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.     

Nucleation of diamond films by ECR-enhanced microwave plasma chemical vapor deposition

A novel nucleation technique based on electron cyclotron resonance microwave plasma was developed to enhance the nucleation of diamond. By choosing a suitable experimental condition, a nucleation density higher than 10⁸ nuclei cm⁻² was achieved on an untreated, mirror-polished silicon substrate. Uniform diamond films were obtained by combining this nucleation method with subsequent growth by the common microwave plasma chemical vapor deposition. Furthermore, the possibility of this new nucleation method to generate heteroepitaxial diamond nuclei on (001) silicon substrates was explored.     

Chemical and structural transformations of silicon submitted to H2 or H2/CH4 microwave plasmas

Silicon substrates are often used to synthesize polycrystalline diamond films by microwave plasma assisted chemical vapour deposition technique (MPCVD). In the case of highly oriented diamond films, several steps are employed to carefully prepare the silicon surface (pre-treatment steps), to nucleate diamond crystals (nucleation step) and to thick the film (growth step). In this study, we characterize {100} silicon substrates and diamond released from its silicon substrate by electronic microscopies (TEM and SEM), by Atomic Force Microscopy (AFM) and by X-ray photoelectron spectroscopy (XPS), to follow the substrate transformations after each step, particularly the formation and the evolution of the silicon carbide and to characterise the diamond films grown on the carburised silicon. We show that according to the experimental conditions and the level of surface/gas contamination by carbon and silicon species, isolated islands or continuous β-SiC compound are formed over the silicon surface and can generate defects such as voids or strip structures that influence the subsequent diamond nucleation and growth.     

Nucleation and Initial Growth Stages of Chemical Vapor Deposition (CVD) Diamond

Nucleation of diamond on non-diamond virgin substrates is characterized by low nucleation densities and long incubation times. Various methods have been developed to enhance nucleation densities and reduce the duration of incubation. This report describes a number of different but related studies of diamond nucleation on silicon and chemically modified silicon surfaces. The effect on the initial stages of deposition of mechanical abrasion with slurries and in-situ sample biasing are especially discussed. Substrate abrasion with diamond results in the embeddying of diamond debris into its surface. Destructive ion implantation into this diamond debris is found to prevent subsequent diamond growth, therefore leading to the conclusion that the diamond debris serves as growth centers. Abrasion of the substrate with mixed metal/diamond slurries is reported to further enhance nucleation relative solely to diamond abrasion. It is suggested that during the chemical vapor deposition (CVD) process some metals alter the composition of the gas phase above the growing surface. Also, the role of surface reactions is emphasized. We also introduce the dc-glow discharge process as a novel, in situ surface pretreatment method for the formation of a precursor for diamond nucleation. Our results show that the promotion of diamond growth by this method is primarily due to formation of nano-size diamond particles during the pretreatment process. It is suggested that, to some extent, graphitic carbon with a high degree of defects may serve as a diamond nucleation center as well.     

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.     

Substrate pre-treatment by ultrasonication with diamond powder mixtures for nucleation enhancement in diamond film growth

Pre-treatment of silicon substrates by ultrasonic abrasion for nucleation enhancement in diamond film formation by hot-filament chemical vapour deposition is discussed. Scanning electron microscopy, atomic force microscopy and visible Raman spectroscopy were employed as analysis techniques. Ultrasonication was applied by suspensions of isopropanol with micro-or nanosized diamond powders, micro-sized metal and alumina particles and mixtures thereof. The root mean square roughness of the ultrasonically pre-treated samples varied from 0.2 to 12.0 nm depending on the applied powder mixture. All samples that were ultrasonically pre-treated had a larger diamond nucleation density than the untreated silicon wafer. As expected, for an effective increment of the diamond nucleation density by several orders of magnitude the application of diamond powder is necessary, since the generation of surface roughness alone is not sufficient to enhance the diamond nucleation kinetics satisfactorily. The simultaneous action of diamond powders and large alumina or titanium particles leads to an increase in diamond nucleation density up to a factor of 10⁶. When nano-diamond powder is used, the embedment of diamond fragments is best and in combination with titanium grains (50–75 µm) a diamond nucleation density of 8 × 10⁹ cm^− 2 is obtained. After 8 h of film growth, the diamond surface grains are significantly smaller for the samples that demonstrated higher nucleation densities, whereas the quality of the diamond layers is equal.     

