Microwave Plasma Source Research Articles

A two-dimensional Plasimo multiphysics model for the plasma–electromagnetic interaction in surface wave discharges: the surfatron source

A novel electromagnetic (EM) model was developed using the Plasimo platform with the aim to study different microwave induced plasmas. In this study, the EM model is coupled to the fluid non-local-thermal-equilibrium (non-LTE) description of a discharge (flow, temperatures and species densities), and it allows the construction of a 2D self-consistent multiphysics model for microwave induced plasmas.The surfatron is a well-known microwave plasma source for the production of surface wave sustained plasmas. The model is used to describe the plasma generated with this source and it permits, for the first time, the study of the differences between the plasma surrounded by the surfatron cavity, i.e. the inside plasma, and the plasma in front of it, i.e. the outside plasma. It also allows the study of the transition from diffusion controlled towards dominant volume recombination electrons losses regime, as is shown with the simulation results with pressure values changing from 660 to 8800 Pa for an argon plasma with a radius of 3 mm and a microwave frequency of 2.45 GHz. The modelling results are compared with experimental measurements of the electron density and temperature and an excellent agreement is found.

Microwave Plasma Sources ‐Applications in Industry

AbstractPlasma, excited with microwave energy, has very interesting properties which are in direct correlation with the excitation frequency. This is due to the reachable high electron density, which itself has an influence on the density of further generated particles like ions, radicals, excited atoms and molecules or UV light. Altogether they are responsible for the high chemical reactivity of microwave plasmas attaining their maximum in the pressure regime between 1 and a few hundred Pa. Therefore, comparatively low end pumping systems can be used to achieve good results in etching, activation and deposition processes. Together with the availability of microwave generators, grown with the market of microwave heating technology, microwave plasma is an interesting tool for industrial applications. Scalability of power up to 100 kW per single magnetron tube and lateral dimensions of a few square meters per single plasma source have opened the field to industrial thin film technology for typical large area applications as solar or other flat panel industries.Roth & Rau Muegge is utilizing these advantages to supply microwave generators and plasma technology for various fields of applications which will be provided as an overview to give insight into running activities in industry. Although the presented applications are part of publicly funded programs and projects together with partners from several research institutes and universities, the technical depth of this contribution has to consider running contracts and non disclosure agreements (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Scalable Microwave Plasma Sources From Low to Atmospheric Pressure

AbstractThere are very specific demands on the plasma processes used in various plasma technological applications. Microwave plasmas offer a wide range of applications for different pressures ranging from very low pressure (<0.1 Pa) over low pressure (0.1‐100 Pa) and medium pressure (103‐104 Pa) up to atmospheric pressure (105 Pa). This contribution is a short review on some microwave based plasma sources at different pressure ranges and a brief introduction into the plasma physics behind them (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Plasma parameters of a small microwave discharge at atmospheric pressure obtained by probe diagnostics

We present the design of a system for probe diagnostics and the results obtained for the plasma parameters in a small portable microwave plasma source operating at atmospheric pressure. The average electron temperature and the argon plasma density in the discharge are estimated from the current-voltage (I-V) characteristics of an asymmetric double probe. The calculated value of the electron temperature is Te ∼1.7±0.3 eV. Three models for the probe ion saturation current in a flowing plasma at atmospheric pressure are applied to calculating the plasma density. The calculated value of the plasma density varies with the input power P = 15 - 25 W and gas flow rate 250 - 350 sccm. The plasma density estimations obtained by the models lie within a wide range and are compared with results from optical diagnostics.

Investigations of a microwave plasma source operating with air, N2, CO2 and argon gases

The present contribution will report on the development of a Microwave Plasma Torch (MPT) at the von Karman institute and on the characterization of subsonic plasma flow of various plasmagene gases (air, N2, CO2 and Ar) at atmospheric pressure. The aim of this work is to explore plasma flow regimes for a large envelope of operating conditions. Using a conventional arrangement for MPT, including cylindrical open-ended dielectric quartz tube, plasma generated with molecular gases have been found to be homogeneous while with argon filamentary structures have been observed. Electrical characterization of the source has been performed for some operating conditions using forwarded and reflected microwave power monitoring. Comprehensive characterization of the flow by means of high speed imaging has been performed to estimate the effect of the oscillation of the delivered power on plasma jet unsteadiness. Analysis in time and frequency domains of the light intensity recorded at 1 kHz rate is presented. Low resolution Optical Emission Spectroscopy (OES) diagnostic was performed in order to identify typical radiative signature of air, N2 and CO2 plasma jet. High resolution OES was performed on pure N2 plasma suitable for thermodynamic characterization using N2+ First Negative and N2 2nd Positive systems. Making the common assumption that the internal energy levels of molecular species are populated according to a Boltzmann distribution and making no assumption concerning the total species concentration similarly to [1], comparisons between measured and calculated radiative signature of molecular systems evidence for the achievement of thermal equilibrium and slight departure to Saha equilibrium.

