into the reactor during TMCVD process was higher than in the HFCVD process. Hydrogen etching can be expectedly more prominent in the TMCVD process than in HFCVD of diamond particles. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) results showed that a proper selection of the diamond grit size for seeding using ultrasounds can lead to enhancement in the nucleation density values of about two orders of magnitude (10 7 to 10 9 cm − 2 ). The TMCVD process using the different seeded substrates can result in high nucleation density values of up to 10 10 cm − 2 .

Silicon carbide films from polycarbosilane and their usage as buffer layers for diamond deposition

Thin films of polycarbosilane (PCS) were coated on a Si (100) wafer and converted to silicon carbide (SiC) by pyrolyzing them between 800 and 1150 °C. Granular SiC films were derived between 900 and 1100 °C whereas smooth SiC films were developed at 800 and 1150 °C. Enhancement of diamond nucleation was exhibited on the Si (100) wafer with the smooth SiC layer generated at 1150 °C, and a nucleation density of 2 × 10 11 cm − 2 was obtained. Nucleation density reduced to 3 × 10 10 cm − 2 when a bias voltage of − 100 V was applied on the SiC-coated Si substrate. A uniform diamond film with random orientations was deposited to the PCS-derived SiC layer. Selective growth of diamond film on top of the SiC buffer layer was demonstrated.

The effects of bias polarity on diamond deposition by hot-filament chemical vapor deposition

Electric voltages with both polarities were applied to silicon substrates in a hot-filament chemical vapour deposition device to study the biasing effects on deposition of diamond and carbon nanocones. It has been found that positive biasing greatly enhanced diamond nucleation density and improved diamond film quality. On the other hand, negative biasing promotes deposition of dense, well-aligned carbon nanocones. The orientation of the carbon nanocones appears to align with the direction of the electric field lines near the substrate surface.PACS Nos.: 81.15.Gh, 81.05.Uw, 81.07.–b

Study on enhancement of diamond nucleation on fused silica substrate by ultrasonic pretreatment

Fused silica substrates were pretreated by the ultrasonic vibration in the diamond powder slurry (UVDS). The influence of UVDS parameters such as the grain size of diamond powder, the liquid medium used to form the slurry, the weight ratio of diamond powder to liquid medium and the pretreatment time on the diamond nucleation density (DND) were systemically investigated. The grain size of diamond powder greatly affected the DND, the larger the grain size the higher the DND in our experiment conditions. The DND was about the same using acetone or ethanol or hexane medium. The best weight ratio of diamond powder (grain size 20–40 μm) to liquid medium was ∼1/60. Under appropriate pretreatment and CVD conditions, the DND of ∼10 10 cm −2 was obtained on fused silica substrates. Continuous ultra-thin diamond films with uniform and smooth surface (diamond grain size: ∼150 nm and surface roughness: ∼6 nm) were synthesized in an improved hot filament chemical vapor deposition (HFCVD) system. Nano-damaged sites on the pretreated surface mainly enhanced the DND and shortened the incubation time of nucleation.

Study on enhancement of diamond nucleation on fused silica substrate by ultrasonic pretreatment

Fused silica substrates were pretreated by the ultrasonic vibration in the diamond powder slurry (UVDS). The influence of UVDS parameters such as the grain size of diamond powder, the liquid medium used to form the slurry, the weight ratio of diamond powder to liquid medium and the pretreatment time on the diamond nucleation density (DND) were systemically investigated. The grain size of diamond powder greatly affected the DND, the larger the grain size the higher the DND in our experiment conditions. The DND was about the same using acetone or ethanol or hexane medium. The best weight ratio of diamond powder (grain size 20–40 μm) to liquid medium was ∼1/60. Under appropriate pretreatment and CVD conditions, the DND of ∼10 10 cm −2 was obtained on fused silica substrates. Continuous ultra-thin diamond films with uniform and smooth surface (diamond grain size: ∼150 nm and surface roughness: ∼6 nm) were synthesized in an improved hot filament chemical vapor deposition (HFCVD) system. Nano-damaged sites on the pretreated surface mainly enhanced the DND and shortened the incubation time of nucleation.

Diamond Nucleation Enhancement on Si by Controlling Ion-Bombardment Energy in Electron Cyclotron Resonance Plasma

A nucleation enhancement technique with defined ion-bombardment energies has been developed for diamond deposition on mirror-polished Si wafers. The substrate was negatively biased at several tens of volts in an electron cyclotron resonance methane-hydrogen plasma at 0.13 Pa. The nucleation density was significantly increased to ∼108 nuclei/cm2 for the bias voltage range of -20 to -50 V. The highest density was obtained with a mean ion energy of around 50 eV. The mechanism of nucleation enhancement was correlated with the formation of sp3 bonds as nucleation sites by energetic ion bombardment.

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.

