Ratio Of Ion Flux Research Articles

First observation of edge impurity behavior with n = 1 RMP application in EAST L-mode plasma

High-Z impurity accumulation suppression and mitigation in core plasma is frequently observed in EAST edge localized mode mitigation experiments by using resonant magnetic perturbations (RMP) coils. To study the individual effects of the RMP field on impurity transport, based on high-performance extreme ultraviolet impurity spectroscopic diagnostics, the effect of the n = 1 (n is the toroidal mode number) RMP field on the behavior of intrinsic impurity ions at the plasma edge, e.g. He+, Li2+, C2+–C5+, O5+, Fe8+, Fe15+, Fe17+, Fe22+, Cu17+, Mo12+, Mo13+ and W27+, is analyzed for the first time in L-mode discharges. Based on the evaluation of the location of these impurity ions, it is found that with the increase in RMP current (I RMP), an impurity screening layer inside the last closed flux surface is formed, e.g. at ρ = 0.74–0.96, which is also the region that the RMP field affects. Outside this screening layer, the impurity ion flux of He+, Li2+, C2+, C3+, O5+, Fe8+, Mo12+ and Mo13+ ions increases gradually, while inside this screening layer, the impurity ion flux of C4+, C5+, Cu17+, W27+, Fe15+, Fe17+ and Fe22+ ions decreases gradually. When I RMP is higher than a threshold value, RMP field penetration occurs, accompanied with m/n = 2/1 mode locking, and the position of this screening layer moves to the plasma core region, i.e. ρ = 0.66–0.76, close to the q = 2 surface, and the opposite behavior of the impurity ion flux at two sides of the screening layer is strengthened dramatically. As a result, significant decontamination effects in the plasma core region, indicated by the factor of ((Γ Imp Z+)w/o–(Γ Imp Z+))/(Γ Imp Z+)w/o (where (Γ Imp Z+)/(Γ Imp Z+)w/o denotes the impurity ion flux ratio with and without RMP), is observed, i.e. 30%–60% for heavy impurity (Fe, Cu, Mo, W), and ∼27% for light impurity of C. In addition, the analysis of the decontamination effects of C and Fe impurities under four different RMP phase configurations shows that it may be related to the strength of the response of the plasma to RMP. These results enhance the understanding of impurity accumulation suppression by the n = 1 RMP field and demonstrate a candidate approach using RMP coils for W control in magnetic confinement devices.

Waste acid recovery utilizing monovalent cation permselective membranes through selective electrodialysis

Waste acid recovery utilizing monovalent cation permselective membranes through selective electrodialysis

The dependence of tungsten fuzz layer thickness and porosity on tungsten deposition rate and helium ion fluence

Fuzz formation on a heated tungsten surface in the presence of a helium-containing plasma and tungsten deposition source was investigated. Tungsten samples were exposed at 1123 K to pure helium plasma with ion incident energy of 76 eV, W/He ion flux ratio of ∼0.4×10−4 , and varied helium ion fluence from 0.18 to 3.4×1026 m−2. Fuzz thickness was measured by cross-sectional scanning electron microscopy to increase from 0.22 to 15 µm with increasing helium ion fluence. No indication of saturation in fuzz thickness at high fluence was observed, in contrast to fuzz produced on a tungsten surface without tungsten deposition. Additional tungsten samples were exposed at 1123 K to pure helium plasma with ion incident energy of 76 eV, helium ion fluence of ∼3.4×1026 m−2, and varied W/He ion flux ratio from 0.26 to 3.0×10−4 . Fuzz thickness increased from 7.5 to 120 µm with increasing W/He ion ratio. A final sample exposed at 1123 K to a mixed helium-deuterium plasma with ion incident energy of 76 eV, helium ion fluence of 0.18×1026 m−2, and W/He ion flux ratio of 2.2×10−4 developed nearly identical fuzz structures to that developed in a pure He plasma. As a function of deposited tungsten fluence, all results were found to trace out a single layer-growth curve given by a power law relation, indicating that fuzz thickness is independent of the W/He ion flux ratio in the range investigated and independent of any deuterium present in the plasma. As a result, for tungsten plasma facing walls in magnetic fusion devices at 1000–2000 K with 10−4 W/He ion flux ratio, fuzz with thicknesses greater than hundreds of microns may form in as little as 104 s (in the absence of ELM-induced erosion or annealing), and may more significantly affect its thermophysical properties than fuzz generated without a tungsten deposition source.

