Hot-filament Chemical Vapor Deposition System Research Articles

The study of diamond/TiC composite film by a DC-plasma–hot filament CVD

Diamond/TiC composite films were prepared on Si substrate by the direct current plasma hot filament chemical vapor deposition (HFCVD) using a gas mixture of methane, hydrogen and Ti[OC 3H 7] 4 carried into the reactor by hydrogen. The films were prepared in an improved HFCVD system that a bias voltage can be added between the substrate and the cathode to form the plasma. The composite films were characterized by filed emission scanning electron microscopy, X-ray diffractometer (XRD) and energy-dispersive X-ray analysis (EDX). It was observed that diamond nucleation together with composite film's deposition rate increased with application of proper bias voltage, as evident from the SEM micrographs. It was also found from EDX analysis that the content of titanium increased with the increase of the flux of the carrier gas. The diffraction peaks of the diamond and titanium carbide can be found from the XRD analysis of the composite films deposited on Si substrate.

The influence of substrate temperature variation on tungsten oxide thin film growth in an HFCVD system

Tungsten oxide (WO 3) thin films were deposited by a modified hot filament chemical vapor deposition (HFCVD) technique using Si (1 0 0) substrates. The substrate temperature was varied from room temperature to 430 °C at an interval of 100 °C. The influence of the substrate temperature on the structural and optical properties of the WO 3 films was studied. X-ray diffraction and Raman spectra show that as substrate temperature increases the film tends to crystallize from the amorphous state and the surface roughness decreases sharply after 230 °C as confirmed from AFM image analysis. Also from the X-ray analysis it is evident that the substrate orientation plays a key role in growth. There is a sharp peak for samples on Si substrate due to texturing. The film thickness also decreases as substrate temperature increases. UV–vis spectra show that as substrate temperature increases the film property changes from metallic to insulating behavior due to changing stoichiometry, which was confirmed by XPS analysis.

Field emission properties of carbon coated Si nanocone arrays on porous silicon

Silicon nanocone arrays are formed on porous silicon substrates by plasma etching in a hotfilament chemical vapour deposition system. The as-formed Si nanocones werecharacterized by means of scanning electron microscopy, high resolution transmissionelectron microscopy, energy dispersive x-ray analysis, and Raman spectroscopy. The resultsindicate that the nanocone is composed of a silicon core coated with a thin amorphouscarbon (a-C) layer produced by carbon-bearing plasma etching. Plasma etching is a keyfactor in the formation of the nanocone arrays, while re-condensation of evaporated siliconatoms on the tip of the as-etched cone also occurs. Field emission measurementsshow that the a-C coating can effectively enhance the field emission ability of thenanocone arrays due to the decrease of the surface work function from 4.15 to 2.37eV.

Synthesis of tungsten oxide (WO 3) nanorods using carbon nanotubes as templates by hot filament chemical vapor deposition

Synthesis of tungsten oxide (WO 3) nanorods using CNTs as templates under typical diamond growing conditions in a hot-filament chemical vapor deposition system is described. Tungsten oxide nanorods were grown on two types of CNT substrates, a Si [100] wafer with dispersed multiwalled CNT and a fresh-grown multiwalled CNT film. The templated growth resulted in an order of magnitude increase in surface area covered by the tungsten oxide compared to a planar Si [100] surface, which when combined with the flexibility of growing CNTs in various forms like pillars and sheets [K. Hata, D. N. Futaba, K. Mizuno, T. Namai, M. Yumura, S. Iijima, Science 306 (2004) 1362–1364.], can be effectively utilized in field emission, gas sensing and catalytic applications.

Growth of diamond films with high surface smoothness

Diamond films with highly smooth backside surface have been deposited by positively biasing the substrate during diamond growth in a hot-filament chemical vapor deposition (HFCVD) system. By bonding the diamond film on the glass and wet etching to remove silicon, the highly smooth diamond surface can be exposed and used directly for the fabrication of diamond devices. Silicon substrate was first treated by diamond powder of 625 nm in an ultrasonic bath. By positively biasing the substrate, electron bombardment during diamond growth increases the nucleation density from 10 8 ∼ 10 9 cm − 2 to 4 × 10 11 cm − 2 . The surface smoothness on the backside of diamond film has thus been improved significantly, inducing root-mean-square roughness of 5 nm. Owing to the extremely high surface smoothness and the high crystalline quality on the backside of diamond film and the high diamond growth rate, the backside surface of the diamond film grown under electron bombardment is particularly suitable for device fabrication.

