Thickness dependent electronic structure of ultra-thin tetrahedral amorphous carbon (ta-C) films

Microstructural properties of ultrathin (1-10 nm) tetrahedral amorphous carbon (ta-C) films are investigated by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, X-ray Photoelectron Spectroscopy, Raman spectroscopy and Atomic Force Microscopy (AFM). The CK-edge NEXAFS spectra of 1 nm ta-C films provided evidence of surface defects (C―H bonds) which rapidly diminish with increasing film thickness. A critical thickness for stabilization of largely sp³ matrix structure distorted by sp² sites is observed via the change of π*C═C peak behavior. Meanwhile, an increase in the film thickness promotes an enhancement in sp³ content, the film roughness remains nearly constant as probed by spectroscopic techniques and AFM, respectively. The effect of thickness on local bonding states of ultrathin ta-C films proves to be the limiting factor for their potential use in magnetic and optical storage devices.     

Metal-doped diamond-like carbon films synthesized by filter-arc deposition

Diamond-like carbon (DLC) thin films are extensively utilized in the semiconductor, electric and cutting machine industries owing to their high hardness, high elastic modulus, low friction coefficients and high chemical stability. DLC films are prepared by ion beam-assisted deposition (BAD), sputter deposition, plasma-enhanced chemical vapor deposition (PECVD), cathodic arc evaporation (CAE), and filter arc deposition (FAD). The major drawbacks of these methods are the degraded hardness associated with the low sp³/sp² bonding ratio, the rough surface and poor adhesion caused by the presence of particles. In this study, a self-developed filter arc deposition (FAD) system was employed to prepare metal-containing DLC films with a low particle density. The relationships between the DLC film properties, such as film structure, surface morphology and mechanical behavior, with variation of substrate bias and target current, are examined. Experimental results demonstrate that FAD-DLC films have a lower ratio, suggesting that FAD-DLC films have a greater sp³ bonding than the CAE-DLC films. FAD-DLC films also exhibit a low friction coefficient of 0.14 and half of the number of surface particles as in the CAE-DLC films. Introducing a CrN interfacial layer between the substrate and the DLC films enables the magnetic field strength of the filter to be controlled to improve the adhesion and effectively eliminate the contaminating particles. Accordingly, the FAD system improves the tribological properties of the DLC films.     

Correlations between substrate bias, microstructure and surface morphology of tetrahedral amorphous carbon films

The microstructure and surface morphology of ta-C films deposited on p-type (1 0 0) single crystal silicon with the substrate negative bias varying from 0 to 2000 V by the filtered cathodic vacuum arc technology have been investigated by means of Raman spectroscopy and atomic force microscope. The optimal deposition process of sp³-rich ta-C films can be confirmed in light of the relations between the coupling coefficients or full-width at half-maximum and the substrate negative bias. The surfaces of these films are uniform and smooth and RMS surface roughness is less than 0.4 nm. At the lower energetic grades, the more the content of sp³ is in the film, the smoother the surface of the film is. The dependence of the surface morphology and the impinging energy of the species can be illustrated according to the subimplantation growth mechanism. Nevertheless at the high energetic grade, the impinging ions with appropriate energy sputter and smoothen the surface so that the roughness might be even lower than the one of the films with the richest sp³ component.     

掺硼四面体非晶碳膜的微观结构及光谱表征

分别采用过滤阴极真空电弧技术制备了不同含硼量四面体非晶碳(ta-C:B)膜, 并用X射线光电子能谱(XPS)、Raman光谱对薄膜的微观结构和化学键态进行了研究. XPS分析表明薄膜中B主要以石墨结构形式存在, 随着B含量的增加, sp³杂化碳的含量逐渐减小, Ta-C:B膜的Raman谱线在含硼量较高时, 其D峰和G峰向低频区偏移, 且G峰的半峰宽变窄, 表明B的引入促进了sp²杂化碳的团簇化, 减小了原子价键之间的变形, 从而降低了薄膜的内应力．     

Influence of argon neutral beam energy on the structural properties of amorphous carbon thin films grown by neutral particle beam assisted sputtering

