Conventional Hot-filament Chemical Vapor Deposition Research Articles

An investigation into low-temperature HFCVD and derivative techniques for the enhancement of diamond-coated cemented carbide tool performance

The performance of diamond-coated cemented carbide tools manufactured via the conventional hot filament chemical vapor deposition (HFCVD) technique exhibits certain limitations, including issues with poor toughness and reduced adhesive strength of film-substrate systems, primarily attributed to the cobalt diffusion at elevated temperatures (> 800 °C). In this study, a pioneering exploration of a low-temperature diamond film deposition technique on cemented carbide is developed, and a comprehensive evaluation of the film properties is conducted. The results indicate that owing to constraints related to growth rate and diamond film purity, it may not serve as an effective approach for enhancing the performance of conventional coated tools. Nonetheless, the inherent capability of this technique in inhibiting cobalt diffusion is strategically harnessed to pioneer the development of a derivative technique referred to as the “low-temperature to high-temperature HFCVD (LT-HT-HFCVD)”. This innovative method leverages a low-temperature deposited diamond layer as an intermediate layer to impede cobalt diffusion while serving as a densely nucleated layer for subsequent diamond growth. Subsequently, the conventional technique is employed to facilitate an effective growth rate and maintain high growth purity. The diamond-coated tools generated by this technique are subject to a comprehensive comparative analysis against those generated by low-temperature HFCVD (LT-HFCVD) and conventional HFCVD (CT-HFCVD) techniques. The results reveal that tools fabricated using the LT-HT-HFCVD combine the advantages inherent to both the LT-HFCVD and CT-HFCVD techniques, thereby manifesting superior toughness, enhanced film-substrate adhesive strength, and outstanding cutting performance.

Diamond Thin Film Coating on WC Substrate by HFCVD Method using Different Seeding Powders and Its Characterization

The formation of diamond thin solid film is studied by Hot Filament Chemical Vapor Deposition (HFCVD) by using different seeding powders on WC substrate. Here, we have deposited microcrystalline diamond (MCD) on SPUN cemented carbide (WC-Co) cutting tool insert using conventional HFCVD technique. The substrates were ultrasonically seeded with titanium, tungsten, molybdenum, diamond, combined diamond and tungsten powder at the same operation parameters to observe the difference in the growth of diamond by different seeding. Nucleation of diamond coating was studied after 40 minutes of coating with seeded substrate. The physical characteristics were studied by X-ray Diffraction (XRD).The Scanning Electron Microscope (SEM) was used to study the surface morphology. The diamond purity was studied with the help of Raman spectroscopy (RS). Reduction of cobalt percentage before and after pretreatment was studied by Energy Dispersive Spectroscopy (EDS). A very dense and uniform coating was deposited on the diamond powder seeded substrate. The nucleation density was found to be highest in diamond powder when compared with molybdenum and tungsten powder individually, because in initial stage the deposited material contains more sp2 content as compared to sp3 content. By using diamond and tungsten mixed powder the coating was uniform rather in case of only tungsten powder coating the discrete crystals were formed.

A novel approach of deposition for uniform diamond films on circular saw blades

Uniform diamond films are highly desirable for cutting industries, due to their high performance and long lifetime used on cutting tools. Nevertheless, they are difficult to obtain on cutting tools with complicated shapes, greatly limiting the applications of diamond films. In this study, a novel approach of deposition for uniform diamond films is proposed, on circular saw blades made of cemented carbide using reflectors of brass sheets. Diamond films are deposited using hot filament chemical vapor deposition (HFCVD). A novel concave structure of brass sheets is designed and fabricated, improving the distribution of temperature field, and overcoming the disadvantages of the conventional HFCVD systems. This increases the energy efficiency of use without changing the structure and increasing the cost of HFCVD. The grains are refined and the intensities of diamond peaks are strengthened obviously, which is confirmed by scanning electron microscopy and Raman spectra respectively.

Carbon nanotubes coated with diamond nanocrystals and silicon carbide by hot-filament chemical vapor deposition below 200 °C substrate temperature

Multi-walled carbon nanotubes (MWCNTs) dispersed onto a silicon substrate have been coated with diamond nanocrystals (DNC) and silicon carbide (SiC) from solid carbon and silicon sources exposed to H2 activated by hot filament chemical vapor deposition (HFCVD) at around 190°C substrate temperature. MWCNT coating by DNC initiates during filament carburization process at 80°C substrate temperature under conventional HFCVD conditions. The hybrid nanocarbon material was analyzed by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, electron energy loss spectroscopy, selected area electron diffraction, X-ray diffraction and Raman spectroscopy. The structure of the MWCNTs is preserved during coating and the smooth DNC/SiC coating is highly conformal. The average grain size is below 10nm. The growth mechanism of DNC and SiC onto MWCNT surface is discussed.

