RF Plasma Reactor Research Articles

Direct Liquid Reactor-Injector of Nanoparticles: A Safer-by-Design Aerosol Injection for Nanocomposite Thin-Film Deposition Adapted to Various Plasma-Assisted Processes

The requirements of nanocomposite thin films, having non-aggregated nanoparticles homogeneously dispersed in the matrix, have been realized using a new method of Direct Liquid Reactor-Injector (DLRI) of nanoparticles. In this approach, unlike conventional aerosol-assisted plasma deposition, the nanoparticles are synthesized before their injection as an aerosol into plasma. In our experiments, we have used two different plasma reactors, namely an asymmetric low-pressure RF plasma reactor and a parallel plate dielectric barrier discharge at atmospheric pressure. Our results have shown that DLRI can be easily coupled with various plasma processes as this approach allows the deposition of high-quality multifunctional nanocomposite thin films, with embedded nanoparticles of less than 10 nm in diameter. Hence, DLRI coupled with plasma processes meets the specifications for the deposition of multifunctional coatings.

Impact of Internal Faraday Shields on RF Driven Hydrogen Discharges

At RF plasma reactors operated at high power, internal Faraday shields are required to shield dielectric vessel or windows from erosion due to isotropic heat and particle fluxes. By utilizing a flexible and diagnostically well-equipped laboratory setup, crucial effects that accompany the application of internal Faraday shields at low-pressure hydrogen (and deuterium) RF discharges are identified and quantified in this contribution. Both an inductively coupled plasma (ICP) utilizing a helical coil and a low-field helicon discharge applying a Nagoya-type III antenna at magnetic fields of up to 12 mT are investigated. Discharges are driven at 4 MHz and in the pressure range between 0.3 and 10 Pa while the impact of the Faraday shields on both the RF power transfer efficiency and spectroscopically determined bulk plasma parameters (electron density and temperature, atomic density) is investigated. Three main effects are identified and discussed: (i) due to the Faraday shield, the measured RF power transfer efficiency is globally reduced. This is mainly caused by increased power losses due to induced eddy currents within the electrostatic shield, as accompanying numerical simulations by a self-consistent fluid model demonstrate. (ii) The Faraday shield reduces the atomic hydrogen density in the plasma by one order of magnitude, as the recombination rate of atoms on the metallic (copper) surfaces of the shield is considerably higher compared to the dielectric quartz walls. (iii) The Faraday shield suppresses the transition of the low-field helicon setup to a wave heated regime at the present conditions. This is attributed to a change of boundary conditions for wave propagation, as the plasma is in direct contact with the conductive surfaces of the Faraday shield rather than being operated in a laterally fully dielectric vessel.

Influence of applied plasma power on degradation of L-proline in an inductively coupled RF plasma reactor

Surface treatments such as heat treatments, paints, plasma treatments and galvanizing are important in metal working industry. In those treatments, the required adhesion characteristics can be limited if contaminants are present, so there is a need to clean these surfaces. Conventional processes can be costly, some generated waste may be environmentally hazardous and, in some cases, due to part geometry, they are not effective. Alternatively, plasma treatments which are considered to be environmentally clean, more efficient and with lower operating costs can overcome those difficulties. Due to the reactional complexity of these treatments, there are many fields to be researched, such as which reactions are due to impacts of (neutral) chemical species and which are due to excited species and radiation. The present work aims to verify the interactions between Ar-10%O2 plasma using amino acid L-proline in an inductively coupled radiofrequency plasma. This study showed that oxidative plasma promotes high degradation of the sample. A gas analyzer (quadrupole mass spectrometer) coupled to the gas outlet was used and many gaseous products are detected during treatment, such: H2, H2O, CO, NO (Nitric Oxide) and CO2 while L-proline undergoes intense degradation. The conclusions allow to state that the interaction between the Ar/O2 plasma and L-proline promotes intense degradation through the breakdown of the heterocyclic ring of the molecule.

