Plasma Source Region Research Articles

Plasma-enhanced atomic layer deposition: Correlating O2 plasma parameters and species to blister formation and conformal film growth

Plasma-enhanced atomic layer deposition has gained a lot of attraction over the past few years. A myriad of processes have been reported, several reviews have been written on this topic, and there is a lot of interest for industrial applications. Still, when developing new processes, often heuristic approaches are used, choosing plasma parameters that worked for earlier processes. This can result in suboptimal plasma process conditions. In order to rationally decide which parameters to use, we systematically studied an inductively coupled RF oxygen plasma source (13.56 MHz) for powers up to 300 W, a pressure range between 10−4 and 10−2 mbar, and a flow range between 10 and 400 sccm. We discerned between chemically active “radical” species (atomic O and excited, metastable O2) and ionic particles (O2+, O+, O2−, and O−), which can have an additional physical effect to the film. Optical emission spectroscopy (OES) was used to study the generation of O2+ and atomic O in the plasma source region. It is shown that the concentration of plasma species increases in a linear way with the plasma power and that the atom-to-ion fraction increases with both the power and the gas flow. To study the effect of plasma species in the remote region, near the sample position, an electrostatic quadrupole analyzer was used to gauge fluxes of O2+, O+, O2−, and O−. Even a moderate increase in pressure can drastically reduce the ion flux toward the substrate. The formation of bubbles or blisters in films can be linked to ion-induced compressive stress, and, hence, it can be mitigated by an increase in the gas pressure. Finally, Al2O3 was deposited in lateral high-aspect ratio structures to investigate the effect of plasma power and gas pressure on the partial pressure of radical species. Simulated profiles were fitted to experimental deposition profiles to estimate trends in the radical partial pressure, and a linear relationship between radical partial pressure and the power was found. This correlated with the density of atomic O species as observed in the OES measurements in the plasma source region. The methods presented in this work are also applicable to characterize other reactor geometries, plasma sources, and gas mixtures.

The design of Multiple Plasma Simulation Linear Device

• The physical design of a plasma linear device called M ultiple P lasma S imulation L inear D evice (MPS-LD) at Dalian University of Technology is presented. • MPS-LD is characterized by replaceable plasma source, plasma auxiliary heating system and flexible target room. • The objective of MPS-LD is to provide the plasma environment to investigate the plasma-material interactions and divertor plasma physics. • The design of a linear plasma device called M ultiple P lasma S imulation L inear D evice (MPS-LD) at Dalian University of Technology is presented. The objective of MPS-LD is to investigate the key scientific issues in tokamaks, i.e. plasma-material interactions and the divertor plasma physics. The distinctive features of MPS-LD are the replaceable plasma sources, the capability of plasma heating and flexible target room. The length of the vacuum chamber is 3 m, and the device consists of two sizes of chambers with diameters of 0.4 m and 0.6 m, respectively. The steady-state magnetic field with maximum strength at axis of 0.3 Tesla (T) is generated by 11 conventional copper magnets, including 8 small coils and 3 big coils. The device is divided into three regions: plasma source region, auxiliary heating region and target region. Initially, the helicon wave plasma source will be installed to produce the plasma with density of 10 18 -10 19 m -3 , and temperature of 1-3 eV. The arrange of magnets and the magnetic field distribution are computed by the finite element method, and the required pumping system for the desired neutral pressure is calculated by the pressure balance method. The design of magnet system, three-stage differentially pumped vacuum system, replaceable plasma sources, cooling circulation system, target chamber system, diagnostic system and auxiliary heating system, are presented in this work.

Plasma flow equilibria in 2D cylindrically symmetric expanding magnetic field

A steady-state plasma flow exiting through an expanding magnetic field is studied by means of 2D particle in cell numerical simulations. The effects of change in the localized plasma source region dimension and the associated plasma transit length in an upstream, uniform-magnetic-field region were examined, simulating cases with different axial lengths of the dielectric plasma source region, distinct from the location of physical expansion. Axial potential profiles at various radial locations show development of a stepwise axial potential drop, producing plasma (ion) acceleration in the corresponding regions. For a narrow source region with a long pre-expansion flow region, nonmonotonic potential variation is recovered where ion phase-space scatters show the trapped region and signatures of chaotic ion trajectories and possible pre-expansion ion heating. Considering a relevance of the studied flow equilibria to the thrust generation schemes for space propulsion, a formal estimate of thrust values associated with the plasma outflow is also done for the cases simulated.

