Helicon-wave Discharge Research Articles

Effect of inhomogeneous magnetic field on blue core in Ar helicon plasma

The effect of the inhomogeneous magnetic field on blue core phenomena in helicon plasma is investigated in this work. The permanent magnets (PMs) are used to provide the magnetic field required for generation of helicon plasma, and three different types of the inhomogeneous magnetic field are constructed by changing the permanent magnets positions, which are PM-top/bottom, PM-top, and PM-bottom, respectively. The maximum magnetic field strengths in these three types of inhomogeneous magnetic fields are at both ends (case A), at the upper end (case B), and at the lower end (case C) of the discharge tube, respectively. Nikon camera, an intensified charge coupled device, optical emission spectrometer (OES), and Langmuir probe are used to diagnose the blue core phenomenon in helicon plasma. The electronic excitation temperature is calculated based on optical emission lines captured by OES. Helicon wave discharge is affirmed by mode transition with three discharge stages corresponding to E-, H- and W-modes, respectively. The blue core could occur in the maximum magnetic field strengths located at both ends at around 200 W, at the upper end at around 300 W, and at the lower end at around 400 W. The formation of blue core in the present work can be attributed to the non-uniformity of the inhomogeneous magnetic field. Meanwhile, the radial distribution of plasma density and electron temperature in blue core is different. The plasma density still maintains the on-axis peak, and electron temperature transforms from on-axis peak to off-axis peak due to the skin effect.

<i>In-situ</i> diagnosis of Ar/CH<sub>4</sub> helicon wave plasma for synthesis of carbon nanomaterials

A variety of carbon-based thin films are prepared by self-developed helicon wave plasma chemical vapor deposition (HMHX, HWP-CVD) through changing the parameters of plasma discharge. The Ar/CH<sub>4</sub> plasma discharge is diagnosed <i>in situ</i> by Langmuir probe, emission spectroscopy and mass spectrometry. The carbon thin films are characterized by scanning electron microscopy (SEM) and Raman spectroscopy (Raman). The results show that under the given parameters, the plasma discharge modes are all helicon wave discharge modes. Under a given CH<sub>4</sub> flow rate, the energy distribution in the plasma is enough to dissociate the methane molecules and form carbon free radicals. The preparation of different carbon-based films is realized by adjusting the CH<sub>4</sub> fluence. The research result shows that when the plasma is rich in CH and H radicals, it is suitable for growing diamond-like carbon films. When the plasma is rich in C<sub>2</sub> radicals and less H, it is favorable for growing vertical graphene nanosheets. According to the results of plasma diagnosis and material characterization, the decomposition mechanism of methane molecules under the action of Ar helicon wave plasma (HWP) is proposed, and the growth model of carbon-based materials is established, the feasibility of Ar/CH<sub>4</sub>-HWP in the preparation of carbon-based nanomaterials is verified, which provides a reference for preparing the carbon-based materials by HWP-CVD technology.

Effects of electron temperature on energy deposition properties of electromagnetic modes propagating in helicon plasma

