Influence of effective pumping speed on oxygen atom density in a plasma post-glow reactor

The density of neutral oxygen atoms in a post-glow reactor was measured with a fibre-optics catalytic probe. The source of O atoms was a microwave discharge generated in Ar-O₂ gas with different flow rates up to 3000 sccm/min. The O density was measured at different power of the microwave generator between 40 and 160 W and different effective pumping speed between 7 and 28 m³/h. It was found to depend on the ratio between O₂ and Ar flow rates. At a constant O₂ flow of 200 sccm/min there was a broad maximum of O density between the Ar flow of 200 and 1000 sccm/min, independent of the effective pumping speed. At a constant O₂ and Ar flow rates of 200 and 1000 sccm/min, respectively, the O density was found to increase both with increasing power and increasing effective pumping speed. The results were explained with collision phenomena in ionized gases and heterogeneous recombination of O atoms on surfaces.     

Hydrogen Balmer-β and Balmer-γ emission profiles in an abnormal glow region of hydrogen plasma

Emission spectroscopy measurements of Balmer-β and Balmer-γ line profiles are carried out to study their line shapes and line intensities as a function of discharge parameters such as fill pressure (3-7 mbar) and input power (200-400 W). A technique based on determining the relative intensities of neutral atomic hydrogen emission lines is used to evaluate the electron temperature, whereas the electron density is extracted from the Stark broadening (FWHM) of the H_β emission profile. It is found that both the emission intensity and the broadening of the Balmer-β and Balmer-γ lines exhibit significant dependence on the fill pressure in the same manner. However, both the emission intensity and broadening of the H_β line exhibit a weak dependence on input power in contrast to the H_γ emission line. Based on optical measurements reported here, plasma discharge parameters can be selected to optimize the electron temperature and density.     

Field emission properties of composite nano-Au/DLC films prepared by CVD technique

Nanocrystalline gold incorporated diamond-like carbon (nano-Au/DLC) films were deposited by capacitively coupled plasma (CCP) r.f. chemical vapour deposition (CVD) technique. Gold content in the DLC matrix was controlled by the amount of argon in the argon + methane mixture in the plasma. Field emission properties of these films were studied critically. Bonding environment (sp²/sp³ ratio) in these films was obtained from Raman measurements. Modification of the surface with the incorporation of gold nanocrystallites and associated modulation of sp²/sp³ ratio in the films culminated in improved field emission properties. Fowler-Nordheim model was used to ascertain the work function (?) which varied between 19 and 64 meV. The field factor (β) varied between 172 and 1050.     

Stability conditions of argon and helium gas mixtures in an atmospheric pressure plasma jet

Non-equilibrium plasmas can be generated by atmospheric pressure glow discharges, amongst others by atmospheric pressure plasma jets (APPJ), which feature a capacitive radio-frequency discharge between bare metallic electrodes.We investigated the stability conditions for discharges in an APPJ operated with helium-argon mixtures. Uniform α-discharges can be sustained in mixtures ranging from pure helium to pure argon. The ignition voltage increases drastically with argon content. There is also an upper limit for the existence of the α-mode, where α-sheath breakdown occurs. Critical plasma parameters for the α-mode were determined by equivalent circuit models and discussed in respect to dependences on the different models. A critical electron density of 2.4×10¹¹ cm⁻³ was obtained for pure helium. It increases steadily with argon content and reaches a value of 1.2×10¹² cm⁻³ for pure argon. Sheath thicknesses for α-sheath breakdown were calculated in the range of 0.17-0.29 mm for helium-argon mixtures.     

Atmospheric pressure plasma jet operated at narrow gap spacings

The atmospheric pressure plasma jet (APPJ) makes use of a dielectric-barrier-free radio-frequency (RF) glow discharge for the production of a non-equilibrium plasma. Usually the APPJ is operated in the alpha mode at gap spacings in the mm range, where the alpha sheath thickness is in the order of 200-300 μm. Narrow gap spacings are experimentally not yet investigated, but it is expected that the bulk region of the alpha discharge should disappear and the discharge should exhibit a sheath-only structure.In order to provide experimental evidence for such situations, APPJs with gap spacings down to 0.1 mm are investigated. The electric properties of the APPJ are studied by measuring the current and voltage characteristics. Time-averaged images of the front view of the discharge are taken with a digital camera. By using an image-intensified gateable video camera the time development of the discharge is studied with nanosecond resolution.It was possible to sustain alpha discharges at gap spacings down to 100 μm, whereby the voltage needed decreases down to an rms voltage of 70 V. A weak indication for a laterally oscillating sheath in the 100 μm gap was found.     