Preliminary analysis of back-streaming electrons on the source plasma region of ion sources for the EAST-NBI

Back-streaming electrons gain significant energy due to the high voltage of the extraction system for a high-current ion source. By theoretical calculation, the particle flux accounts for 13.88% of the total beam current, and the power flux accounts for about 7.5% of the total beam power. This shows that back-streaming electrons are very destructive to the plate of electron absorption that is installed opposite of the accelerator. At the same time, as particles impinge on grids, the energy level that the grids absorb will be really high. Compared with the water flow calorimetry data of ion sources on the ASIPP-NBI testbed, it can be found that, as the high voltage of the extraction system rises, the particle flux and the power flux of the back-streaming electrons are essentially in the same proportions. Therefore, the corresponding energy deposited on the components of the ion source will grow by the same percentage with the increase in high voltage, which demonstrates strong inhibition to improving the neutral beam power injected into a tokamak.

Transverse motion of a plasma column in a sheet plasma

In this paper, we show the dynamic behaviors of a sheet plasma and the corresponding release of blob‐like plasmas to the periphery of the sheet based on observation with a high‐speed camera. We found that the plasma column moves in a transverse direction reciprocally when injecting a small amount of Ar gas to the He plasma from the plasma source region. The motion continued periodically at a frequency of ∼5 kHz. The frequency of the plasma column motion decreased with increasing Ar gas flow rate and increasing magnetic field. The mechanism of the transverse motion is discussed. Furthermore, in response to the reciprocating motion, release of a blob‐like plasma was observed. The differences in the behavior of the released plasma compared with the other linear devices are discussed.

An atmospheric‐pressure microplasma array produced by using graphite coating electrodes

In this work, a novel non‐thermal atmospheric‐pressure microplasma array (APMPA) has been characterized. The APMPA employs 16 parallel capillaries with an inner diameter of 200 µm and arranged in a square configuration. Our measurements show that the stable plasma plumes can be generated by the APMPA and then spread along the dielectric surface. Both the power in the plasma source region and the one transferred onto the dielectric surface are calculated by plotting Lissajous figures. The power strongly depends on the applied voltage, frequency, and the distance between the nozzles and the dielectric surface. In addition, the temperature of N2 molecules in APMPA plasmas is calculated by using optical emission spectroscopy. The temporally resolved images of the APMPA plasmas show that the total emission intensities for the case of positive half‐period is much higher than that of negative half‐period. In order to describe the performance of the APMPA, the optical emission of the APMPA is compared with a single tube plasma jet with the same configuration and cross section area, and the result illustrates the APMPA can provide a higher density plasma and larger treatment area which may have a beneficial effect on various applications.

Enhancement of helium exhaust by resonant magnetic perturbation fields at LHD and TEXTOR

The ability to exhaust helium as the fusion born plasma impurity is a critical requirement for burning plasmas. We demonstrate in this paper that resonant magnetic perturbation (RMP) fields can be used to actively manipulate helium exhaust characteristics. We present results from puff/pump studies at TEXTOR as example for a tokamak with a pumped limiter and from the Large Helical Device (LHD) with the closed helical divertor as example for a heliotron/stellarator device. For LHD, the effective helium confinement time is a factor of 7–8 higher in the low and high density regimes explored when compared to TEXTOR discharges. This is attributed to ion root impurity transport which is one particular impurity transport regime assessed experimentally at LHD and which facilitates helium penetration to the plasma core. However, when an edge magnetic island is induced by externally applied RMP fields, is decreased by up to and hence values closer to those of TEXTOR can be established. The combination of TEXTOR and LHD results suggest that a magnetic island induced by the RMP field in the plasma source region is an important ingredient for improving helium exhaust. The reduction in seen is caused by a combination of improved helium exhaust due to an enhanced coupling to the pumping systems, increased outward transport and a reduced fueling efficiency for the helium injected and recycling from the wall elements.