Understanding the power deposition characteristic of high density helicon wave plasma source is critical for further investigating into the discharge mechanism of helicon wave discharge. Based on the warm plasma dielectric tensor model which contains both the particle thermal effect and temperature anisotropy and using the insulting boundary condition, the eigenmode dispersion relation of helicon wave and Trivelpiece-Gould (TG) wave propagating in radially uniform plasma column are numerically obtained. Then based on the eigenmode dispersion relation and exact field distribution in the plasma column, the mode coupling properties between the helicon wave and TG wave, the parametric dependence of the cyclotron damping properties of the electron cyclotron wave (TG wave) and power deposition properties of the <i>m</i> = –1, 0, +1 modes under moderate plasma density and low magnetic fields conditions are theoretically investigated in typical helicon plasma parameter range. The detailed investigations are shown below. Under typical helicon plasma parameter conditions, i.e. wave frequency <i>ω</i>/2π = 13.56 MHz and the ion temperature is one-tenth of the electron temperature, there exist a critical magnetic field value <i>B</i><sub>0,c</sub> and a critical electron temperature value <i>T</i><sub>e,c</sub> for which under the conditions of <i>B</i><sub>0</sub> < <i>B</i><sub>0,c</sub> the helicon wave becomes an evanescent wave and the TG wave becomes an evanescent wave when <i>T</i><sub>e</sub> < <i>T</i><sub>e,c</sub>. The cyclotron damping of the TG wave dramatically increases as the wave frequency approaches to the electron cyclotron frequency. The TG wave becomes a growth wave when the ratio of perpendicular electron temperature to parallel electron temperature is above a certain value. For the high magnetic field, i.e. <i>ω</i>/<i>ω</i><sub>ce</sub> = 0.1, most of the power deposition is deposited in the central core region, while for the low magnetic field, i.e. <i>ω</i>/<i>ω</i><sub>ce</sub> = 0.9, the power is deposited mainly in the outer region of plasma column. For typical helicon plasma electron temperature range, <i>T</i><sub>e </sub>∈ (3 eV, 5 eV), the energy depositions induced by the collisional damping and Landau damping of the TG wave are dominant for different electron temperature ranges, which implies that different damping mechanisms have different heating intensities for electrons. Under current parameter condition, compared with the <i>m</i> = +1 mode, the <i>m</i> = –1 and <i>m</i> = 0 mode of the TG wave play major role in the power deposition process, although the cyclotron damping of the TG wave dominates the power deposition in this typical electron temperature range. All these conclusions provide some useful clues for us to better understand the high ionization mechanism of helicon wave discharge.

Parametric analysis of mode coupling and liner energy deposition properties of helicon and Trivelpiece-Gould waves in helicon plasma

Based on the finite temperature plasma dielectric tensor model which contains the particle thermal effect, by numerically solving the eigenmode dispersion relation of electromagnetic waves propagating in radially uniform and magnetized warm plasma column which is surrounded by conducting boundary, the mode coupling characteristic and liner damping mechanism induced wave power deposition properties of helicon and Trivelpiece-Gould (TG) waves are parametrically analyzed. The detailed investigations show as follows. Under typical helicon plasma parameter conditions, i.e. wave frequency <i>ω</i>/(2π) = 13.56 MHz, ion temperature is much smaller than electron temperature, for the helicon wave, there exist a cut-off magnetic field <i>B</i><sub>0,H,cutoff</sub> and a cut-off plasma density <i>n</i><sub>0,H,cutoff</sub>, for which under the conditions of <i>B</i><sub>0</sub> > <i>B</i><sub>0,H,cutoff</sub> or <i>n</i><sub>0</sub> < <i>n</i><sub>0,H,cutoff</sub>, the helicon wave becomes an evanescent wave. When the magnetic field intensity changes from 48.4 to 484 G, i.e., <i>ω</i>/<i>ω</i><sub>ce</sub> ranges from 0.01 to 0.1, for the power deposition intensity, Landau damping of TG wave dominates for the <i>m</i> = 0 mode, meanwhile, for the <i>m</i> = 1 mode, which wave, i.e. helicon wave or TG wave, plays a major role in power deposition mainly depends on the magnitude of the magnetic field. On the other hand, for a given magnetic field <i>B</i><sub>0</sub> = 100 G, when <i>ω</i><sub>pe</sub>/<i>ω</i><sub>ce</sub> changes from 3 to 100, for both the <i>m</i> = 0 mode and the <i>m</i> = 1 mode, the power deposition induced by Landau damping of TG wave plays a major role, further, one may notice that the power deposition of TG wave decreases while the power deposition of the helicon wave increases as plasma density increases. Finally, for both the <i>m</i> = 0 mode and the <i>m</i> = 1 mode, the power deposition due to the Landau damping plays a dominant role. All these conclusions provide us with some useful clues to better understanding the high ionization mechanism of helicon wave discharges.