Low residual stress diamond coatings on titanium

Diamond coatings on pure titanium substrates are of interest for tribological and biomedical implants, among others, however, due to the different thermal expansion coefficients of the two materials, it is difficult to grow adherent thick diamond layers on Ti. The aim of this work was to investigate the influence of cooling down the substrate from the deposition temperature to the room temperature and then repeat the deposition process several times, on the quality and stress of the deposited diamond. This process intended to allow a partial relaxation of the diamond/Ti thermal stresses by accommodating the diamond nuclei onto the titanium carbide formed interlayer without detachment from the substrate. A hot-filament chemical vapor deposition system fed with ethanol, highly diluted in hydrogen was employed. Coatings of complete surface coverage with faceted grains of lateral size of about 4 μm and 0.38 GPa have been obtained.

Time-Modulated Chemical Vapor Deposition of Diamond Films

This article investigates the role of substrate temperature in the deposition of diamond films using a newly developed time-modulated chemical vapor deposition (TMCVD) process. TMCVD was used to deposit polycrystalline diamond coatings onto silicon substrates using hot-filament chemical vapor deposition system. In this investigation, the effect of (a) substrate temperature and (b) methane (CH4) content in the reactor on diamond film deposition was studied. The distinctive feature of the TMCVD process is that it time-modulates CH4 flow into the reactor during the complete growth process. It was noted that the substrate temperature fluctuated during the CH4 modulations, and this significantly affected some key properties of the deposited films. Two sets of samples have been prepared, in each of which there was one sample that was prepared while the substrate temperature fluctuated and the other sample, which was deposited while maintaining the substrate temperature, was fixed. To keep the substrate temperature constant, the filament power was varied accordingly. In this article, the findings are discussed in terms of the CH4 content in the reactor and the substrate temperature. It was found that secondary nucleation occurred during the high timed CH4 modulations. The as-deposited films were characterized for morphology, diamond-C phase purity, hardness, and surface roughness using scanning electron microscopy, Raman spectroscopy, Vickers hardness testing, and surface profilometry, respectively.

Growth kinetics of diamond film with bias enhanced nucleation and H 2/CH 4/Ar mixture in a hot-filament chemical vapor deposition system

A study on the effect of growth kinetics and properties of diamond film obtained by addition of argon (∼7 vol%) into the methane/hydrogen mixture is carried out using a hot-filament chemical vapor deposition system. A negative bias was used as a nucleation enhancement method in addition to the argon dilution. The scanning electron microscopic images show well faceted crystallites with a predominance of angular shapes, corresponding to 〈1 0 0〉 and 〈1 1 0〉 crystalline surfaces. The surface nucleation density and growth rate with argon dilution is found to be two orders of magnitude higher than those without argon dilution. The micro-Raman spectra show a good-quality film whereas X-ray photoelectron spectra show existence of only carbon phase (C 1s peak). The effect of argon dilution on the microstructure of the diamond is discussed in the view of reported literature.

Effects of heavy-ion radiation on the electron field emission properties of sulfur-doped nanocomposite carbon films

The effects of heavy-ion radiation over the electron field emission (EFE) properties of sulfur-doped nanocomposite carbon (n-C:S) films were investigated. Two identical sets of n-C:S films were prepared in a hot-filament chemical vapor deposition system using CH 4, H 2 and H 2S. Films with various sp 3 C and sp 2 C fractions were present within each set. Raman spectroscopy revealed the composite nature of the films. One set of films was submitted to a 20 krad dose of energetic Si and Fe ions (GeV/amu) at Brookhaven National Laboratory's alternating gradient synchrotron. Films originally having poor EFE properties showed a dramatic emission enhancement as a result of irradiation, while those having good EFE properties before irradiation showed a small emission improvement upon irradiation. This improved EFE behavior is explained as due to the formation of conductive trigonal carbon nano-channels within the tetragonally-bonded carbon matrix. The diameter of these trigonal carbon nano-channels is small enough to produce large local field enhancement, and large enough to withstand the relatively large emitted current densities. A mechanism is proposed for the creation of this graphitic carbon nano-wire network within the tetragonal carbon matrix upon energetic irradiation. The results suggest that radiation-hardened cold cathodes can be fabricated using nanocomposite carbon materials that would remain stable in their EFE properties while exposed to ionizing radiation.