Sensitivity Enhancement in Plasma Polymer Films for Surface Acoustic Wave Based Sensor Applications

Plasma polymer films (PPF), widely used as sensing layers in surface acoustic wave (SAW) based gas and liquid phase sensors, have a major drawback: high concentrations of the sensed analytes easily drive these films into saturation, where accurate measurements are no longer possible. This work suggests a solution to this problem by modifying the PPF with the sensed chemical compound to improve the overall sorption properties and sensor dynamic range. Thin polymer films were synthesized from hexamethyldisiloxane (HMDSO) and triethylsilane (TES) monomers in a plasma-enhanced chemical vapor deposition (PECVD) process using a RF plasma reactor. We used these Si-containing compounds because they are known for their excellent sensing properties. In this work, the layers were deposited onto the active surface of high-Q 438 MHz Rayleigh SAW two-port resonators, used as mass sensitive sensor elements. We call these devices quartz surface microbalances (QSM). In a second step, ammonia plasma modification was applied to the HMDSO and TES films, in order to achieve a higher sensitivity to NH3. The sensors were probed at different NH3 gas concentrations in a computer controlled gas probing setup. A comparison with unmodified films revealed a 74% to 85% improvement in both the sensitivity and sorption ability of the HMDSO sensing layers, and of about 8% for the TES films.

Preparation of graphene-based nanomaterials by pulsed RF discharges on liquid organic compounds

A new pulsed RF plasma reactor has been used for the synthesis of graphene-based nanomaterials under atmopheric pressure conditions. This is an environmental-friendly system, not using catalyst and where no hazardous components are generated. Nanographenes formed upon plasma treatment of some hydrocarbons (cyclohexane, cyclohexene, heptane, hexane, pentane, and toluene), while N-containing organic compounds (cyclohexylamine, pyridine, dipropylamine, and triethylamine) resulted in N-doped graphenes. Synthesis took place in a very fast way, mediated by plasma reactive species.

Pyrolysis and Oxidation of Methane in a RF Plasma Reactor

The chemical kinetic effects of RF plasma on the pyrolysis and oxidation of methane were studied experimentally and computationally in a laminar flow reactor at 100 Torr and 373 K with and without oxygen addition into He/CH4 mixtures. The formation of excited species as well as intermediate species and products in the RF plasma reactor was measured with optical emission spectrometer and Gas Chromatography and the data were used to validate the kinetic model. The kinetic analysis was performed to understand the key reaction pathways. The experimental results showed that H2, C2 and C3 hydrocarbon formation was the major pathways for plasma assisted pyrolysis of methane. In contrast, with oxygen addition, C2 and C3 formation dramatically decreased, and syngas (H2 and CO) became the major products. The above results revealed oxygen addition significantly modified the chemistry of plasma assisted fuel pyrolysis in a RF discharge. Moreover, an increase of E/n was found to be more beneficial for the formation of higher hydrocarbons while a small amount of oxygen was presented in a He/CH4 mixture. A reaction path flux analysis showed that in a RF plasma, the formation of active species such as CH3, CH2, CH, H, O and O (1D) via the electron impact dissociation reactions played a critical role in the subsequent processes of radical chain propagating and products formation. The results showed that the electronically excitation, ionization, and dissociation processes as well as the products formation were selective and strongly dependent on the reduced electric field.

Hydrophobic coating of surfaces by plasma polymerization in an RF plasma reactor with an outer planar electrode: synthesis, characterization and biocompatibility

This paper presents the plasma polymerization of poly(hexafluorobutyl acrylate) (PHFBA) thin films on different substrates in an RF plasma reactor with an outer planar electrode. This reactor configuration allows large area uniformity and fast processing times. Deposition rates of up to 60 nm min−1 were observed. The influence of plasma power and substrate temperature on the deposition rate, structure and wettability of the as-deposited films was investigated. It was observed that better hydrophobicity was obtained at high plasma power and in low temperature conditions. PHFBA thin films deposited on electrospun poly(acrylonitrile) fiber mats under such conditions resulted in superhydrophobic surfaces with contact angle values greater than 150°. In vitro cell studies using human epithelial cells demonstrated the non-toxic nature of the plasma-polymerized PHFBA films.