Diamond growth reactor chemistry and film nucleation enhancement using chlorinated hydrocarbons

The chemistry of diamond film growth from chlorinated hydrocarbons has been investigated using a hot filament reactor coupled to an orifice sampling mass spectrometer. The relative concentrations of the species present near the growth surface have been determined as a function of filament temperature for dilute mixtures of CH 4, CH 3Cl, CH 2Cl 2 and CHCl 3 in H 2. Mass spectral analysis indicated that chlorinated hydrocarbons are sequentially dechlorinated in the presence of hydrogen at moderate reactor temperatures. A dark film was deposited on all surfaces of the reactor during studies of this dechlorination of CHCl 3. Raman analysis indicated that these deposits are small particle polycrystalline graphite. Pretreatment of a Si 〈111〉 substrate under conditions that create the graphite deposit is seen to produce a significant nucleation enhancement of diamond when followed by growth at a higher temperature. Chemical mechanisms for some of these processes are proposed.

Enhanced nucleation of diamond films assisted by positive dc bias

Diamond nucleation on untreated silicon was enhanced by positive dc biasing in a horizontal microwave plasma chemical vapor deposition system. The effects of process parameters of bias voltage, methane content, bias time, working pressure and microwave power on the diamond nucleation were investigated. A nucleation density higher than 10 10 cm −2 was achieved on an untreated, mirror-polished silicon substrate by positive dc biasing. With a bias voltage between the range of +200 and +300 V, it was found that high methane content is effective in the enhancement of diamond nucleation. It was also found that amorphous carbon was formed under the positive dc bias conditions. Uniform diamond films were obtained by combining the positive dc bias enhanced nucleation process with subsequent growth under the usual microwave plasma chemical vapor deposition diamond conditions. Diamond films deposited were characterized by scanning electron microscopy and Raman spectroscopy. The effect of positive dc biasing on nucleation at a high methane content to generate oriented diamond nuclei on (100) Si substrate was explored.

The influence of surface roughness and chemical modification of silicon surfaces on dc-glow discharge enhanced diamond nucleation

In this study the effect of surface roughness and damage, as well as chemical modification with Ti additives, in different dispersion states, on the dc glow discharge enhanced diamond nucleation on silicon were investigated. It was established that the relative sp 3 content in the carbon film produced by the dc-glow discharge process is enhanced as a result of surface damage. Consequently, diamond DPD (deposited particle density) obtained in the subsequent deposition is substantially enhanced. Also it was found that Ti surface coverage influences the efficiency of nucleation. Continuous Ti films have a deleterious effect on nucleation, while particles catalyze precursor film generation. This phenomenon was explained as a balance between Ti aided accumulation of carbon on the one hand, and Ti induced dissolution of diamond nucleation centers in the hydrogen rich atmosphere. As a result of surface abrasion the optimal temperature for nano-diamond formation during the dc glow discharge stage is shifted to lower temperatures relative to that observed for virgin silicon.

Nucleation-enhancing treatment for diamond growth over a large-area using magnetoactive microwave plasma chemical vapor deposition

A novel pretreatment enhancing diamond nucleation has been developed for diamond growth over a large area using a magnetoactive microwave plasma chemical vapor deposition method. After the predeposition of carbon films on Si(100) substrates using CH4/CO2/He gas mixtures, diamond films with high nucleation densities were obtained after a subsequent 2 h growth process commonly employed using a CH4/CO2/H2 gas mixture. In the present study, especially, the effect of CO2 concentration in the CH4/CO2/He gas mixture in the pretreatment process has been examined on the carbon film growth. The results show that the diamond nucleation with densities as high as ∼109/cm2 was attained for small CO2 concentrations of 1%–2% during the pretreatment process, while no successful enhancement was enabled for Si substrates pretreated at high CO2 concentrations beyond 3.7%. The structural property of the predeposited carbon films significantly influenced the diamond nucleation. This was evidenced by in situ data of optical emission spectroscopy and quadrupole mass spectroscopy during the pretreatment process, as well as by ex situ data of morphology and composition of the specimens. The volume density of the carbon films obtained after the pretreatment was maximized at a CO2 concentration of 1.9%. The bonding nature of the carbon atoms deduced from the related Raman scattering spectra apparently changed with CO2 concentration. The role of the predeposited carbon films is discussed in relation to etching and agglomeration phenomena during the subsequent diamond growth process.

Enhancement of diamond nucleation using the solid-liquid-gas interface energy

We demonstrate the enhancement of diamond nucleation through the use of the equilibrium forces at the solid-liquid-gas interface on a substrate wetted with droplets of an oil of low vapor pressure. Such a process is shown to produce well-faceted grains with densities of roughly 107 nuclei cm−2 (boundary), 105 nuclei cm−2 (oil-coated area), and 104 nuclei cm−2 (uncoated area) without the need for scratching or seeding the substrate. Diamond deposition was undertaken on silicon using ethanol and hydrogen in the feed of a hot-filament chemical vapor deposition reactor. The oil-covered regions, in addition to showing higher nucleation densities, have the merit that the intergrain spaces are covered with diamond structures, while the parts uncovered with oil exhibit intergrain spaces covered with diamond-like carbon.

