Effect of titanium metal in the prenucleation of ultrananocrystalline diamond film growth at low substrate temperature

Ultrananocrystalline diamond (UNCD) film is usually grown in methane–argon plasma unlike methane–hydrogen plasma conventionally used to deposit microcrystalline diamond film. The prenucleation and growth mechanism of these two types of diamond films are different as well. The present study introduces titanium metal powder during ultrasonication of silicon substrate to enhance the nucleation density of UNCD. A titanium thin film was also used at the interface to find the effect of metal on the growth of diamond film. The nucleation density of as-grown film was estimated from the FE-SEM images. After 20 min of growth, nucleation density reaches to 10¹¹/cm² on a surface pretreated by titanium mixed nanodiamond powder. Raman study was carried out for qualitative analysis of different carbon phase present in the UNCD films. X-ray photoelectron spectroscopy (XPS) was used to understand the growth mechanism by detecting the formation of carbon phase and metal carbide formation at the surface after stopping the growth at different time intervals.     

Transparent ultrananocrystalline diamond films on quartz substrate

Highly transparent ultrananocrystalline diamond (UNCD) films were deposited on quartz substrates using microwave plasma enhanced chemical vapor deposition (MPECVD) method. Low temperature growth of high quality transparent UNCD films was achieved by without heating the substrates prior to the deposition. Additionally, a new method to grow NCD and microcrystalline diamond (MCD) films on quartz substrates has been proposed. Field emission scanning electron microscopy (FESEM) and Raman spectroscopy were used to analyze the surface and structural properties of the films. The surface morphology of UNCD film shows very smooth surface characteristics. The transparent property studies of UNCD film on quartz substrate showed 90% transmittance in the near IR region. The transparent and dielectric properties of UNCD, NCD, and MCD films on quartz substrates were compared and reported.     

Synthesis of diamond films and nanotips through graphite etching

A hot filament CVD process based on hydrogen etching of graphite has been developed to synthesize diamond films and nanotips. The graphite sheet was placed close to the substrate and only hydrogen was supplied during deposition. No hydrocarbon feed gases are required for this process. High quality diamond films were synthesized with high growth rate on P-type (1 0 0)-oriented silicon wafers without discharge or bias. The diamond growth rate is approximately five times higher than that through conventional hot filament chemical vapor deposition using a gas mixture of methane and hydrogen (1 vol.% methane) under similar deposition conditions. The diamond films synthesized in this process exhibit smaller crystallites and contain smaller amount of non-diamond carbon phases. Synthesis of well-aligned diamond nanotips with various orientation angles was achieved on the CVD diamond-coated Si substrate when the substrate holder was negatively biased in a DC glow discharge. The nanotips grown at locations far enough from the sample edges are aligned vertically, while those around the sample edges are tilted and point away from the sample center. The alignment orientation of the nanotips appears to be determined by the direction of the local electric field lines on the sample surfaces.     

Pre-nucleation techniques for enhancing nucleation density and adhesion of low temperature deposited ultra-nanocrystalline diamond

Effect of pre-nucleation techniques on enhancing nucleation density and the adhesion of ultra-nanocrystalline diamond (UNCD) deposited on the Si substrates at low temperature were investigated. Four different pre-nucleation techniques were used for depositing UNCD films: (i) bias-enhanced nucleation (BEN); (ii) pre-carburized and then ultrasonicated with diamond powder solution (PC-U); (iii) ultrasonicated with diamond and Ti mixed powder solution (U-m); (iv) ultrasonicated with diamond powder solution (U). The nucleation density is lowest for UNCD/U-substrate films ( 10⁸ grains/cm²), which results in roughest surface and poorest film-to-substrate adhesion. The UNCD/PC-U-substrate films show largest nucleation density ( 1 × 10¹¹ grains/cm²) and most smooth surface (8.81 nm-rms), whereas the UNCD/BEN-substrate films exhibit the strongest adhesion to the Si substrates (critical loads =  67 mN). Such a phenomenon can be ascribed to the high kinetic energy of the carbon species, which easily form covalent bonding, Si–C, and bond strongly to both the Si and diamond.     