Diamond nucleation and growth on zeolites

In this work, we report the use of zeolites as substrates for the deposition of porous diamond films. Films were deposited in a hot-filament chemical vapor deposition (HFCVD) apparatus. The HFCVD system was fed with a mixture of methane (0.8%) with the balance being hydrogen. A series of depositions were done in the pressure range 20–120 Torr and at substrate temperature 880 °C. The morphologies of the as-deposited films were analyzed by scanning electron microscopy and show isolated diamond grains in the initial nucleation stages, which develop into a microporous film in the next stage and form a continuous film after long time deposition. Raman spectroscopy was used to investigate the crystal morphology, structure and non-diamond impurities in the films deposited at various growth conditions. The nature of the hydrogen bonding with sp 3 and sp 2 network and the quantitative analysis were done by Fourier transform infrared spectroscopy.

Diamond films grown on cemented WC–Co dental burs using an improved CVD method

A modified hot-filament chemical vapour deposition system was used to deposit adherent diamond films onto cutting edges of cemented tungsten carbide (WC–Co) dental burs. Generally, the as-grown films were found to be polycrystalline and displayed good coverage. Micro-Raman spectroscopy was used to characterise the stress distribution at three positions: (i) bur tip, (ii) middle of the bur and (iii) end of bur. The stress at the back, in the middle and at the tip of the bur was calculated to be −1.7, −2.3 and −3.4 GPa in compression, respectively. It was noted that the filament temperature, as measured by the two colour optical pyrometer, varied along the coils of the filament. The coiled filament was at higher temperatures around the central region as compared to the filament edges. The temperature of the cutting edges was higher at the tip of the bur compared to the middle and back of the bur. The cutting teeth were closer to the filament and therefore expected to be at a slightly higher temperature. Consequently, a thicker coating was deposited on the edges of the cutting teeth as compared to the flat bur base material.

High‐Quality Ultra‐Fine GaN Nanowires Synthesized Via Chemical Vapor Deposition

GaN nanowires with diameters in the quantum‐confinement size regime (4–10 nm, ∼10 nm in Figure) are prepared on large‐area substrates through catalytic reaction of Ga and NH3 in a hot‐filament chemical vapor deposition system. The synthetic method is reproducible and could be applied to the growth of other semiconductor nanostructures.

Growth of self-supported diamond tubes in the hot-filament chemical vapor deposition system

A hot-filament chemical vapor deposition system fed with ethanol highly diluted in hydrogen was employed to grow self-supported diamond tubes with external diameter to wall thickness aspect ratio between ∼5 and ∼60, and 30 mm in length. Tungsten wires of diameters between 238 and 586 μm were used as substrates for the deposition, and were subsequently completely removed by etching, to leave stress-free diamond tubes, as demonstrated by micro-Raman spectroscopy. Scanning electron microscopy revealed cross-sections with diamond grains growing radially, forming well-faceted top-surface grains on the tube's external surface.

