Carbon-rich Film Research Articles

Friction-induced reconstruction of sliding interface and low friction mechanism of WC/a-C films

Tungsten carbide reinforced amorphous carbon (WC/a-C) films exhibit significantly lower coefficient of friction (COF) than typical a-C films in the atmospheric environment; thus, they are promising candidates for use as solid lubricants. However, the low friction mechanism of WC/a-C films remains poorly understood. Here, we revealed that the friction-induced formation of WO3-rich transfer film changed the real sliding interface between the initial friction pair materials into an interface between a WO3-rich transfer film and carbon-rich worn film surface (WO3/C interface). First-principles calculations indicated that the WO3/C interface showed weaker interactions than the C/C interface of the a-C film under the same conditions, particularly when the dangling carbon bonds at the sliding interface were not completely passivated. This resulted in a reduced COF of the WO3/C interface. Thus, friction-induced reconstruction of the sliding interface to an intrinsically weak-interacting interface is proposed as a new path to further improve the tribological performance of a-C-based films.

Conformal and Stoichiometric Chemical Vapor Deposition of Silicon Carbide onto Ultradeep Heterogeneous Micropores by Controlling the Initial Nucleation Stage.

Conformal chemical vapor deposition (CVD) of silicon carbide (SiC) from methyltrichlorosilane (MTS) and hydrogen (H2) onto high-aspect-ratio (HAR; typically >100:1) three-dimensional features has been a challenge in the fabrication of ceramic matrix composites. In this study, the impact of heterogeneous underlayers on the initial nucleation of SiC-CVD was studied using HAR (1000:1) microchannels with a tailored wetting underlayer of Si(100) and dewetting underlayers of thermally formed amorphous silicon dioxide (a-SiO2) and turbostratic boron nitride (t-BN). Incubation periods were distributed in the microchannels on a-SiO2 and t-BN underlayers, with the longest period of 70 min found at the feature-bottom due to a decreased concentration (C) of film-forming species. The longer incubation periods with more dewetting underlayers arose due to demoted initial nucleation. Prolonged incubation at the feature bottom led to poor conformality because thick films had already formed at the inlet when film formation began at the feature bottom. The incubation periods were eliminated by increasing the supply of MTS/H2, in accordance with classical heterogeneous nucleation theory. In the meantime, carbon-rich SiC films formed in the vicinity of dewetting a-SiO2 and t-BN underlayers at the feature bottoms, with greater carbon segregation on more dewetting underlayers. This was probably due to the deposition of pyrocarbons (CH4, C2H2, and/or C2H4) generated from MTS/H2 in the gas phase. Decreasing the temperature (T) from 1000 to 900 °C prevented carbon-rich film formation, and the expected deposition rate of pyrocarbon decreased to 0.6% for the case of CH4. A higher C of MTS/H2 combined with a lower T enabled conformal and stoichiometric film formation on the heterogeneous HAR features.

Tribological performance of a novel wide-temperature applicable a-C/(WC/a-C) film against M50 steel

To improve the wide-temperature applicability of amorphous carbon-based films, a novel a-C/(WC/a-C) film compositing of a-C nano-layers and super-latticed WC/a-C nano-multilayers was proposed and successfully fabricated on M50NiL substrate by an unbalanced magnetron sputtering system. For comparison, a monolayer a-C film and a super-latticed WC/a-C film were also prepared on the same substrates. The microstructures, mechanical properties and thermal stability of the as-deposited films were investigated. Particularly, the tribological behaviors of films against M50 counterpart were evaluated at the temperatures ranging from 25 °C to 350 °C. The results showed that because of the excellent thermal stability of super-latticed WC/a-C sublayers, the a-C/(WC/a-C) film has effectively overcome the poor tribological performance of a-C film at high temperatures (≥200 °C). While benefiting from the good toughness of a-C sublayers, the a-C/(WC/a-C) film showed significant better wear resistance than the WC/a-C film at the temperatures of 25–200 °C. The best wide-temperature applicability of the novel a-C/(WC/a-C) film should be attributed to the favorable mechanical properties, together with the formations of carbon-rich transfer film at 25–150 °C and WO3-rich transfer film at 200–350 °C.

