Microwave-excited Plasma Research Articles

Electron cyclotron resonance (ECR) enhanced diverging magnetic field for controlled particle flux in a microwave-excited plasma column – a numerical investigation

The present work is an investigation of the effect of an externally applied diverging magnetic field on a surface microwave-sustained plasma column. Microwaves (2.45 GHz) are allowed to propagate through a tapered waveguide system containing a discharge tube made up of quartz. Argon gas flows down the tube from top to bottom maintaining a pressure of 1 Torr and a plasma is ignited within the tube owing to the surface microwave propagation. In the absence of a magnetic field, the plasma column exhibits discrete regions of overdense plasma near its center where the electric field of the incident microwaves is observed to be high. As the gas flows down the tube, the plasma density is also found to decrease and the resulting plasma profile is asymmetric about its length. However, in the presence of an axially applied diverging magnetic field , an axial force acts on the plasma, and the discrete overdense plasma regions are found to get symmetrically arranged along the plasma axis. Interesting results are observed when the diverging magnetic field includes a region of electron cyclotron resonance (ECR) corresponding to the microwave frequency. In the presence of an ECR, the electrons are expected to experience resonant heating by microwaves other than the direct heating by these waves. Under such conditions, the discharge dynamics are governed by the resonance mechanism, and the bright spots of overdense plasma regions get shifted to the ECR positions. As the magnetic field strength increases, the overdense plasma moves axially away from the center. These results are a clear indication of a magnetically controlled particle flux over a target and can be exploited in various material processing applications, particularly for surface cleaning applications in the semiconductor industries.

Plasma-Activated Media Produced by a Microwave-Excited Atmospheric Pressure Plasma Jet Is Effective against Cisplatin-Resistant Human Bladder Cancer Cells In Vitro.

Media exposed to atmospheric pressure plasma (APP) produce reactive oxygen and nitrogen species (RONS), with hydrogen peroxide (H2O2), nitrite (NO2-), and nitrate (NO3-) being among the most detected species due to their relatively long lifetime. In this study, a standardized microwave-excited (ME) APP jet (APPJ) source was employed to produce gaseous RONS to treat liquid samples. The source was a commercially available plasma jet, which generated argon plasma utilizing a coaxial transmission line resonator at the operating frequency of 2.45 GHz. An ultraviolet-visible spectrophotometer was used to measure the concentrations of H2O2 and NO3- in plasma-activated media (PAM). Three different types of media (deionized water, Hank's balanced salt solution, and cell culture solution Dulbecco's modified eagles medium [DMEM]) were utilized as liquid samples. Among these media, the plasma-treated DMEM was observed to have the highest levels of H2O2 and NO3-. Subsequently, the feasibility of using argon ME-APPJ-activated DMEM (PAM) as an adjuvant to enhance the therapeutic effects of cisplatin on human bladder cancer cells (T-24) was investigated. Various cancer cell lines, including T-24 cells, treated with PAM were observed in vitro for changes in cell viability using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. A viability reduction was detected in the various cancer cells after incubation in PAM. Furthermore, the study's results revealed that PAM was effective against cisplatin-resistant T-24 cells in vitro. In addition, a possible connection between HER expression and cell viability was sketched.

Scaling up microwave excited plasmas—An alternative technology for industrial decarbonization

AbstractThe use of electricity for the industry's transition to a decarbonized economy requires optimization of the energy transfer to deliver efficient, cost‐effective processes. Industrial demand for high‐density plasmas over a wide pressure range has stimulated the use of microwave plasma (MWP) applications. In high‐temperature processing, when microwaves of the correct frequency are absorbed by the plasma, the plasma particles absorb energy from the electromagnetic field and transfer it to the other particles through collisions, heating them. This rapid heating gives MWP properties that can be exploited to increase the conversion, selectivity, and/or energy efficiency of chemical processes. Here, we address questions raised by industrial users who wish to better understand the limitations of MWP applications.