Characteristics of a magnetron cold cathode gauge

The discharge characteristics of a cold-cathode gauge of the non-inverted magnetron type were studied in ultra-high vacuum. The experimental magnetron cell of length 56 mm and diameter 32 mm was made of stainless steel. The cathode with a diameter of 6 mm was placed along the anode axis. The diameter of the anode was 25 mm and the length was 50 mm. Discharge current versus voltage and magnetic field was measured in the pressure range between 1×10⁻⁸ and 1×10⁻⁶ mbar. It was found that the current at first slowly increased with increasing voltage, reached a maximum at a certain voltage, and decreased rapidly with further increase of the voltage. The voltage, at which the current reached the maximum, depended on the magnetic field density and slightly on the pressure. A novel type of a cold cathode gauge with a self-adjusting power supply is suggested.     

Pressure/current characteristics of a magnetron cold cathode gauge

The gauge characteristics for a magnetron gauge in the pressure range 10⁻⁸-10⁻⁵ mbar with nitrogen as a test gas have been studied. The discharge current versus pressure was measured at different magnetic fields from 0.10 to 0.15 T and anode voltages from 4.0 to 7.0 kV. The results showed that a change of slope appeared at pressures between 5×10⁻⁸ and 5×10⁻⁷ mbar. At higher magnetic fields, a change of slope appeared at lower pressures. The slope of the curve below the inflection was higher than that above the inflection. At the lowest magnetic field (0.10 T), a divergence of the curves was observed at pressures below the inflection. The slope of the curve showed no clear dependence on applied voltage.     

A low-voltage Penning cell for vacuum measurement and vacuum technology

This paper presents new results on the performance of a special cold-cathode gauge. The use of an AC and DC voltage simultaneously as the power source makes it possible to ignite and sustain a discharge at a significantly lower voltage than normal over a wide pressure range (from 5 to 3×10⁻⁶ Pa). The Penning discharge voltage can be as low as 20 V at a pressure of 0.3 Pa and 300 V in high vacuum. For low values of pressure, the discharge current has a monotonously increasing dependence on the gas pressure. If a low RF voltage is applied to a high-voltage DC discharge, the discharge current increases in many times. These discharge properties can be used to improve the performance of devices that use the Penning and other magnetic discharges (total and partial pressure gauges, leak detectors, ion pumps, ion sources and hot plasma sources). A vacuum gauge of this type combines the best features of magnetic discharge and hot cathode ionization gauges.     

Radial structure of low pressure rf capacitive discharges

This paper studies the glow intensity distribution of the discharge plasma against the tube radius and reports the radial profiles of electron temperature and plasma concentration in the rf capacitive discharge registered with a Langmuir probe. An abrupt increase of electron temperature and glow intensity near the tube wall in the weak-current α-mode of the rf capacitive discharge is revealed, the radial distribution of plasma concentration and ion flow to the electrodes possessing a maximum near the radial sheath boundary. In the γ-mode of the rf capacitive discharge the electron temperature decrease in the total plasma volume leads to an electric field weakening and the peak of the glow intensity near the tube wall vanishes. The radial sheath thickness in the α-mode of the rf capacitive discharge obtained with 2D simulation experiences pulsations during the rf field period, the changing radial electric field heating electrons and increasing the plasma concentration near the boundary of the radial sheath.     

