Nitrogenated amorphous carbon films synthesized by electron cyclotron resonance plasma enhanced chemical vapor deposition

Nitrogenated amorphous carbon (a-CN_x:H) films were investigated as protective overcoats for industrial applications. Thin a-CN_x:H films have been deposited on silicon by electron cyclotron resonance plasma-enhanced chemical vapor deposition. The substrate bias was found to play an important role in determining the chemical compositions and mechanical properties of the films. The surface roughness and hardness of the films can reach 1.4 Å and 20 GPa, respectively. The influence of mechanical properties by hydrogen was studied. A correlation exists between the background slope of Raman spectra and the hydrogen content as determined by elastic recoil detection analysis.     

Deposition of carbon nitride films using an electron cyclotron wave resonance plasma source

Hydrogenated amorphous carbon nitride (a-C:N:H) has been synthesised using a high plasma density electron cyclotron wave resonance (ECWR) technique using N₂ and C₂H₂ as source gases, at different ratios and a fixed ion energy (80 eV). The composition, structure and bonding state of the films were investigated and related to their optical and electrical properties. The nitrogen content in the film rises rapidly until the N₂/C₂H₂ gas ratio reaches 2 and then increases more gradually, while the deposition rate decreases steeply, placing an upper limit for the nitrogen incorporation at 30 at%. For nitrogen contents above 20 at%, the band gap and sp³-bonded carbon fraction decrease from 1.7 to 1.1 eV and ∼65 to 40%, respectively. The transition is due to the formation of polymeric CN, CN and NH groups, not an increase in CH bonds. Films with higher nitrogen content are less dense than the original hydrogenated tetrahedral amorphous carbon (ta-C:H) film but, because they have a relatively high band gap (1.1 eV), high resistivity (10⁹ Ω cm) and moderate sp³-bonded carbon fraction (40%), they should be classed as polymeric in nature.     

Secondary electron amplification using single-crystal CVD diamond film

The current amplification characteristics of an unbiased 8.3-μm-thick single-crystal CVD diamond film are examined using secondary-electron-emission measurements. In particular, the intensity and energy distribution of transmitted and reflected secondary electrons are measured and used to examine the transport and emission properties that govern the current amplification process. Overall, the measurements confirm the excellent transport and emission properties of single-crystal CVD diamond, as compared to polycrystalline CVD diamond films studied previously. Specifically, the transmitted and reflected energy distributions measured from the single-crystal diamond are nearly identical, with a sharp, narrow (FWHM = 0.35 eV) emission peak dominating the spectra. However, the transmitted distributions are more fully thermalized as a result of the longer transport distances. In fact, transmitted electrons are detected even after traveling more than 8 μm through the film, which demonstrates the potential for excellent transport efficiency. Maximum transmission gains of 3–4 are obtained, which is encouraging under such field-free conditions. However, the results of the study indicate that the transmission process is being limited by diffusive transport in the unbiased diamond film.     

Effect of microwave power on diamond-like carbon films deposited using electron cyclotron resonance chemical vapor deposition

Diamond-like carbon (DLC) films have been deposited using electron cyclotron resonance chemical vapor deposition (ECR-CVD) under various microwave power conditions. Langmuir probe measurement and optical emission spectroscopy (OES) were used to characterize the ECR plasma, while the films were characterized using Raman and infrared (IR) spectroscopies, hardness and optical gap measurements. It was found that the ion density and all signal peaks in the optical emission (OE) spectra increased monotonously following the increase in microwave power. Raman spectra and optical gap measurements indicate that the films become more graphitic with lower content of sp³-hybridized carbon atoms as the microwave power was increased. IR and hardness measurements indicate a reduction in hydrogen content and decrease in hardness for the film produced at relatively high microwave powers. A deposition mechanism is described which involved the ion bombardment of film surfaces and hydrogen–surface interactions. The deposition rate of DLC film is correlated to the ion density and CH₃ density.     

Molybdenum-containing carbon films deposited using the screen grid technique in an electron cyclotron resonance chemical vapor deposition system

A novel technique involving the incorporation of two molybdenum (Mo) screen grids embedded in an electron cyclotron resonance chemical vapor deposition (ECR–CVD) system is presented in this paper. A comprehensive set of film deposition experiments based on this screen grid sputtering technique has been carried out. The Mo-containing carbon films were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS). The film resistivity, optical bandgap and hardness were evaluated as a function of the gas flow ratio (CH₄/Ar). XPS analysis showed that the fraction of Mo incorporated in the carbon film decreased drastically from 15.11 to 0.32% following an increase in the CH₄/Ar flow ratio. The optical absorption also decreases strongly and the film with the lowest Mo fraction has a bandgap of 2.0 eV. The film resistivity was found to increase by 11 orders of magnitude following the decrease in the metal fraction. It is found that Mo can exist in the forms of MoC, Mo₂C, Mo, and even MoO₃ in the films, the last being mainly due to air exposure. The results showed that our ECR-based screen grid technique for Me-C:H deposition is highly effective and flexible with good control over the amount of metal incorporated.     

