Microfabrication issues in constructing freestanding membranes of near-frictionless carbon and diamond-like films

The near-frictionless carbon (NFC) film developed at Argonne National Laboratory has excellent mechanical and tribological properties, such as the super low friction coefficient of 0.001 and the wear rates of 10^− 11–10^− 10 mm³/N m. In this study, microfabrication procedures are developed to fashion the NFC thin films into freestanding structures which are necessary for many MEMS sensor devices. The entire microfabrication process is outlined in detail for use by other researchers. The NFC membranes were characterized with optical, scanning electron, and atomic force microscopy and Raman spectroscopy both before and after the fabrication process to observe any structural changes. Several surface and bulk micromachining issues, such as reactive ion notching effect and NFC film releasing, were studied and mitigated.     

Lubrication performance of diamond-like carbon and fluorinated diamond-like carbon coatings for intravascular guidewires

Diamond-like carbon (DLC) and fluorinated DLC (F-DLC) films were deposited on SUS316L guidewires using radio frequency (RF) plasma enhanced chemical vapor deposition (CVD), and the lubrication performance of DLC- or F-DLC-coated guidewires was then evaluated under in vitro conditions using a novel friction simulator developed for this study. Scanning electron microscopy (SEM) demonstrated that DLC or F-DLC film completely coated the specimens (SUS316L guidewires) and that polishing scars were substantially reduced. In the torturous vessel model, DLC- or F-DLC-coated guidewires exhibited significantly improved lubrication performance (by approximately 30% over that of uncoated wires). DLC and F-DLC films are thus promising candidates for lubricious coating of intravascular guidewires.     

A diamond-like carbon film as a self-alignment layer for LCDs

Diamond-like carbon (DLC) films without H deposited with a DC magnetron in-line sputtering system have shown sufficient self-alignment properties towards liquid crystals (LC). The DLC film was successfully used as an alignment layer for LC without any alignment processes such as rubbing or atomic beam bombardment or UV irradiation. From the observations of the test cells, the LC director was aligning parallel to the substrate movement direction of the in-line sputtering system. The alignment property of the DLC films has been demonstrated by a contrast ratio value of close to 200. It appears that DLC film may have anisotropic structure that is interacting with LC to align.     

Annealing effects on the mechanical properties of near-frictionless carbon thin films

In this work, the near-frictionless carbon (NFC) thin films developed at Argonne National Laboratory were annealed at 100 °C, 150 °C, 200 °C, 400 °C, and 600 °C in nitrogen atmosphere. The changes of the NFC mechanical properties were measured with both static and dynamic nanoindentation methods. It was found that the Young's modulus and hardness decreased with increasing annealing temperatures. Raman spectroscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM) were used to characterize the film's structural change before nanoindentation testing. Raman characterization indicated that the G peak shifted upwards as the annealing temperature was increased above 150 °C, which indicated decreasing sp³ content. The intensity of the D peak was shown to increase with annealing temperature indicating that the NFC film became more graphite-like. AFM analysis showed an increase of sp² clustering with annealing temperature, which resulted in an increase in surface roughness. SEM characterization indicated that as the films were annealed large cracks and numerous pinholes were generated. The characterization results were in good agreement with the measured mechanical properties.     

Photoconductivity and recombination in diamond-like carbon

The steady-state photoconductivity of tetrahedral amorphous carbon (ta-C) and hydrogenated ta-C (ta-C:H) has been studied as a function of temperature, light intensity, and photon energy, in order to understand the transport and recombination process in diamond-like carbon. It is found that the levels demarking the recombination states can span only part of the gap, so that the recombination centres can vary from every defect, to some defects, to some tail states, according to conditions.     