The effects of argon neutral beam (NB) energy on amorphous carbon (a-C) films were investigated, the a-C films were deposited by a neutral particle beam assisted sputtering (NBAS) system. The energy of the neutral particle beam can be directly controlled by a reflector bias voltage as a unique operating parameter of the system. The results from the analysis by Raman spectra, Fourier transform infrared (FT-IR), UV-visible spectroscopy and electrical conductivity indicate the properties of the amorphous carbon films can be manipulated by simply adjusting the NB energy (or reflector bias voltage) without changing any other process parameters. By increasing the reflector bias voltage, the amount of cross-linked sp² clusters as well as the sp³ bonding in the a-C film coating from the NBAS system can be increased effectively and the composition of carbon thin films can be changed from a nano-crystalline graphite phase to an amorphous carbon phase. In addition, the deposition rate increases with reflector bias voltage due to additional sputtering at the carbon reflector without any variation of physical and electrical properties of the a-C film.     

Multilayer DLC coatings via alternating bias during magnetron sputtering

To combat the high residual stress problem in monolayer diamond-like carbon coatings, this paper fabricated multilayer diamond-like carbon coatings with alternate soft and hard layers via alternating bias during magnetron sputtering. The surface, cross sectional morphology, bonding structures and mechanical properties are investigated. The atomic force microscopy images indicate low bias results in rougher surface with large graphite clusters and voids suggesting low coating density. The multilayered coatings demonstrate relatively smooth surface stemming from higher bias. The cross sectional images from field emission scanning electron microscopy indicate coating thickness decreases as substrate bias increases and confirm that higher bias results in denser coating. Delamination is observed in monolayer coatings due to high residual stress. The trend of sp³/sp² fraction estimated by X-ray photoelectron spectroscopy is consistent with that of I_D/I_G ratios from Raman spectra, indicating the change of bonding structure with change of substrate bias. Hardness of multilayer diamond-like carbon coating is comparable to the coatings deposited at low constant bias but the adhesion strength and toughness are significantly improved. Alternately biased sputtering deposition provides an alternative when combination of hardness, toughness and adhesion strength is needed in an all diamond-like carbon coating.     

Laser-induced transformation of a-C:H thin films

In this work, we report the laser irradiation effects on the properties of various types of amorphous hydrogenated carbon (a-C:H) films. The influence of the initial carbon film (hydrogen concentration, sp³/sp² ratio, and sp² clustering) is studied. The results show that a loss of hydrogen and an increase of the sp² phase are the main processes in the laser power range between 1.8 and 5 MW/cm². Only these processes are stronger for “more polymer-like” and “graphite-like” films than for “more diamond-like” films.     

XPS and IR study of carbon thin films prepared under negative substrate DC voltage bias

Hydrogenated amorphous carbon (a-C:H) thin films were prepared on glass substrates at different applied DC voltage bias by the HF-CVD method. Other factors of deposition were kept constant. The IR and XPS spectra of the films were obtained. By the deconvolution of the IR and XPS spectra sp³/sp² ratio calculated. The sp³/sp² ratio varies nonlinearly with bias voltage and it has a minimum and maximum in the 0–70 V range of the bias voltage.     

Relative fraction of sp bonding in boron incorporated amorphous carbon films determined by X-ray photoelectron spectroscopy

Boron incorporated amorphous carbon (a-C:B) films were deposited by a filtered cathodic vacuum arc system using various percentage of boron mixed graphite cathodes. X-ray photoelectron spectroscopy (XPS) was employed to determine the properties of the films as a function of boron concentration. Deconvolutions of the XPS C 1s core level spectra were carried out using four different components. The relative fraction of sp³ bonding was then evaluated from the area ratio of the peaks at 285.0, 284.1 eV which were individually attributed to sp³ C-C, sp² CC hybridizations. The results showed that the sp³ content of a-C:B film decreases from 73.8 to 58.6% for the films containing boron from 0.59 to 2.13 at.%, and then gradually reduced to 42.5% at a slower rate with boron concentration up to 6.04 at.%. Furthermore, a series of a-C:B films with fixed boron content (2.13 at.%) were prepared to identify the relationship between sp³ bonding and substrate bias. It was found that the fraction of sp³ bonding increased from 50.28% at the bias voltage of 0 V and reached a maximum value of 66.3% at −150 V. As the bias voltage increased up to −2000 V, the sp³ content decreased sharply to 43.9%.     