Role of substrate temperature during diamond film growth using the newly developed time-modulated chemical vapor deposition process

Diamond has many excellent properties that make it useful for many industrial and consumer applications [1]. Researchers have successfully deposited diamond films onto a variety of substrate materials using a range of techniques [2–4]. The most successful and the most widely used method for depositing diamond coatings is chemical vapor deposition (CVD). It is generally accepted that the morphology, crystal size/orientation, surface roughness and coating adhesion, are critical in determining the suitability of the coating being used for a particular application. Films grown using conventional CVD processes display rough surfaces, which become pronounced with increasing film thickness. Therefore, this limits their potential for use in microelectronic, optical and biomedical applications. Researchers have employed numerous techniques to produce smoother looking diamond films [5–12]. In addition, smooth nano-crystalline diamond films have also been deposited using a range of methods [13–20]. Recently, we proposed a new time-modulated CVD (TMCVD) for depositing smooth diamond films [21]. The key feature of this process is that it involves CH4 pulse cycles during film growth. The process of dynamic film growth using CH4 modulations constitutes an interesting concept in CVD diamond technology. However, the TMCVD process is not yet fully understood, for example, the mechanisms involved during film growth remain un-investigated. In this communication, results relating to the effects of substrate temperature, during CH4 pulses, on the properties of diamond films grown using TMCVD have been presented. Diamond films were deposited onto silicon (100) substrates (5 mm × 5 mm × 0.5 mm). The substrates were abraded with diamond powder prior to film deposition in order to enhance the nucleation density. A conventional hot-filament CVD system [22] was used to deposit the diamond films. Prior to the depositions the filament was pre-carburized in order to prevent filament poisoning. In this investigation, two types of samples were prepared, namely, sample A and sample B. The conditions employed during film growth were: hydrogen flow, 150 sccm; pressure, 30 Torr; growth time, 126 mins; filament-substrate distance, 4 mm. The TMCVD process pulsed CH4 throughout the growth process. The substrate temperature was measured using a K-type thermocouple. A Hitachi 4100 scanning electron microscope (SEM) was used to characterize the films for morphology. A surface profiler (Hommelwerke, T1000) was used to measure the surface roughness of the films. In addition, a Renishaw 2000 micro-Raman system with a 514 nm He-Ne laser was used to characterize the diamond films for diamond-carbon phase purity. Fig. 1 shows the variations in substrate temperature and CH4 flow during sample A (a) and sample B (b) preparation using the TMCVD process. It is important to note that during sample B preparation, although the CH4 flow was modulated, the substrate temperature was maintained constant throughout the growth process. It may be of interest to note that the substrate temperature was kept constant during the high/low CH4 pulse cycles merely by adjusting the filament power accordingly. The temperature of the silicon substrates at 3 and 4.5 sccm CH4 flow was noted to be 802 ◦C and 776 ◦C, respectively. The fluctuations in substrate temperature, resulting from the CH4 pulses during TMCVD, can significantly alter the diamond film growth process. Fig. 2a displays the graph relating substrate temperature to CH4 concentration under typical diamond CVD growth conditions. Generally, the substrate temperature decreased with increasing CH4 concentration. It should be noted that the observed temperature trend was obtained in the presence of two gases, namely, CH4 and hydrogen, present in the reactor chamber during film growth. Since the TMCVD process pulses CH4 at 3% and 2% concentrations, the fluctuation in the substrate temperature, from 776 to 802 ◦C, respectively, needs to be considered, as it is a critical factor influencing film growth during diamond CVD. In order to investigate the independent affect of CH4 on substrate temperature, only CH4, without any hydrogen, was introduced into the vacuum chamber. It is clear from Fig. 2b that the substrate temperature remains almost constant with varying CH4 flow. Therefore, it can be stated that CH4 flow has no significant affect on the substrate temperature. However, once excess of hydrogen ions are present in the CVD reactor then the resulting hydrogen/CH4 plasma plays a critical role in governing the substrate temperature. There are three main factors that can contribute to the substrate temperature changes observed during CH4 pulses, namely, (i) gas species thermal motion, (ii) chemical reactions between carbon-containing species and hydrogen, and (iii) the carbon-phase transitions. It is important to realize that generally during diamond CVD, these three factors come into play

Stress distribution in diamond films grown on cemented WC–Co dental burs using modified hot-filament CVD

A modified hot-filament chemical vapour deposition system has been used to deposit diamond films onto dental burs. The as-grown films were found to be polycrystalline and displayed good coverage. Micro-Raman spectra were taken at different positions of the coating to characterise stress distribution in the deposited film. The Raman diamond peak positions were used to calculate the stress along and across the diamond coating. It was found that the stress across the coating remained constant. However, the stresses along the coating displayed some interesting results. The stress at the base, in the middle and at the tip of the bur was calculated to be −1.7, −2.3 and −3.4 GPa in compression, respectively. The temperature of the bur during deposition was found to be different at different positions on the bur. At the base of the cutting area; at the middle of the bur; and at the tip of the bur the temperature was found to be 842, 908 and 952 °C, respectively. The stress in the films increased as the temperature of the bur increased. Scanning electron microscopy analysis was used to characterise the as-grown films for morphology and crystallinity.

