Discharge mode transitions in low-frequency inductively coupled plasmas with internal oscillating current sheets

Transitions between the two discharge modes in a low-frequency (∼460 kHz) inductively coupled plasma sustained by an internal oscillating radio frequency (rf) current sheet are studied. The unidirectional rf current sheet is generated by an internal antenna comprising two orthogonal sets of synphased rf currents driven in alternately reconnected copper litz wires. It is shown that in the low-to-intermediate pressure range the plasma source can be operated in the electrostatic (E) and electromagnetic (H) discharge modes. The brightness of the E -mode argon plasma glow is found remarkably higher than in inductively coupled plasmas with external flat spiral "pancake" coils. The cyclic variations of the input rf power result in pronounced hysteretic variations of the optical emission intensity and main circuit parameters of the plasma source. Under certain conditions, it appears possible to achieve a spontaneous E→H transition ("self-transition"). The observed phenomenon can be attributed to the thermal drift of the plasma parameters due to the overheating of the working gas. The discharge destabilizing factors due to the gas heating and step-wise ionization are also discussed. © 2005 American Vacuum Society.

Suzumushi : the silent swarm

This silent swarm of stylized crickets is downloading data from Internet and catalogue searches being undertaken by the public at the State Library Queensland. These searches are being displayed on the screen on their backs. Each cricket downloads the searches and communicates this information with other crickets. Commonly found searches spread like a meme through the swarm. In this work memes replace the crickets’ song, washing like a wave through the swarm and changing on the whim of Internet users. When one cricket begins calling others, the swarm may respond to produce emergent patterns of text. When traffic is slow or of now interest to the crickets, they display onomatopoeia. The work is inspired by R. Murray Schafer’s research into acoustic ecologies. In the 1960’s Schafer proposed that many species develop calls that fit niches within their acoustic environment. An increasing background of white noise dominates the acoustic environment of urban human habitats, leaving few acoustic niches for other species to communicate. The popularity of headphones and portable music may be seen as an evolution of our acoustic ecology driven by our desire to hear expressive, meaningful sound, above the din of our cities. Similarly, the crickets in this work are hypothetical creatures that have evolved to survive in a noisy human environment. This speculative species replaces auditory calls with onomatopoeia and information memes, communicating with the swarm via radio frequency chirps instead of sound. Whilst these crickets cannot make sound, each individual has been programmed respond to sound generated by the audience, by making onomatopoeia calls in text. Try talking to a cricket, blowing on its tail, or making other sounds to trigger a call.

A hybrid approach for resolving the electromagnetic fields inside a waveguide loaded with a lossy medium

In this work a novel hybrid approach is presented that uses a combination of both time domain and frequency domain solution strategies to predict the power distribution within a lossy medium loaded within a waveguide. The problem of determining the electromagnetic fields evolving within the waveguide and the lossy medium is decoupled into two components, one for computing the fields in the waveguide including a coarse representation of the medium (the exterior problem) and one for a detailed resolution of the lossy medium (the interior problem). A previously documented cell-centred Maxwell’s equations numerical solver can be used to resolve the exterior problem accurately in the time domain. Thereafter the discrete Fourier transform can be applied to the computed field data around the interface of the medium to estimate the frequency domain boundary condition in-formation that is needed for closure of the interior problem. Since only the electric fields are required to compute the power distribution generated within the lossy medium, the interior problem can be resolved efficiently using the Helmholtz equation. A consistent cell-centred finite-volume method is then used to discretise this equation on a fine mesh and the underlying large, sparse, complex matrix system is solved for the required electric field using the iterative Krylov subspace based GMRES iterative solver. It will be shown that the hybrid solution methodology works well when a single frequency is considered in the evaluation of the Helmholtz equation in a single mode waveguide. A restriction of the scheme is that the material needs to be sufficiently lossy, so that any penetrating waves in the material are absorbed.

Hydrogen gas sensing properties of Pt/Ta2O5 Schottky diodes based on Si and SiC substrates

In this paper, we fabricated Pt/tantalum oxide (Ta2O5) Schottky diodes for hydrogen sensing applications. Thin (4 nm) layer of Ta2O5 was deposited on silicon (Si) and silicon carbide (SiC) substrates by radio frequency (RF) sputtering technique. We compared the performance of these sensors at different elevated temperatures of 100 ∘C and 150 ∘C. At these temperatures, the sensor based on SiC exhibited a larger sensitivity while the sensor based on Si exhibited a faster response toward hydrogen gas. We discussed herein, the responses exhibited by the Pt/Ta2O5 based Schottky diodes demonstrated a promising potential for hydrogen sensing applications.