Computational investigation on the disturbed thermal plasma for improving the reliability of electrostatic diagnosis

For measuring the current-voltage (I-V) characteristic curve with respect to flowing plasmas, a Langmuir probe, a conducting object, should be inserted to the region directly, and the interaction between the probe and the flowing plasma is not avoidable. This interaction can cause the serious problem for plasma diagnostics due to aerodynamic and thermal disturbances generated by inserting the metallic probe. In spite of the problem related to the disturbances, the conventional probe theory assumes a connection between the measured probe currents and parameters of the undisturbed plasma. Unfortunately, the simplicity of the operating scheme is not reflected by the appropriate theories for the disturbed plasma. Also, the disturbed region should be affected by the size of the inserted probe, and this calls for a small probing object not to perturb the global state of the plasma. The main purpose of this paper is to know the disturbances of the original temperature and velocity fields, which are called as thermal and aerodynamic disturbances, of the entire free-burning arc disturbed by the inserted metallic probe with the turbulent model.     

RF reactive magnetron sputtering for Fe-doped titania films deposited from ceramic targets

Fe-doped titania films are prepared by RF magnetron sputtering on Si wafers with specifically designed TiO₂ targets containing different amounts of Fe₂O₃ powder as a dopant source. The physical properties of the films are investigated in terms of the preparation conditions, such as Fe₂O₃ content in the target, RF power, substrate temperature and working pressure. The films show the typical crystallographic orientation. The growth rate increases with increasing RF power, but decreases with working pressure. Films with 40 nm and the transmittance over 90% at the visible region are prepared by using Fe-doped titania target.     

Synthesis of functional nanoassemblies in reactive plasmas

Synthesis of various functional nanoassemblies, by using a combination of low-pressure reactive plasma-enhanced chemical deposition and plasma-assisted rf magnetron sputtering deposition is reported. This paper details how selective generation and manipulation of the required building blocks and management of unwanted nanoparticle contaminants, can be used for plasma-aided nanofabrication of carbon nanotip microemitter structures, ultra-high aspect ratio semiconductor nanowires, ordered quantum dot arrays, and microporous hydroxyapatite bioceramics. Emerging challenges of the plasma-aided synthesis of functional nanofilms and nanoassemblies are also discussed.     

Temperature dependence of InN film deposition by an RF plasma-assisted reactive ion beam sputtering deposition technique

Indium nitride (InN) films were deposited on Si(100) substrates using a radiofrequency (RF) plasma-assisted reactive ion beam sputtering deposition technique at various substrate temperatures. The X-ray diffraction patterns of the InN films suggest that the InN films deposited at substrate temperatures up to 370 °C were cubic crystalline InN; and at 500 °C, the InN film was hexagonal crystalline InN. In a scanning electron microscope image of the InN film surface, facets of cubic single-crystalline InN grains were clearly observed on the InN film deposited at 370 °C. The inclusion of metallic indium appeared on the InN film deposited at 500 °C.     

Femtosecond pulsed laser ablation and deposition of titanium carbide

In this work we have evaporated a titanium carbide target by an Nd:glass laser with 250 fs pulse duration. The plasma produced from the ablation has been characterized by Intensified Coupled Charge Device (ICCD) fast imaging, optical emission spectroscopy and quadrupole mass spectrometry, while X-ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDX), X-ray Photoelectron Spectroscopy (XPS) and Scanning and Transmission Electron Microscopy have been used to study the deposited film morphology and composition. The plume shape and front velocity are very similar to those found in other systems and are typical of femtosecond ablation. In particular the front velocity is 1.1 × 10⁷ cm s^− 1 at a laser fluence of 1.9 J cm^− 2, while the value of the cosine exponent is 4.5 in the same conditions. In the TiC system a delayed emission, found by ICCD imaging and emission spectroscopy, is also present. In fact, although the emission involved in the “traditional” plume ends after about 1 μs, the target is still hot and gives origin to another emission, expanding with a velocity that is about two orders of magnitude lower compared to that of the traditional plume (2.2 × 10⁴ cm s^− 1 at a laser fluence of 1.9 J cm^− 2).The results of the analysis of both the gaseous plume and the deposited films seem to indicate that in the case of TiC system the presence of a large number of particles ejected from the target is responsible for the formation of the films. XPS and EDX data indicate that the stoichiometry of the target is preserved in the films, while XRD analysis shows that the films are amorphous in structure.     