Ion flux and energy virtual sensor for measuring ion flux and energy distribution at a RF biased electrode in ICP reactor (RIE-mode)

The modern technology of micro- and nanoelectronics involves a great number of steps, such as pattern transfer, where Reactive Ion Etching (RIE) in rf plasma reactors is widely used. To control the etching process, the ion flux and ion energy distribution should be managed precisely. However, the measurements of these parameters during the process in the real-time operation are impossible. This paper is devoted to the construction of a virtual diagnostics of the Ion Energy Distribution (IED) function. This method for the determination of the ion energy spectrum on the surface of rf biased electrode is based on model calculations using in-situ measured discharge parameters. The results of IED virtual diagnostics were compared with data, obtained by Retarded Field Energy Analyzer (RFEA). This was done for Ar- and H2-plasmas operated under low-pressure rf plasma conditions. The good agreement between the model and the experimental justifies the conclusion that the IED virtual diagnostics can be applied successfully. This enables the in-situ monitoring of the IED at the electrode surface in RIE reactors.

Diagnostics of low-pressure hydrogen discharge created in a 13.56 MHz RF plasma reactor

A 13.56 MHz RF discharge in hydrogen was studied within the pressure range of 1–10 Pa, and at a power range of 400–1000 W. The electron energy distribution function and electron density were measured by a Langmuir probe. The gas temperature was determined by the Fulcher-α system in pure H2, and by the second positive system of nitrogen using N2 as the probing gas. The gas temperature was constant and equal to 450 ± 50 K in the capacitively coupled plasma (CCP) mode, and it increased with pressure and power in the inductively coupled plasma (ICP) mode. Also, the vibrational temperature of the ground state of hydrogen molecules was determined to be around 3100 and 2000 ± 500 K in the ICP and CCP mode, respectively. The concentration of atomic hydrogen was determined by means of actinometry, either by using Ar (5%) as the probing gas, or by using H2 as the actinometer in pure hydrogen (Q1 rotational line of Fulcher-α system). The concentration of hydrogen density increased with pressure in both modes, but with a dissociation degree slightly higher in the ICP mode (a factor 2).

Apparatus and method for plasma processing of SRF cavities

An apparatus and a method are described for plasma etching of the inner surface of superconducting radio frequency (SRF) cavities. Accelerator SRF cavities are formed into a variable-diameter cylindrical structure made of bulk niobium, for resonant generation of the particle accelerating field. The etch rate non-uniformity due to depletion of the radicals has been overcome by the simultaneous movement of the gas flow inlet and the inner electrode. An effective shape of the inner electrode to reduce the plasma asymmetry for the coaxial cylindrical rf plasma reactor is determined and implemented in the cavity processing method. The processing was accomplished by moving axially the inner electrode and the gas flow inlet in a step-wise way to establish segmented plasma columns. The test structure was a pillbox cavity made of steel of similar dimension to the standard SRF cavity. This was adopted to experimentally verify the plasma surface reaction on cylindrical structures with variable diameter using the segmented plasma generation approach. The pill box cavity is filled with niobium ring- and disk-type samples and the etch rate of these samples was measured.

Enhanced mechanical properties of low-surface energy thin films by simultaneous plasma polymerization of fluorine and epoxy containing polymers

Thin films of poly(2,2,3,4,4,4 hexafluorobutyl acrylate-glycidyl methacrylate) (P(HFBA-GMA) were deposited on different surfaces using an inductively coupled RF plasma reactor. Fluorinated polymer was used to impart hydrophobicity, whereas epoxy polymer was used for improved durability. The deposition at a low plasma power and temperature was suitable for the functionalization of fragile surfaces such as textile fabrics. The coated rough textile surfaces were found to be superhydrophobic with water contact angles greater than 150° due to the high retention of long fluorinated side chains. The hydrophobicity of the surfaces was observed to be stable after many exposures to ultrasonification tests, which is attributed to the mechanical durability of the films due to their epoxide functionality. FTIR and XPS analyses of the deposited films confirmed that the epoxide functionality of the polymers increased with increasing glycidyl methacrylate fraction in the reactor inlet. The modulus and hardness values of the films also increase with increasing epoxide functionality.