Energetic ion variations during substorm intervals using the Van Allen Probes data

The study investigates ion flux variations for the substorms in the inner magnetosphere. The effect of substorm-induced magnetic field dipolarization on the O+ and H+ ion flux is analysed for 22 events from the year 2018 using the Helium, Oxygen, Proton, and Electron (HOPE) Mass Spectrometer data on board the Van Allen Probes (VAP/RBSP) satellite. The clear dipolarization signatures are observed using the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) suite from the VAP. These observations provide evidence of the ion flux enhancement at 1–50 keV energy after the substorm onset, in particular, the energy range of 20–50 shows dominance. The typical characteristics of magnetic field dipolarization and its time scales are investigated. It is found that the O+ ion flux enhances greater in magnitude than H+ ion flux at energies 20–50 keV after the substorm onset. In addition to it, the correlation of these enhanced oxygen fluxes with the related interplanetary magnetic field (IMF) Bz, solar wind velocity (Vsw), and the auroral electrojet (AE) index are found good. The new results reports that the ion flux variation ratio shows the MLT dependence for different energy ranges and found that the peak of the O+/ H+ ion flux ratio centred near midnight for the energy range of 8–20 keV, whereas the O+/ H+ ion flux ratio of the energy 20–50 keV is high within the post-midnight. The possible mechanisms for the enhanced ion flux are discussed.

Effects of high energy electrons on electron temperature measurements of asymmetric double Langmuir probes

In this work, electron temperature was measured with both the asymmetric double Langmuir probe (ADLP) and the single Langmuir probe to investigate the reliability of the ADLP on the electron temperature measurement in multi-temperature Maxwellian plasmas. A series of I–V traces of the ADLP were obtained at various plasma conditions with different area ratios and analyzed with different methods including conventional ADLP analysis and two-temperature Maxwellian fitting with results measured by a single planar Langmuir probe analyzed with three-temperature Maxwellian fitting as reference. The measured Te of the ADLP is found to reflect that of the temperature of the degraded primary electrons when the area ratio of the probe tips is close to ∼16 and approaches the real effective electron temperature as the area ratio increases to a value of ∼30% higher than that measured by a single Langmuir probe, which occurs even when the area ratio is higher than the flux ratio of electrons and ions entering their respective sheaths. This effect is consistent with the distortion effect of Langmuir probe I–V traces caused by the presence of hotter electron species, which was computationally reconstructed and agreed well with the experimental observations. This result implies that an area ratio, possibly ∼20 times much larger than what was conventionally assumed, is needed for an ADLP to be reliably treated as a single Langmuir probe in practical settings, where electron energy distribution functions of plasmas are generally expected to be multi-temperature Maxwellian. This effect is also analogous to the current balance between a single Langmuir probe and the device wall, implying that this effect would also affect the application of the single Langmuir probe in plasmas, where the ion loss to the device wall can be reduced, such as plasmas in miniaturized devices, strong magnetic fields, or a highly ion-neutral collisional environment.

Effect of pulse width on deposition of diamond-like carbon on high-power pulsed magnetron sputtering

A diamond-like carbon (DLC) film was deposited using high-power pulsed magnetron sputtering. The effect of pulse width on the deposition of DLC film was investigated under constant peak power density or average power density to clarify the densification mechanism of DLC film. The maximum hardness of 25 GPa analyzed by nanoindentation was obtained using Ar gas without negative substrate bias voltage at pulse width 30 μs and a peak power density of 1.5 kW cm−2. The flux and energy of C+ and Ar+ incident to the DLC film was evaluated by using energy-resolved and time-resolved mass spectrometry to clarify the relation between the input power to the target and the behavior of produced ions. The change in hardness is well correlated with the ion flux ratio C+/Ar+. This result indicates that a flux and energy of Ar+ as well as C+ is a key parameter to characterize the microstructure of DLC film.

