Direct Current Glow Discharge Plasma Research Articles

High rate growth of thick diamond films by high-current hot-cathode PCVD

On the basis of a conventional direct-current glow discharge plasma chemical vapor deposition (DC-PCVD), we succeeded in raising the deposition rate of diamond films to a large extent by increasing its discharge power. The maximum deposition rate reached about 20 μm/h. The quality of diamond films was also much improved. The maximum thermal conductivity of diamond films prepared by our process was 15.1 W/K cm. Diamond films with such high thermal conductivity can meet the need of many thermal management applications. In this paper, the influences of the deposition techniques on the characteristics of diamond films were studied experimentally from the viewpoint of discharge current.

Characterization of PECVD boron carbonitride layers

BCN films on silicon substrates were deposited with two different PECVD techniques. A microwave plasma with RF-bias enhancement (MW-PECVD) and a direct current glow discharge plasma system (GD-PECVD) was used with N-trimethylborazine (TMB) and triethylamine borane (TEAB) as precursors and with benzene as an additional carbon source. Argon and nitrogen were used as plasma gases. Substrate temperature, substrate bias and gas composition were varied. ERDA (elastic recoil detection analysis) measurements yield information on the layer composition regarding the concentrations of the elements boron, carbon, nitrogen and hydrogen. Depth profiles are also available. The hydrogen content in the produced BCN layers strongly depends on the substrate temperature and increases up to 35 at.%. Depth profiles show a homogeneous distribution of the elements B, C , N and H over the entire layer thickness. Further, the layers were examined regarding their structure (FTIR spectroscopy) and their mechanical properties (nanoindentation measurements).

Hydrogen content and density in nanocrystalline carbon films of a predominant diamond character

Nanocrystalline carbon films possessing a prevailing diamond or graphite character, depending on substrate temperature, can be deposited from a methane hydrogen mixture by the direct current glow discharge plasma chemical vapor deposition method. While at a temperature of ∼880 °C, following the formation of a thin precursor graphitic film, diamond nucleation occurs and a nanodiamond film grows, at higher and lower deposition temperatures the films maintain their graphitic character. In this study the hydrogen content, density and nanocrystalline phase composition of films deposited at various temperatures are investigated. We aim to elucidate the role of hydrogen in nanocrystalline films with a predominant diamond character. Secondary ion mass spectroscopy revealed a considerable increase of the hydrogen concentration in the films that accompanies the growth of nanodiamond. It correlates with near edge x-ray adsorption spectroscopy measurements, that showed an appearance of spectroscopic features associated with the diamond structure, and with a substantial increase of the film density detected by x-ray reflectivity. Electron energy loss spectroscopy showed that nanocrystalline diamond films can be deposited from a CH4/H2 mixture with hydrogen concentration in the 80%–95% range. For a deposition temperature of 880 °C, the highest diamond character of the films was found for a hydrogen concentration of 91% of H2. The deposition temperature plays an important role in diamond formation, strongly influencing the content of adsorbed hydrogen with an optimum at 880 °C. It is suggested that diamond nucleation and growth of the nanodiamond phase is driven by densification of the deposited graphitic films which results in high local compressive stresses. Nanodiamond formation is accompanied by an increase of hydrogen concentration in the films. It is suggested that hydrogen retention is critical for stabilization of nanodiamond crystallites. At lower deposition temperatures an excess of hydrogen in the deposited layers helps to prevent the densification of the films and accumulation of microstresses and consequently the films maintains its graphitic character. At higher temperatures the hydrogen content in the films is relatively low and the film maintains its graphitic character.

The dynamics of macroparticles in a direct current glow discharge plasma undermicro-gravity conditions

The dynamics of large-sized(70–180 µm)spherical bronze particles in a direct current glow discharge plasma was studiedexperimentally under microgravitation conditions. The temperatures, velocities,pair correlation functions and self-diffusion coefficients of macroparticles weremeasured at various discharge currents. The charges of dust particles (on the order of106 e)corresponded to high surface potentials of about 30–40 V.

The dynamics of macroparticles in a direct current glow discharge plasma under microgravitation conditions

The dynamics of large-sized (70-180 μm) spherical bronze particles in a direct current glow discharge plasma was studied experimentally under microgravitation conditions. The temperatures, velocities, pair correlation functions, and self-diffusion coefficients of macroparticles were measured at various discharge currents. The charges of dust particles (on the order of 106 e) corresponded to high surface potentials of about 30–40 V. The experimental data were in close agreement with the simulation data on Yukawa systems with weak screening of dust charges. The influence of macroparticles on equilibrium ionization in a dense dust cloud was considered.

Microstructure and phase composition evolution of nano-crystalline carbon films: Dependence on deposition temperature