Reflectance and photoluminescence spectra of as grown and hydrogen and nitrogen incorporated tetrahedral amorphous carbon films deposited using an S bend filtered cathodic vacuum arc process

The study of reflectance and photoluminescence (PL) spectra of as grown and also hydrogen and nitrogen incorporated tetrahedral amorphous carbon (ta-C) films, deposited using an S bend filtered cathodic vacuum arc process is reported here. First the effect of negative substrate bias on the properties of as grown ta-C films and next the effect of varying hydrogen and nitrogen partial pressure at a high substrate bias of − 300 V on the properties of hydrogen and nitrogen incorporated ta-C (ta-C:H and ta-C:N) films are reported for the first time. The values of the optical band gap (E_g) evaluated using the reflectance spectra were found to decrease with the increase of the substrate bias in the as grown ta-C films. Hydrogen incorporation up to 1.9 × 10^− 2 Pa partial pressure in as grown ta-C films increased the values of E_g and beyond which the values of E_g decreased while the nitrogen incorporation up to 3.0 × 10^− 1 Pa partial pressure has no effect on the E_g values. The PL spectra indicated a strong peak at ∼2.66 eV in as grown ta-C films deposited at − 20 V substrate bias. This main peak was found to shift to higher energy with the increase of the substrate bias up to − 200 V and thereafter the PL peak shifted towards the lower energy. Other peak at 3.135 eV starts appearing and this is found to start shifting to higher energy for films deposited at higher substrate bias. The intensity of the main PL peak was enhanced at low temperature and several other peaks started appearing in place of the broad peak at ∼3.16 eV. The peak width and area of both the main peak were found to decrease with the increase of substrate bias in as grown ta-C films and with the increase of the hydrogen and nitrogen partial pressure used in depositing ta-C:H and ta-C:N films. The current models on the source of luminescence in amorphous carbon have been discussed.     

Wafer Scale Solventless Adhesive Bonding with iCVD Polyglycidylmethacrylate: Effects of Bonding Parameters on Adhesion Energies

Initiated chemical vapor deposition (iCVD) polyglycidylmethacrylate (PGMA) thin films are investigated as adhesives for wafer‐scale bonding of 300 mm silicon substrates and demonstrated to form highly uniform, void‐free bond interfaces. The effects of bonding temperature and pressure on critical adhesion energy (G_c) between iCVD PGMA and silicon are studied using the four‐point bend technique. G_c values can be varied over an order of magnitude (0.59–41.6 J m⁻²) by controlling the bonding temperature and the observed dependence is attributed to changes in the physical (diffusion) and chemical (crosslinking) properties of the film. Thermal degradation studies using spectroscopic ellipsometry reveal that the iCVD PGMA films can crosslink when annealed above 120 °C in air. Further, changes in polymer behavior associated with annealing temperature are demonstrated to influence the crack propagation interface between the bonded substrates. These findings demonstrate the feasibility of iCVD polymer films for both temporary “thermoplastic,” and permanent “thermoset” bonding with potential applications in 3D integrated circuit technologies.     

Diffusion in Au-Ni thin films

The objective of this work was to determine experimentally the values of the surface diffusion parameters of Au-Ni thin films obtained by vacuum evaporation and by sputter deposition. Thin film diffusion couples with an edge-to-edge interface arrangement were employed in order to define the surface diffusion mechanisms better. Experimental results show that the frequency factor (diffusion constant) for evaporated films (1.5×10^-8 cm² sec^-1) is higher than that for sputtered films (1.1×10^-8 cm² sec^-1) and bulk material (bulk diffusion data). The values obtained for the thermal activation energy in evaporated films were one order of magnitude less than those obtained for bulk material. Sputtered thin films were found to have an activation energy over 20% higher than that for evaporated films. This discrepancy apparently occurs because of partial incorporation of sputtered atoms into the glass substrate. Measurements of thin film adhesion showed the same effect.Examination of the structural characteristics of the specimens showed that both sputtered and evaporated films 300 Å and more in thickness become completely microscopically continuous. Some variations in grain size were also observed. Sputtered films were found to have crystallite grains twice as large as those in films prepared by evaporation. Microphotographs showed that for films 300 Å thick the “evaporation-condensation” effect occurs in the overlapping zone.     