Charge and energy transferred from a plasma jet to liquid and dielectric surfaces

A key parameter in using plasma jets for biomedical applications is the transferred energy to the living tissues. The objective of this paper is to understand which parameters control the energy transfer from the plasma jet to a liquid or a dielectric surface. The plasma jet is flown with helium and ignited by a 600 Hz ac high voltage (up to 15 kV). Capacitors are connected to two measurement electrodes placed in the plasma source region, and under the sample. Charge and energy transferred are estimated by plotting Lissajous cycles; the number of bullets and the charge probability density function are also calculated. It is shown that the applied voltage and the gap (distance between the end of the tube and the sample) have a dramatic influence on the energy deposition on the sample as well as on the charge probability density function. Surprisingly, both gap distance and voltage have very little influence on the number of bullets reaching the sample per cycle. It is also shown that the conductivity of the liquid sample has almost no influence on the energy deposition and charge probability density function.

Multi‐instrument, high‐resolution imaging of polar cap patch transportation

AbstractTransionospheric radio signals in the high‐latitude polar cap are susceptible to degradation when encountering sharp electron density gradients associated with discrete plasma structures, or patches. Multi‐instrument measurements of polar cap patches are examined during a geomagnetic storm interval on 22 January 2012. For the first time, we monitor the transportation of patches with high spatial and temporal resolution across the polar cap for 1–2 h using a combination of GPS total electron content (TEC), all‐sky airglow imagers (ASIs), and Super Dual Auroral Radar Network (SuperDARN) HF radar backscatter. Simultaneous measurements from these data sets allow for continuous tracking of patch location, horizontal extent, and velocity despite adverse observational conditions for the primary technique (e.g., sunlit regions in the ASI data). Spatial collocation between patch‐like features in relatively coarse but global GPS TEC measurements and those mapped by high‐resolution ASI data was very good, indicating that GPS TEC can be applied to track patches continuously as they are transported across the polar cap. In contrast to previous observations of cigar‐shaped patches formed under weakly disturbed conditions, the relatively narrow dawn‐dusk extent of patches in the present interval (500–800 km) suggests association with a longitudinally confined plasma source region, such as storm‐enhanced density (SED) plume. SuperDARN observations show that the backscatter power enhancements corresponded to the optical patches, and for the first time we demonstrate that the motion of the optical patches was consistent with background plasma convection velocities.

Helicon plasma thruster discharge model

By considering particle, momentum, and energy balance equations, we develop a semi-empirical quasi one-dimensional analytical discharge model of radio-frequency and helicon plasma thrusters. The model, which includes both the upstream plasma source region as well as the downstream diverging magnetic nozzle region, is compared with experimental measurements and confirms current performance levels. Analysis of the discharge model identifies plasma power losses on the radial and back wall of the thruster as the major performance reduction factors. These losses serve as sinks for the input power which do not contribute to the thrust, and which reduce the maximum plasma density and hence propellant utilization. With significant radial plasma losses eliminated, the discharge model (with argon) predicts specific impulses in excess of 3000 s, propellant utilizations above 90%, and thruster efficiencies of about 30%.

A plasma source driven predator-prey like mechanism as a potential cause of spiraling intermittencies in linear plasma devices

Three-dimensional global drift fluid simulations are carried out to analyze coherent plasma structures appearing in the NAGDIS-II linear device (nagoya divertor plasma Simulator-II). The numerical simulations reproduce several features of the intermittent spiraling structures observed, for instance, statistical properties, rotation frequency, and the frequency of plasma expulsion. The detailed inspection of the three-dimensional plasma dynamics allows to identify the key mechanism behind the formation of these intermittent events. The resistive coupling between electron pressure and parallel electric field in the plasma source region gives rise to a quasilinear predator-prey like dynamics where the axisymmetric mode represents the prey and the spiraling structure with low azimuthal mode number represents the predator. This interpretation is confirmed by a reduced one-dimensional quasilinear model derived on the basis of the findings in the full three-dimensional simulations. The dominant dynamics reveals certain similarities to the classical Lotka-Volterra cycle.