The discharge characteristics in nitrogen helicon plasma

Discharge characteristics of helicon plasma in nitrogen and argon-nitrogen mixtures were investigated experimentally by using a Langmuir probe, a B-dot probe, and an optical emission spectrum. Helicon wave discharge is confirmed by the changes of electron density and electromagnetic signal amplitude with the increasing RF power, which shows three discharge stages in nitrogen, corresponding to E-mode, H-mode, and W-mode discharges in helicon plasma, respectively. Discharge images in the radial cross section at different discharge modes through an intensified charge coupled device (ICCD) show a rapid increase in luminous intensity along with the RF power. When the nitrogen discharge is in the W-mode, the images show that the strongest luminance locates near the plasma boundary and no blue core appears in the axial center of tube, which is always observed in argon W-mode discharge. The “big blue” or blue core is a special character in helicon plasma, but it has not been observed in nitrogen helicon plasma. In nitrogen-argon mixtures, a weak blue core is observed in ICCD images since the nitrogen content is increased. The electric field turns to the periphery in the distribution of the radial field and the electron temperature decreases with the increasing nitrogen content, especially when the blue core disappears. The different behaviors of the electron impact and the energy consumption in nitrogen helicon plasma are suggested to be responsible for the decrease in electron energy and the change in the electric field distribution.

Synthesis of SiON films by helicon-wave plasma and their characteristic analysis

SiON becomes a kind of new functional material because it has many excellent mechanical and chemical properties such as high hardness, corrosion resistance, oxidization resistance. It is reported that the coating of SiON has many important applications in optical device. For example, it can be used as the protective layer of Edmund optics, touch panel of mobile phone, waveguide tube et al. the Mohs hardness of SiON is about 9H. In addition, the transmittance of the coating is 98% in the visible region. Lately, lots of researchers are interested in the biomedical applications of SiON coatings due to its favourable mechanical properties and the good hemocompatibility of the material. Such as hip implants, it can help patient recover soon because the coating is smooth and the Si–N and Si–O–N chemical bonding are more to the hydrophilicity than Si–O or Si–Si bonds. There is also lots of significant application of SiON films in the field of microelectronics. According to the reports, SiON has the inhibiting effect on hot-carrier injection, impurity diffusion and water vapor permeability. As the dielectric layer, SiON also possesses better electronic mobility and the trap densities of electron. So far, the main synthetic method of SiON is plasma enhanced chemical vapor deposition (PECVD) including inductive coupled plasma source (ICP), capacitive coupled plasma source (CCP) and microwave electron cyclotron resonance (ECR). It is found that selectivity of PECVD is bad along with the further study of it. In addition, the SiON dielectric layer contains impurities unavoidably, for example, it will affect the insulation and dielectric properties if adding H atom into the dielectric layer, which can restrict application in large scale integrated circuit. Helicon wave plasma source (HWP) comes to the notice of many researchers because of its high density (about 1013 cm-3) at low pressure (0.1~10 Pa). Chen said that “Interest in helicon discharges stems from their unusually high ionization efficiency: plasma densities n achieved are almost an order of magnitude higher than in other discharges at comparable pressures and input powers.” During the discharge, high-energy electron collides with gas particle adequately. In this experiment, it can divide N2 into N2+ efficiently along with steady-state helicon wave discharge. In addition, there is no impurity element mixed in the SiON (such as C, H) which can improve the quality of the SiON films. In this experiment, SiON films were synthesized by nitridation on the surface of Si sputtered by N2/Ar helicon-wave plasma. During the mixed helicon-wave discharge, the mass flow ratio of N2 and Ar was different. X-ray photoelectron spectroscopy (XPS) showed that the SiON films were constituted of Si–O–N and Si–N. Atom force microscopy (AFM) indicated that the surface of the film was very flat and smooth, and the RMS was less than 1.2 nm. We collected Plasma discharge parameters by optical emission spectroscopy (OES) when the Si wafer was sputtered using N2/Ar helicon-wave plasma, and studied the relationship between the particle in the plasma and structure of SiON films.