Removal of epoxy-attached thin-sections from glass slides by plasma-etching

Thin-sectioned samples mounted on glass slides with common petrographic epoxies cannot be easily removed (for subsequent ion-milling) by standard methods such as heating or dissolution in solvents. A method for the removal of such samples using a radio frequency (RF) generated oxygen plasma has been investigated for a number of typical petrographic and ceramic thin sections. Sample integrity and thickness were critical factors that determined the etching rate of adhesive and the survivability of the sample. Several tests were performed on a variety of materials in order to estimate possible heating or oxidation damage from the plasma. Temperatures in the plasma chamber remained below 138°C and weight changes in mineral powders etched for 76 hr were less than ±4%. A crystal of optical grade calcite showed no apparent surface damage after 48 hr of etching. Any damage from the oxygen plasma is apparently confined to the surface of the sample, and is removed during the ion-milling stage of transmission electron microscopy (TEM) sample preparation.

Pt/Nanostructured RuO2/SiC Schottky Diode based hydrogen gas sensors

In this paper, we report the development of novel Pt/nanostructured RuO2/SiC Schottky diode based sensors for hydrogen gas applications. The nanostructured ruthenium oxide thin films were deposited on SiC substrates using radio frequency sputtering technique. Scanning electron microscopy revealed the sputtered RuO2 layer consists of nano-cubular structures with dimensions ranging between 10 and 50 nm. X-ray diffraction confirmed the presence of tetragonal ruthenium (IV) oxide, with preferred orientation along the (101) lattice plane. The current-voltage characteristics of the sensors were investigated towards hydrogen gas in synthetic air at different temperatures from 25 °C to 240 °C. The dynamic responses of the sensors were studied at an optimum temperature of 240 °C and a voltage shift of 304 mV was recorded toward 1% hydrogen gas.

Hydrogen gas sensing properties of Pt/Ta2O5 Schottky diodes based on Si and SiC substrates

We developed Pt/tantalum oxide (Ta2O5) Schottky diodes for hydrogen sensing applications. Thin layer (4 nm) of Ta2O5 was deposited on silicon (Si) and silicon carbide (SiC) substrates using the radio frequency sputtering technique. We compared the performance of these sensors at different temperatures of 100 °C and 150 °C. At these operating temperatures, the sensor based on SiC exhibited a larger sensitivity, whilst the sensor based on Si exhibited a faster response toward hydrogen gas. We discussed herein, the experimental results obtained for these Pt/Ta2O5 based Schottky diodes exhibited that they are promising candidates for hydrogen sensing applications.

Designing atmospheric-pressure plasma sources for surface engineering of nanomaterials

Atmospheric-pressure plasma processing techniques emerge as efficient and convenient tools to engineer a variety of nanomaterials for advanced applications in nanoscience and nanotechnology. This work presents different methods, including using a quasi-sinusoidal high-voltage generator, a radio-frequency power supply, and a uni-polar pulse generator, to generate atmospheric-pressure plasmas in the jet or dielectric barrier discharge configurations. The applicability of the atmospheric-pressure plasma is exemplified by the surface modification of nanoparticles for polymeric nanocomposites. Dielectric measurements reveal that representative nanocomposites with plasma modified nanoparticles exhibit notably higher dielectric breakdown strength and a significantly extended lifetime.

Silica nanoparticles treated by cold atmospheric-pressure plasmas improve the dielectric performance of organic–inorganic nanocomposites

We report on the application of cold atmospheric-pressure plasmas to modify silica nanoparticles to enhance their compatibility with polymer matrices. Thermally nonequilibrium atmospheric-pressure plasma is generated by a high-voltage radio frequency power source operated in the capacitively coupled mode with helium as the working gas. Compared to the pure polymer and the polymer nanocomposites with untreated SiO2, the plasma-treated SiO2–polymer nanocomposites show higher dielectric breakdown strength and extended endurance under a constant electrical stress. These improvements are attributed to the stronger interactions between the SiO2 nanoparticles and the surrounding polymer matrix after the plasma treatment. Our method is generic and can be used in the production of high-performance organic–inorganic functional nanocomposites.

Copper-capped carbon nanocones on silicon : plasma-enabled growth control

Controlled self-organized growth of vertically aligned carbon nanocone arrays in a radio frequency inductively coupled plasma-based process is studied. The experiments have demonstrated that the gaps between the nanocones, density of the nanocone array, and the shape of the nanocones can be effectively controlled by the process parameters such as gas composition (hydrogen content) and electrical bias applied to the substrate. Optical measurements have demonstrated lower reflectance of the nanocone array as compared with a bare Si wafer, thus evidencing their potential for the use in optical devices. The nanocone formation mechanism is explained in terms of redistribution of surface and volumetric fluxes of plasma-generated species in a developing nanocone array and passivation of carbon in narrow gaps where the access of plasma ions is hindered. Extensive numerical simulations were used to support the proposed growth mechanism.