Effect of strain and grain boundaries on dielectric properties in La0.7Sr0.3MnO3 thin films

High dielectric constant and its dependence on structural strain and grain boundaries (GB) in La_0.7Sr_0.3MnO₃ (LSMO) thin films are reported. X-ray diffraction, magnetization, and magneto-transport measurements of the LSMO films, made by pulsed laser deposition on two different substrates—MgO and SrTiO₃ (STO), were compared to co-relate magnetic properties with dielectric properties. At room temperature, in the ferromagnetic phase of LSMO, a high dielectric constant (6 × 10⁴) was observed up to 100 kHz frequency for the films on MgO, with polycrystalline properties and more high-angle GB related defects, while for the films on STO, with single-crystalline properties but strained unit cells, high dielectric constant (≈10⁴) was observed until 1 MHz frequency. Also, a large dielectric relaxation time with significant broadening from the Debye single-dielectric relaxation model has been observed in samples with higher GB defects. Impedance spectroscopy further shows that large dielectric constant of the single-crystalline, strained LSMO film is intrinsic in nature while that in the polycrystalline films are mainly extrinsic due to higher amount of GBs. The presence of high dielectric constant value until high frequency range rules out the possibility of “apparent giant dielectric constant” arising from the sample-electrode interface. Coexistence of ferromagnetism and high dielectric constant can be very useful for different microelectronic applications.     

Thickness limit of BaTiO3 thin film capacitors grown on SUS substrates using aerosol deposition method

We fabricated BaTiO₃ thin films with 2.2-0.1 μm thickness on hard stainless steel (SUS) substrates by using the ADM to confirm the causes of dielectric thickness limit showing in BaTiO₃ thin films prepared on SUS substrates and suggest key factors which can overcome the limit. Then, from the measurements of thickness dependence of their dielectric properties, the thickness limit of 0.2 μm was confirmed and to confirm the reason why their dielectric properties could not be measured in the thickness below 0.2 μm, the thickness dependence of leakage current mechanisms in BaTiO₃ films were investigated. As a result, by decreasing the thickness of films from 2.2 to 0.2 μm, the mechanism changed from Poole-Frenkel emission to modified-Schottky emission indicating increase of interface effects. Especially, in the case of 0.2 μm thickness, it was confirmed that the dominant mechanism was Fowler-Nordheim tunneling based on electric field concentration at a high electric field. Consequently, from this investigation of leakage current mechanism, it can be expected that the cause of thickness limits was electric field concentration at rough BaTiO₃/SUS interfaces forming in AD process, and to get over the thickness limit and decrease level of leakage currents, the hard substrates are required to reduce the interface roughness and oxygen vacancies acted as donors should be decreased.     

Mechanism of nucleation and growth of cubic boron nitride thin films

The mechanism and the crystallography of the nucleation and growth of cubic boron nitride (c-BN) films deposited on 〈100〉-oriented silicon substrate by RF bias sputtering have been studied by means of cross-sectional high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. Both methods provide experimental information showing no sp²-bonded BN layer formation in the subsurface region of c-BN phase. This is clear evidence for layer-by-layer homoepitaxial growth of cubic boron nitride without graphitic monolayers in the near-surface region of the film. The turbostratic boron nitride (t-BN) consists of thin sub-layers, 0.5–2 nm thick, growing in such a way that a sub-layer normal is almost parallel to the growth direction. t-BN also comprises a large volume fraction of the grain boundaries with high interface energies. The present result and the finding by Shtansky et al. [Acta Mater. 48, 3745 (2000)], who showed that an individual sub-layer consists of parallel lamellae in both the hexagonal +h-BN) and rhombohedral (r-BN) configurations, demonstrate that high intrinsic stress in the films is due to the complex structure of sp²-bonded BN. The crystallography of c-BN films indicates heteroepitaxial nucleation of cubic phase on the graphitic BN structural precursor. The present results are consistent with stress-induced c-BN formation.     