用于大面积薄膜沉积的线形等离子体源研究进展

本文系统介绍了几种线形等离子体源的特点及其应用，根据激励源的不同分别称为直流、射频、甚高频、微波、双频线形等离子体源，有磁场辅助的称为磁场增强线形等离子体源。与传统的大面积等离子体源不同，线形等离子体源仅需在一维方向实现均匀、稳定的等离子体，采用多个线形结构并排，或者与被镀样品在水平/垂直方向以适当速度运动，即可获得大面积均匀的薄膜沉积。近年来，研究人员采用磁场约束技术，减少带电粒子在器壁的复合损失，进一步提高等离子体的密度、均匀度和稳定性。线形等离子体及磁场增强线形等离子体密度 > 1011 cm−3，不均匀度 1 m)，被广泛用于Si3N4、SiO2、光伏电池硅本征层、纳米金刚石等薄膜的大面积沉积，具有很重要的应用价值和科学意义。 The characteristics and application of several kinds of linear plasma sources are introduced. According to the power sources for plasma systems, there are DC, RF, VHF, microwave and dual frequency linear plasma sources, which with a magnet system are called magnetic field enhanced linear plasma sources. Compared with the conventional large scale plasma sources, linear plasma sources take the advantage that they just need to obtain uniform and stable plasma in one dimension. Through the linear plasma array or moving the substrate in horizontal and vertical direction, large scale uniform thin film can be deposited. Recent years, researchers tried to use the magnetic field to confine plasma so as to reduce the recombination loss in plasma and improve the density, uniformity and stability of plasma. The linear plasma sources with the density above1011 cm−3, dis-uniformity below ±5% (L >1 m), have been applied to fabricate large scale Si3N4, SiO2, intrinsic Si and nano-diamond thin film.

Rapid thermal annealing of sputter-deposited ZnO/ZnO:N/ZnO multilayered structures

Rapid thermal annealing (RTA) of sputter-deposited ZnO/ZnO:N/ZnO multilayered structures formed by a combination of radio-frequency magnetron sputtering and a microwave plasma source was investigated for the fabrication of highly-crystallized ZnO:N films. The assistance of the microwave plasma source resulted in the enhancement of nitrogen incorporation into the ZnO films and the deterioration of film crystallization. On the other hand, crystallization of the ZnO:N layer was improved by RTA with no significant effusion and diffusion of N atoms using a ZnO/ZnO:N/ZnO multilayered structure. The role of the front and bottom ZnO layers during RTA of ZnO/ZnO:N/ZnO multilayered structures is demonstrated.

Generation of cold electrons in the downstream region of a microwave plasma source with near boundary resonances for production of negative ions

A microwave driven multicusp plasma based volume negative ion source equipped with a magnetic filter is developed. Instead of employing any electrodes or current carrying filaments, microwaves of frequency 2.45 GHz is used to generate plasma by resonance heating mechanisms namely the electron cyclotron resonance (ECR) and upper hybrid resonances (UHR), occurring near the boundary plasma layers. The principal process of negative ion production in hydrogen is dissociative attachment of low energy (0.5–1.0 eV) electrons to vibrationally excited neutral molecules generated from high energy (15–20 eV) electron impact. The source therefore necessitates two distinct spatial regions (a) production and (b) attachment chambers; which would contain electrons with optimum cross section for the aforementioned processes. A biased grid after the magnetic filter further helps to lower down the electron temperature to ≤1 eV which is favorable for the dissociative attachment process.

Measurement of axial neutral density profiles in a microwave discharge ion thruster by laser absorption spectroscopy with optical fiber probes

In order to reveal the physical processes taking place within the "μ10" microwave discharge ion thruster, internal plasma diagnosis is indispensable. However, the ability of metallic probes to access microwave plasmas biased at a high voltage is limited from the standpoints of the disturbance created in the electric field and electrical isolation. In this study, the axial density profiles of excited neutral xenon were successfully measured under ion beam acceleration by using a novel laser absorption spectroscopy system. The target of the measurement was metastable Xe I 5p(5)((2)P(0) (3/2))6s[3/2](0) (2) which absorbed a wavelength of 823.16 nm. Signals from laser absorption spectroscopy that swept a single-mode optical fiber probe along the line of sight were differentiated and converted into axial number densities of the metastable neutral particles in the plasma source. These measurements revealed a 10(18) m(-3) order of metastable neutral particles situated in the waveguide, which caused two different modes during the operation of the μ10 thruster. This paper reports a novel spectroscopic measurement system with axial resolution for microwave plasma sources utilizing optical fiber probes.

