Characterization of diamond fluorinated by glow discharge plasma treatment

The surface fluorination of diamond by treatment in glow discharge plasmas of CF₄ for different times has been investigated. High quality diamond films were deposited onto silicon substrates using hot filament chemical vapor deposition (HFCVD). Subsequently, the films were exposed to a radiofrequency glow discharge plasma of CF₄ for times ranging from 5 min to 1 h. The effects of the plasma treatment on the surface morphology, diamond quality and elemental composition were investigated using atomic force microscopy (AFM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. Differences in film roughness caused by the plasma treatment were detected by AFM and confirmed by scanning electron microscopy (SEM). Raman spectroscopic analyses showed that the original diamond was of high quality and that the bulk of each film was unchanged by the plasma treatment. Analyses using XPS revealed increased surface fluorination of the films at longer treatment times. In addition, the density of free radicals in the films was probed using electron paramagnetic resonance spectroscopy (EPRS), revealing that untreated diamond possesses an appreciable density of free radicals (6×10¹² g⁻¹) which initially falls with treatment time in the CF₄ plasma but increases for long treatment times.     

Plasma etching treatment for surface modification of boron-doped diamond electrodes

Boron-doped diamond (BDD) thin film surfaces were modified by brief plasma treatment using various source gases such as Cl₂, CF₄, Ar and CH₄, and the electrochemical properties of the surfaces were subsequently investigated. From X-ray photoelectron spectroscopy analysis, Cl and F atoms were detected on the BDD surfaces after 3 min of Cl₂ and CF₄ plasma treatments, respectively. From the results of cyclic voltammetry and electrochemical AC impedance measurements, the electron-transfer rate for Fe(CN)₆^3−/4− and Fe^2+/3+ at the BDD electrodes was found to decrease after Cl₂ and CF₄ plasma treatments. However, the electron-transfer rate for Ru(NH₃)₆^2+/3+ showed almost no change after these treatments. This may have been related to the specific interactions of surface halogen (C-Cl and C-F) moieties with the redox species because no electrical passivation was observed after the treatments. In addition, Raman spectroscopy showed that CH₄ plasma treatment of diamond surfaces formed an insulating diamond-like carbon thin layer on the surfaces. Thus, by an appropriate choice of plasma source, short-duration plasma treatments can be an effective way to functionalize diamond surfaces in various ways while maintaining a wide potential window and a low background current.     

Pulsed laser surface modifications of diamond thin films

In view of practical applications requiring diamond films, plates and membranes with very smooth surfaces, ArF excimer laser polishing treatments were applied to thin (30 μm) diamond films grown by CVD on silicon substrates. The as-prepared diamond surfaces and the laser-treated parts of the samples were characterised by SEM analysis, Raman and micro-Raman spectroscopy. The presence on the laser-treated surface of a thin amorphous carbon layer responsible for the higher surface electrical conductivity and for the different optical reflectivity properties was evidenced. Using confocal micro-Raman spectroscopy a comparative depth profile analysis of the phase quality, below the surface in different regions of the films, was carried out. After short (10 min) treatment by H₂ plasma etching in the CVD chamber the graphitic top layer was completely removed from the samples.     

Cycle of two-step etching process using ICP for diamond MEMS applications

Inductively coupled plasma (ICP) etching was performed to etch diamonds, and a new diamond etching technique was established in order to obtain the high selective etching rate of diamond with respect to the silicon oxide etching mask and the smooth etched surface. Scanning electron microscope (SEM) observations and energy dispersive X-ray (EDX) characterizations of the etched diamond surfaces show that the silicon oxide etching mask particles were re-deposited on the fresh etched diamond surface. This results in un-intentional whisker formation. In order to obtain both the high selectivity and the smooth etched surface, a sequential cycle of ICP etching was, for the first time, applied for diamond. Diamond is etched with O₂ plasma during the first step and, in the second step, silicon oxide was removed using (CF₄ + O₂) plasma. Both the selective etching rate of diamond to silicon oxide was improved and the smooth surface was obtained. By applying the two-step etching process, diamond gene-surgery tips for atomic force microscopy (AFM) have been fabricated.     