Morphological changes of carbon thin films with nitrogen doping synthesized by microwave-excited surface wave plasma CVD

Nitrogen (N) doped graphene-based materials are of significant interest for various applications, such as, catalytic reactions, tribological energy generation, sensor devices and other electronic device applications. Most recently, the catalytic and noncatalytic growth of graphene structures at a low substrate temperature were explored by using the microwave-excited surface wave plasma (MW-SWP) chemical vapor deposition (CVD) technique. In this study, growth of carbon thin films is investigated by the MW-SWP CVD technique with nitrogen doping at various temperatures on the insulating substrates. It is obtained that partially graphitized carbon thin films can be obtained at a temperature as low as 150 °C on the quartz substrates. Interestingly, the morphological changes are observed with nitrogen doping and change in synthesis temperature for the carbon thin films grown by the MW-SWP CVD method. The partially graphitized carbon thin films form irregular morphologies at a lower temperature, while the morphology transforms with increase in temperature. At a certain growth temperature (500 and 700 °C), the carbon film shows granular morphology without addition of nitrogen, whereas the addition of nitrogen reduces the granular morphology and forming planar structures. Thus, these studies reveal the growth of carbon films with control morphology with respect to incorporation of nitrogen, which can be significant to develop carbon films and nanostructures for diverse fields of applications.

Numerical investigation of hydrogen production from low-pressure microwave steam plasma

Hydrogen is recognized as a suitable energy carrier that can facilitate the storage and transport of renewable energy. In this context, hydrogen production from microwave-excited steam plasma is one of the least researched techniques and lacks numerical study due to the complex plasma kinetics and difficullty of the simulation process. Therefore, in this study, a kinetic model is developed for steam plasma, considering forty-one reactions and fourteen species. Two-dimensional microwave steam plasma is modeled using COMSOL Multiphysics software for the first time. At a microwave power of 800 W, plasma formation and hydrogen production from low pressure (1 Torr) and high temperature (150 °C) steam plasma are numerically studied within the time domain of 10−10 to 10−4 s. The presented results demonstrate that the maximum electron density reaches 5 1017m−3 at 10−4s, and 16.8% of the water molecules dissociate to form various species. The conversion rate of water molecules to hydrogen is calculated as 24%. According to the thermodynamic evaluations, the proposed system's energy and exergy efficiencies are 10.31% and 10.14%, respectively, with a hydrogen production rate of 0.68 μg/s. Furthermore, the effect of the applied microwave power on plasma properties and hydrogen production is parametrically studied. Despite the proportional relationship between the input power and hydrogen production, no correlation is found between microwave power and system efficiency.

Investigation of Pressure Dependence in Photoresist Ashing Process using Microwave Excited Water Vapor Plasma

Microwave-excited water plasma asher (WPA) is our laboratory's new technology for the photoresist (PR) ashing process in semiconductor manufacturing which uses water vapor as process gas. It has the potential to solve many problems of conventional PR ashing technologies including high temperature, oxidation of metal structure, low ashing rate for ion-implanted PR, and high costs of chemicals. In this study, the pressure dependence of various aspects of plasma in water plasma asher, including light emission area, emission intensity, and substrate temperature was investigated. The ashing rate of the novolak PR film on the silicon wafer at the various condition of process pressure was also investigated. Average emission intensity and emission area of plasma significantly increased as process pressure in the chamber were decreased, and they were the highest when the process pressure was 0.3 kPa. The average ashing rate of PR film was only 0.42 μm/min when process pressure was 2.14 kPa, and it gradually increased as process pressure was decreased. It is also found that the ashing rate was improved after reducing the temperature of the water. The ashing rate when using air-rich condition of plasma was observed to be higher than when only water vapor is used as process gas.

(Invited) Role of Trace Impurities in Microwave-Excited Atmospheric Pressure Plasmas: Application to 3D Nano-Printing