Effect of standing wave on the uniformity of a low magnetic field helicon plasma

Helicon wave discharge has higher coupling efficiency than capactively coupled and inductively coupled discharge in low static magnetic field. In the wave sustained mode, a large volume and large area plasma can be produced at lower pressure by using comparable discharge power, and thus it expands the helicon wave plasma applications in material surface modification, thin film deposition, dry etching and thruster usage. However, the application of helicon wave source still faces challenges, such as the controversial power coupling mechanism, operation stability and the plasma distribution uniformity in the experiment. The wave mode existing in bounded helicon wave plasma column generally consists of helicon and Trivelpiece-Gould (TG) components, and their mode transitions and different transverse wave field distribution regions, and the propagating characteristic of the helicon wave are directly related to the power coupling and plasma density distribution in the source region, then affect the uniformity of material processing and film deposition in the diffusion chamber. In this paper, the plasma azimuthal non-uniformity, with using Doubble Saddle antenna, 100 G static magnetic field in helicon wave plasma source, is studied by electrical characteristic (power-current) curve, intensified charge coupled device (ICCD) image and magnetic probe measurements. The electrical characteristic curve indicates two discharge stages with different effective resistances. Meanwhile, in the second stage, the higher effective resistance would result in higher coupling efficiency and higher plasma density. But the ICCD image demonstrates the azimuthal non-uniformity of plasma, indicating that the main heating points at the diagonal edge are linked to the stationary transverse electrical field line pattern of azimuthal mode number m=+1 helicon wave, and the magnetic probe is used to measure the helicon wave magnetic field Bz component along the quartz source tube axially. The magnetic probe results show that the standing wave appearing below the antenna even though in the upper region of the antenna is characteristic of the traveling wave. Furthermore, at the plasma boundary, the standing wave can be coupled to the TG wave, and not like travelling wave it has no angular rotation of the electric field and may cause the non-uniform coupling between the helicon and TG components. The TG wave then has azimuthal non-uniform electron heating. Therefore, the standing helicon wave below the antenna is the key factor to the plasma non-uniformity problem. Changing the propagating characteristics of the helicon wave further in the plasma column will be of positive significance for optimizing the discharge efficiency of the plasma source and controlling the plasma distribution uniformity, stability and other operations as well.

Low temperature deposition of hydrogenated nanocrystalline SiC films by helicon wave plasma enhanced chemical vapor deposition

Hydrogenated nanocrystalline silicon carbide (nc-SiC:H) films have been deposited by using helicon wave plasma enhanced chemical vapor deposition technique at low substrate temperature. The influences of radio frequency (rf) power and substrate temperature on the properties of the deposited nc-SiC:H films were investigated. It is found that hydrogenated amorphous SiC films were fabricated at a low rf power, while the nc-SiC:H films with a microstructure of SiC nanocrystals embedded in amorphous counterpart could be deposited when the rf power is 400 W or more. The plasma transition from the capacitive dominated discharge to the helicon wave discharge with high plasma intensity influences the film microstructure and surface morphology. The analysis of the films deposited at various substrate temperatures reveals that the onset of SiC crystallization occurs at the substrate temperature as low as 150 °C. The low temperature deposition of nc-SiC:H films enables the fabrication of silicon-based thin-film solar cells onto flexible plastic substrates using nc-SiC:H film as a window layer.

Negative ion densities in high-density, low-temperature recombining hydrogen plasmas

The objective of this work is to investigate the density of negative hydrogen ions in high-density, low-temperature recombining hydrogen plasmas. The recombining hydrogen plasmas were obtained in a compact divertor simulator excited by helicon-wave discharges. The electron density and the electron temperature of the recombining plasmas were (1–4) × 1012 cm−3 and 0.1–0.2 eV, respectively, which were evaluated by comparing the population distribution of the excited states of atomic hydrogen with the Saha–Boltzmann equation. The ratio of the negative ion to electron density ranged between 0 and 0.02 in the recombining plasmas, which was one order of magnitude lower than that observed in a plasma with an electron density on the order of 1011 cm−3. The less efficient production of negative hydrogen ions may be attributed to significant dissociation of molecular hydrogen in high-density plasmas.