Funnelling of rf current via a plasmoid through a grid hole in an rf capacitive plasma reactor

The boundaries of capacitively coupled radio-frequency (rf) plasma reactors generally include at least one grounded metal grid or perforated plate for purposes of gas flow or diagnostic access. When increasing the rf power, an intense localized plasma (a plasmoid) can spontaneously ignite in a hole of a grounded surface. Experiments described here show that the plasmoid funnels rf current through the hole to the other side of the grounded plate, thereby increasing the effective grounded area in contact with the plasma. Hence, plasmoid ignition is always accompanied by a drop in the dc self-bias voltage of the rf electrode.The small area of the plasmoid aperture means that the rf current density passing through the plasmoid is very high, causing intense optical emission and strong local heating. Plasmoid ignition can therefore cause a loss of process reproducibility and potentially lead to melting and eventual destruction of reactor components.

Design and operation of a rotating drum radio frequency plasma reactor for the modification of free nanoparticles

A rotating drum rf plasma reactor was designed to functionalize the surface of nanoparticles and other unusually shaped substrates through plasma polymerization and surface modification. This proof-of-concept reactor design utilizes plasma polymerized allyl alcohol to add OH functionality to Fe2O3 nanoparticles. The reactor design is adaptable to current plasma hardware, eliminating the need for an independent reactor setup. Plasma polymerization performed on Si wafers, Fe2O3 nanoparticles supported on Si wafers, and freely rotating Fe2O3 nanoparticles demonstrated the utility of the reactor for a multitude of processes. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy were used to characterize the surface of the substrates prior to and after plasma deposition, and scanning electron microscopy was used to verify that no extensive change in the size or shape of the nanoparticles occurred because of the rotating motion of the reactor. The reactor design was also extended to a non-depositing NH3 plasma modification system to demonstrate the reactor design is effective for multiple plasma processes.

Control of the ion flux and ion energy in CCP discharges using non-sinusoidal voltage waveforms

Using particle-in-cell simulations we perform a characterization of the ion flux and ion energy in a capacitively coupled rf plasma reactor excited with non-sinusoidal voltage waveforms. The waveforms used are positive Gaussian type pulses (with a repetition frequency of 13.56 MHz), and as the pulse width is decreased, three main effects are identified that are not present in typical symmetric sinusoidal discharges: (1) the ion flux (and plasma density) rapidly increases, (2) as the pressure increases a significant asymmetry in the ion fluxes to the powered and grounded electrodes develops and (3) the average ion energy on the grounded electrode cannot be made arbitrarily small, but in fact remains essentially constant (together with the bias voltage) for the pressures investigated (20–500 mTorr). Effects (1) and (3) potentially offer a new form of control in these types of rf discharges, where the ion flux can be increased while keeping the average ion energy on the grounded electrode constant. This is in contrast with the opposite control mechanism recently identified in Donkó et al (2009 J. Phys. D: Appl. Phys. 42 025205), where by changing the phase angle between applied voltage harmonics the ion flux can be kept constant while the ion energy (and bias voltage) varies.

Cleaning of Organic Contamination from EUV Optics Surfaces Using Hydrogen-based Plasmas

The efficiency of optics used in extreme ultraviolet (EUV) range suffers from reflectivity degradation due to oxidation and carbon contamination of mirror surfaces. Therefore, an efficient cleaning procedure should be ascertained in order to facilitate applications of EUV lithography tools. Carbon contamination removal from the mirror surface has been reported in Hydrogen plasmas, and laser-induced Hydrogen plasmas in EUV tools. Here, we performed measurements in a helicon-type RF plasma reactor with hydrogen pressures similar to the ones used in laser-induced plasmas in EUV tools. The method of optical actinometry was used to determine hydrogen atom concentration and the degree of dissociation. The results are compared to those obtained by pressure-rise measurements. Preliminary results of plasma etching of thin organic films (PMMA) were also obtained to assess the cleaning efficiency.

Deposition of cobalt oxide thin films by plasma-enhanced chemical vapour deposition (PECVD) for catalytic applications