Effect of basic physical parameters to control plasma meniscus and beam halo formation in negative ion sources

Our previous study shows that the curvature of the plasma meniscus causes the beam halo in the negative ion sources: the negative ions extracted from the periphery of the meniscus are over-focused in the extractor due to the electrostatic lens effect, and consequently become the beam halo. In this article, the detail physics of the plasma meniscus and beam halo formation is investigated with two-dimensional particle-in-cell simulation. It is shown that the basic physical parameters such as the H− extraction voltage and the effective electron confinement time significantly affect the formation of the plasma meniscus and the resultant beam halo since the penetration of electric field for negative ion extraction depends on these physical parameters. Especially, the electron confinement time depends on the characteristic time of electron escape along the magnetic field as well as the characteristic time of electron diffusion across the magnetic field. The plasma meniscus penetrates deeply into the source plasma region when the effective electron confinement time is short. In this case, the curvature of the plasma meniscus becomes large, and consequently the fraction of the beam halo increases.

Impact of static magnetic fields on the radial line slot antenna plasma source

The radial line slot antenna plasma source is used in semiconductor device fabrication. As is the case for all plasma sources, ever more strict uniformity control requirements are driven by the precision demands of new device technologies. Large volume diffusion plasmas, of which the radial line slot antenna source is one type, must overcome transport effects or diffusion modes that tend to “center peak” the plasma density near the wafer being processed. One way to resolve problematic transport effects is the insertion of magnetic fields into the plasma region. In this paper, the impact of the magnetic field on plasma properties is parameterized as a function of slot configuration. The magnetic field orientation and the magnitude of magnetic field are varied in a computational study in which the source is modeled as a two-dimensional axisymmetric quasineutral plasma. This work employs a finite element model simulation. The magnitude of magnetic fields considered is 50 Gauss maximum with a microwave power of 3000 W at a pressure of 20 mTorr. 20 mTorr is chosen as this is a condition where diffusion effects are challenging to counteract. The study showed that there are specific conditions for slot configuration and magnetic field that improve the plasma controllability and some that do not. Plasma property modulation is most effective when the plasma source region is placed at large radius with axial magnetic fields. There are synergistic effects between the slot location and magnetic field that are important and placing large magnetic fields at the chamber edge alone does not result in improved uniformity. Electron cyclotron resonance (ECR) heating and the impact of pulsing the magnetic fields are presented. ECR heating is not important for the conditions relevant to this paper and pulsing is shown to have benefit.

Energetic electron observations of Rhea’s magnetospheric interaction

Saturn’s moon Rhea is thought to be a simple plasma absorber, however, energetic particle observations in its vicinity show a variety of unexpected and complex interaction features that do not conform with our current understanding about plasma absorbing interactions. Energetic electron data are especially interesting, as they contain a series of broad and narrow flux depletions on either side of the moon’s wake. The association of these dropouts with absorption by dust and boulders orbiting within Rhea’s Hill sphere was suggested but subsequently not confirmed, so in this study we review data from all four Cassini flybys of Rhea to date seeking evidence for alternative processes operating within the moon’s interaction region. We focus on energetic electron observations, which we put in context with magnetometer, cold plasma density and energetic ion data. All flybys have unique features, but here we only focus on several structures that are consistently observed. The most interesting common feature is that of narrow dropouts in energetic electron fluxes, visible near the wake flanks. These are typically seen together with narrow flux enhancements inside the wake. A phase-space-density analysis for these structures from the first Rhea flyby (R1) shows that Liouville’s theorem holds, suggesting that they may be forming due to rapid transport of energetic electrons from the magnetosphere to the wake, through narrow channels. A series of possibilities are considered to explain this transport process. We examined whether complex energetic electron drifts in the interaction region of a plasma absorbing moon (modeled through a hybrid simulation code) may allow such a transport. With the exception of several features (e.g. broadening of the central wake with increasing electron energy), most of the commonly observed interaction signatures in energetic electrons (including the narrow structures) were not reproduced. Additional dynamical processes, not simulated by the hybrid code, should be considered in order to explain the data. For the small scale features, the possibility that a flute (interchange) instability acts on the electrons is discussed. This instability is probably driven by strong gradients in the plasma pressure and the magnetic field magnitude: magnetometer observations show clearly signatures consistent with the (expected) plasma pressure loss due to ion absorption at Rhea. Another potential driver of the instability could have been gradients in the cold plasma density, which are, however, surprisingly absent from most crossings of Rhea’s plasma wake. The lack of a density depletion in Rhea’s wake suggests the presence of a local cold plasma source region. Hybrid plasma simulations show that this source cannot be the ionized component of Rhea’s weak exosphere. It is probably related to accelerated photoelectrons from the moon’s negatively charged surface, indicating that surface charging may play a very important role in shaping Rhea’s magnetospheric interaction region.