Superior wear resistance of diamond and DLC coatings

As the hardest known material, diamond and its coatings continue to generate significant attention for stringent applications involving extreme tribological conditions. Likewise, diamond-like carbon (DLC, especially the tetragonal amorphous carbon, ta-C) coatings have also maintained a high level interest for numerous industrial applications where efficiency, performance, and reliability are of great importance. The strong covalent bonding or sp³-hybridizaiton in diamond and ta-C coatings assures high mechanical hardness, stiffness, chemical and thermal stability that make them well-suited for harsh tribological conditions involving high-speeds, loads, and temperatures. In particular, unique chemical and mechanical nature of diamond and ta-C surfaces plays an important role in their unusual friction and wear behaviors. As with all other tribomaterials, both diamond and ta-C coatings strongly interact with the chemical species in their surroundings during sliding and hence produce a chemically passive top surface layer which ultimately determines the extent of friction and wear. Thick micro-crystalline diamond films are most preferred for tooling applications, while thinner nano/ultranano-crysalline diamond films are well-suited for mechanical devices ranging from nano- (such as NEMS) to micro- (MEMS and AFM tips) as well as macro-scale devices including mechanical pump seals. The ta-C coatings have lately become indispensable for a variety of automotive applications and are used in very large volumes in tappets, piston pins, rings, and a variety of gears and bearings, especially in the Asian market. This paper is intended to provide a comprehensive overview of the recent developments in tribology of super-hard diamond and DLC (ta-C) films with a special emphasis on their friction and wear mechanisms that are key to their extraordinary tribological performance under harsh tribological conditions. Based on the results of recent studies, the paper will also attempt to highlight what lies ahead for these films in tribology and other demanding industrial applications.     

Field emission characteristics of fast grown nanocrystalline diamond/amorphous carbon composite films by microwave plasma-enhanced chemical deposition method

This study synthesized the nanocrystalline diamond/amorphous carbon (NCD/a-C) composite films by the microwave plasma-enhanced chemical vapor deposition (MPCVD) system with Ar/CH₄/N₂ mixtures. A localized rectangular-type jet-electrode with high density plasma was used to enhance the formation of NCD/a-C films, and a maximum growth rate of 105.6 µm/h was achieved. The content variations of sp² and sp³ phases via varying nitrogen gas flow rates were investigated by using Raman spectroscopy. The NCD/a-C film which synthesized with 6% nitrogen concentration and no hydrogen plasma etching treatment possessed a low turn-on electric field of 3.1 V/µm at the emission current of 0.01 µA.     

Effect of bias voltage on growth property of Cr-DLC film prepared by linear ion beam deposition technique

Cr-containing diamond-like carbon films were deposited on silicon wafers by a combined linear ion beam and DC magnetron sputtering. The influence of the bias voltage on the growth rate, atomic bond structure, surface topography and mechanical properties of the films were investigated by SEM, XPS, Raman spectroscopy, AFM, and nano-indentation. It was shown that the chromium concentration of the films increased with negative bias voltage and that a carbide phase was detected in the as-deposited films. The surface topography of the films evolved from a rough surface with larger hillocks reducing to form a smoother flat surface as the bias voltage increased from 0 to −200 V. The highest hardness and elastic modulus were obtained at a bias voltage of about −50 V, while the maximum sp³ bonding fraction was acquired at −100 V. It was suggested that the mechanical properties of the films not only depended on the sp³ bonding fraction in the films but also correlated with the influence of Cr doping and ion bombardment.     

Synthesis of sp2 Carbon-Conjugated Covalent Organic Framework Thin-Films via Copper-Surface-Mediated Knoevenagel Polycondensation

sp² carbon-conjugated covalent organic framework (sp²c-COF) featured with high π-conjugation, high chemical stabilities, and designable chemical structures, are thus promising for applications including adsorption and separation, optoelectronic devices, and catalysis. For the most of these applications, large-area and continuous films are required. However, due to the needs of harsh conditions in the formation of CC bonds, classical interfacial methodologies are challenged in the synthesis of sp²c-COFs films. Herein, a novel and robust interfacial method namely copper-surface-mediated Knoevenagel polycondensation (Cu-SMKP), is shown for scalable synthesis of sp²c-COF films on various Cu substrates. Using this approach, large-area and continuous sp²c-COF films could be prepared on various complicated Cu surfaces with thickness from tens to hundreds of nanometers. The resultant sp²c-COF films on Cu substrate could be used directly as functional electrode for extraction of uranium from spiked seawater, which gives an exceptionally uptake capacity of 2475 mg g⁻¹. These results delineate significant synthetic advances in sp²c-COF films and implemented them as functional electrodes for uranyl capture.