Localized subsurface modification of materials using micro-low-energy multiple ion beamlets

Generation of focused multiple ion beamlets from an intense microwave plasma source is investigated for the creation of localized subsurface modification of materials. Unlike conventional single element focused ion beam (FIB) systems, the plasma source is capable of providing ion beams of multiple elements. Two types of plasma electrodes (PE) are employed, one with a honeycomb structure with notched apertures and another with a 5×5 array of through apertures, both attached to the plasma source and are capable of generating focused ion beamlets (50 - 100 μm diameter) in a patterned manner. Measurements of ion saturation current near the PE indicate that the plasma is uniform over an area of ∼ 7 cm2, which is further confirmed by uniformity in extracted beam current through the apertures. The ion beams are applied to investigate change in electrical sheet resistance Rs of metallic thin films in a controlled manner by varying the ionic species and beam energy. Results indicate a remarkable increase in Rs with beam energy (∼ 50 % at 1 keV for Ar ions), and with ionic species (∼ 90% for Krypton ions at 0.6 keV), when 80 nm thick copper films are irradiated by ∼2 cm diameter ion beams. Ion induced surface roughness is considered as the main mechanism for this change as confirmed by atomic force microscopy (AFM) measurements. Predictions for micro-beamlet induced change in Rs are discussed. The experimental results are verified using TRIM and AXCEL-INP simulations.

Numerical Analysis of Tuning Procedure of a Waveguide-Based Microwave Plasma Source

Numerically obtained electric field module distributions inside a waveguide-based high-flow microwave plasma source (MPS) with a plasma column inside during a tuning procedure that employs a microwave plunger are presented. Standing-wave patterns have been obtained for each plunger position. The optimal position of the plunger has been found. Strong skin effect is seen in the plasma generated in the presented MPS.

Coaxial microwave plasma source

Physical principles underlying the operation of a pulsed coaxial microwave plasma source (micro-wave plasmatron) are considered. The design and parameters of the device are described, and results of experimental studies of the characteristics of the generated plasma are presented. The possibility of application of this type of plasmatron in gas-discharge physics is discussed.

Numerical Analysis and Optimization of Power Coupling Efficiency in Waveguide-Based Microwave Plasma Source

Three-dimensional electric field distributions in a waveguide-based microwave plasma source (MPS) have been determined numerically. Tuning characteristics of the MPS have been calculated using a direct and newly proposed two-port network method. A method for easy assessment of the quality of a set of the tuning characteristics is presented. Optimization of the plasma source has been performed using these characteristics to ensure good power coupling and stability of the plasma source operation. The calculated tuning characteristics have been compared with experimental ones. Good agreement has been found.

Effects of water addition on OH radical generation and plasma properties in an atmospheric argon microwave plasma jet

Water vapor was added to the feeding gas of a continuous atmospheric argon (Ar) microwave plasma jet to study its influence on plasma shape, plasma gas temperature, and OH radical concentrations. The plasma jet was created by a 2.45 GHz microwave plasma source operating at constant power of 104 W with H2O-Ar mixture flow rate of 1.7 standard liter per minute (slm). With an increase in the H2O/Ar ratio from 0.0 to 1.9%, the plasma jet column length decreased from 11 mm to 4 mm, and the plasma jet became unstable when the ratio was higher than 1.9%; elevation of plasma gas temperature up to 330 K was observed in the plasma temperature range of 420-910 K. Optical emission spectroscopy showed that the dominant plasma emissions changed from N2 in the pure Ar plasma jet to OH with the addition of water vapor, and simulations of emission spectra suggested non-Boltzmann distribution of the rotational levels in the OH A-state (v'=0). Spatially resolved absolute OH number densities along the plasma jet axis were measured using UV cavity ringdown spectroscopy of the OH (A-X) (0-0) band in the H2O/Ar ratio range of 0.0–1.9%. The highest OH number density is consistently located in the vicinity of the plasma jet tip, regardless of the H2O/Ar ratio. OH number density in the post-tip region follows approximately an exponential decay along the jet axis with the fastest decay constant of 3.0 mm in the H2O/Ar ratio of 1.5%. Given the low gas temperature of 420-910 K and low electron temperature of 0.5-5 eV along the jet axis, formation of the OH radical is predominantly due to electron impact induced dissociation of H2O and dissociative recombination of H2O+ resulting from the Penning ionization process.