Influence of CF4 addition for HFCVD diamond growth on silicon nitride substrates

This work presents a study of CVD diamond growth on silicon nitride-based ceramics with the addition of carbon tetrafluoride (CF₄) in a hot filament-assisted reactor (HFCVD). Silicon nitride substrates were hot pressed under a nitrogen atmosphere for 90 min at 1750°C, giving specimens of very high density and good mechanical properties. The CF₄ addition is known to bring several advantages to diamond growth and, in particular, in this work, an important interaction of the CF₄-containing gas phase with the silicon nitride (Si₃N₄) substrates has been proven to be very beneficial for nucleation, growth and adherence of the diamond films. A basic gas mixture of H₂/1.5 vol.% CH₄/0.5 vol.% CF₄ was used in the growth experiments. The nucleation study reveals a strong interaction of the halogen-containing gas phase with the vitreous phase on the substrate surface. A strong erosion of the surface has been observed, which induced a high nucleation density (N_d) of the order of 10⁸ particles cm⁻², without any surface pre-treatment. Silicon nitride surface analysis was performed with Raman and infrared specular reflectance spectroscopy. Results suggest the erosion of the vitreous phase, mainly the silica (SiO₂) component, and the formation of silicon carbide, prior to diamond growth. Raman spectra and scanning electron microscopy (SEM) show better quality film grown with CF₄ addition. Indentation tests with a Rockwell C tip, at variable charge, show a better film adherence if grown with CF₄ addition.     

Diamond nucleation suppression in chemical vapor deposition process

A systematic study of the effect of different pre-treatments of the Si substrate surface in suppressing diamond nucleation was performed to investigate the nature of the nucleation centers in chemical vapor deposition (CVD) of diamond. The Si substrates were initially scratched with diamond powder and then submitted to one of the following pre-treatments: thermal annealing in high vacuum and in air, deposition of an amorphous silicon film, and ⁸⁴Kr⁺ ion implantation. The pre-treated substrates were used in a hot filament CVD diamond process, and the diamond films obtained were analyzed by different techniques. The results suggest that the observed nucleation reduction under certain pre-treatment conditions is related to modifications induced on the original topographical features of the scratched substrate surface, which would be responsible for the CVD diamond nucleation. The dimensions of these surface features are estimated to be of the order of 5 nm.     

A kinetic model of diamond nucleation and silicon carbide interlayer formation during chemical vapor deposition