Production and application of reactive plasmas using helicon-wave discharge in very low magnetic fields

In order to explore new application fields of a helicon-wave discharge, we have investigated the production of helicon-wave plasmas in very low magnetic fields (0–10 mT) and the resultant nanocarbon creation using methane and/or hydrogen. The reactive plasmas are effectively produced by the helicon wave around 3 mT independently of gas species in the wide range of pressures (0.1–10 Pa), where hydrocarbons and atomic hydrogens are generated. Using the helicon-wave reactive plasma as a precursor source for plasma-enhanced chemical vapor deposition, well-aligned carbon nanotubes and nanowalls are found to be formed even in a very low gas pressure of 0.7 Pa.

Measurements of Gas Temperature in High-Density Helicon-Wave H2 Plasmas by Diode Laser Absorption Spectroscopy

The gas temperatures in high-density H2 plasmas excited by helicon-wave discharges were measured by absorption spectroscopy using a diode laser as the light source. The gas temperature was evaluated from the Doppler broadening of the absorption line profile at Hα. The gas temperature increased with rf power from 0.05 to 0.18 eV at a gas pressure of 50 mTorr. The temporal variations of the gas temperature after the initiation of discharge and the termination of the rf power were investigated. The power consumed by heating the gas was evaluated using the temperature and the time constant of the temporal variation.

Investigation of Helicon-Wave Discharge in a Low Magnetic Field Using Molecular and Rare Gases

AbstractThe helicon-wave discharge is performed in a low magnetic field B

Development of a Compact Divertor Simulator Excited by Helicon-Wave Discharge

We have developed a high-density H2 plasma source excited by helicon-wave discharge. By optimizing the antenna shape, the diameter of the discharge tube, and the magnetic field strength, a high electron density close to 1×1013 cm-3 was achieved at an rf power of 3 kW and a H2 gas pressure of 30 mTorr. The high-density H2 plasma source can be used as a compact divertor simulator in nuclear fusion research.

Development of a compact nitrogen radical source by helicon-wave discharge employing a permanent magnet

We have developed a nitrogen radical source excited by helicon-wave discharge in N 2 gas. The major features of the present radical source are: (1) high electron and radical densities; (2) low operational gas pressure below 1 mtorr; and (3) compactness by employing a permanent magnet. This radical source has potential applications in production of various nitride films such as GaN and β-C 3N 4.

Contribution of slow waves on production of high-density plasmas by m=0 helicon waves

The contribution of the slow waves (SWs) on high-density plasma production by m=0 helicon waves is investigated in H2, D2, He, N2, Ne, Ar, and Xe plasmas. The density-jump is observed in all the gases used at optimum external magnetic fields B0. The measured plasma density np as a function of B0 peaks at a condition close to the lower-hybrid resonance in H2, D2, He, N2, and Ne, whereas, the measured dispersion relation indicates the excitation of helicon wave. Dispersion relation of electromagnetic waves is solved using dielectric tensor elements of a cold uniform plasma including ion terms, which shows two branches: the SW and fast (helicon) wave. The transition from the ionization by SWs, which are excited in the low-density plasma produced by an antenna induction field, to that by helicon waves has been predicted to explain the mechanism to produce the high-density plasma in the helicon wave discharges.

Model for relaxation oscillations in a helicon discharge

Relaxation oscillations observed in the large-volume, helicon plasma experiment WOMBAT (Waves on Magnetized Beams and Turbulence) [R. W. Boswell and R. K. Porteous, Appl. Phys. Lett. 50, 1130 (1987)] are modeled. These oscillations have a period of several milliseconds and have been identified as transitions between a low-density, inductive discharge and a high-density, helicon-wave discharge. In the model, it is assumed that the mode transitions are triggered by variations in the neutral density in the source region. The neutral density decreases due to ionization augmented by ion pumping and increases due to refilling of the source chamber from the much larger diffusion chamber. The system is modeled using two, coupled, nonlinear, ordinary differential equations that describe the neutral and plasma densities in the source chamber. Ionization by inductively-coupled fields and ionization due to electrons accelerated by helicon waves with phase velocities near the threshold electron velocity for ionization are considered. The model is found to reproduce experimentally measured variations of the plasma density and helicon wave phase velocity with rf power, neutral pressure and magnetic field. The negative impedance needed for the existence of a relaxation oscillation is provided by the helicon-wave coupling mechanism.