Surface acoustic wave properties of natural smooth ultra-nanocrystalline diamond characterized by laser-induced SAW pulse technique

Diamond is one of the best SAW substrate candidates due to its highest sound velocity and thermal conductivity. But conventional diamond films usually express facet structure with large roughness. Ultra-nanocrystallined diamond (UNCD) films grown in a 2.45 GHz IPLAS microwave plasma enhanced chemical vapor deposition (MPECVD) system on Si (100) substrates in CH₄-Ar plasma possess naturally smooth surface and are advantageous for device applications. Moreover, highly C-axis textured aluminum nitride (AlN) films can be grown by DC-sputtering directly on UNCD coated Si substrate. However, properties of UNCD films are much complex than microcrystalline diamond films, that is because this is a very complex material system with large but not fixed portion of grain boundaries and sp²/sp³ bonding. Properties of UNCD films could change dramatically with similar deposition condition and with similar morphologies. A simple and quick method to characterize the properties of these UNCD films is important and valuable. Laser-induced SAW pulse method, which is a fast and accurate SAW properties measuring system, for the investigation of mechanical and structure properties of thin films without any patterning or piezoelectric layer.     

Experimental study of nucleation and quality of CVD diamond adopting two-step deposition approach using MPECVD

Effects of the deposition conditions on quality and nucleation density of CVD diamond were investigated using a microwave plasma enhanced chemical vapor deposition (MPECVD) method with methane-hydrogen gas mixtures. Diamond films were deposited at pressures of 665–4000 Pa, temperatures of 660–950 °C, and methane concentrations of 0.5–5 vol.%. Deposited diamond films were characterized by scanning electron microscopy, field emission scanning electron microscopy, micro-Raman spectroscopy, and X-ray diffraction. Diamond quality and nucleation density significantly affected by the deposition pressure, substrate temperature, and methane concentration. The findings of this work were discussed in terms of the effects of deposition conditions on the plasma composition. A two-step deposition approach was applied to improve nucleation density and quality of CVD diamond films. Polycrystalline diamond films were grown using the two-step deposition process changing a combination of parameters in the two steps. Growth and quality of the deposited diamond films were improved altering the deposition pressure and substrate temperature in the two steps.     

Growth and characterization of diamond micro and nano crystals obtained using different methane concentration in argon-rich gas mixture

Diamond with different grain sizes and nanographite films were grown on silicon and diamond substrate using 90 vol.% argon in hydrogen and methane gas mixtures by hot filament chemical vapor deposition method (HFCVD). In current study, the methane volume concentration was varied from 0.125 to 2 vol. % in order to estimate its effect on film morphology. The substrate temperature was varied from 550 to 850 °C by external heating independently of other CVD parameters, in order to estimate the activation energy. Characterization techniques have involved Raman spectroscopy, high resolution X-ray difractometry and scanning electron microscopy. The CHEMKIN computer package has also been used to simulate the experiments. The results obtained here indicate a single mechanism for diamond growth but with a high competition with sp² phase's growth.     

Scaling the microwave plasma-assisted chemical vapor diamond deposition process to 150–200 mm substrates

The scale up of two microwave plasma assisted chemical vapor deposition processes from 75 mm to 200 mm substrates is investigated. A thermally floating 2.45 GHz reactor is scaled up by increasing its physical size by a factor of 2.7 and exciting the reactor with 915 MHz microwave energy. Two processes are investigated, 1) the deposition of ultananocrystalline diamond films (UNCD) and 2) the deposition of polycrystalline diamond films (PCD). Gas chemistries of argon/methane/hydrogen were used for UNCD deposition and hydrogen/methane was used for PCD deposition. Experimental pressures range from 40–110 Torr while microwave power input ranged from 1.9–7 kW resulting in steady state substrate temperatures from 630–950 °C. Uniform deposition was demonstrated over 150–200 mm substrates, i.e. thickness variations of 4% over 150 mm and 6% over 200 mm were achieved with deposition rates ranging from 30–460 nm/h. Low temperature deposition at 633 °C was achieved and thereby demonstrated the potential of integrating the process with temperature sensitive materials. A comparison of the power densities between the two reactors indicates that the large reactor operates at five to nine times lower discharge power densities than smaller reactors suggesting improved deposition efficiencies.     

Improvement of field emission performance on nitrogen ion implanted ultrananocrystalline diamond films through visualization of structure modifications

The relationship between the electron field emission properties and structure of ultra-nanocrystalline diamond (UNCD) films implanted by nitrogen ions or carbon ions was investigated. The electron field emission properties of nitrogen-implanted UNCD films and carbon-implanted UNCD films were pronouncedly improved with respect to those of as-grown UNCD films, that is, the turn-on field decreased from 23.2 V/μm to 12.5 V/μm and the electron field emission current density increased from 10E−5 mA/cm² to 1 × 10E−2 mA/cm². The formation of a graphitic phase in the nitrogen-implanted UNCD films was demonstrated by Raman microscopy and cross-sectional high-resolution transmission electron microscopy. The possible mechanism is presumed to be that the nitrogen ion irradiation induces the structure modification (converting sp³-bonded carbons into sp²-bonded ones) in UNCD films.     