A superhard diamond-like carbon film

A superhard hydrogen-free amorphous diamond-like carbon (DLC) film was deposited by pulsed arc discharge using a carbon source accelerator in a vacuum of 2×10⁻⁴ Pa. The growth rate was about 15 nm/min and the optimum ion-plasma energy was about 70 eV. The impact of doping elements (Cu, Zr, Ti, Al, F(Cl), N) on the characteristics of DLC films deposited on metal and silicon substrates was studied aiming at the choice of the optimum coating for low friction couples. The microhardness of thick (≥20 μm) DLC films was studied by Knoop and Vickers indentations, medium thick DLC films (1–3 μm) were investigated using a ‘Fischerscope’, and Young's module of thin films (20–70 nm) was studied by laser induced surface acoustic waves. The bonds in DLC films were investigated by electron energy loss spectroscopy (EELS), X-ray excited Auger electron spectroscopy (XAES), and X-ray photoelectron spectroscopy (XPS). The adhesion of DLC films was defined by the scratch test and Rockwell indentation. The coefficient of friction of the Patinor DLC film was measured by a rubbing cylinders test and by a pin-on-disk test in laboratory air at about 20% humidity and room temperature. The microhardness of the Patinor DLC film was up to 100 GPa and the density of the film was 3.43–3.65 g/cm³. The specific wear rate of the Patinor DLC film is comparable to that of other carbon films.     

Stresses in diamond-like carbon films

Internal stresses have been measured in diamond-like carbon (DLC) films deposited by d.c. plasma assisted chemical vapor deposition from methane, acetylene, or cyclohexane, and in nitrogen containing DLC films deposited from acetylene, or cyclohexane and nitrogen. The total hydrogen content in the films and the fraction of bound hydrogen have been analyzed by forward recoil elastic scattering and Fourier transform infrared spectroscopy respectively. It was found that in pure DLC films the stresses increase with increasing fraction of unbound hydrogen. The highest compressive stresses were obtained in the films deposited from methane and the lowest stresses in films deposited from cyclohexane. In the nitrogen containing DLC films the stresses decrease with increasing nitrogen content in the films. Stresses as low as 0.22 GPa were obtained in the films deposited from cyclohexane and nitrogen at a ratio of 1/15 in the plasma.     

Growth and characteristics of metal-containing diamond-like carbon using a self-assembled process

A recently developed self-assembled method, involving the use of a single metal target and a fixed substrate in a sputter deposition chamber, was used for the growth of various metal-containing diamond-like carbon thin films. The metals used include, Cu, Pt, and Ni. The resulting films consist of self-assembled alternating dark and bright layers, both of which constitute a period. The dark layer was found to always have a higher metal concentration than the bright layer in a period. Effects of the growth condition on the growth rate, periodicity, and film structure were examined. The relation between the growth condition and the film characteristics is discussed.     

Structural characterization of diamond-like carbon films for ultracold neutron applications

We have produced hydrogen-free diamond-like carbon (DLC) films by vacuum arc deposition for use as wall coating material in ultracold neutron (UCN) applications. The sp³ fraction, the main quality factor for DLC used in UCN applications, was varied from 0.4 to 0.9, the coating thickness between 10 nm and 120 nm. The samples were characterized by using X-ray Absorption Near-Edge Spectroscopy (XANES), X-ray induced Photoelectron Spectroscopy (XPS), Laser induced surface Acoustic Waves (LAwave), cold neutron reflectometry and Raman spectroscopy at visible excitation wavelength. We observe reasonable agreement between the different results for film thicknesses below 20 nm. For larger thickness, we find that the surface-sensitive methods XPS and XANES yield smaller sp³ fractions (by up to 20%) than the bulk-sensitive LAwave, being consistent with the assumption of a lower-density surface layer on a nominal-density bulk layer.     

Study on the diamond-like carbon multilayer films for tribological application

In this paper, DLC multilayer films consisting of alternating layers of soft and hard carbon films were deposited on Si wafer by a plasma CVD deposition system. Different DLC multilayer films were prepared by varying the sub-layer thickness (from 1000 to 25 nm) and the ratio of hard to soft sub-layer (H/S) thickness (from 1:1 to 4:1). By using a ball-on-disk tribo-tester, the friction and wear properties of the DLC multilayer films were measured in vacuum, O₂ and dry-air environments respectively. By comparing with single-layer DLC film, the change of the multilayer structure has little influence on friction coefficient of the multilayer films. However, the wear rate of the DLC multilayer films is restricted effectively by constructed the multilayer structure in the film. The wear rate of the multilayer films is lower than that of the single film in reactive (O₂ and dry-air) environments. An DLC multilayer film with excellent wear resistance, approximately in the level of 10⁻⁸ mm³/Nm in different environments (dry-air, O₂ and vacuum), is obtained as the DLC multilayer film at a certain sub-layer thickness and ratio.     