Influence of periodic magnetic field on the growth of CVD diamond films at lower temperature

A periodic magnetic field (PMF) of B= B 0 sin ωt, where B 0=106 Guass and ω=40,000 rpm, was designed and first applied in a special hot-filament chemical vapor deposition (HFCVD) system for diamond synthesis. Different to the conventional HFCVD, an external PMF was introduced for diamond thin film deposition on a Si (1 1 1) substrate. The morphology and cross-section of the film were analyzed by field emission scanning electron microscopy (FESEM) while the bond structure was investigated using Raman spectroscopy. Compared with the conventional HFCVD, the growth rate of diamond film defined as the ratio of film thickness vs deposition time increases by about 7 times at 520 °C with PMF. The I Dia/ I G ratio in Raman spectra indicates that the quality of the diamond film grown with PMF is much higher than that of the film grown by conventional HFCVD. The research shows that the novel technology using PMF may have great practical application potentiality in CVD diamond growth at low temperature and in some relevant fields.

Developments in hot-filament metal oxide deposition (HFMOD)

Hot-filament metal oxide deposition (HFMOD) is a variant of conventional hot-filament chemical vapor deposition (HFCVD) recently developed in our laboratory and successfully used to obtain high-quality, uniform films of MO x , WO x and VO x . The method employs the controlled oxidation of a filament of a transition metal heated to 1000 °C or more in a rarefied oxygen atmosphere (typically, of about 1 Pa). Metal oxide vapor formed on the surface of the filament is transported a few centimetres to deposit on a suitable substrate. Key system parameters include the choice of filament material and diameter, the applied current and the partial pressures of oxygen in the chamber. Relatively high film deposition rates, such as 31 nm min − 1 for MoO x , are obtained. The film stoichiometry depends on the exact deposition conditions. MoO x films, for example, present a mixture of MoO 2 and MoO 3 phases, as revealed by XPS. As determined by Li + intercalation using an electrochemical cell, these films also show a colouration efficiency of 19.5 cm 2 C − 1 at a wavelength of 700 nm. MO x and WO x films are promising in applications involving electrochromism and characteristics of their colouring/bleaching cycles are presented. The chemical composition and structure of VO x films examined using IRRAS (infrared reflection–absorption spectroscopy), RBS (Rutherford backscattering spectrometry) and XPS (X-ray photoelectron spectrometry) are also presented.

Nano-crystalline diamond films synthesized at low temperature and low pressure by hot filament chemical vapor deposition

Nano-crystalline diamond (NCD) films on silicon were synthesized at a low temperature of 450–550 °C and a low pressure of ∼7 Torr by a hot filament chemical vapor deposition (HFCVD) in a mixture of CH 4 and H 2. The synthesis process included pretreatment, nucleation, growth and in situ annealing steps. The Si wafers were ultrasonically pretreated in a suspension of acetone and diamond powder (20–40 μm in diameter). It was found that the key parameter to obtain smooth NCD films by conventional HFCVD without Ar was the high nucleation density (ND). Optimal ultrasonic pretreatment with coarser diamond powder and nucleation at 2.5% CH 4 greatly increased the ND to about 1.5 × 10 11 cm − 2 . The NCD films with average grain sizes in the range of 40–70 nm, thickness less than 500 nm and surface roughness of ∼19 nm were grown at 1.5% CH 4, 550 °C and 7 Torr, their optical transmittance in the near infrared (NIR) range was as high as 85%.