The presence of thin silicon carbide intermediate layers on silicon substrates during nucleation and the early stages of diamond deposition have been frequently reported. It is generally accepted that the intermediate layer is formed by the bulk diffusion of carbon atoms into the silicon carbide layer and the morphology and orientation of the diamond film subsequently grown on the intermediate layer are strongly affected by that layer. While there have been considerable attempts to explain the mechanism for intermediate layer formation, limited quantitative data are available for the layer formation under the operating conditions conducive to diamond nucleation.This study employs a kinetic model to predict the time evolution of a β-SiC intermediate layer under the operating conditions typical of diamond nucleation in hot filament chemical vapor deposition reactors. The evolution of the layer is calculated by accounting for gas-phase and surface reactions, surface and bulk diffusions, the mechanism for intermediate layer formation, and heterogeneous diamond nucleation kinetics and of its dependence on the operating conditions such as substrate temperature and inlet gas composition. A comparison between the time scales for intermediate layer growth and diamond nuclei growth is also performed. Discrepancies in published adsorption energies of gaseous hydrocarbon precursors on the intermediate layer—ranging from 1.43 to 4.61 eV—are examined to determine the most reasonable value of the adsorption energy consistent with observed saturated thicknesses, 1–10 nm, of the intermediate layer reported in the literature. The operating conditions that lead to intermediate layer growth followed by diamond deposition vs. those that yield heteroepitaxial diamond nucleation without intermediate layer formation are discerned quantitatively. The calculations show that higher adsorption energies, 3.45 and 4.61 eV, lead to larger surface number densities of carbon atoms, lower saturated nucleation densities, and larger intermediate layer thicknesses. The observed saturated thicknesses of the intermediate layer may be reproduced if the true adsorption energy is in the range of 3.7–4.5 eV. The intermediate layer thickness increases by increasing substrate temperature and inlet hydrocarbon concentration and the dependence of the thickness on substrate temperature is especially significant. Heteroepitaxial diamond nucleation without intermediate layer formation reported in experimental results can be readily explained by the significant decrease of the intermediate layer thickness at lower substrate temperatures and at higher diamond nucleation densities. Further, the present model results indicate that the intermediate layer thickness becomes saturated when growing diamond nuclei cover a very small surface area of that layer.     

Influence of H2S addition during diamond deposition on hardmetal substrates

Diamond deposition on hardmetal substrates is an industrial process to increase the wear resistance of tools during machining operations, but till now an increase in diamond layer adhesion is desirable. The main problem during diamond deposition on hardmetal substrates is the Co content in the binder phase.H₂S was used for immobilizing the cobalt on the substrate surface. The H₂S should react with the metallic Co covering its surface with CoS. Because of this the diamond nucleation occurs easier and the Co vapour pressure is also reduced. Similar mechanisms were observed using silicon and boron vapour during substrate pre-treatment.Positive effects of H₂S addition were achieved if the H₂S is added only during the diamond nucleation period. The experiments with continuous H₂S addition were not successful.For comparison diamond deposition on Murakami/Carrot pre-treated substrates were carried out.     

The orientation effect of silicon grains on diamond deposition

Diamond deposition on mirror-polished polycrystalline silicon substrates which have grains in various orientations has been investigated using electron backscatter diffraction (EBSD) method with scanning electron microscopy (SEM). Diamond was deposited by microwave plasma chemical vapor deposition with application of a negative bias voltage on the substrate. The evidence from systematic SEM observations shows that silicon orientation determined by EBSD has a strong effect on diamond nucleation. In general, the diamond nucleation density on Si grains oriented close to <100> is the highest, while it is the lowest for those grains close to <111>, under the same experimental conditions for deposition. The same phenomena have been observed in the range of methane concentration from 2% to 4% in hydrogen.     

Heteroepitaxial diamond nucleation and growth on silicon by microwave plasma-enhanced chemical vapor deposition synthesis

Heteroepitaxial diamond films were successfully nucleated and deposited on 1-inch diameter Si(001) substrates by microwave plasma-enhanced chemical vapor deposition (MPECVD). The precursor gases for the synthesis were methane and hydrogen. Before the application of a negative d.c. bias to the substrate, an in-situ carburization pre-treatment on the silicon was found to be an indispensable step towards the heteroepitaxial diamond on the silicon. Morphologies of the films were characterized by scanning electron microscopy (SEM). Interface observations based on the cross-sectional HRTEM directly reveal the heteroepitaxial diamond nucleation phenomena in detail. No interlayers of silicon carbide and/or amorphous carbon phases were observed. Tilt and azimuthal misorientation angles between the heteroepitaxial diamond crystals and the substrate were determined by combining the Ewald sphere construction in the reciprocal lattice space and the selected area diffraction (SAD) patterns taken across the interface.     