Nitrogenation of boron doped diamond: Comparison of an electrochemical treatment in liquid ammonia and a NH3/N2 plasma

In the present work, two different nitrogenation of diamond surface were studied onto moderately doped BDD samples (10¹⁹ at cm^? 3). The effects of an electrochemical treatment in liquid ammonia have been compared to those produced by NH₃/N₂ plasma. The evolution of surface terminations from “CH” to “CN” has been studied by XPS analyses for both nitrogenation methods. Angle contact measurements complete this work, estimating the effect of such treatments on BDD wettability properties. Finally, cyclic voltammetry in presence of redox systems have been carried out to evidence the impact of “CN” bonds on BDD electrochemical behavior. In both cases the formation of “CN” terminations was evidenced by XPS analyses. Physico-chemical properties of BDD electrodes, deduced from contact angle and electrochemical measurements, were strongly modified according the process. The galvanostatic treatment in liquid ammonia led to a more hydrophobic diamond surface and an improvement of the charge transfer with Ce^4+/3+ redox couple, while a more hydrophilic surface and a notable decrease of the electrochemical response were generated by NH₃/N₂ plasma treatment.     

Termination of diamond surfaces with hydrogen,oxygen and fluorine using a small,simple plasma gun

We have formed and characterized polycrystalline diamond films with surfaces having hydrogen terminations, oxygen terminations, or fluorine terminations, using a small, simple and novel plasma gun to bombard the diamond surface, formed by plasma assisted CVD in a prior step, with ions of the wanted terminating species. The potential differences between surface regions with different terminations were measured by Kelvin Force Microscopy (KFM). The highest potential occurred for oxygen termination regions and the lowest for fluorine. The potential difference between regions with oxygen terminations and hydrogen terminations was about 80 mV, and between regions with hydrogen terminations and fluorine terminations about 150 mV. Regions with different terminations were identified and imaged using the secondary electron signal provided by scanning electron microscopy (SEM), since this signal presents contrast for surfaces with different electrical properties. The wettability of the surfaces with different terminations was evaluated, measuring contact angles. The sample with oxygen termination was the most hydrophilic, with a contact angle of 75°; hydrogen-terminated regions with 83°, and fluorine regions 93°, the most hydrophobic sample.     

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.     

Dependence of diamond nucleation and growth through graphite etching at different temperatures

Diamond films have been grown on silicon substrate from graphite etching as a carbon source with atomic hydrogen instead of using conventional hydrocarbon in the feed gas. A graphite plate was used as sample support in a hot filament chemical vapor deposition reactor. Graphite temperature demonstrated to have a strong dependence with the diamond nucleation and growth rate. Scanning electron microscopy (SEM) images of graphite targets associated with their Raman spectra were used to analyze their graphite morphology and structural properties before and after etching process for each graphite temperature studied. The results showed that chemical erosion intrinsically induces graphite surface changes that influence Raman spectra. The disorder behavior from the I_D/I_G ratio presented a maximum value at 900 °C, for 60 min of etching time, when compared with graphite surface at room temperature before atomic hydrogen etching. SEM diamond images were also used to analyze the nucleation rate. Diamonds grown during 15 min at 600 and 700 °C presented the higher nucleation rate. For growth time of 30 min the diamonds are continuous, covering the entire Si substrate surface, with submicrometer grain size. Raman spectra showed good quality diamond coating. Diamond content or diamond purity values, evaluated for growth time of 15, 30 and 60 min increases with the increase of the graphite temperature confirming the high carbon content in the first stage of diamond growth.     