Because rare gases like helium, neon or argon have first-excited levels at high energy (respectively 19.8, 16,62 and 11.5 eV), impurities with much lower energy levels are more easily excited. Thus, if their concentrations are high enough (hundreds of ppm or more), these impurities drive the behavior of the discharge, leading for instance to emission spectra where no lines of rare gas are visible.Modelling these discharges is a challenge because of the complexity of the chemical behavior of these trace elements. In microwave discharges, several problems arise among which one finds the role of electron-electron collisions, super-elastic collisions and the possible non-local heating of the electron energy distribution function. Based on a collisional–radiative model proposed to describe the behavior of a helium plasma sustained in a resonant cavity at atmospheric pressure, we will discussed these different aspects. The role of impurities will be described next. The level of dry air used as impurity typically varies from 0 to 1500 ppm, which corresponds to the most commonly encountered range in atmospheric pressure discharge experiments. Results clearly show that the plasma chemistry and consequently the discharge evolution is highly affected by the concentration level of impurities (from tens of thousands of ppm) in the mixture.Next, the use of these plasmas for 3D-nanoprinting will be detailed. This technology, named DISCRIBE, might be used for drawing electrical contacts, building pillars as part of metasurfaces or even creating 3D parts of clocks in watchmaking industry. A plasma-assisted printer, operated at atmospheric pressure, was developed to write patterns with sub-micrometric resolution. A reactive gas is injected through a needle within an atmospheric argon microwave plasma. Forming sub-micrometric needles (down to 100 nm in diameter) by pulling glass capillaries heated locally by laser is simple as this technology is readily available in the medical field. By plasma-enhanced chemical vapor deposition, a coating is formed on a substrate (conveniently a silicon wafer, but any other substrate can be used) with a spot size that almost corresponds to the needle aperture. Then, when the substrate moves, a pattern is formed accordingly. The writing speed is dependent on the dynamic deposition rate, which can be as high as 2 nm·m·s-1.In the specific case of acetylene used as precursor, the shape of the hydrogenated-carbon spot is strongly dependent on the amount of oxygen-containing impurities present in the microwave argon plasma. The influence of the plasma composition and the local hydrodynamic flow pattern are essential to control the deposition conditions and these aspects will be discussed. In particular, the distance between the needle exit and the substrate turns out to be a key parameter in the deposition process. DISCRIBE offers the advantage of all PECVD processes: it allows deposition at atmospheric pressure and reduced temperature of multi-materials (semiconductors, metals, ceramics, plasma polymers) ony any kind of substrate.

Synthesis of Graphene and Related Materials by Microwave-Excited Surface Wave Plasma CVD Methods

Several kinds of chemical vapor deposition (CVD) methods have been extensively used in the semiconductor industries for bulk crystal growth, thin film deposition, and nanomaterials synthesis. In this article, we focus on the microwave-excited surface wave plasma CVD (MW-SWP CVD) method for growth of graphene and related materials. The MW-SWP CVD system consisting of waveguide, slot antenna, and dielectric windows is significant for generating high density plasma with low electron temperature, enabling low temperature growth of materials without damaging the surface of base substrates. The synthesis of graphene and hexagonal boron nitride (hBN) films has been achieved on metals, semiconductors, insulators, and dielectric substrates for application in photovoltaics, sensors, batteries, supercapacitors, fuel cells, and various other electronic devices. The details of the synthesis process for graphene films, vertically-oriented graphene, doped-graphene, and hBN films by the MW-SWP CVD method are summarized to understand the growth mechanism, which will enable further development of the plasma CVD process for material synthesis at a low temperature for industrial applications.

Effective reduction and doping of graphene oxide films at near-room temperature by microwave-excited surface-wave plasma process

The effective reduction of graphene oxide (GO) and incorporation of dopant elements on flexible soft substrates, such as plastics, polymers, papers etc. are of significant importance for energy-related devices and flexible wearable electronics. Here, we demonstrate a novel microwave-excited surface-wave plasma (MW-SWP) process to reduce insulating GO to highly conducting reduced graphene oxide (rGO) on the polyethylene terephthalate (PET) substrate as well as achieving reduction of free-standing GO paper at a near-room temperature. It is also observed that the GO coated on the both top and back surface of a PET substrate reduce simultaneously to obtain conducting rGO films. Most effective reduction was achieved using an argon-hydrogen gas mixture, whereas addition of nitrogen in the gas mixture enabled to incorporate nitrogen dopant in the rGO layers. The GO coated PET and freestanding GO films showed a sheet resistance in the range of MΩ/□, which reduced to an average value of 1.075 kΩ/□ and 134 Ω/□, respectively after the plasma treatment. Thus, the developed MW-SWP process can be significant to produce highly conducting rGO-based graphene films on a soft substrate as well as enabling fabrication of highly conducting freestanding rGO papers.