Incorporation of hydrogen in diamond thin films

In this investigation, diamond thin films with grain size ranging from 50 nm to 1 µm deposited using hot filament chemical vapor deposition (HFCVD) have been analyzed by elastic recoil detection analysis (ERDA) for determining hydrogen concentration. Hydrogen concentration in diamond thin films increases with decreasing grain size. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) results showed that part of this hydrogen is bonded to carbon forming C–H bonding. Raman spectra also indicated the increase of non diamond phase with the decrease in crystallite size. Incorporation of hydrogen in the samples and increase of hydrogen content in nanocrystalline sample are discussed. Large separation between filament and substrate used for the synthesis of nanocrystalline film helped to understand the large incorporation of hydrogen in nanocrystalline diamond films during growth. The study addresses the hydrogen trapping in different samples and higher hydrogen concentration in nanocrystallites by considering the synthesis conditions, growth mechanisms for different grain sized diamond films and from the quality of CVD diamond films.     

Fabrication and field emission properties of ultra-nanocrystalline diamond lateral emitters

Field emission characteristics of ultra-nanocrystalline diamond (UNCD) have recently caught much attraction due to its importance in technological applications. In this work, we have fabricated lateral-field emitters comprised of UNCD films, which were deposited in CH₄/Ar medium by microwave plasma-enhanced chemical vapor deposition method. The substrates, silicon-on-insulator (SOI) or SiO₂-coated silicon, were pre-treated by mixed-powders-ultrasonication process for forming diamond nuclei to facilitate the synthesis of UNCD films on these substrates. Lateral electron field emitters can thus be fabricated either on silicon-on-insulator (SOI) or silicon substrates. The lateral emitters thus obtained possess large field enhancement factor (β = 1500–1721) and exhibit good electron field emission properties, regardless of the substrate materials used. The electron field emission can be turned on at 5.25–5.50 V/μm, attaining 5500–6000 mA/mm² at 12.5 V/μm (100 V applied voltage).     

Nanocrystalline diamond containing hydrogels and coatings for acceleration of osteogenesis

In the present study, we have compared the effects of ultrananocrystalline diamond/amorphous carbon composite films (UNCD/a-C) and nanocrystalline diamond (NCD) containing hydrogels to support the osteogenesis of endothelial progenitor cells (EPCs). The course of EPCs osteogenic differentiation was followed 21 days and assayed by measuring cell-associated alkaline phosphatase activity, calcium deposition, and expression of fibronectin. We found that EPCs were capable to adhere to both surfaces in flattened and elongated morphology. The attachment and spreading on the UNCD/a-C films were faster as compared to the hydrogels containing NCDs (by day 7), and this was connected with the release and adsorption of fibronectin to the surfaces. During the process of EPCs differentiation, the release of fibronectin was favored by hydrogels + NCD (day 21). The formation of calcium nodules, characteristic of osteoblastic mineralization, was detected by Alizarin Red S staining. Differentiation-induced calcium nodules were detected in EPCs growing on both surfaces. The EPCs cultured on hydrogels containing NCD deposited more extracellular calcium in comparison with those on UNCD/a-C films on day 21. These results were consistent with the data about the alkaline phosphatase activity on the same day and verified that an active EPC transformation to osteoblast phenotype occurred on both substrates. Our results could have direct implications in the use of biomaterials in tissue engineering strategies, and this work might be useful for the improvement of the methodologies for substrate preparation (including scaffolds). Thus both surfaces studied could be used for modification of bone implants (bone-anchoring parts of joint prostheses or bone replacements) in order to improve their integration with the surrounding bone tissue, for which improved cell-substrate adhesion is also needed.     

Plasma amination of ultrananocrystalline diamond/amorphous carbon composite films for the attachment of biomolecules

Ultrananocrystalline diamond/amorphous carbon nanocomposite films (UNCD/a-C) have been deposited by microwave plasma chemical vapour deposition at 600 °C from 17% CH₄/N₂ mixtures. The as-grown films turned out to be hydrogen terminated and very stable. Photochemical amination of H-terminated diamond is a well-established route to attach functional groups to such surfaces for applications in biosensors. Here we report on experiments to aminate UNCD surfaces directly by exposure to ammonia plasmas. Thereafter the surfaces were reacted with the heterobifunctional crosslinker molecule SSMCC bearing a N-hydroxysuccinimide (NHS) ester group which should react with the surface NH₂ groups. By means of X-ray photoelectron spectroscopy (XPS), contact angle measurements and fluorescence microscopy it is shown that both steps, plasma amination and SSMCC attachment lead to the desired aims. On the other hand, experiments to attach a thiol-bearing fluorescein molecule directly to H-terminated UNCD films turned out to be partially successful although according to literature such a reaction should be very unlikely.     