Corrosion of One-Step Superhydrophobic Stainless-Steel Thermal Spray Coatings.

As most superhydrophobic coatings are made of soft materials, the need for harder, more robust films is evident in applications where erosional degradation is of concern. The work herein describes a methodology to produce superhydrophobic stainless-steel thermal spray coatings using the high-velocity oxygen fuel technique. Due to the use of a kerosene fuel source, a carbon-rich film is formed on the surface of the thermal spray coatings, lowering the surface energy of the high-energy metallic substrates. The thermal spray process generates a hierarchical micro-/sub-micro-structure that is needed to sustain superhydrophobicity. The effect of spray parameters such as particle velocity and temperature on the coating's hydrophobicity state was explored, and a high particle velocity was shown to cause superhydrophobic characteristics. The coatings were characterized using scanning electron microscopy, profilometry, X-ray photoelectron spectroscopy, static water contact angle measurements, water droplet roll-off measurements, and water droplet bouncing tests. The corrosion behavior of the coatings was studied using potentiodynamic polarization measurements in order to correlate water repellency with corrosion resistance; however, all coatings demonstrated active corrosion without passivation. This study describes an interesting phenomenon where superhydrophobicity does not guarantee corrosion resistance and discusses alternative applications for such materials.

Lubricating behavior of Submicrometer carbon spheres as lubricant additives

The purpose of this paper is to evaluate carbon spheres as a green lubricant additive in 5W30 engine oil to reduce friction and wear. Carbon spheres with a diameter of ∼400 nm are synthesized from glucose by using a hydrothermal process. Lubrication is studied using a ball-on-disk tribometer by measuring the friction coefficient and wear in the boundary lubrication regimes. The worn surfaces were analyzed by Raman spectroscopy. Carbon spheres are found to be stably dispersed at 0.5 wt% concentration in base oil. The experiments revealed that carbon spheres are effective in reducing both friction and wear when lubricating by using base oil. Raman results show that the enhanced tribological performance is owing to a carbon-rich boundary film, the formation of which is attributed to the deposit of carbon particles during the sliding process.

Influence of Composition of SiCN as Interfacial Layer on Plasma Activated Direct Bonding

Surface activated bonding is more and more attractive as a key technology to realize higher performance CMOS devices independent of scaling. The major challenge of dielectric bonding is to decrease the process temperature in order to be compatible with CMOS processing. In the past, we demonstrated low temperature bonding using SiCN as interfacial dielectric layer, where we have obtained a bond energy above 2.2 J/m2 with a post bond annealing process of 250°C. In this work, the composition of SiCN was varied aiming at the identification of the key elements taking part in the bonding mechanism. The film density, roughness, CMP outcome, water contact angle and impact of plasma activation have been investigated on three different compositions of SiCN. Bond energy above 2.5 J/m2 is obtained for the carbon rich SiCN film.

Tribological characteristics of self-lubricating nanostructured aluminum reinforced with multi-wall CNTs processed by flake powder metallurgy and hot pressing method

Ultrafine-grained Al-CNT (2 and 4 vol%) composites were successfully fabricated using the flake powder metallurgy and hot pressing route, and the microstructure and tribological properties of the produced composites were studied. The results showed that the coefficient of friction and the wear rate of Al decrease with the addition of the CNT reinforcement. A carbon-rich film was formed on the worn surfaces during wear test, which prevented the Al oxidation and yielded the self-lubricating effect for the composites. This improvement in the wear behavior of the composites can be attributed to the simultaneous effects of the ultrafine-grained matrix and strengthening and self-lubricating properties of the uniformly dispersed and undamaged CNT in the flake powder metallurgy method.