Adsorption of Proteins and Cell Adhesion to Plasma Treated Polymer Substrates

Plasma treatment is often used to alter cell interaction with polymer surfaces used in biomedical application. The influence of surface hydrophilicity/hydrophobicity on human mammary epithelial cell (HMEC) proliferation and adhesion of protein albumin to plasma treated polystyrene (PS) was studied. The PS surface was made hydrophilic or hydrophobic by treatment either in O₂ or CF₄ plasma. The rate of protein adhesion was studied by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) after incubation of PS in albumin solution for different periods, while cell viability and morphology was studied by MTT assay and scanning electron microscope (SEM). XPS measurements have shown that the quantity of adsorbed protein was higher for both plasma treated samples than for the untreated one. No significant difference regarding protein adhesion on hydrophilic or hydrophobic plasma treated surface was found by XPS. Contrary, the results for cell proliferation showed much better proliferation on hydrophilic surface.     

Hydrogen incorporation,bonding and stability in nanocrystalline diamond films

The hydrogen concentration in hot filament and microwave plasma CVD nanocrystalline diamond films is analysed by secondary ion mass spectrometry and compared to the film grain size. The surface and bulk film carbon bonds are analysed respectively by X-ray photoelectron spectroscopy (XPS) and ultra-violet Raman spectroscopy. XPS results show the presence of the hydrogenated p-type surface conductive layer. The respective intensities of the 1332 cm^− 1 diamond peak, of the G and D bands related to sp² phases, and of the 3000 cm^− 1 CH_x stretching mode band, are compared on Raman spectra. The samples are submitted to thermal annealing under ultra-high vacuum in order to get hydrogen out-diffusion. XPS analysis shows the surface desorption of hydrogen. Thermal annealing modifies the sp² phase structure as hydrogen out diffuses.     

Effect of anodic and cathodic treatments on the charge transfer of boron doped diamond electrodes

This paper is concerned with the study of the influence of electrochemical pre-treatments on the behavior of highly boron doped diamond electrodes. Anodic and cathodic preconditioning, performed during 10 s either with 10^− 4 A/cm^− 2 (10^− 3 C cm^− 2) or 10^− 1 A/cm^− 2 (1 C cm^− 2), has been studied. Cyclic voltammetry at as-deposited, anodically and cathodically treated electrodes, in presence of 2 redox couples serving as electrochemical probes is analyzed in the light of the surface characterization given by XPS chemical analysis. Ce^4+/3+ redox couple in 0.5 M H₂SO₄ medium and Fe(CN)₆^3−/4− redox couple in 0.1 M KOH medium, have been studied before and after the different treatments. The results of Mott–Schottky plots and current voltage curves are reported and show that the electrochemical response of BDD electrodes is very dependent on the current density involved in the electrochemical preconditioning. The modification of surface bond termination – either hydrogen or oxygen – studied by XPS analyses is also strongly dependent on electrochemical pre-treatment. In particular, it is evidenced that the most important conversion of surface functionalities from hydrogen to oxygen is obtained when the anodic treatment is performed with the smallest current density. Finally, a correlation between surface terminations and charge transfer is evidenced.     

Surface energy of the plasma treated Si incorporated diamond-like carbon films

Surface energy and surface chemical bonds of the plasma treated Si incorporated diamond-like carbon films (Si-DLC) were investigated. The Si-DLC films were prepared by r.f. plasma assisted chemical vapor deposition using benzene and diluted silane (SiH₄/H₂ = 10:90) as the precursor gases. The Si-DLC films were subjected to plasma treatment using various gases like N₂, O₂, H₂ and CF₄. The plasma treated Si-DLC films showed a wide range of water contact angles from 13.4° to 92.1°. The surface energies of the plasma treated Si-DLC films revealed a high polar component for O₂ plasma treated Si-DLC films and a low polar component for CF₄ plasma treated Si-DLC films. The CF₄ plasma treated Si-DLC films indicated the minimum surface energy. X-ray photoelectron spectroscopy (XPS) revealed that the polarizability of the bonds present on the surface explains the hydrophilicity and hydrophobicity of the plasma treated Si-DLC films. We also suggest that the O₂ plasma treated surface can provide an excellent hemocompatible surface from the estimated interfacial energy between the plasma treated Si-DLC surface and human blood.     