Mechanical and tribological properties of diamond-like carbon coatings films deposited from an electron cyclotron resonance plasma

The friction coefficients have been investigated in amorphous diamond-like carbon (DLC) films deposited by a dual ECR–r.f. method, as a function of r.f. substrate bias in relation with the H content and bonding. Combined infrared absorption, elastic recoil detection analysis and tribological tests are used to characterize fully the films in their as-deposited state. Friction coefficients (μ) of the coatings against sapphire balls are determined in air at room temperature. The results indicate clearly that the samples exhibit high compressive stresses and the friction coefficients are found to be low and are affected by the magnitude of the biaxial stress and the microstructure of the films.     

Superhydrophobic carbon nanotube/amorphous carbon nanosphere hybrid film

Fabrication of superhydrophobic surfaces has been widely investigated due to their wide range of applications. Here, synthesis of self-assembled aligned carbon nanotubes (ACNT)/amorphous carbon (a-C) nanosphere hybrid film is reported. Carbon plasma produced by FCVA was used to deposit a-C nanospheres on the ACNT films fabricated by PECVD. The superhydrophobic properties of the surface was investigated by static contact angle (CA) measurement. It is found that the surface morphology of the film which depends on the size of the a-C nanospheres, has a great influence on the hydrophobic properties of the surface. The hydrodynamic properties of the surface is discussed in terms of both Cassie and Wenzel mechanisms. The microstructure of the films is also investigated by XPS and HRTEM. It is shown that the bombardment of the CNTs with high energy carbon ions will damage the crystalline structure of the CNT walls as well.     

The role of pressure on thin amorphous carbon films deposited using microwave surface wave plasma CVD

We report the effects of gas composition pressure (GCP) on the optical, structural and electrical properties of thin amorphous carbon (a-C) films grown on p-type silicon and quartz substrates by microwave surface wave plasma chemical vapor deposition (MW SWP CVD). The films, deposited at various GCPs ranging from 50 to 110 Pa, were studied by UV/VIS/NIR spectroscopy, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and current–voltage characteristics. The optical band gap of the a-C film was tailored to a relatively high range, 2.3–2.6 eV by manipulating GCPs from 50 to 110 Pa. Also, spin density strongly depended on the band gap of the a-C films. Raman spectra showed qualitative structured changes due to sp³/sp² carbon bonding network. The surfaces of the films are found to be very smooth and uniform (RMS roughness < 0.5 nm). The photovoltaic measurements under light illumination (AM 1.5, 100 mW/cm²) show that short-circuit current density, open-circuit voltage, fill factor and photo-conversion efficiency of the film deposited at 50 Pa were 6.4 μA/cm², 126 mV, 0.164 and 1.4 × 10^− 4% respectively.     

Characterization of amorphous carbon films deposited by surface wave plasma CVD

Many dangling bonds in hydrogenated amorphous carbon (a-C:H) films are usually generated by bombardments of high-energy ion precursors in typical chemical vapor deposition (CVD). To generate low dangling bonds, a-C:H films should be deposited from low-energy radical species. Surface wave plasma (SWP) generates low-energy and high-density radicals. We prepare a-C:H films using SWP and investigate the relationship between the plasma characteristics and structures of a-C:H films. The microwave of the TM01 mode was introduced through the dielectric window and SWP generate under the dielectric window. An Ar and C₂H₂ plasma mixture mainly consists of neutral radical species, and the electron temperature is as low as 1 eV. Electron density significantly decreases with increasing distance from the dielectric window. The a-C:H films are prepared from these hydrocarbon and carbon low-energy radicals as main precursors. The sp² bonded network cluster size in a-C:H films increase with electron density in SWP. This structure change is the influence of the termination structure of clusters changing to CH from CH₃ and CH₂.     

Control of ion energy using a screen-grid in an electron cyclotron resonance chemical vapour deposition system

A new technique that incorporates a biased screen grid above the substrate was used in conjunction with an ECR plasma for the deposition of diamond-like carbon (DLC) thin films. The biased screen grid established an electric field that accelerates the ions towards the substrate and provides the energy required for the formation of DLC films. The use of a direct d.c. bias, instead of an r.f.-induced bias, has an important advantage of providing a narrower ion energy distribution. The results show that the screen grid technique is much more effective in accelerating the ions and, in addition, also overcomes the dependence of the ion energy on the geometry of the substrate holder and the magnetic field configuration. The effects of the screen grid method on the characteristics of the films such as Raman scattering, IR absorption and hardness were investigated and compared with the case of deposition using a d.c. bias applied directly to the substrate holder. The effects of changing pressure on the properties of the films under a constant direct d.c. bias were also studied and it was found that higher pressure leads to films with increasing DLC characteristics in an ECR-CVD system.     

Local field electron emission from grain boundaries of CVD diamond film

In this article, we have investigated local field electron emission from grain boundaries of diamond films with (100) preferential orientation by double-probe scanning electron microscopy (SEM). Compared with the field emission from the plane area of diamond film, local field emission from grain boundary area is greatly enhanced at the same applied field, and further increased with the increasing of grain boundary number density. This result provides a direct evidence that grain boundary plays an important role in field emission from diamond film because a great deal of sp² graphitic carbon phases exists in grain boundary areas as electron transport channels for the surface field emission process.     