Selective Fabrication of SiC/Si Heterojunction Diodes by Local Laser Annealing Process for Microwave Circuit Applications

This paper presents the RF characterization of SiC/Si heterojunction diodes fabricated using a novel localized laser process. Selective growth of SiC on Si is carried out by utilizing excimer laser-based annealing in the presence of carbon source. Use of ambient conditions (atmospheric pressure and room temperature) during growth makes this process simple and low cost. The Si wafer is first coated with carbon rich film and then locally annealed using high power laser which causes dissociation of solid carbon sources and a simultaneous melting of the silicon leading to the growth of SiC. The Raman spectrum shows peaks for acoustical and optical phonon modes for ${\beta }$ -SiC between 940 and 980 cm $^{-1}$ . The fabricated diodes show high breakdown voltage (>200 V) and low leakage current density of 3- $5~\mu \text{A}$ /cm2 (−5 V). Experimental results for the dc characteristics, microwave rectification and frequency multiplication are presented. The measured results show good RF rectification (e.g., 21 V/W at 3 GHz) up to 6 GHz using a large diode ( $70\times 70\mu \text{m}^{2}$ ). The diodes also work well as frequency doublers over a wide frequency range of 2–6 GHz. The diode area can be scaled down to design circuits operating at higher frequencies.

Atomic layer etching of silicon dioxide using alternating C4F8 and energetic Ar+ plasma beams

Atomic layer etching (ALE) of SiO2 was studied by alternating exposure of a 5 nm-thick SiO2 film on Si substrate to (1) a plasma beam emanating from a c-C4F8 inductively coupled plasma (ICP), to grow a fluorocarbon (FC) film composed mainly of CF2, and (2) an energetic (130 eV) Ar+ ion beam extracted from a separate Ar ICP. In situ x-ray photoelectron spectroscopy was used to analyze the chemical composition of the near-surface region, and to quantify the thickness of the FC and SiO2 films. A very thin (3–6 Å), near self-limiting thickness CF2-rich FC film was found to deposit on the SiO2 surface with exposure to continuous or pulsed power C4F8 plasma beams, under conditions that generated a large relative flux of CF2. Following this, a FC film of similar composition grew at ~10 times slower rate. Exposure of the thin film to the Ar+ beam led to removal of 1.9 Å SiO2. An estimated yield of 1.3 SiO2 molecules-per-Ar+ was found for a single ALE step. The rate of 1.9 Å/cycle persisted over multiple ALE cycles, but a carbon-rich residual film did build up. This film can be removed by a brief exposure to an O2-containing plasma beam.

(Invited) Advanced Plasma Etching Processing: Atomic Layer Etching for Nanoscale Devices

Plasma processes for deposition and etching are required for development of the next generation semiconductor devices. To achieve high performance of nanoscale devices, the continuous development of fabrication processes is necessary. An atomic layer etching (ALE) process achieves this continuous development, since the process has advantages such as more precise, higher controllability and repeatability. We have developed ALE process for SiO2 to achieve high controllability and repeatability. The ALE process is a cyclic process composed of two steps: a first step which deposits a fluorocarbon film on the SiO2 surface using an Ar/C4F8 plasma, followed by an O2 plasma etching step. The O2 plasma etching suppresses forming a carbon-rich film on the target material surface and maintains the chamber conditions by removing the fluorocarbon on the chamber walls. The ALE process for SiO2 exhibits high reproducibility and has the potential to allow uniform EPC values over large wafer surfaces.

(Invited) Advanced Plasma Etching Processing: Atomic Layer Etching for Nanoscale Devices