Development of compact high voltage switched mode power supply for microwave plasma sources supply for low pressure plasma

Although microwave induced plasmas are well known as high efficiency plasma sources, their uses in laboratories are limited since the microwave power systems are complicated and expensive. The output power of commercially available low-cost microwave ovens is fixed and discontinuous resulting from the high voltage doubler topology of the magnetron tube power supply. In this paper, a high voltage switched mode power supply of forward topology has been developed for continuous microwave power radiation. The forward converter can generate a no-load high voltage output maximum of 7 kV. When driving the magnetron tube, the microwave output power could be varied from 0 to 35 W while the high voltage output level was constantly regulated at -3.4 kV. A microwave induced plasma system was setup to investigate the plasma produced. A low pressure argon plasma was produced with only 2 W over a wide range of pressures.

Design of a compact quarter wave coaxial cavity resonator for plasma ignition applications

Atmospheric and higher pressure RF and microwave plasma sources have numerous applications including material processing and spectroscopy. More recently, advantages in using such discharges for combustion ignition are being investigated. A particularly simple and compact microwave discharge generating device is the quarter wave coaxial cavity resonator (QWCCR). This paper presents a new, compacted design of such a device. A simple approximate analysis of the quality factor, Q, which is a measure of the resonant electromagnetic potential step-up capability is given, and compared to experimentally measured quality factors showing reasonable agreement. Analytic results indicate that the foreshortened folded cavity quality factors are comparable to tapered coaxial cavity designs.

Production of hydrogen via conversion of hydrocarbons using a microwave plasma

In this paper, results of hydrogen production from hydrocarbons in an atmospheric pressure microwave plasma are presented. As sources of hydrogen, both methane CH4 and tetrafluoroethane C2H2F4 were tested. A new waveguide-based nozzleless cylinder-type microwave plasma source was used to convert hydrocarbons into hydrogen. The processed gaseous hydrocarbons were introduced into the plasma by four gas ducts which formed a swirl flow in the plasma reactor. The absorbed microwave power was up to 5 kW. The gas flow rate was up to 212 L min−1. The hydrogen mass yield rate and the corresponding energetic hydrogen mass yield were up to 866 g[H2] h−1 and 577 g [H2] kWh−1 of microwave energy absorbed by the plasma, respectively. These parameters are better than our previous results when nitrogen was used as a swirl gas and much better than those typical for other plasma methods of hydrogen production (electron beam, gliding arc, plasmatron).

Note: Flowing ion population from a resonance cavity source

The experimental low energy plasma for hemispherical analyzers nominal testing thermal plasma facility of Dartmouth College uses a microwave plasma source which generates an ionosphere-like plasma through a two-step process. The plasma is initially generated inside a cylindrical, insulated, resonance cavity. This initial plasma must pass through a sheath in order to enter the main experimental region. This process imparts a significant flow velocity to the ions which has been neglected in previous analysis of this plasma source. We predict the flow energy of the ions to be between 12-15 eV depending on conservation laws and show agreement with experimental results.

Effect of temperature and V/III ratio on the initial growth of indium nitride using plasma-assisted metal-organic chemical vapor deposition

The growth of Indium nitride (InN) was studied in the nucleation stage by metal-organic chemical vapor deposition technique using atomic nitrogen from an RF microwave plasma source. Deposition was carried out through a range of substrate temperatures from 375 to 550 °C and at varying V/III ratios from 950 to 3150. We found that the diffusion lifetime of In atoms on the substrate becomes maximized at the growth temperature 475 °C, in which low temperature photoluminescence exhibits the excellent optical properties of the materials with a bandgap of 0.69 eV and a width of 34 meV. In addition, we observed that nitrogen cracking efficiency is significantly improved by using plasma so that high quality InN crystallites were grown with a very low V/III ratio around 950.

A biresonant plasma source based on a gapped linear microwave vibrator

The operating principle of a novel microwave plasma source—a linear microwave vibrator with a gap—is discussed. The source is placed on a microwave-transparent window of a chamber filled with a plasma-forming gas (argon or methane). The device operation is based on the combination of two resonances—geometric and plasma ones. The results of experimental tests of the source are presented. For a microwave frequency of 2.45 GHz, microwave power of ≤1 kW, and plasma-forming gas pressure in the range 5 × 10−2–10−1 Torr, the source is capable of filling the reactor volume with a plasma having an electron density of about 1012 cm−3 and electron temperature of a few electronvolts.