Properties of diamond films grown from CH 4CF 4H 2 gas mixture using the hot-filament chemical vapor deposition technique

Here, we report a study of the effect of intentional incorporation of fluorine on the growth and properties of diamond films prepared by adding CF 4 gas to the source gas mixture of CH 4+H 2 in a hot-filament chemical vapor deposition system. The films were characterized by using the scanning electron microscopy, Raman and photoluminescence (PL) spectroscopy, and X-ray photoelectron spectroscopy (XPS) measurements as a function of fluorine fraction in the gas phase. The films with fluorine fractions above a critical value were found to be composed of clean cubo-octahedral diamond crystals. The XPS measurements confirmed fluorine bonding at the diamond crystallite surfaces while the increase of width of 1332 cm −1 Raman band attributed to a possible incorporation of fluorine in the bulk of the diamond crystallites. The PL spectra indicated intensity enhancement of the 1.73 eV band while the 1.683 eV band intensity decreased drastically with an increase in fluorine fraction. These PL bands are proposed to originate from the defect centers formed, respectively, by multi-vacancies and mono-vacancies in the diamond crystallites while the fluorine incorporation in the bulk is proposed to induce reduction in the mono-vacancies with proportionate increase in the multi-vacancies.

Fabrication of tubes of diamond with micrometric diameters and their characterization

The fabrication and characterization of ‘self-supporting’ diamond tubes grown by chemical vapor deposition (CVD) are reported. Diamond layers were deposited onto tungsten wires with diameters of 238 μm; the tungsten cores were subsequently completely removed by etching to leave ‘self-supporting’ diamond tubes with a diameter of approximately 400 μm and length of 20 mm. A hot-filament CVD system fed with ethanol highly diluted in hydrogen was employed. Growth rates of 7.8 μm h −1 have been measured and incubation times >3 h have been estimated. Scanning electron microscopy of cross-sections revealed columnar structures, which terminate on sharp (111) facets on the tube's external surface. Raman spectroscopy showed that the tube structure is predominantly composed of CC sp 3 bonds, with intrinsic tensile stresses.

Nucleation of diamond on silicon wafers using C 60 in the hot filament chemical vapour deposition system

Scanning electron microscopy and Raman shifts were used to study the process of diamond nucleation and growth using C60 in the hot filament chemical vapour deposition (HFCVD) system. The process of nucleation and growth of diamond films on silicon wafer using C60 as intermediate layer in HFCVD system is described. In order to increase the density of diamond nuclei on the wafers, it is not necessary to use negative bias. The UV-light pre-treatment is not beneficial for improving the diamond nucleation. The multi-layers of C60 molecules, but not a monolayer, can increase the density of diamond nuclei in the presence of H atoms.

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.

Development of tubes of micro-crystalline diamond and diamond-like carbon

Tungsten wires were used as templates for the deposition of tubes of micro-crystalline diamond or of ‘spongy’ carbon nano-structures. A hot-filament chemical vapor deposition system fed with highly diluted ethanol in hydrogen and argon was used. Argon was intentionally added to the feed at different concentrations to affect the crystalline growth. As the argon concentration was increased beyond 65 vol.% in the reactor feed, the morphology drastically changed from micro-crystalline diamond to complex carbon nano-structures. While the samples produced at argon concentrations of 65 vol.% or less resulted in a radial columnar structure of micro-crystalline diamond, the samples produced at higher argon concentrations showed radial flaws and ‘spongy’ structures. Each type of morphology is of potential interest for the use of these tubes as molecular filters or in electron emission devices.