Erosive wear resistance of NCD coatings produced by pulsed microwave discharges

Erosion tests on nanocrystalline diamond (NCD) films are relevant not only for the evaluation of the erosive wear resistance, anticipating applications where coated materials are exposed to particle impacts, but also as a way to evaluate their adhesion to the substrates. NCD films were grown on Si₃N₄ ceramic by microwave plasma assisted deposition in continuous (CW) and pulsed (PW-50 Hz and PW-500 Hz) discharge modes in argon-rich gas mixture. The films grown in PW modes presented lower crystallite size and lower surface roughness than those grown in CW one, while the use of CF₄ plasma pre-treatment of the substrate lead to further film homogeneity. The erosive wear resistance of NCD was evaluated by solid particle impact using SiC (45–250 μm size) as erodent material, with selected parameters accordingly to Hertzian stress field calculations. Film weight loss was undetectable until delamination took place. When tested with 150 μm SiC particles, the CF₄ plasma pre-treated substrates yield a three-fold increase (15 min) in delamination time comparing to untreated specimens, while samples coated under PW-50 Hz conditions presented a six times lower erosion rate compared to CW ones. It is believed that the improved nucleation behaviour by the use of PW mode and its higher homogeneity on the CF₄ plasma pre-treated samples decrease the flaw population on the diamond/substrate interface, leading to improved adhesion levels.     

Surface properties of polymers treated with tetrafluoromethane plasma

Polymer films of poly(ethylene terephthalate), polypropylene, and cellophane were surface treated with tetrafluoromethane plasma under different time, power, and pressure conditions. Contact angles for water and methylene iodide and surface energy were analyzed with a dynamic contact angle analyzer. The stability of the treated surfaces was investigated by washing them with water or acetone, followed by contact angle measurements. The plasma treatments decreased the surface energies to 2–20 mJ/m² and consequently enhanced the hydrophobicity and oleophobicity of the materials. The treated surfaces were only moderately affected after washing with water and acetone, indicating stable surface treatments. The chemical composition of the material surfaces was analyzed with X-ray photoelectron spectroscopy (XPS) and revealed the incorporation of about 35–60 atomic % fluorine atoms in the surfaces after the treatments. The relative chemical composition of the C ls spectra's showed the incorporation of —CHF— groups and highly nonpolar —CF₂— and —CF₃ groups in the surfaces and also —CH₂—CF₂— groups in the surface of polypropylene. The hydrophobicity and oleophobicity improved with increased content of nonpolar —CF₂—, —CF₃, and —CH₂—CF₂— groups in the surfaces. For polyester and polypropylene, all major changes in chemical composition, advancing contact angle, and surface energy are attained after plasma treatment for one minute, while longer treatment time is required for cellophane. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1591–1601, 1997     

In situ study of the initial stages of diamond deposition on 3C–SiC (100) surfaces: Towards the mechanisms of diamond nucleation

The mechanisms involved in the diamond nucleation on 3C–SiC surfaces have been investigated using a sequential in situ approach using electron spectroscopies (XPS, XAES and ELS). Moreover, diamond crystals have been studied by HRSEM. The in situ nucleation treatment allows a high diamond nucleation density close to 4 × 10¹⁰ cm^− 2. During the in situ enhanced nucleation treatment under plasma, a negative bias was applied to the sample. The formation of an amorphous carbon phase and the roughening of the 3C–SiC surface have been observed. The part of these competing mechanisms in diamond nucleation is discussed.     