Polymer‐based nucleation for chemical vapour deposition of diamond

Chemical vapour deposition of diamond on foreign substrates is hindered due to its high surface energy. Therefore, nucleation treatment has to be employed to initialize the formation of diamond crystals. This article deals with diamond growth on silicon substrates coated with three types of polymers: (i) polystyrene (PS), (ii) polylactic‐co‐glycolic acid (PLGA), and (iii) polyvinyl alcohol (PVA) were applied in different forms, i.e., microspheres (PS, PLGA), monolayers (PLGA), multilayers (PLGA, PLGA/PS), and composites with embedded diamond nanoparticles (PLGA, PVA). Thin polymers and microsphere monolayers did not contribute to the diamond nucleation and/or growth. A thicker continuous polymer film (>750 nm) or thin polymer/microsphere layer led to a homogeneous and dense formation of diamond grains. In the case of nucleation using polymer composites, where the thin polymer film serves as a 3D carrier matrix for embedded diamond nanoparticles, a comparable nucleation density to the well‐established ultrasonic seeding method was achieved. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43688.     

Influences of WC-Co hard metal substrate pre-treatments with boron and silicon on low pressure diamond deposition

Diamond coatings were produced on WC-Co hard metal substrates. To improve the adhesion between the diamond coating and the substrate a substrate surface pre-treatment with boron or with silicon vapor was applied. This surface pre-treatment resulted in an increase in both the diamond nucleation density and the diamond growth rate. Simple adhesion tests confirmed an improved adhesion of thin diamond layers as compared with those on untreated hard metal substrates.Secondary ion mass spectroscopy (SIMS) depth profiles revealed an enrichment of B or of Si at the substrate-diamond interface due to the pre-treatment procedure. The correlation of the Co and W depth profiles in samples coated for 12 and 24 h supports the theory of diamond dissolution into the substrate. Co was detected only in the interface regions and on the surface of the diamond layers but not in the bulk of the thick layers. The SIMS results confirm X-ray examinations of the hard metal Co binder phase.     

Microwave plasma enhanced chemical vapor deposition of diamond in silicon pores

In this work, the feasibility of growing boron-doped diamond coatings, approximately 0.3 μm thick, on thin silicon substrates that have 50-μm diameter pores etched 125 μm deep has been demonstrated using deep reactive ion etching (DRIE) in combination with chemical–mechanical polishing (CMP). Using a microwave plasma enhanced chemical vapor deposition (MPECVD) cyclic growth process consisting of carburization, bias-enhanced nucleation, diamond growth and boron-doped diamond growth, uniform diamond coatings throughout the pores have been obtained. The coatings were characterized by Raman spectroscopy and scanning electron microscopy and the secondary electron emission coefficients were found to increase from 4 to 10 between 200 and 1000 V, in agreement with reported values for thicker polycrystalline diamond films grown under similar conditions.     

Deposition of diamond films on sapphire: studies of interfacial properties and patterning techniques

Polycrystalline diamond films were grown on single crystal sapphire substrates using hot filament chemical vapour deposition (CVD). Problems with poor adhesion, stress and film cracking became severe for deposited areas greater than about (100 μm)². Scanning electron microscopy analysis showed the films to be failing both at the interface and in the diamond layer itself. Transmission electron microscopy cross-sections of the interface showed that the interface was clean and free from non-diamond carbon impurities. Spallation problems in the diamond film could be reduced by introducing a barrier layer of epitaxial silicon grown on the sapphire prior to the diamond CVD step. Patterned silicon-on-sapphire wafers were then used as substrates for CVD of diamond in order to define features of linewidth more than 10 μm in the diamond films. Two methods were used: selective nucleation and lift off.     