Plasma processes for atomic scale deposition and etching are required for nanoscale devices such as Fin-field effect transistors (FETs), nanowires, 3D NAND-type flash memory units, and others 3D devices. To achieve high performance of such nanoscale devices, the continuous development of fabrication processes is necessary. The solution is usually based on a trial and error approach where some external parameters such as RF power, feed gas, gas mixture ratio, pressure, and electrode temperature have been optimized to obtain desired etching characteristics. This spends long development time with high cost. Moreover, it is not a robust process because internal parameters such as electron and radical densities, ion energy, and wafer temperature, which define the etching performance directly, can change over time even if external parameters are kept constant.We will introduce history of atomic layer etching (ALE) and conventional plasma etching. ALE achieves this continuous improvement, since the process has advantages such as more precise, higher controllability and repeatability compared to conventional plasma etching. The ALE concept starts with adsorption of an etchant gas or deposition of an etchant film on a target material, followed by a removal step to react between the etchant species and the target material. The reaction is promoted by using accelerated ions in plasma. An ALE process for Si by alternating subnanometer-scale fluorocarbon film deposition and Ar+ ion etching in two different chambers, and developed self-limiting, layer-by-layer etching by controlling the thickness of the fluorocarbon film deposited on the surface Si.Fluorine atoms in the fluorocarbon film are known to react with Si atoms in the SiO2 during Ar+ ion etching, while carbon atoms will react with oxygen atoms in the SiO2 to generate gas phase COx molecules. The unreacted carbon atoms will form a carbon-rich film on the SiO2 surface, and the thickness of this film increases with the number of cycles, eventually disturbing the etching reaction between the fluorocarbon and the SiO2.The carbon-rich film formed on the SiO2 surface and the residual fluorocarbon film on the chamber walls result in difficulty in controlling the process and affect the repeatability of the etch rate per cycle (EPC) during the ALE. In industrial applications, moreover, the ALE process is required to perform both deposition and etching process in the same chamber. Therefore, we must consider the effects of fluorocarbons deposited on the chamber walls.We will show our suggested ALE process for SiO2 to achieve high controllability and repeatability. The ALE process is a cyclic process composed of two steps: a deposition step which forms a fluorocarbon film using an Ar/C4F8 plasma, followed by an O2 plasma etching step that performs the reaction between the fluorocarbon and the SiO2. Simultaneously, the unreacted carbon atom is evaporated as COx molecules. Therefore, O2 plasma suppresses forming a carbon-rich film on the target material surface and maintains the chamber wall conditions by removing the fluorocarbon on the chamber walls.We demonstrated that novel ALE process for SiO2 exhibits high reproducibility and has the potential to allow uniform EPC values over large wafer surfaces. The surface chemistry and etched thickness of SiO2 during this ALE process were investigated.

Impact of structured catalysts in amine oxidation under mild conditions

A structured graphene/graphite catalyst grown on a commercial austenitic stainless steel sheet providing a micromonolith was obtained by submitting the nude stainless steel structure to a carbon-rich atmosphere (first 300mL/min of a reductive H2/N2 (1:1) flow, then to 180mL/min of a CH4/H2 (1:5)) at high temperature (900°C) for 2h. The preparation procedure resulted in a homogenous surface coated with a carbon-rich film as observed by EDX and SEM images. Further characterizations by Raman spectroscopy revealed characteristic Raman lines of graphene and crystalline graphite disposed in a hierarchical organization. The disposal of the obtained surface layers was also confirmed by grazing incidence X-ray diffraction. Besides this, XRD indicated the overlapping diffraction lines of graphite, cementite and M7C3 carbides. The graphene nature of the outermost layer was also confirmed by XPS. The catalytic behavior of the structured graphene/graphite catalyst was evaluated in the selective oxidation of heptylamine. At 200°C it afforded a total conversion with a combined selectivity in heptanonitrile and N-heptylidene-heptylamine of 67% (10% heptanonitrile) that corresponds indeed to a very efficient system in the absence of any metal. Kinetic experiments with the scope to calculate the activation energies were also performed.

Low-Temperature Performance Improvement of Graphite Electrode by Allyl Sulfide Additive and Its Film-Forming Mechanism

This work describes the beneficial effects given by the allyl sulfide (AS)-derived surface film on the low-temperature performances of graphite electrode and the film-forming mechanism. Adding a small amount of allyl sulfide as an electrolyte additive into a Li/graphite cell increases the reversible capacity of graphite electrode to three times larger than that of the AS-free cell at −30°C. Lithium plating is also suppressed by adding AS into the background electrolyte. An impedance analysis reveals that the charge transfer resistance is significantly lower in the AS-added cell at low-temperatures. When the graphite electrode is soaked in the AS-added electrolyte, allyl sulfide is spontaneously oxidized to produce the sulfur-containing surface film. The as-generated film is then electrochemically reduced during the first lithiaiton period to produce another type of the sulfur-containing film. After repeated cycling, a carbon-rich sulfur-containing film is generated near the graphite surface, while the decomposition products of background electrolyte are deposited in the outer surface region. The presence of the carbon-rich sulfur-containing film near the graphite surface seems to be responsible for the facilitation of charge transfer reaction at low temperatures.