Determination of fluorine atom density in reactive plasmas by vacuum ultraviolet absorption spectroscopy at 95.85 nm

Vacuum ultraviolet absorption spectroscopy was developed for the measurement of absolute fluorine (F) atom density in reactive plasmas. In order to minimize the influence of radiation trapping (self-absorption) in the light source, fluorescence at a wavelength of 95.85 nm from the F atoms in an electron–cyclotron resonance (ECR) CF4 plasma, which was operated with a low microwave power (0.1 kW) and a low gas pressure (1 mTorr), was employed as the probe emission. A windowless transmission system for the probe emission was constructed by connecting the ECR light source with the target plasma and the detection system using vacuum tubes having small slits. The connection tubes were differentially evacuated with turbomolecular pumps to prevent neutral particles from passing through between the ECR and target plasmas. The present method was applied to high-density CF4 and C4F8 plasmas produced by helicon-wave discharges. The accuracy of the measurement was examined carefully by evaluating various sources of error. In the present article, we have emphasized the evaluation of the radiation trapping effect in the light source plasma.

Surface Kinetics of CFx Radicals and Fluorine Atoms in the Afterglow of High-Density C4F8 Plasmas

Temporal variations of absolute densities of CF, CF2 and atomic fluorine (F) were measured in the afterglow of high-density C4F8 plasmas generated by helicon-wave discharges. Laser-induced fluorescence (LIF) spectroscopy was adopted for CF and CF2 radicals, while vacuum ultraviolet (VUV) absorption spectroscopy was employed for the F atom. CF and F densities gradually decreased for 20–80 ms after the extinction of the rf power, while CF2 density steadily increased during the same period. This slow increase in CF2 density can be explained by surface kinetics of the radicals. In the afterglow of discharges with a high degree of dissociation, the increase in CF2 density is approximately equal to CF density at the beginning of the afterglow. The mechanism for the surface production of CF2 in the afterglow is discussed based on the close relationships between the temporal variations of CF and CF2 densities.

Spatial and temporal variations of CF and CF2 radical densities in high-density CF4 plasmas studied by laser-induced fluorescence

Laser-induced fluorescence spectroscopy has been employed for the measurements of the ground-state CF and CF2 radical densities in low-pressure and high-density CF4 plasmas generated by helicon wave discharges. In the pulsed operation (5 Hz, 10 ms duration), the radial profiles of the CF and CF2 densities were hollow shape in the discharge phase, which indicates that both radicals were desorbed from the wall and were lost in the plasma column. In the continuous-wave operation, roughly uniform radial profiles were observed for both radicals. Therefore the desorbed radicals in the pulsed discharge seem to originate from the adsorbed species in the afterglow of the previous discharge.

Efficient production of O+ and O− ions in a helicon wave oxygen discharge

High density oxygen plasmas (ne=1010–1013 cm−3) are produced by a helicon wave discharge source with rf powers of 0.1–3 kW. Positive and negative ion species in the plasmas are measured by a time-of-flight mass spectrometer. The intensity ratio of O+ to O+2 increases with the electron density ne and is almost proportional to it in the region of ne=1010–1012 cm−3. When the electron density increases up to 8×1012 cm−3, the ratio becomes about 4. In a high density plasma of 1.3×1013 cm−3 obtained by mixing Ar gas, about 83% of the positive oxygen ions becomes O+. By using pulse modulation of the rf power, O− ions are mainly observed with remarkable increase in the afterglow. The maximum density of O− is about 3×1011 cm−3 at 30 μs after turning off the rf power of 0.85 kW and the decay time of O− is about 60 μs.