Nano-crystalline carbon films possessing a prevailing diamond or a graphite character, depending solely on substrate temperature and deposition time, can be deposited from a methane–hydrogen mixture by the direct current glow discharge plasma chemical vapor deposition method. In this study we investigate the evolution of nano-crystalline carbon films deposited in the 800–950 °C temperature range onto silicon substrates aiming to enlight the physicochemical processes leading to the formation of nano-diamond films. While at a deposition temperature of ∼880 °C the formation of a thin precursor graphitic film is followed by deposition of a film of diamond character, at higher and lower temperatures the films maintain their graphitic character. The morphology of the films and their growth rate vary with deposition temperature: slower growth rates and higher film roughness are obtained at lower temperatures suggesting the importance of kinetic effects during the growth process. For deposition times longer than ∼60 min, similar morphologies are obtained irrespectively of the deposition temperature. A preferred spatial alignment of the basal planes of the graphitic film at the interface with the silicon substrate was determined. The alignment was found to differ with deposition temperature: at 800 and 880 °C the alignment occurs along the graphitic â axis perpendicular to the silicon substrate, while at 950 °C the ĉ axis is aligned perpendicular to the silicon substrate. However, it was determined that for films a few hundred nm thick close to the evolving surface the films display a preferred alignment of the basal planes vertical to the surface, irrespectively of their orientation at the interface. The reason for this alignment is suggested to be associated with a stress relaxation mechanism in the graphitic films. It was determined that film growth is accompanied by the evolution of large local stresses which obtain a maximum value for the films deposited at 880 °C. The relaxation of these stresses is suggested to lead to the transformation of the graphitic material into the diamond phase. The narrow range of temperatures (880+/−10 °C) which enables the formation of the diamond phase indicates the importance of hydrogen adsorption/desorption processes in the nucleation and growth of the nano-crystalline diamond films. The morphological evolution of the films was analyzed by atomic force microscope. By electron diffraction and high-resolution transmission electron microscopy the phase composition of the films and their microstructure were examined. The alignment of the graphitic films within the near-surface region of the evolving films as a function of the deposition time and temperature was investigated by angle-resolved near edge x-ray absorption fine structure measurements. Raman spectroscopy was applied to determine the presence of stresses within the films and their phase composition.

Mechanism of nanodiamond film formation by stress relaxation on a preferentially oriented vertical basal plane graphitic precursor

Nanocrystalline carbon films possessing a prevailing diamond or a graphite character, depending solely on the substrate temperature, can be deposited from a methane–hydrogen mixture by the direct current glow discharge plasma chemical vapor deposition method. While in a narrow temperature window around 880 °C a nanodiamond film composed of an agglomerate of diamond particles 3–5 nm in size embedded in an amorphous matrix is obtained, at higher and lower deposition temperatures the films maintain their graphitic character throughout. The nanodiamond film forms on top of a thin graphitic precursor layer of 150–200 nm thickness (critical thickness of the precursor). It was also found that the formation of the nanodiamond phase is initially accompanied by an increase in surface roughness which decreases with film growth. The graphitic precursor film displays a preferred spatial alignment of its basal planes perpendicular to the silicon substrate surface. The reason for this alignment is suggested to be associated to a stress relaxation mechanism in the graphitic films during growth. Beyond a “critical thickness” where compressive stress has built up in the layer to an extent that it must be relaxed, stress relaxation is governed by the formation of a nanodiamond film. By cross sectional and high resolution transmission electron microscopy analysis the microstructure of the films as a function of distance from the silicon substrate interface was investigated. The alignment of the graphitic precursor within the surface near region of the films as a function of deposition time was investigated by angle-resolved near edge x-ray adsorption fine structure. Atomic force microscopy was applied to study the morphological evolution of the films.

Charging of silica particles in an argon dusty plasma

The Fisk Plasma Source (FPS) is a plasma device operating at Fisk University. Dusty plasmas in the FPS device are produced by suspending 40 μm diameter silica (SiO2) particles in an argon dc (direct current) glow discharge plasma. Experiments are performed at pressures ranging from 90 to 150 mTorr. A 20 mW HeNe laser is used to illuminate the dust particles. Langmuir probes and video images are used to diagnose the charging and confinement of the dust particles in the plasma. A modified version of the dust grain charging model is developed and is used to estimate the charge. The silica particles are shown to become negatively charged in the plasma with up to 105 electrons. The presence of the negatively charged dust grains is correlated to a reduction in the electron saturation current in the plasma.

Mechanism of dust-acoustic instability in a direct current glow discharge plasma

An observation of low frequency waves spontaneously excited in a dc glow discharge dusty plasma is reported. To analyze possible reasons for the instability observed, a linear dispersion relation which takes into account collisions with neutrals, dust grain charge variations, ion drift, and forces acting on dust particles is derived. Numerical analysis of the dispersion relation shows that the observed instability is the result of dust charge variations in the presence of external charge-dependent forces together with the ion drift effect.

Two-dimensional fluid simulation of plasma reactors for the immobilization of krypton

Direct current glow discharge plasma sources are being developed as reactors for immobilization of radioactive krypton gas recovered from the off-gas stream of a nuclear fuel reprocessing plant. In order to design these reactors, two-dimensional simulations have been performed using a fluid model coupled with Poisson's equation. Nonlinear governing equations were expressed by a fully implicit scheme and the equations were solved using the Newton–Krylov method. We investigated algorithm performance of two methods from the Krylov projection techniques, namely the Bi-CGSTAB method and the transpose-free quasi-minimal residual (TFQMR) method. The results showed that the Bi-CGSTAB method converged considerably faster than the TFQMR method. Direct current glow discharge plasmas in three types of krypton immobilization apparatus were simulated to study the differences in electrode geometry. Simulation results agreed qualitatively with measurements of electron density.

Basic processes in glow discharge plasmas

Direct current and radio frequency glow discharge plasmas are described in terms of their evolution, mechanism, spatial characteristics, and voltage-current relationship. The basic plasma processes such as excitation, ionization/dissociation and recombination are reviewed using examples of the chemically reactive H 2 + O 2/H 2O and N 2 + H 2/NH 3 plasmas. This is followed by a discussion of some of the heterogeneous processes occurring at the walls and substrates, namely film formation, etching, nitriding and analogous reactions.