Different nanostructures from different plasmas : nanoflowers and nanotrees on silicon

Using the advanced radio-frequency plasma-assisted magnetron deposition system, various nanostructures such as nanoflowers of carbon nanotubes, ZnO nanobelts, and silicon nanotrees were successfully synthesized. In this paper, we present the photographs of ICP and magnetron discharges, the photograph of a complex plasma structure, and the SEM images of various nanostructures synthesized in the system with magnetron and ICP sources operating simultaneously.

Real-time monitoring of nucleation-growth cycle of carbon nanoparticles in acetylene plasmas

Quantum cascade laserabsorption spectroscopy was used to measure the absolute concentration of acetylene in situ during the nanoparticle growth in Ar + C2H2 RF plasmas. It is demonstrated that the nanoparticle growth exhibits a periodical behavior, with the growth cycle period strongly dependent on the initial acetylene concentration in the chamber. Being 300 s at 7.5% of acetylene in the gas mixture, the growth cycle period decreases with the acetylene concentration increasing; the growth eventually disappears when the acetylene concentration exceeds 32%. During the nanoparticle growth, the acetylene concentration is small and does not exceed 4.2% at radio frequency (RF) power of 4 W, and 0.5% at RF power of 20 W. An injection of a single acetylene pulse into the discharge also results in the nanoparticlenucleation and growth. The absorption spectroscopy technique was found to be very effective for the time-resolved measurement of the hydrocarbon content in nanoparticle-generatingplasmas.

Structural, optical and electrical properties of Al-doped ZnO transparent conducting oxide for solar cell applications

Al-doped zinc oxide (AZO) thin films are deposited onto glass substrates using radio-frequency reactive magnetron sputtering and the improvements in their physical properties by post-synthesis thermal treatment are reported. X-ray diffraction spectra show that the structure of films can be controlled by adjusting the annealing temperatures, with the best crystallinity obtained at 400°C under a nitrogen atmosphere. These films exhibit improved quality and better optical transmittance as indicated by the UV-Vis spectra. Furthermore, the sheet resistivity is found to decrease from 1.87 × 10-3 to 5.63 × 10-4Ω⋅cm and the carrier mobility increases from 6.47 to 13.43 cm2 ⋅ V-1 ⋅ s-1 at the optimal annealing temperature. Our results demonstrate a simple yet effective way in controlling the structural, optical and electrical properties of AZO thin films, which is important for solar cell applications.

Silicon on silicon : self-organized nanotip arrays formed in reactive Ar+H2 plasmas

The formation of arrays of vertically aligned nanotips on a moderately heated (up to 500 degrees C) Si surface exposed to reactive low-temperature radio frequency (RF) Ar+H(2) plasmas is studied. It is demonstrated that the nanotip surface density, aspect ratio and height dispersion strongly depend on the substrate temperature, discharge power, and gas composition. It is shown that nanotips with aspect ratios from 2.0 to 4.0 can only be produced at a higher RF power density (41.7 mW cm(-3)) and a hydrogen content of about 60%, and that larger aspect ratios can be achieved at substrate temperatures of about 300 degrees C. The use of higher (up to 500 degrees C) temperatures leads to a decrease of the aspect ratio but promotes the formation of more uniform arrays with the height dispersion decreasing to 1.5. At lower (approximately 20 mW cm(-3)) RF power density, only semispherical nanodots can be produced. Based on these experimental results, a nanotip formation scenario is proposed suggesting that sputtering, etching, hydrogen termination, and atom/radical re-deposition are the main concurrent mechanisms for the nanostructure formation. Numerical calculations of the ion flux distribution and hydrogen termination profiles can be used to predict the nanotip shapes and are in a good agreement with the experimental results. This approach can be applied to describe the kinetics of low-temperature formation of other nanoscale materials by plasma treatment.

Disentangling fluxes of energy and matter in plasma-surface interactions : effect of process parameters

The possibility to discriminate between the relative importance of the fluxes of energy and matter in plasma-surface interaction is demonstrated by the energy flux measurements in low-temperature plasmas ignited by the radio frequency discharge (power and pressure ranges 50-250 W and 8-11.5 Pa) in Ar, Ar+ H2, and Ar+ H2 + CH4 gas mixtures typically used in nanoscale synthesis and processing of silicon- and carbon-based nanostructures. It is shown that by varying the gas composition and pressure, the discharge power, and the surface bias one can effectively control the surface temperature and the matter supply rates. The experimental findings are explained in terms of the plasma-specific reactions in the plasma bulk and on the surface.