Plasma control by modification of helicon wave propagation in low magnetic fields

By making use of nonuniform magnetic fields, it is shown experimentally that control of helicon wave propagation can be achieved in a low pressure (0.08 Pa) expanding plasma. The m=1 helicon waves are formed during a direct capacitive to wave mode transition that occurs in a low diverging magnetic field (B0<3 mT). In this initial configuration, waves are prevented from reaching the downstream region, but slight modifications to the magnetic field allows the axial distance over which waves can propagate to be controlled. By changing the effective propagation distance in this way, significant modification of the density and plasma potential profiles can be achieved, showing that the rf power deposition can be spatially controlled as well. Critical to the modification of the wave propagation behavior is the magnetic field strength (and geometry) near the exit of the plasma source region, which gives electron cyclotron frequencies close to the wave frequency of 13.56 MHz.

Mechanism of cone-shaped carbon nanotube bundle formation by plasma treatment

Formation of the cone-shaped multi-walled carbon nanotube (MWCNT) bundles was investigated with the consideration of the induced dipole moments of the MWCNTs interaction under the ion irradiation which is accelerated by the applied sheath electric field for the various argon, hydrogen, nitrogen, and oxygen plasmas. Vertically grown MWCNTs were irradiated by energetic ion whose energy and dose were controlled by the sheath formed on the MWCNT substrate. Plasma irradiation was carried out in a downstream region separated from the plasma source region, providing that the irradiated ion density and energy could be controlled precisely with the sheath electric field. In argon and hydrogen plasmas, the cone-shaped MWCNT bundle was not fabricated, while it was formed successfully in nitrogen and oxygen plasmas. Especially, the oxygen plasma was the most effective in the formation of the bundle. The mechanism of the bundle formation could be explained by a model explaining the interaction between the induced dipole moment of the MWCNT and the sheath electric field. For the nitrogen and oxygen plasma irradiated MWCNT, the induced dipole moment could be enhanced by C–N and C–O bonds so the size of the bundle is proportional to the ion irradiation and the sheath electric field.

Double Temperature and Density Phenomenon in Grid Enhanced Plasma Source Ion Implantation for Inner Surface Modification of Tubes

Inner surface coating for tubular samples was realized by the grid enhanced plasma source ion implantation (GEPSII) method. In the GEPSII system, two electrodes, a central rod electrode and a coaxial grid electrode were coaxially assembled inside the tube. Plasma was generated between the electrodes by a radio-frequency (RF) oscillating power source. Plasma then diffused through the grid and realized inner surface ion implantation by a negative high voltage applied to the tube. The plasma was then divided, by the grid, into two regions, namely the source plasma region and the diffused plasma region. The plasma's self-bias between two RF power source electrodes was measured. At the same time, the electron temperature and plasma density in the GEPSII system were measured by a scattering spectrometer. Results showed that the plasma properties of the two regions were entirely different; the plasma self-bias, which might greatly affect the sputtering rate of the central titanium electrode, depended on the electrode structure, gas pressure and RF power.