Synthesis and Tribology of Micro-Carbon Sphere Additives for Enhanced Lubrication

Poor or inefficient lubrication often gives rise to high friction and wear losses in machine components, which adversely affect their performance, efficiency, and durability. Many approaches are being explored to enhance the antifriction and antiwear properties of sliding machine components. In this study, the antifriction and antiwear properties of carbon spheres, synthesized from plastic waste by an autogenic process, were investigated as an additive to a poly-alpha-olefin (PAO-4 grade) oil. When dispersed at 1 wt% concentration, the carbon spheres reduced both friction and wear under boundary-lubricated sliding conditions. In particular, the reduction in wear was quite dramatic and appeared to be enabled by the formation of a fairly thick (≈200 nm) carbon-rich boundary film, the formation of which is attributed to tribochemical interactions between the carbon particles and sliding contact surfaces.

Carbon tribo-layer for super-low friction of amorphous carbon nitride coatings in inert gas environments

Super-low friction of amorphous carbon nitride (CNx) coating in inert gas environment has generally been attributed to the formation of a uniform and homogeneous carbon-rich transfer film (denoted as tribo-film) on the mating ball surface in the ball-on-disk tribo-system. However, the role of tribo-film in terms of its appearance (visibility and amount), composition and structure in the low friction mechanisms of CNx coatings is still not clearly understood. In this study, the key parameters of tribo-film for achieving low friction coefficients of CNx coatings in inert gas environments are systematically investigated by classifying the wear scars on the ball surfaces. The wear scars are classified into three modes according to the amount of tribo-film, such as no, partial, and almost full, accumulated inside of the contact area on the ball surface. As a result, it is confirmed that the amount as well as composition and structure of tribo-film on the wear scar of the ball surface can control the friction behavior of CNx coatings. Most importantly, a thin amorphous carbon layer (denoted as carbon tribo-layer) with a thickness of approximately 10nm has been found as sub-surface of the wear scar on the ball surface where tribo-film is not observed using naked eye as well as confocal microscopy. It is strongly argued that the formation of this carbon tribo-layer on the wear scar of the ball surface is enough for achieving stable and low friction coefficients (less than 0.05) of CNx coatings in inert gas environments. On the other hand, the tribo-film is not necessary and should be avoided from both viewpoints of low and stable friction.

Improvement of Uniformity and Reliability of Scaled-Down Cu Interconnects with Carbon-Rich Low-k Films

Highly selective dry-etching processes are developed for conventional via-first (VF) pattering sequences to fabricate reliable Cu dual-damascene interconnects (DDI) in carbon-rich low-k films, such as a molecular-pore-stack (MPS) SiOCH film (k = 2.55). The carbon-rich MPS film, which had excellent endurance against plasma-processes, acted as etching stopper during hard-mask (HM)-etching on it, and the high selectivity of trench-HM etching reduced variability of over-etching depth in the MPS film. This effect reduced variability in trench-depth in the MPS film, or interconnect characteristics such as capacitance–resistance (C–R) time delay. The via yield and reliability were influenced also by via-etch selectivity of MPS against SiCN cap underlain. We found that the SiCN thickness remained after the via etch should be greater than 10 nm to prevent Cu from oxidation by O2 ashing step followed. Chemical-reaction-enhanced gas chemistry in N2–CFX–Ar system, i.e., high N2/Ar ratio under limited CFX supply, increased the etching selectivity of MPS to keep enough thickness of SiCN. Early-failure-mode in electro-migration test was suppressed by the high selective via-etch. Precise selectivity control for robust carbon-rich low-k films was very important to achieve the low variability and high reliability of scaled-down Cu interconnects.

Contamination-Free TEM for High-Resolution Imaging of Soft Materials

Element-selective imaging and analysis at atomic resolution have become possible by the recent advancements in TEM and STEM. However, the spatial resolution in images of soft materials can be limited by electron beam damage and/or contamination. This contamination is a carbonaceous layer deposited on the specimen surface as a result of electron bombardment. Beam-induced specimen contamination is caused by polymerization of hydrocarbons that are present in a TEM specimen chamber. The electron beam reacts with stray hydrocarbons in the beam's path to create hydrocarbon ions, which then condense and form carbon-rich polymerized film on the area being irradiated. Figure 1a shows contamination spots created on a carbon thin foil by illuminating a beam with an intensity of 5.6 × 104 el/nm·s at an accelerating voltage of 200 kV. The thickness of the contamination spots can be estimated by electron energy-loss spectroscopy (EELS). With increase in the irradiation period, the intensity of the zero-loss peak decreases, but the overall intensities in the energy-loss regions of the spectrum increase (Figure 1b). The thickness (D) can be estimated using the equation, D = Λ·ln(It/I0), where Λ is the total mean free path for inelastic scattering, and It and I0 are the integral intensities of the overall spectrum and the zero-loss peak, respectively. Using this equation, the thickness of the contamination was found to be about 600 nm with a 10-minute irradiation.