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

Nucleation and growth of diamond on silicon substrate coated with polymer

Using conventional hot-filament chemical vapor deposition system at the substrate temperature of 800°C, diamond crystals and films were deposited on mirror-polished silicon substrate coated with polymer, scratched silicon substrate coated with polymer and scratched silicon substrate, respectively. The [C 6H 5C] n polymer, poly(phenylcarbyne), consists of randomly constructed rigid network of tetrahedral hybridized phenylcarbyne units. The nucleation density of diamond deposited on these substrates coated with polymer is estimated to be about 10 8–10 11 cm −2 and higher than that on the substrate only scratched by diamond paste. Moreover, it is found that diamond can grow on the polymer at low substrate temperature (500°C), while diamond crystals are not observed on other polymers, such as pump oil, polyethylene, etc., The morphology and structure of diamond crystals and films are characterized by scanning electron microscopy (SEM) and Raman spectrum. Some interpretation for the results are given.

An improved method for large-area oriented nucleation of diamond during bias process via hot-filament chemical vapor deposition

A hollow cathode technique has been applied as a new bias-enhanced nucleation (BEN) process prior to diamond growth in hot-filament chemical vapor deposition (HF-CVD). A large-area plasma is produced above a silicon substrate by glow discharge during the new BEN process from the beginning of the diacharge. A uniform nucleation over the substrate surface can be obtained even under the condition of lower bias voltage. The plasma sheath plays an important role for diamond nucleation. Diamond films are deposited by conventional HF-CVD after the new BEN pretreatment. Nomarski micrography has revealed that the uniformity of as-grown film pretreated by this method is improved. Scanning electron microscopy results have shown that the area of the oriented nucleation is enhanced considerably. The micro-Raman spectrum of as-grown film has shown a strong crystalline diamond peak at 1333 cm −1.

High efficiency deposition of diamond film by hot filament chemical vapor deposition

A new designed reaction chamber with new relative distribution of filament and substrates has been adopted in order to increase the deposition area of diamond films and thus increase the deposition efficiency in conventional hot filament chemical vapor deposition (HFCVD) systems. The relatively small reaction chamber was cuboid shaped (50 × 25 × 25 mm3) and composed of molybdenum wafers. It was established in the vacuum chamber. A tungsten filament was hung up vertically in the center of the small chamber and parallel to the gas flow path. At the four inner sides of the reaction chamber, four Si(100) substrates (30 × 10 × 0.5 mm3) were installed to grow diamond films. The deposition results indicate that uniform diamond films can be obtained on the four substrates, and the film growth rate is the same at both ends of the substrates. The diamond film growth rate was about 1−2 μm/h, which is similar to those of the conventional HFCVD method. Thus, the deposition area and efficiency can be increased four times in the case without the filament number, gas flow rate, and power consumption.

Seeding With a Diamond Suspension for Growth of Smooth Polycrystalline Diamond Surfaces

AbstractA number of techniques have been used to smooth polycrystalline diamond films. Recent work in substrate seeding with nanocrystalline diamond powder, alone or in a carrier fluid, has shown that diamond seeding improves nucleation density and reduces diamond surface roughness. In this work, silicon substrates were seeded using a commercially available waterbased 0.1 micrometer diamond polishing suspension. Growth was achieved using conventional hot-filament chemical vapor deposition (HFCVD). Films were characterized using optical microscopy, scanning electron microscopy (SEM), x-ray diffraction (XRD), and surface profilometry. The resulting diamond films exhibited well-faceted crystals, small grain size and minimal surface roughness. Additionally, the silicon substrate was chemically etched in order to permit examination of the backside of the diamond film. Results show that the diamond surface at the silicon-diamond interface is ultra-smooth. Comparison of the backside of these surfaces with those prepared using conventional diamond grit abrasion indicates that a significant improvement in surface quality is achieved using this diamond seeding technique.

Nucleation enhancement of diamond using diamond-like carbon film synthesized from polymer as an intermediate layer

Using a conventional hot-filament chemical vapor deposition system, diamond crystals and films were deposited on diamond-like carbon (DLC) films which were synthesized from a [C 6H 5C] n polymer on silicon substrates under a temperature of 1000 °C in an argon atmosphere. The nucleation density of the diamond on DLC-Si substrates was estimated to be about 10 8 to 10 10 cm −2, equivalent to or higher than the best values for scratched silicon substrates. During the deposition process, most of the amorphous carbon phases of the DLC films were etched away, while most of the crystalline carbon phases remained, and homoepitaxial growth on the crystals in the DLC films was observed which suggested that the amorphous phase mainly consists of sp 2-hybridized carbon and the crystalline phase mainly consist of sp 3-hybridized carbon. It is proposed that the diamond phase carbon (sp 3 carbon) of the DLC films act as a nucleation site and enhances the diamond nucleation.