Identification of Radial Distance of Plasma Dispersionless Injection Boundary from the Injection Source

Measurements of energetic particles obtained by the two geosynchronous satellites (1991-080 and LANL-97A) are performed to investigate the plasma injection boundary and source region during the magnetospheric substorms. The measurement method is developed to allow remote sensing of the plasma injection time and the radial distance of injection boundaries by using measured energy dispersion and modelling particle drifts within the Volland–Stern electric field and the dipole magnetic field model. The radial distance of the injection boundary deduced from a dispersion event observed by the LANL-97A satellite on 14 June 1998 is 7.1RE, and the injection time agrees well with the substorm onset time identified by the Polar Ultraviolet Imager. The method has been applied to an event happened at 22.9 UT on 11 March 1998, when both the satellites (1991-080 and LANL-97A) observed the dispersionless character. The results indicate that the radial distance of injection source locates at 8.1RE at magnetotail, and particles move earthward from magnetotail into inner magnetosphere at 22.5 UT.

Characterization of remote inductively coupled CH4–N2 plasma for carbon nitride thin-film deposition

We investigated reaction characteristics in a CH4∕N2 plasma for deposition of amorphous CNx thin films (a-CNx) by evaluating the change in electron density using the wave cutoff method, and the behavior of ions and radicals with an optical emission spectroscopy (OES). An inductively coupled plasma source that was 30cm away from the substrate stage was used for the discharge. The change in electron density in the substrate region and OES spectra in the plasma-source region were evaluated to investigate both the reaction mechanism and the remote effect while varying process conditions such as rf power, pressure, and gas-mixing ratio. We found that the electron density in the remote CH4∕N2 plasma was closely related to recombination reactions of major ions such as N2+, CH4+, CH3+, and H2+ during diffusion from the plasma source to the substrate. The electron density and optical emission of major ions and radicals in the CH4∕N2 plasma increase at higher rf power. The ratio [N]∕([N]+[C]) in a-CNx films, as measured by auger electron spectroscopy, also increases with rf power since more excited N and C species are generated. For increasing pressure, the change in electron density and emission spectra showed different behavior, which arose from recombination of ions that generated more CH4, Nx (x=1,2), and CN radicals. The majority of positive ions generated from N2 species are greatly affected by the remote effect, while the majority of positive ions generated from CH4 species are not significantly influenced, since each species has different losses dependent on the pressure. A higher N2 gas fraction in the gas mixture generated more CN radicals, which resulted not only in more N incorporated into a-CNx films but also to a reduction of H passivation that retards formation of hybrid bonding between C and N in the films. These results suggest that efficient H abstraction is required to achieve more NC triple bonding in CH4∕N2 plasma deposition.

An Alfvén wave maser in the laboratory

A frequency selective Alfvén wave resonator results from the application of a locally nonuniform magnetic field to a plasma source region between the cathode and anode in a large laboratory device. When a threshold in the plasma discharge current is exceeded, selective amplification produces a highly coherent (δω∕ω&amp;lt;5×10−3), large amplitude shear Alfvén wave that propagates out of the resonator, through a semitransparent mesh anode, into the adjacent plasma column where the magnetic field is uniform. This phenomenon is similar to that encountered in the operation of masers∕lasers at microwave and optical frequencies. The current threshold for maser action is found to depend upon the confinement magnetic field strength B0. Its scaling is consistent with the condition for matching the drift speed of the bulk plasma electrons with the phase velocity of the mode in the resonator. The largest spontaneously amplified signals are obtained at low B0 and large plasma currents. The magnetic fluctuations δB associated with the Alfvén maser can be as large as δB∕B0≈1.5% and are observed to affect the plasma current. Steady-state behavior leading to coherent signals lasting until the discharge is terminated can be achieved when the growth conditions are well-above threshold. The maser is observed to evolve in time from an initial m=0 mode to an m=1 mode structure in the transition to the late steady state. The laboratory phenomenon reported is analogous to the Alfvén wave maser proposed to exist in naturally occurring, near-earth plasmas.