Flexible diamond-like carbon film coated on rubber

Dynamic rubber seals are major sources of friction of lubrication systems and bearings, which may take up to 70% of the total friction. The solution we present is to coat rubbers with diamond-like carbon (DLC) thin films by which the coefficient of friction is reduced to less than one tenth. Coating rubber is very challenging because the film must be flexible and strongly adhered to the surface of rubber substrate. Our novel approach is depositing flexible DLC films on rubbers via self-segmentation. By making use of the substantial thermal mismatch between DLC film and rubber substrates a dense crack network forms in DLC films and contributes to flexibility. The size of film micro-segments can be tuned by varying the bias voltage of pulsed-DC plasma CVD, which governs the amplitude of the substrate temperature variation during deposition. The formation mechanism of crack network and its effect on the flexibility and friction of DLC film coated rubbers are scrutinized. This paper provides generic design rules for the deposition of flexible and ultra-low friction DLC films on rubber seals.     

Iodine doping of amorphous diamond-like carbon films

Amorphous diamond-like carbon (a:DLC) films have been doped by incorporation of iodine during the films deposition. XPS and AES analysis shows the existence of iodine atoms with constant concentration of 0.9% along the iodine doped DLC film (a:I-DLC). The optical and electronic properties of the doped films were studied. Optical measurements in the visible light show that iodine affects the interband absorption of the a:DLC films. Iodine causes decreasing of the optical energy gap, from 1.07 to 0.78 eV and affects the density of states at the conducting band. Like the optical measurements, electrical measurements show that iodine also decreases the activation energy of the films from 0.34 to 0.22 eV. This shows that although both gaps decrease, the optical energy gap remains different from that of electrical gap, also after doping.     

Thermal gas effusion from diamond-like carbon films

Good-quality diamond-like carbon films (6 at.% H₂, 2400 kgf/mm² microhardness, 2.7 eV bandgap, higly insulating) have been obtained by the DC glow discharge decomposition of acetylene. Mass spectroscopic thermal effusion measurements were carried out on the films deposited under different deposition conditions. Analyses of hydrogen in conjunction with hydrocarbon effusing species yield information on the microstructure and nature of C---H bonding configurations. It is shown to be a useful analytical tool to study hydrogenated amorphous carbon films of different microstructures varying from polymer-like to diamond-like.     

Modeling of optical constants of diamond-like carbon

A new model of optical constants of diamond-like carbon parameterizing joint density of states (PJDOS) is introduced and compared with a previously published model based on the parameterization of density of states (PDOS). Both the models employ only six parameters: three parameters for π → π^⁎ and three parameters for the σ → σ^⁎ transitions. These parameters have a clear physical meaning. One describes the minimum energy needed for the band transition, i.e. band gap, the second is the maximum energy of transitions and the third is proportional to the total density of the corresponding electronic states. An inherent disadvantage of the PDOS model is in the necessity of a numerical calculation. On the other hand the PJDOS model is analytical and, therefore, it can be implemented in a simple manner. The efficiency of both the models for the study of DLC structure is demonstrated in several examples.     

Fast deposition of diamond-like hydrogenated carbon films

Diamond-like hydrogenated carbon films have been formed at low temperatures using methane and acetylene as precursor gases. The source used was of a cascaded arc type employing Ar and Ar/H₂ as carrier gases. Energies of ion species and ion densities in the plasma were measured with a mass energy probe and a Langmuir probe.The films produced were characterized in terms of sp³ content, refractive index, relative hydrogen content, hardness and adhesion. The variation of these parameters is presented as functions of precursor gas flow, process pressure, and surface temperature.Deposition rates up to 30 nm/s have been achieved using acetylene as precursor gas at substrate temperatures below 100 °C. Experiments with acetylene showed deposition rates seven times greater than with methane. The typical sp³ content of 55–78% in the films was determined by X-ray-Excited Auger Electron Spectroscopy (XAES) technique. The hardness and reduced modulus were determined by nanoindentation. Preliminary Atomic Force Microscopy (AFM) studies of the films showed a roughness below 3 nm (R_a).     