Plasma-enhanced chemical vapour deposition (PECVD) was used to prepare thin films of cobalt oxide. Cobalt oxide-based (CoO and Co3O4) catalysts were chosen due to their efficiency in mineralisation of organic pollutants achieved by catalytic ozonation. In this work, two types of PECVD processes were used for the production of cobalt oxide thin films. In the first one, a solution of nitrate salt of cobalt was sprayed into a RF low pressure plasma discharge (40MHz, 600Pa, 200W) to obtain CoxOy layers. In the second MOPECVD (metal organic plasma-enhanced chemical vapour deposition) process, cobalt oxide thin films were deposited using a capacitive coupled external electrodes RF plasma reactor (13.56MHz, 100Pa, 200W) with cobalt carbonyl Co2(CO)8 dissolved in hexene as precursor sprayed in a gas carrier (argon and oxygen).In the case of coatings produced from a solution of cobalt nitrate salt, a layer of 1μm of Co3O4 in crystalline form was obtained after annealing. Considering the thin films obtained from cobalt carbonyl precursor, analyses confirmed the presence of cobalt oxide in a polymeric layer on the surface of the substrate. XRD investigation showed the presence of a crystalline phase of Co3O4 (crystallite size of about 40nm).

Nano-columnar grain growth structure of boron-compensated silicon thin films by transmission electron microscopy

Here in we present a study of the influence of Boron concentration upon the growth, and structural properties of hydrogenated nano and microcrystalline silicon thin films. The films were deposited in a RF plasma reactor using a mixture of silane and diborane as reactive gas, both highly diluted in hydrogen. The Boron concentration in the reactive gas was modified from 0 to 100ppm. Increased concentration of Boron induces a change in the amorphous–crystalline transition, showing an increase in the crystal volume fraction (Xc) of the samples with concentrations from 0–75ppm; whereas, there is a decrease of this parameter for the Boron concentration at 100ppm. This fact is related to amorphization of the material for higher doping concentrations. A grain type with columnar structure embedded in an amorphous matrix was observed via electron microscopy micrographs and SEM images. A correlation between morphological properties and defects in the material at the grain boundaries of the columnar grains were also studied.

Surface Hardening of Aluminium-Copper Alloy 2011 by RF Plasma Nitriding Process

In this work, an inductively-coupled rf plasma reactor was utilized in the nitriding process for surface hardness improvement of aluminium-copper alloy 2011. Substrate bias at 400V was used in the pre-sputtering step to eliminate the aluminium oxide on the samples. Plasma nitriding was carried out in a N2-H2 admixture at total pressure of 1 torr. The process length was varied from 9 to 36 hours while the input rf power and substrate temperature were varied from 100 to 300 W and kept at 400 oC, respectively. A negative bias voltage up to 400 V was used in the nitriding process. Glancing incident-angle x-ray diffraction (GIXRD) results showed the hexagonal crystal structure of AlN on samples. The roughness increased slightly when the voltage increase up to 400V and was investigated by Scanning Electron Micrograph (SEM). Electron Probe Microscopy Analysis (EPMA) and Energy Dispersive X-ray Analysis (EDX) were used to detect the N atoms in specimens. Significant increases of surface hardness are observed after plasma nitriding.

Acid/Base Gas/Liquid Adsorption Properties of Carbon Black Modified in NH3/O2 RF Glow Discharges

AbstractCarbon Black granules were modified in a rotating RF (13.56 MHz) plasma reactor fed with NH3/O2 gas mixtures, in order to graft acid (O‐containing) and/or basic (N‐containing) chemical groups and tune their surface acid/base character. X‐rays photoelectron spectroscopy was used to evaluate the surface chemical composition of the modified carbon surface. Boehm titrations, in OH−/H+ water solutions, and NH3/HCl vapour adsorption tests were performed on plasma treated Carbon Black, with the aim to check its capability of adsorbing acid and basic compounds from solution and in gas/vapour phase, respectively.magnified image

Formation and Distribution of Silver Nanoparticles in a Functional Plasma Polymer Matrix and Related Ag+ Release Properties

AbstractPlasma polymer coatings with embedded Ag nanoparticles were deposited in a low pressure RF plasma reactor using an asymmetrical setup with an Ag electrode. The plasma polymer was deposited from a reactive gas/monomer mixture of CO2/C2H4 yielding a functional hydrocarbon matrix. In addition, Ar was simultaneously used to sputter Ag atoms from the Ag electrode, forming nanoparticles within the growing polymer matrix. The influence of the power input, gas ratio and coating thickness on both, the Ag content and the Ag nanoparticle morphology, as well as the distribution in the polymer matrix were investigated. It was found that both increasing the power input and the CO2 ratio result in a higher incorporation of Ag into the matrix.magnified image