On the origin and evolution of hotspots in multipatterning processes

Understanding the origins and propagation of defects and hotspots in patterning processes used for semiconductor fabrication is of paramount importance in managing yield. In this paper, results from physics-based simulators to model lithography and dry etch processes are presented and compared to experimental results. These models are used to study different types of hotspots and defects observed in a litho-etch-litho-etch (LELE) multipatterning process. At each pass of the LELE flow, patterns are printed into a SiO2 collecting layer using a trilayer film stack comprised of a negative tone photoresist layer, a spin-on-glass layer (SOG), and a spin-on-carbon layer (SOC). After both passes of the LELE process, the patterns in the SiO2 collecting layer will be transferred to a TiN hardmask prior to final etch into an underlying dielectric. The SOG and SiO2 layers are etched using fluorocarbon plasma, while the SOC layer is etched with an H2/N2 plasma generated in a capacitively coupled plasma source. A pinching hotspot is observed during the single litho-etch pass in a region where two features are placed very close and the image contrast is low. However, for some lithography process conditions, this hotspot is rectified by subsequent etch steps and does not always transfer as a defect into the SiO2 layer. The quenching of the hotspot occurs primarily during the etching of the SOC layer due to the aspect ratio-dependent etching (ARDE) effect. A bridging hotspot is also observed at lithography during the single litho-etch pass at high exposure doses. This hotspot, on the other hand, is exacerbated by the etch steps because of the ARDE effect. Hotspots are also identified that originate from overlay errors between photomasks exposed during first and second passes of the LELE process. The etch bias generated during etching of the SOG layer is crucial to ensure that the overlay-related hotspot does not translate to the SiO2 layer. The extent of etch bias in the SOG etch step is critical and can be tuned by adjusting the neutral to ion flux ratio during that etch step. Increasing the flux ratio improves the process window for the overlay defect; however, when the ratio is higher than approximately 20% of the nominal value, a different defect type is formed in the SOG layer due to the inverse ARDE effect that propagates downstream to the SiO2 layer.

Investigation into SiO2 Etching Characteristics Using Fluorocarbon Capacitively Coupled Plasmas: Etching with Radical/Ion Flux-Controlled.

SiO2 etching characteristics were investigated in detail. Patterned SiO2 was etched using radio-frequency capacitively coupled plasma with pulse modulation in a mixture of argon and fluorocarbon gases. Through plasma diagnostic techniques, plasma parameters (radical and electron density, self-bias voltage) were also measured. In this work, we identified an etching process window, where the etching depth is a function of the radical flux. Then, pulse-off time was varied in the two extreme cases: the lowest and the highest radical fluxes. It was observed that increasing pulse-off time resulted in an enhanced etching depth and the reduced etching depth respectively. This opposing trend was attributed to increasing neutral to ion flux ratio by extending pulse-off time within different etching regimes.

Direct measurement of ion and electron flux ratio at their respective sheath-edges and absence of the electron Bohm criterion effects

A recent theory suggests that electrons enter electron sheaths at an electron Bohm velocity given by (T e/m e)1/2 instead of the electron thermal velocity as conventionally assumed. To test this theory, the flux density ratio Γe,se/Γi,se of electrons and ions entering their respective sheaths was directly measured via an almost continuous A i/A e area ratio scanning. The measured value agrees with the predictions assuming electrons entering the electron sheaths at their thermal velocity. The predictions associated with the electron Bohm criterion have not been found. If the predictions of such theories are true, the electron or ion presheath density drops will be very different from conventionally expected values.

Transient behavior in quasi-atomic layer etching of silicon dioxide and silicon nitride in fluorocarbon plasmas

The mechanism for atomic layer etching (ALE) typically consists of two sequential self-limited half-reactions—passivation and ion bombardment—which provide unique control over the process. Some of the possible benefits of this control include increased selectivity, reduced plasma induced damage, improved uniformity and aspect ratio independence. To achieve the greatest benefit from ALE, both half-reactions should be fully self-limited. In the experimental demonstration of ALE of SiO2 using fluorocarbon plasmas, the passivation phase typically consists of deposition of fluoropolymer on the SiO2 surface. This passivation step is not a self-limited reaction as the final polymer thickness depends on the passivation time. In this paper, results are presented from a computational investigation of the ALE of SiO2 and Si3N4 focusing on the implications of this nonself-limited passivation phase. The polymer overlayer was found to be critically important to the ALE performance, providing the main mechanism for selectivity between SiO2 and Si3N4. The polymer overlayer acts as a fuel for etching SiO2, which couples the etch depth per ALE cycle to the passivation time. Due to the inherently pulsed nature of the ALE mechanism, the polymer overlayer requires a finite number of cycles to reach a pulsed periodic steady-state thickness. Since the thickness of the polymer overlayer largely determines selectivity between SiO2 and Si3N4, the initial formation of an overlayer results in a transient period at the beginning of etching where high selectivity may not be achieved. For the etching of thin films, or applications which require very high selectivity, this transient etching period may be a limiting factor. Results are also presented using ALE to etch high aspect ratio self-aligned contacts which could not be cleared using continuous plasma etching with similar ion energies and flux ratios.