Wettability enhancement of polystyrene with electron cyclotron resonance plasma with argon

Polystyrene cell‐culture substrates were treated with argon glow discharge to make their surfaces hydrophilic. The process was novel in that it used a microwave electron cyclotron resonance (ECR) source for polymer surface modification. The substrates were processed at different microwave powers and time periods, and the surface modification was assessed with by measurement of the water contact angle. A decrease in contact angle was observed with increasing microwave power and processing time. Beyond a certain limit of power and duration of exposure, however, surface deterioration occurred. The optimum conditions for making the surfaces hydrophilic without deterioration of the samples were identified. The plasma parameters were assessed by Langmuir probe measurement. Fourier transform infrared spectroscopy with attenuated total reflectance showed evidence for the induction of hydrophilicity on the surface. The surface micromorphology was examined with scanning electron microscopy. The results prove that the ECR glow discharge was an efficient method for enhancing the wettability of the polymer surfaces. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1618–1623, 2003     

An optical absorption and electron spin resonance study in hydrogenated amorphous carbon prepared by radio frequency sputtering

In this work, we investigated different hydrogenated amorphous carbon (a-C:H) films grown by radio frequency (RF) magnetron sputtering covering a large variety of films from polymer-like carbon (PLC) to diamond-like carbon (DLC). Photothermal deflection spectroscopy (PDS), transmission spectroscopy in the visible and near infrared (IR) region were combined with electron spin resonance (ESR) measurements. The optical absorption spectra were analysed assuming a distribution of two overlapping Gaussian bands near the Fermi level in order to deduce the density of states (DOS) at the Fermi level. From the DOS and the density of paramagnetic states a correlation energy for midgap states is derived.     

CVD金刚石薄膜技术发展现状及展望(下)

简要描述了CVD金刚石薄膜技术的发展历程。介绍了纳米特别是超纳米金刚石膜、CVD金刚石大单晶的技术特点及其应用。超纳米金刚石膜在MEMS(微机电系统)、电化学和生物医学上的应用和CVD金刚石大单晶是当前的研究热点。简言之,金刚石的发展向着更大或者更小的方向深入进行,即"非大即小"。     

CVD金刚石薄膜技术发展现状及展望(上)

简要描述了CVD金刚石薄膜技术的发展历程.介绍了纳米特别是超纳米金刚石膜、CVD金刚石大单晶的技术特点及其应用.超纳米金刚石膜在MEMS(微机电系统)、电化学和生物医学上的应用和CVD金刚石大单晶是当前的研究热点.简言之,金刚石的发展向着更大或者更小的方向深入进行,即“非大即小”.     

Microscopic spreading of nonreactive perfluoropolyalkylether film on amorphous carbon surfaces

A spreading mechanism of nonfunctional perfluoropolyalkylehter (PFPE) on carbon surfaces is proposed. For the thin thin-film regime, adsorption-desorption is a main driving force for spreading, and the surface diffusion coefficients increase as the film thickness increases. A two-dimensional virial equation is employed to explain the dependency of surface diffusion coefficient on the film thickness. For the thick thin-film regime, the spreading characteristic is determined by the disjoining pressure gradient. We adopt a slip boundary condition to analyze the thick thin-film regime. This modification of the boundary condition reasonably explains the dependence of surface diffusion coefficients on film thickness.     

Characteristic defects in CVD diamond: optical and electron paramagnetic resonance study

Constant photocurrent method (CPM), electron paramagnetic resonance (EPR), and infra-red optical absorption (FTIR) techniques are used to study characteristic defects in the gap of free-standing optical-quality CVD diamond. It is shown that the gap density of states (DOS) is very sensitive to oxidation, hydrogenation and annealing treatments. The room-temperature (RT) EPR and CPM measurements reveal a well-defined single substitutional nitrogen defect (P1). The photoionization energy of this defect is E_i=2.2 eV. The presence of another defect state, denoted previously as D1 (E_i=1.2 eV) is discussed. This (D1) defect level is stable up to high annealing temperatures. The changes observed in the EPR spectra after oxidation/hydrogenation are discussed. IR absorption in the CH-stretch band and in one-phonon absorption regions is investigated.     

Growth of tetrahedral amorphous carbon film: Tight-binding molecular dynamics study

Molecular dynamics simulation using tight-binding potential has been performed to examine the growth and performance of tetrahedral amorphous carbon during ion deposition. The sp³ hybrid atom content, density, and compressive stress of the tetrahedral amorphous carbon film depend on the growing conditions such as substrate temperature, ion energy, ion dose, and annealing temperature. The critical temperature for sp³ transition to sp² decreases with ion energy (40, 80, and 120 eV). At low temperatures (<300 K) and low ion energies, the sp³ fraction increases up to 82%. At the annealing temperature less than 1200 K or with a few ions (<20) implanted into the film, its sp³ content and density have only slight changes while the compressive stress has a large reduction with the annealing temperature and the number of implanted ions. This large reduction in the compressive stress is due to a structural relaxation.