Carbon in palladium catalysts: A metastable carbide

The catalytic activity of palladium toward selective hydrogenation of hydrocarbons depends on the partial pressure of hydrogen. It has been suggested that the reaction proceeds selectively toward partial hydrogenation only when a carbon-rich film is present at the metal surface. On the basis of first-principles simulations, we show that carbon can dissolve into the metal because graphite formation is delayed by the large critical nucleus necessary for graphite nucleation. A bulk carbide Pd(6)C with a hexagonal six-layer fcc-like supercell forms. The structure is characterized by core level shifts of 0.66-0.70 eV in the core states of Pd, in agreement with experimental x-ray photoemission spectra. Moreover, this phase traps bulk-dissolved hydrogen, suppressing the total hydrogenation reaction channel and fostering partial hydrogenation.

Characterization of the Running-in Period in Total Hip Resurfacing Arthroplasty: An in Vivo and in Vitro Metal Ion Analysis

Metal-on-metal total hip resurfacing arthroplasty is increasingly being performed in young and active patients. Preclinical in vitro testing of implants is usually performed with use of hip simulators, and the serum metal ion concentration is determined for the purpose of monitoring the patients. The goal of this study was to characterize the early running-in period in vivo and in vitro by characterizing metal ion levels. A well-functioning total hip resurfacing prosthesis was implanted in fifteen consecutive patients, and the serum metal ion concentrations in these patients were then determined preoperatively and at intervals during the first postoperative year (at one, six, twelve, twenty-four, and fifty-two weeks). The number of walking cycles was measured with use of a computerized accelerometer in order to compare walking cycles to hip simulator cycles. In vitro, five similar components were investigated for 3 million cycles with use of a hip simulator. Serum samples were obtained at different time points, and wear was measured by quantifying wear particles and ions in the samples. All patient and simulation serum samples were analyzed with use of inductively coupled plasma-mass spectrometry. One simulator implant was investigated with use of scanning electron microscopy. The serum chromium and cobalt levels of the patients continuously increased during the first six months and showed an insignificant decrease thereafter. The molybdenum concentration was unchanged compared with preoperative values. In contrast, the simulator measurements showed a different wear pattern with a high-wear running-in period and a low-wear steady-state phase. The running-in period was delayed by 300,000 cycles and lasted up to 1 million cycles. Scanning electron microscopic analysis showed a carbon-rich protein film predominantly in the early phases of simulation. Scratches were detected originating from pits filled with aluminum oxide and silicon oxide and from pulled-out carbides that were causing third-body wear. The simulator study allowed an exact characterization of the running-in period and showed a delayed onset of running-in wear. In contrast, the clinical data showed a slow increase in measured ion concentrations. These different wear patterns are probably due to the effects of distribution, accumulation, and excretion of particles and ions in vivo.

Room-temperature deposition of highly-insulating SiOCH films by plasma-enhanced chemical vapor deposition using tetraethoxysilane

Highly-insulating silicon oxide which contained hydrocarbon (CH n ) groups (SiOCH) was deposited at room temperature by plasma-enhanced chemical vapor deposition using tetraethoxysilane. The carbon-rich SiOCH film deposited at 50W, 27 °C showed the high resistivity of 6 × 10 15Ω cm at 0.5MV/cm. The insulating properties strongly depended on the deposition temperature and radio frequency (RF) power. The SiOCH films deposited at 300W, 53 °C showed the low carbon content and low resistivity due to the incorporation of hydroxyl (OH) groups generating electron traps. The room-temperature deposition at low RF power enabled the high-CH n addition, resulting in the suppression of OH incorporation into the films. The highly-insulating properties of the room-temperature deposited SiOCH originated from the suppression of OH incorporation.