From micro to nanocrystalline transition in the diamond formation on porous pure titanium

A novel composite material of nanocrystalline (NCD) and/or microcrystalline (MCD) diamond films grown on porous titanium (Ti) substrate was obtained by hot filament chemical vapor deposition technique. Diamond films were grown using 1.5 vol.% CH₄ in a balanced mixture of Ar/H₂. The grain size control was obtained by varying the argon concentration from 0 up to 90 vol.% at substrate temperature of 870 K. Porous Ti substrates were obtained by powder metallurgy and presented an inter-connected open porosity. Scanning electron microscopy images of diamond/Ti exposed the substrate covered by a continuous textured coating which changed from MCD to NCD morphology; depending on the amount of Ar concentration in the feed gas. Micro-Raman spectra showed the characteristic t-polyacethylene peaks around 1150 cm^− 1 and 1470 cm^− 1, associated to NCD formation for samples grown with Ar concentration higher than 40 vol.%. X-ray diffraction patterns identified the diamond and TiC peaks, where the crystallinity of (111) TiC phase decreased as the Ar amount increased. This behavior was associated to diamond (220) peak increase for films grown with Ar concentration higher than 70 vol.%. Diamond crystallite size was also evaluated from Sherrer's formula in the range of 11 up to 20 nm.     

Nucleation and Growth of Diamond Films on Ni-Cemented Tungsten Carbide: II, Effects of Deposition Conditions

Diamond films were deposited by hot-filament chemical vapor deposition (HFCVD) on substrates made of WC sintered with 6 wt% of Ni. The as-ground substrates were scratched with diamond powder (S samples) or scratched and wet-etched (SE samples). Diamond synthesis was carried out at substrate temperatures ranging between 600° and 1050°C, and using 1.0% or 2.0% CH₄ in H₂. The diamond nucleation density, as measured by scanning electron microscopy (SEM) and automatic image analysis (AIA), did not significantly change in the 600°-900°C temperature range, while at substrate temperatures higher than 900°C a steep decrease of the density of nuclei was observed and attributed to the thermal annealing of nucleation sites. The activation energy of the growth process was measured and found to be 21 ± 2 kcal/mol. Neither nucleation density nor growth rate were affected by an increase of CH₄ concentration in the feed gas, while a lack of crystallinity was observed at the higher methane concentration. Raman analysis showed that phase purity of the films was affected mainly by the substrate temperature: the lower the temperature, the better the film quality. The presence of Ni on the substrate surface did not induce the preferential formation of non-diamond carbon phases, as confirmed by comparing the Raman spectra obtained from both S and SE substrates. As a comparison, continuous films were deposited on scratched WC-5 wt% Co substrates under the same experimental conditions. The results indicated that the use of Ni as a binder is preferable to Co.     

Status review of the science and technology of ultrananocrystalline diamond (UNCD™) films and application to multifunctional devices

This review focuses on a status report on the science and technology of ultrananocrystalline diamond (UNCD) films developed and patented at Argonne National Laboratory. The UNCD material has been developed in thin film form and exhibit multifunctionalities applicable to a broad range of macro to nanoscale multifunctional devices. UNCD thin films are grown by microwave plasma chemical vapor deposition (MPCVD) or hot filament chemical vapor deposition (HFCVD) using new patented Ar-rich/CH₄ or H₂/CH₄ plasma chemistries. UNCD films exhibit a unique nanostructure with 2–5 nm grain size (thus the trade name UNCD) and grain boundaries of 0.4–0.6 nm for plain films, and grain sizes of 7–10 nm and grain boundaries of 2–4 nm when grown with nitrogen introduced in the Ar-rich/CH₄ chemistry, to produce UNCD films incorporated with nitrogen, which exhibit electrical conductivity up to semi-metallic level. This review provides a status report on the synthesis of UNCD films via MPCVD and integration with dissimilar materials like oxides for piezoactuated MEMS/NEMS, metal films for contacts, and biological matter for a new generation of biomedical devices and biosensors. A broad range of applications from macro to nanoscale multifunctional devices is reviewed, such as coatings for mechanical pumps seals, field-emission cold cathodes, RF MEMS/NEMS resonators and switches for wireless communications and radar systems, NEMS devices, biomedical devices, biosensors, and UNCD as a platform for developmental biology, involving biological cells growth on the surface. Comparisons with nanocrystalline diamond films and technology are made when appropriate.     