Facilitation of adhesion and spreading of endothelial cells on silicone oxide-coated dacron material by microwave-excited low-pressure plasma.

ObjectivesAutologous transplants are still the means of choice for bypass surgery. In addition to good tolerability, there is a reduced thrombogenicity and fewer neointima hyperplasia compared to artificial materials. However, since viable transplants are limited, attempts are being made to improve existing artificial vascular prosthesis material. Next to the reduction of thrombogenicity, a rapid endothelialization of the vascular graft should reduce intimal hyperplasia and thus prevent stenoses. The effect of newly developed silicon oxide coatings on the growth of endothelial cells was therefore the goal of this work in a cell culture study.MethodsA woven, uncoated polyethylene terephthalate (PET) vessel prosthesis was used. The coating process was carried out in a low-pressure plasma reactor in a multi-step process. After preparation of the vacuum chamber hexamethyldisiloxane (HDMSO) with oxygen was evaporated using argon plasma. By this an approx. 1 nm thin adhesion promoter layer was separated from plasma and HMDSO. The silicone oxide barrier layer was applied to the PET vessel samples. The carbon content of the layer could be selectively altered by changing the HMDSO oxygen flow ratio, resulting in coatings of 100 nm, 500 nm, and 1,000 nm. In addition, two different oxygen-to-HMDSO ratios were used. To achieve a carbon coating as low as possible, the ratio was set to 200:1. A carbon-rich layer was obtained with the 1:1 setting. The various coatings were then examined for their surface texture by scanning electron microscopy (SEM) as well as by cell culture experiments for cell viability and growth using EA.hy 926 cells.ResultsSEM showed no changes in the surface morphology; however a layer thickness of 1,000 nm showed peeled off coating areas. Alamar blue assays showed a significantly higher metabolic activity (p=0.026) for the coating 500 nm, ratio 200:1 compared to untreated control samples and a significantly lower metabolic activity (p=0.037) of the coating 500 nm, ratio 1:1 compared to the coating 500 nm, ratio 200:1. This underlines the apparent tendency of the 1:1 coating to inhibit the metabolic activity of the cells, while the 200:1 coating increases the activity. Fluorescence microscopy after calcein acetoxymethyl ester (AM) staining showed no significant difference between the different coatings and the uncoated PET material. However, a tendency of the increased surface growth on the coating 500 nm, ratio 200:1, is shown. The coatings with the ratio 1:1 tend to be less densely covered.ConclusionsThe results of this work indicate a great potential in the silicon coating of vascular prosthesis material. The plasma coating can be carried out easy and gently. Cell culture experiments demonstrated a tendency towards better growth of the cells on the 200:1 ratio coating and a poorer growth on the carbon-rich coating 1:1 compared to the uncoated material. The coating with silicon oxide with a thickness of 500 nm and an oxygen-HMDSO ratio of 200:1, a particularly low-carbon layer, appears to be a coating, which should therefore be further investigated for its effects on thrombogenicity and intimal hyperplasia.

Graphene growth in microwave-excited atmospheric pressure remote plasma enhanced chemical vapor deposition

Graphene growth on Cu foil was demonstrated by plasma enhanced chemical vapor deposition (PECVD) with a microwave-excited non-equilibrium atmospheric pressure remote plasma (MNAPP) using CH4 and H2 gases diluted by He gas. The density of graphene grain decreased and their average sizes increased with decreasing the CH4 flow rate. Moreover, the sizes were almost constant at each condition. In the MNAPP, emission intensities of CH and C2 radicals normalized by that of He atom decreased with decreasing the CH4 flow rate. From the results, nucleation of graphene on Cu surface caused by the precursors such as CH x and C2 radicals occur only at the early period of PECVD, and after the initial nucleation, these precursors are mainly consumed for the planar growth of graphene without inducing subsequent nucleation. Therefore, flux control of precursors at the early period is important for synthesizing graphene with more large size in PECVD using MNAPP.