Transmission electron microscopy study of diamond nucleation and growth on smooth silicon surfaces coated with a thin amorphous carbon film

Abstract Amorphous carbon (a-C) films with high contents of tetrahedral carbon bonding (sp3) were synthesized on smooth Si(100) surfaces by cathodic arc deposition. Before diamond growth, the a-C films were pretreated with a low-temperature methane-rich hydrogen plasma in a microwave plasma-enhanced chemical vapor deposition system. The evolution of the morphology and microstructure of the a-C films during the pretreatment and subsequent diamond nucleation and initial growth stages was investigated by high-resolution transmission electron microscopy (TEM). Carbon-rich clusters with a density of ∼1010 cm−2 were found on pretreated a-C film surfaces. The clusters comprised an a-C phase rich in sp3 carbon bonds with a high density of randomly oriented nanocrystallites and exhibited a high etching resistance to hydrogen plasma. Selected area diffraction patterns and associated dark-field TEM images of the residual clusters revealed diamond fingerprints in the nanocrystallites, which played the role of diamond nucleation sites. The presence of non-diamond fingerprints indicated the formation of Si–C-rich species at C/Si interfaces. The predominantly spherulitic growth of the clusters without apparent changes in density yielded numerous high surface free energy diamond nucleation sites. The rapid evolution of crystallographic facets in the clusters observed under diamond growth conditions suggested that the enhancement of diamond nucleation and growth resulted from the existing nanocrystallites and the crystallization of the a-C phase caused by the stabilization of sp3 carbon bonds by atomic hydrogen. The significant increase of the diamond nucleation density and growth is interpreted in terms of a simple three-step process which is in accord with the experimental observations.

Diamond coatings deposited on tool materials with a 915 MHz scaled up surface-wave-sustained plasma

An essential difficulty in coating tool materials with polycrystalline diamond is the film poor adhesion. To solve this problem we have developed a generic pretreatment method for the substrate surface, prior to diamond deposition, which essentially consists of submitting it to ion bombardment by carbon species through plasma immersion in a methane d.c. glow discharge. This procedure leads to carbon supersaturation of the substrate surface. Under diamond deposition conditions, it ensures carbon saturation of the substrate surface with respect to the gas phase and provides a diffusion barrier preventing migration, from the substrate bulk to the surface, of species which exert a catalytic effect on sp 2 carbon phases, thereby enhancing diamond nucleation and ensuring good adhesion of the diamond film. The diamond films were deposited with a 915 MHz surface-wave-sustained plasma scaled up from a 2.45 GHz system, enabling us to eventually coat large areas (100 mm in diameter) of cobalt sintered WC. With such a material, as shown by secondary ion mass spectroscopy, the diffusion barrier obtained through carbon supersaturation is effective in preventing cobalt atoms from migrating to the surface where they would favor the formation of non-diamond phases. Raman spectroscopy and scanning electron microscopy measurements confirmed the good quality of the obtained diamond films. A scratch test analysis was used to demonstrate the improved adhesion strength resulting from this novel pretreatment method.

X-ray photoelectron spectroscopy investigation of substrate surface pretreatments for diamond nucleation by microwave plasma chemical vapor deposition

The effects of surface pretreatments on the nucleation of diamond on silicon substrates have been studied by quantitative X-ray photoelectron spectroscopy (XPS), SEM and Raman spectroscopy. The methods of surface pretreatments including ultrasonic abrasion, scratching, and DC biasing were used for diamond nucleation enhancement. Diamond films grown by different surface pretreatments and selected intervals in bias-enhanced nucleation were analyzed for the surface composition in the C 1s and Si 2p regions. Before diamond growth XPS analysis showed that the Si substrate surface is covered by a layer of SiO 2 and carbonaceous residue. It was found that methods of surface pretreatment all introduce a substantial amount of carbon species on the substrate surface that was the primary reason for the enhancement of diamond nucleation. In the biasing process, it reduced and suppressed the formation of oxide that further contributed to the enhanced nucleation density of diamond, and it was observed that the presence of SiC in the initial stage of nucleation is due to the carbon interaction with the Si substrate.

Diamond nucleation enhancement by direct low-energy ion-beam deposition

Direct ion beam deposition was successfully applied for the nucleation of nanodiamond crystallites on mirror-polished Si(001) substrates. Low-energy (80--200 eV) argon, hydrocarbon, and hydrogen ions from a Kaufman ion source were used. An amorphous carbon film was deposited on the substrate after ion bombardment. The films were characterized by high-resolution transmission electron microscopy, selected area electron diffraction, secondary electron microscopy, and micro-Raman spectroscopy. At ion doses above $1\ifmmode\times\else\texttimes\fi{}{10}^{18}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2},$ nanocrystalline diamond particles of 50--100 \AA{} in diameter were formed in a matrix of amorphous carbon. These diamond nanocrystals served as nucleation centers for subsequent diamond growth by conventional hot filament chemical vapor deposition. The nucleation density depended strongly on the ion dosage, and a nucleation density of $3\ifmmode\times\else\texttimes\fi{}{10}^{9}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ could be achieved under optimized conditions. These results were found very helpful for the evaluation of the mechanism of ion-bombardment-induced nucleation of diamond.