Effects of CF4 plasma on the field emission properties of aligned multi-wall carbon nanotube films

We present a simple method to functionalize the surface and to modify the structures of aligned multi-wall carbon nanotube (CNT) arrays grown on silicon substrates using CF₄ plasma produced by reactive ion etching (RIE). Field emission (FE) measurements showed that after 2 min of plasma treatment, the emission currents were enhanced compared with as-grown CNTs; however, extended treatment over 2 min was found to degrade the FE properties of the film. Scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy have been employed to investigate the mechanism behind the modified FE properties of the CNT film. The FE enhancement after 2 min of etching could be attributed to favorable surface morphologies, open-ended structures and a large number of defects in the aligned CNT films. On the other hand, deposition of an amorphous layer comprising carbon and fluorine during extended CF₄ plasma treatment may hamper the field emission of CNT films.     

Wettability modification of zirconia by laser surface texturing and silanization

This paper presents the modification of wettability by nanosecond laser surface textured followed by silanization to fabricate the superhydrophobic zirconia surface. Surface modification by varying the pitch between channels leads to micro-channel and micro-grid pattern with different surface roughness. The generated morphological and metallurgical modifications of the surface are measured by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Numerous micro-pits and cracks in the laser-treated areas can be observed from SEM, which indicates crack propagation dominating the process of laser ablation of zirconia. The surface is superhydrophilic with laser-texturing instantly, whose wettability is modified over time. By analyzing the XPS, carbon content, especially C-C (H) groups, is important for the time-dependent wettability. The hydrophobicity of all laser-textured surfaces is improved after silanization. Laser texturing with smaller pitch (50 μm and 70 μm) leads to superhydrophobic surfaces after silanization, which may be due to the modification of physicochemical properties of substrate by very rapid local heating and cooling on the thick surface layer. Overall, the investigations indicate that wettability modifications can be attributed to the surface's microstructure, which depend on laser processing parameters, and chemical composition, especially in terms of −CF₃, −CF₂, and C-C (H).     

X-ray photoelectron spectroscopy study of Schottky junctions based on oxygen-/fluorine-terminated (100) diamond

In this study, investigation of Schottky junctions based on oxygen-/fluorine-terminated (100) diamond (O-/F-diamond) film has been carried out. Both of the O-/F-diamond surfaces have been formed on different areas of one (100) diamond sample by O₂ and CF₄ plasma. Metals of Au, Pd, and Cu have been evaporated on the diamond surfaces to form Schottky junctions, whose barrier heights on O-/F-diamond have been investigated by X-ray photoelectron spectroscopy technique, the results of which indicate that the barrier heights of the metals on O-diamond are about 1.70 eV, and those on F-diamond are about 2.30 eV, respectively.     

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.     

Effect of diamond surface orientation on the thermal boundary conductance between diamond and aluminum

[100] and [111] oriented diamond substrates were treated using Ar:H and Ar:O plasma treatments, and 1:1 HNO₃:H₂SO₄ heated at 200 °C. Subsequent to these treatments, an aluminum layer was either evaporated or sputteredon the substrates. The thermal boundary conductance (TBC) as well as the interfacial acoustical reflection coefficient between this layer and the diamond substrate was then measured using a Time Domain ThermoReflectance (TDTR) experiment. For the Ar:H plasma treated surfaces the [111] oriented faces exhibited conductances 40% lower than the [100] oriented ones with the lowest measured TBC at 32 ± 5 MWm^− 2 K^− 1. The treatments that led to oxygen-terminated diamond surfaces (extiti.e. acid or Ar:O plasma treatments) showed no TBC anisotropy and the highest measured value was 230 ± 25 MWm^− 2 K^− 1 for samples treated with Ar:O plasma with a sputtered Al layer on top. Sputtered layers on oxygen-terminated surfaces showed systematically higher TBC than their evaporated counterparts. The interfacial acoustic reflection coefficient correlated qualitatively with TBC when comparing samples with the same type of surface terminations (O or H) but this correlation failed when comparing H and O terminated interfaces with each other.