Dynamic Measurement of Relative Complex Permittivity of Microwave Plasma at Atmospheric Pressure

Complex permittivity is one of the most important parameters to characterize the interaction between microwave and medium, especially for microwave-excited plasma. It is convenient to study plasma’s dielectric properties and microwave propagation characteristics by measuring its complex permittivity. A dynamic measurement method of equivalent relative complex permittivity of microwave-excited plasma at atmospheric pressure is proposed in this paper. Firstly, a cavity based on WR-430 at a frequency of 2.45 GHz was specially designed in COMSOL. Then, the samples with different real parts of complex permittivity and loss tangent were simulated in the designed cavity to obtain their corresponding S parameters, and they were used to train the BP neural network until the error was lower than 0.001. A two-port network was built to excite the plasma. The input power, reflected power, and transmitted power could be measured by the transmission reflection method. Finally, the measured power values were converted into S parameters and used as inputs in the BP neural network. The plasma’s real parts of complex permittivity and loss tangent were obtained by inversion. The variation of plasma complex permittivity conforms to the interaction principles between microwave and plasma, which verifies the accuracy of the method.

Temporal Variations of Optical Emission Spectra in Microwave-Excited Plasma in Saturated Water Vapor under Reduced Pressure during Photoresist Removal

Mechanisms of photoresist removal were investigated using microwave-excited plasma in saturated water vapor under a reduced pressure condition (approx. 6 kPa). A few seconds of white emission were observed at the beginning of photoresist removal. To ascertain the species of spectra, optical emission spectroscopy was used for photoresist removal from a Si wafer. Emission spectra of OH (A2Σ – X2Π), Hα (656 nm), and O (777 nm) derived from dissociation of water molecules were observed. Other emission spectra attributed to CO molecules were also detected and identified as the Ångström system (B1Σ – A1Π). This result indicates CO molecule generation during photoresist removal. The CO molecule emission intensity reached a maximum value at 1.1 s after plasma ignition, whereas those of OH, Hα, and O reached a minimum value at almost identical time. Furthermore, normalized emission spectra shape from CO molecules during photoresist removal were almost identical for Δv = +1, 0, -1, -2, -3, and -4. Rotational and vibrational temperatures of the CO molecules during photoresist removal were found to be almost constant and to be 1500 and 2500 K, respectively. These results suggest increased CO molecular density above the Si wafer at 1.1 s.

Improved Uniformity of Photoresist Ashing for a Half-Inch Wafer with Double U-shaped Antenna Structure in a Microwave-Excited Water Vapor Plasma

Improved Uniformity of Photoresist Ashing for a Half-Inch Wafer with Double U-shaped Antenna Structure in a Microwave-Excited Water Vapor Plasma

Dual microwave-excited atmospheric-pressure plasma jets with a single power source

In this paper, we propose dual microwave-excited atmospheric-pressure plasma jets (DME-APPJs) that can be generated with a single power source. A T-junction power divider that evenly distributes the input power to two ports is used, and two coaxial transmission line resonators are used as plasma generators for the DME-APPJs. The power divider distributes the incoming power in port 1 and supplies 46% of this power to port 2 and port 3, respectively. Surface treatments are conducted by exposing the multiple plasma jets to polypropylene (PP) specimens and then confirming the degree of hydrophilic change. Using the proposed DME-APPJs, it is possible to perform surface treatments on areas that are up to 50% larger compared to using two single ME-APPJs with the same power consumption. The DME-APPJs thus show more advanced performance for surface treatments by comparison with the single ME-APPJ.

Plasma Diagnosis and Biomedical Application Using Linear Microwave Atmospheric- Pressure Plasma Generator

A new type of microwave-excited atmospheric-pressure plasma (APP) generator that uses the principle of a strip-line transmission line resonator is presented. The microwave plasma has a length of 30 mm, so it can treat a wide area at the same time. The impedance of the resonator was matched to $50~\Omega $ without additional matching equipment by exploiting characteristic of quarter wavelength transmission line. The design of the resonator was double-checked by computer simulation for the purpose of generating stable microwave APP. Microwave plasma discharge of the resonator was operated with argon gas flow of 1 slm, net power of 2.3 W, and resonance frequency of 2.41 GHz. The plasma gas temperature was maintained below 40 °C for 30 min, so it is safe to use in biomedical applications. The intensity of optical emission spectrum was higher at the center than edges. The average of electron excitation temperature was 0.78 eV with a standard deviation of 0.03 eV (3.85%) in 20-mm range. Increase in the amount of reactive oxygen and nitrogen species expressed in the normal human dermal fibroblast cells after plasma treatment was confirmed using fluorescent reagent. This new type of microwave atmospheric plasma resonator is expected to be useful in various biomedical fields.