Experimental study of the behaviour of leather under vacuum conditions

A characterization of the device-pumping behaviour of our vacuum reactor for plasma treatment of materials at low pressure is presented. The analysis of the pumpdown curves measured both in the viscous and in the molecular regimes was made to study the outgassing flowrates of several leather materials. Estimates of the relevant flowrates in different pressure ranges and measurements of the flow composition have been obtained. Such data can be interpreted in the framework of simplified desorption models and could be used in order to design the pumping system needed for vacuum treatment of such materials. We have also measured other related quantities such as the moisture uptake and the weight loss.     

Generation of nitrogen beams with very high N/N2 ratio using hollow cathode discharge

Nitrogen beams with very high N⁺/N₂⁺ ratio were generated using hollow-cathode discharge. The dependence of N⁺/N₂⁺ ratio in the extracted beams on the discharge parameters was studied by mass spectroscopy. Very high N⁺ fractions (up to 90% and the maximum is about 98%) can be easily achieved without mass separation under typical operating conditions. The extracted beams with discharges using mixture precursors, such as N₂/NH₃, or N₂/Ar were also investigated.     

Estimate of the negative ion density in reactive gas plasmas

The negative ion density in a SF₆/Ar double plasma is estimated. Here, the density ratio of negative ions to positive ions is evaluated from the reduction rates of the ion and electron saturation current. Furthermore, the negative ion density obtained with this method is confirmed to agree with that calculated from the measured phase velocity of the ion acoustic wave (fast mode) when <0.6, where the positive and negative ion mass are obtained from the spectrum analysis with a quadruple mass spectrometer (QMS) system. The negative ion density in fluorocarbon ECR plasma is also estimated by means of the Langmuir probe method. It is found that in fluorocarbon ECR plasma is less than 0.5.     

Electromagnetic sources of nonuniformity in large area capacitive reactors

Theory and experiment show that two electromagnetic modes are necessary and sufficient to determine the field nonuniformity within a parallel-plate rf capacitive plasma reactor. These two modes give rise to the standing wave effect and the telegraph effect. The standing wave effect is associated with high frequencies in large reactors where the reactor size is larger than about a tenth of the vacuum wavelength of the rf excitation. The telegraph effect is associated with asymmetric electrode areas, which necessitates the redistribution of rf current along the plasma to maintain rf current continuity.     

Role of carbon atoms in plasma-enhanced chemical vapor deposition for carbon nanotubes synthesis

The role of carbon atoms in a dc plasma-enhanced chemical vapor deposition for carbon nanotubes (CNTs) synthesis was investigated. It was observed that at 1.33 kPa pressure of CH₄ gas in plasma, a high value of the ratio between the intensities of the graphite peak (G peak) and the disorder peak (D peak) in the Raman spectrum corresponds to the maximum value of the excited C number density in the vicinity of the Si substrate. It was found that a CH₄ gas pressure higher than 1.33 kPa leads to an increase of the relative density of the C₂, C₃ molecules and the clusters, and to a decrease of the C excited atom number density in plasma. The presence of a high amount of sp²-graphite in the composition of CNTs observed in Raman spectrum was also confirmed by the measurement of the IR-active G peak at 1584 cm^- 1 in the transmission spectrum.     

Effect of bias in patterning diamond by a dual electron cyclotron resonance-radio-frequency oxygen plasma

Arrays of microhole patterns are fabricated on the surfaces of diamond films through a physical mask in a dual microwave electron cyclotron resonance/radio-frequency oxygen plasma. It is found that nanotips with high aspect ratio form in the microholes, and then through-holes are fabricated with a further increase of etching time. Optical emission spectroscopy was employed to calculate oxygen atom density and evaluate the variation of the plasma excitation temperature. The plasma excitation temperature and the O atom density present significant dependences on the voltage of rf bias V_b at a high frequency of 13.56 MHz, suggesting that the application of the rf bias not only strengthens ion bombardment on the material surface, but also induces the variations of the bulk plasmas including the increase of O atom density. Whereas, both the plasma excitation temperature and the O atom density remain nearly unchanged with V_b under the bias frequency of 400 kHz. The etching process depends on the rf-bias frequency and voltages, which are correlated with the measured plasma characteristics.