In situ TEM studies of tribo-induced bonding modifications in near-frictionless carbon films

Direct in situ TEM evidence is presented for a mechanically-induced increase in sp² bond content in the tribolayers formed on near-frictionless carbon (NFC) films. An in situ TEM nanomanipulation holder is used to perform sliding experiments between a nano-sized tungsten probe and electron transparent NFC samples. Electron energy loss spectra (EELS) were acquired between sliding events to show an incremental increase in the 1s-π peak, suggesting that a graphitization-like process occurs which can lead to a change in the tribological performance of a variety of hard carbon films. Presently, this behavior is only observed for one of the two varieties of NFC, while supporting TEM imaging evidence is in qualitative agreement with macroscopic friction and wear behavior.     

Bacterial adhesion on silicon-doped diamond-like carbon films

In this paper the surface properties of silicon-doped diamond-like carbon films with various Si contents on 316 stainless steel substrate by a magnetron sputtering technique were investigated. X-ray photoelectron spectroscopy was applied to determine the surface chemical composition of the films. Atomic force microscopy was used for the determination of surface roughness and topography. The sp² contents in the films were determined with Auger electron spectroscopy, which were 67.1%, 34.2% and 25.0% for silicon contents 1%, 2% and 3.8%. The sp³/sp² ratio increases with increasing the silicon contents in the films. Contact angles of three test liquids on the films were obtained with a Dataphysics OCA-20 contact angle analyzer. Surface free energies of the films and their dispersive and polar components were calculated using van Oss acid–base approach. Staphylococcus aureus was used for bacterial adhesion test. The experimental results showed that bacterial adhesion decreased with increasing the silicon content or with increasing sp³/sp² ratio in the films.     

Heat resistance of fluorinated diamond-like carbon films

The heat resistance of fluorinated diamond-like carbon (F-DLC) films produced by Plasma Immersion Ion Processing (PIIP) technique was investigated by annealing F-DLC coatings in a vacuum furnace. The growth rate for the F-DLC films was approximately 0.6 μm/h. In order to see the possible change in the composition and properties of the F-DLC films, Rutherford Backscattering Spectrometry (RBS), nanoindentation and contact angle measurements were performed before and after the heat treatments. The results show that the composition and properties of the F-DLC films were unchanged up to heat treatment at 300°C for up to 30 min. Blistering and film delamination occurred for samples treated at 400°C.     

Atmospheric plasma deposition of diamond-like carbon coatings

The atmospheric pressure plasma-enhanced chemical vapor deposition of diamond-like carbon (DLC) has been investigated. The DLC coatings were grown with a mixture of acetylene, hydrogen and helium that was fed through a linear plasma source. The plasma was driven with radio frequency power at 27.12 MHz. Deposition rates exceeded 0.10 µm/min at substrate temperatures between 155 and 200 °C. Solid-state carbon-13 nuclear magnetic resonance revealed that the coatings contained approximately 43% sp²-bonded carbon and 57% sp³-bonded carbon. Coefficient of friction values for the coatings were found to be 0.24 ± 0.02, which is within the range observed for vacuum deposited DLC.     

Synthesis and mechanical properties of carbon nanotube/diamond-like carbon composite films

Diamond-like carbon (DLC) coatings were successfully deposited on carbon nanotube (CNT) films with CNT densities of 1 × 10⁹/cm², 3 × 10⁹/cm², and 7 × 10⁹/cm² by a radio frequency plasma-enhanced chemical vapor deposition (CVD). The new composite films consisting of CNT/DLC were synthesized to improve the mechanical properties of DLC coatings especially for toughness. To compare those of the CNT/DLC composite films, the deposition of a DLC coating on a silicon oxide substrate was also carried out. A dynamic ultra micro hardness tester and a ball-on-disk type friction tester were used to investigate the mechanical properties of the CNT/DLC composite films. A scanning electron microscopic (SEM) image of the indentation region of the CNT/DLC composite film showed a triangle shape of the indenter, however, chippings of the DLC coating were observed in the indentation region. This result suggests the improvement of the toughness of the CNT/DLC composite films. The elastic modulus and dynamic hardness of the CNT/DLC composite films decreased linearly with the increase of their CNT density. Friction coefficients of all the CNT/DLC composite films were close to that of the DLC coating.