Interpretation of the Raman spectra of ultrananocrystalline diamond

It has long been known that by slightly altering the deposition conditions for diamond in plasma-enhanced chemical vapor deposition (PECVD), a transition from a microcrystalline to a nanocrystalline diamond morphology can be affected. The method of this transition, however, is not clear. This work investigates that transition by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Raman spectroscopy. These experiments show that far from being a continuous transition, there is competitive growth between microcrystalline and nanocrystalline diamonds. Additionally, this work confirms the interpretation that certain peaks in the Raman spectrum previously attributed to “nanocrystalline diamond” are indeed due to the presence of hydrogen at the grain boundaries. For ultrananocrystalline diamond (UNCD) films, we verify that none of the spectral features observed using visible Raman spectroscopy can be attributed to sp³-bonded carbon, although the sample is composed of ∼95% sp³-bonded carbon. Thus, the Raman signal in UNCD can be considered to be solely due to the disordered sp²-bonded carbon at the grain boundaries.     

Substrate temperature effects on the electron field emission properties of nitrogen doped ultra-nanocrystalline diamond

For the purpose of improving the electron field emission properties of ultra-nanocrystalline diamond (UNCD) films, nitrogen species were doped into UNCD films by microwave plasma chemical vapor deposition (MPCVD) process at high substrate temperature ranging from 600° to 830 °C, using 10% N₂ in Ar/CH₄ plasma. Secondary ion mass spectrometer (SIMS) analysis indicates that the specimens contain almost the same amount of nitrogen, regardless of the substrate temperature. But the electrical conductivity increased nearly 2 orders of magnitude, from 1 to 90 cm^− 1 Ω^− 1, when the substrate temperature increased from 600° to 830 °C. The electron field emission properties of the films were also pronouncedly improved, that is, the turn-on field decreased from 20 V/μm to 10 V/μm and the electron field emission current density increased from less than 0.05 mA/cm² to 15 mA/cm². The possible mechanism is presumed to be that the nitrogen incorporated in UNCD films are residing at grain boundary regions, converting sp³-bonded carbons into sp²-bonded ones. The nitrogen ions inject electrons into the grain boundary carbons, increasing the electrical conductivity of the grain boundary regions, which improves the efficiency for electron transport from the substrate to the emission sites, the diamond grains.     

Wettability and protein adsorption on ultrananocrystalline diamond/amorphous carbon composite films

Ultrananocrystalline diamond/amorphous carbon (UNCD/a-C) composite films have been prepared by microwave plasma chemical vapour deposition (MWCVD) from 17% CH₄/N₂ mixtures and modified with O₂ and CHF₃ plasmas, which changed the surface termination from hydrogen to oxygen and fluorine, respectively. X-ray photoelectron spectroscopy (XPS) showed that successful oxidation and fluorination of the UNCD surface has been achieved with surface O or F concentrations of ca. 12 at.%. None of the plasma modification processes led to a change of the film topography as studied by atomic force microscopy (AFM); for all samples the rms roughness was in the range of 10–12 nm. The UNCD/a-C films with different terminations were characterized by contact angle measurements with water, formamide and benzyl alcohol; from the results obtained the surface energy was calculated. The adsorption of albumin and fibrinogen to the different UNCD/a-C samples was assessed by an inverted enzyme-linked immunosorbent assay (ELISA). The determined albumin/fibrinogen ratios, which could be used to evaluate the tendency of thrombus formation, are correlated with the surface properties of as-deposited and modified UNCD/a-C films.     

Investigation in the role of hydrogen on the properties of diamond films grown using Ar/H2/CH4 microwave plasma

The transition of diamond grain sizes from micron- to nano- and then to ultranano-size could be observed when hydrogen concentration is being decreased in the Ar/CH₄ plasma. When grown in H₂-rich plasma (H₂ = 99% or 50%), well faceted microcrystalline diamond (MCD) surface with grain sizes of less than 0.1 μm are observed. The surface structure of the diamond film changes to a cauliflower-like geometry with a grain size of around 20 nm for the films grown in 25% H₂-plasma. In the Ar/CH₄ plasma, ultrananocrystalline diamond (UNCD) films are produced with equi-axed geometry with a grain size of 5-10 nm. The H₂-content imposes a more striking effect on the granular structure of diamond films than the substrate temperature. The induction of the grain growth process, either by using H₂-rich plasma or a higher substrate temperature increases the turn-on field in the electron field emission process, which is ascribed to the reduction in the proportion of grain boundaries.