Effect of discharge tube temperature on and densities in remote nitrogen plasma

We examined the effect of the discharge tube temperature on the densities of and in microwave-excited remote nitrogen plasma. The density of and the rotational temperature of were constant against the discharge tube temperature between 30 °C and 400 °C, whereas we observed the decrease in the density. In addition, we observed the decreases in the density and the vibrational temperature of with the discharge tube temperature. The experimental results indicated the decrease in the production rates of the electronic excited states in the spatial afterglow, which may be caused by the lower density of vibrational excited states of molecular nitrogen in the heated discharge tube.

Anticancer Effects of Plasma-Activated Medium Produced by a Microwave-Excited Atmospheric Pressure Argon Plasma Jet.

Cold atmospheric plasma (CAP) has been reported to have strong anticancer effects in vitro and in vivo. CAP has been known to induce apoptosis in most cancer cells by treatment to cells using direct and indirect treatment methods. There are many reports of apoptosis pathways induced by CAP, but for indirect treatment, there is still a lack of fundamental research on how CAP can cause apoptosis in cancer cells. In this study, we applied an indirect treatment method to determine how CAP can induce cancer cell death. First, plasma-activated medium (PAM) was produced by a 2.45 GHz microwave-excited atmospheric pressure plasma jet (ME-APPJ). Next, the amounts of various reactive species in the PAM were estimated using colorimetric methods. The concentration of NO2– and H2O2 in PAM cultured with cancer cells was measured, and intracellular reactive oxidative stress (ROS) changes were observed using flow cytometry. When PAM was incubated with A549 lung cancer cells, there was little change in NO2– concentration, but the concentration of H2O2 gradually decreased after 30 min. While the intracellular ROS of A549 cells was rapidly increased at 2 hours, there was no significant change in that of PAM-treated normal cells. Furthermore, PAM had a significant cytotoxic effect on A549 cells but had little effect on normal cell viability. In addition, using flow cytometry, we confirmed that apoptosis of A549 cells occurred following flow cytometry and western blot analysis. These results suggest that among various reactive species produced by PAM, hydrogen peroxide plays a key role in inducing cancer cell apoptosis.

A Single-chip Resonance-frequency Tracking Microwave-excited Plasma Driver IC for Portable Biomedical Applications

This paper presents a first-time single-chip microwave-excited plasma driver IC with resonance-frequency tracking and output-power-regulation scheme for biomedical applications. The power module includes a driver IC and a power amplifier, and is designed to provide stable microwave power that is required for generation of a plasma jet. A 2.4- GHz coaxial transmission line resonator is used to generate the plasma jet using input microwave power. The proposed system continuously monitors the resonance frequency, and controls it by adaptively changing the microwave frequency. To prevent the plasma jet from becoming high enough to damage human skin, the system monitors the power entering the resonator and keeps it constant. The proposed microwave plasma driver IC is implemented in a 0.18-μm complementary metal-oxide-semiconductor process. The system generates stable 1 W with a frequency tracking.

Cell viability and measurement of reactive species in gas- and liquid-phase exposed by a microwave-excited atmospheric pressure argon plasma jet

A microwave-excited atmospheric pressure plasma jet (ME-APPJ) was generated utilizing coaxial transmission line resonators with the resonance frequency of 788 MHz. We investigated the effects of the operating parameters such as input power and gas flow rate on the electron excitation temperature and electron density using optical emission spectroscopy. The amounts of various reactive species in the plasma-activated media (PAM) exposed by the ME-APPJ were estimated using colorimetric methods. The PAM was applied to A549 cultured cells and subsequently incubated for a given duration. The concentrations of NO2− and H2O2 in the PAM incubating A549 cells were monitored until 24 h. The reduced cell viability rate was observed on A549 cells incubated in the PAM. The concentrations of NO2− and H2O2 and the viability of A549 cells depended on the operating conditions of ME-APPJ. The cell viability rate exhibited a decrease with increasing power and decreasing gas flow rate. Correlation between the concentrations of gaseous reactive species in the plasma and in the PAM and the cell viability is discussed.