Two routes for HFCVD diamond nucleation on Si(1 1 1) studied by HRTEM

The early stages of diamond nucleation on Si(1 1 1) thinned areas have been studied by several TEM techniques. Both top-view and cross-section investigations have been carried out to better characterise the nature of the diamond precursor as well as the diamond/silicon interface. Two different routes for diamond nucleation have been underlined using the same CVD experimental conditions. One involves the formation of a β-SiC interlayer while the second is a direct nucleation on silicon. Moreover, the size of the smallest detectable diamond particles is a few nanometers according to nanodiffraction experiments.     

The effect of ion bombardment on the nucleation of CVD diamond

The nucleation effect of CVD diamond by ion bombardment was studied by a two-step process. In the first step, hydrocarbon and hydrogen ion bombardment was used to induce nucleation on mirror-polished (001) Si substrates. In the second step, diamond films were subsequently deposited on the ion-bombarded substrates by a conventional hot filament chemical vapor deposition. It was found that after the ion bombardment, an amorphous layer embedded with nano-crystalline diamond particles formed on the Si substrate. These nano-crystalline diamond particles were proposed to serve as the nucleation centers for the growth in the second step. The nucleation density depended strongly on the ion dosage and a nucleation density of up to 2×10⁹ cm⁻² could be achieved under optimized conditions.     

Diamond nucleation and growth under very low-pressure conditions

The synthesis of thin diamond films using various chemical vapor deposition methods has received significant attention in recent years due to the unique characteristic of diamond, which make it an attractive candidate for a wide range of applications. In order to grow diamond epitaxially, the proper control of diamond nucleation on mirror-polished Si is essential. Adding the negative bias voltage to the substrate is the most popular method. This paper has proposed a new method to greatly enhance the nuclear density. Under very low pressure (1 torr), the high-density nucleation of diamond is achieved on mirror-polished silicon in a hot-filament chemical vapor deposition (HFCVD). Scanning electron microscopy has demonstrated that the nuclear density can be as high as 10¹⁰–10¹¹ cm⁻². Raman spectra of the sample have shown a dominant diamond characteristic peak at 1332 cm⁻¹. The pressure effect has been discussed in detail and it has been shown that the very low pressure is a very effective means to nucleate and grow diamond films on mirror-polished silicon. Extraordinary pure hydrogen (purity=99.9999%) was used as the source. Compared with the highly pure hydrogen (purity=99.99%), we found that the density of nucleation was greatly increased. The residual oxygen in the hydrogen displayed a very obvious negative effect on the nucleation of diamond, although it can accelerate the growth of diamond. Based on these results, it was suggested that the enhanced nucleation at very low pressure should be attributed to an increased mean free path, which induced a high density of atomic hydrogen and hydrocarbon radicals near the silicon surface. Atomic hydrogen can effectively etch the oxide layer on the surface of silicon and so greatly enhance the nucleation density.     

Diamond deposition on chromium, cobalt and nickel substrates by microwave plasma chemical vapour deposition

Only after a relatively long incubation time (which is necessary to saturate the substrate and its surface with carbon by diffusion or formation of an intermediate layer) did diamond nucleation and deposition occur on Cr, Co and Ni, Also, prior to the onset of the diamond formation, non-diamond carbon layers can be formed with too high a concentration of CH₄. However, most of the experimental facts observed during the diamond depositions on Cr, Co and Ni surfaces can be explained by interactions occurring between the reaction gases and the substrates.

Chromium substrates form an intermediate carbide layer prior to diamond deposition. Diamond nucleation did not occur readily. Cobalt has only a low solubility for C. At low CH₄ concentrations, diamond was deposited on pure Co. No deposition of amorphous carbon was observed. Nickel has a certain C solubility. Diamond nucleation occurred only after the substrate and its surface had been carbon saturated. The length of the interval until saturation was reached depended on the substrate thickness.

During the time needed to cover the substrate fully with a diamond layer, the metal vapour from substrate interacted with the diamond growth. Large growth steps developed on the diamond crystal facets. Also refractory metal substrates placed near to the Cr, Co or Ni substrates were contaminated and their diamond coatings exhibited the same growth step features.