Correlation of HPPMS plasma and coating properties using artificial neural networks

The development of industrial coating processes for tool coatings by means of physical vapor deposition (PVD) is usually extremely complex. This is caused by the large number of necessary coating batches and associated coating analyses until suitable process parameters are found. Artificial neural networks (ANN) are basically capable of describing complex relationships between various characteristic process values. Hence, within the scope of this paper the capability of describing complex correlations was tested on the example of a reactive high power pulsed magnetron sputtering (HPPMS) (Cr,Al)ON process. Selected process parameters pulse frequency and process gas composition were chosen, since they exhibit strongly non-linear cause-effect relationships. The ANN was used in order to correlate selective results from efficient substrate-oriented plasma diagnostics and coating analyses. Regarding the plasma properties the Al/Cr ratio and the metal-to-gas ion flux ratio were considered. With respect to the coating properties the Al/Cr ratio and the universal hardness were examined. From the correlation of these results, conclusions on the process parameters for desired coating properties were deduced and successfully proven for the investigated HPPMS (Cr,Al)ON process. Hence, the ANN exhibits a great potential to supplement the fundamental understanding of PVD processes in order to contribute to a simplification of the development of industrial coating processes.

Smart DNA Hydrogel Integrated Nanochannels with High Ion Flux and Adjustable Selective Ionic Transport.

Nanochannels based on smart DNA hydrogels as stimulus-responsive architecture are presented for the first time. In contrast to other responsive molecules existing in the nanochannel in monolayer configurations, the DNA hydrogels are three-dimensional networks with space negative charges, the ion flux and rectification ratio are significantly enhanced. Upon cyclic treatment with K+ ions and crown ether, the DNA hydrogel states could be reversibly switched between less stiff and stiff networks, providing the gating mechanism of the nanochannel. Based on the architecture of DNA hydrogels and pH stimulus, cation or anion transport direction could be precisely controlled and multiple gating features are achieved. Meanwhile, G-quadruplex DNA in the hydrogels might be replaced by other stimulus-responsive DNA molecules, peptides, or proteins, and thus this work opens a new route for improving the functionalities of nanochannel by intelligent hydrogels.

Smart DNA Hydrogel Integrated Nanochannels with High Ion Flux and Adjustable Selective Ionic Transport

AbstractNanochannels based on smart DNA hydrogels as stimulus‐responsive architecture are presented for the first time. In contrast to other responsive molecules existing in the nanochannel in monolayer configurations, the DNA hydrogels are three‐dimensional networks with space negative charges, the ion flux and rectification ratio are significantly enhanced. Upon cyclic treatment with K+ ions and crown ether, the DNA hydrogel states could be reversibly switched between less stiff and stiff networks, providing the gating mechanism of the nanochannel. Based on the architecture of DNA hydrogels and pH stimulus, cation or anion transport direction could be precisely controlled and multiple gating features are achieved. Meanwhile, G‐quadruplex DNA in the hydrogels might be replaced by other stimulus‐responsive DNA molecules, peptides, or proteins, and thus this work opens a new route for improving the functionalities of nanochannel by intelligent hydrogels.

Fundamental study of an industrial reactive HPPMS (Cr,Al)N process

In this work, a fundamental investigation of an industrial (Cr,Al)N reactive high power pulsed magnetron sputtering (HPPMS) process is presented. The results will be used to improve the coating development for the addressed application, which is the tool coating for plastics processing industry. Substrate-oriented plasma diagnostics and deposition of the (Cr,Al)N coatings were performed for a variation of the HPPMS pulse frequency with values from f = 300 Hz to f = 2000 Hz at constant average power P = 2.5 kW and pulse length ton = 40 μs. The plasma was investigated using an oscilloscope, an intensified charge coupled device camera, phase-resolved optical emission spectroscopy, and an energy-dispersive mass spectrometer. The coating properties were determined by means of scanning electron microscopy, glow discharge optical emission spectroscopy, cantilever stress sensors, nanoindentation, and synchrotron X-ray diffraction. Regarding the plasma properties, it was found that the average energy within the plasma is nearly constant for the frequency variation. In contrast, the metal to gas ion flux ratio is changed from JM/JG = 0.51 to JM/JG = 0.10 for increasing frequency. Regarding the coating properties, a structure refinement as well as lower residual stresses, higher universal hardness, and a changing crystal orientation from (111) to (200) were observed at higher frequencies. By correlating the plasma and coating properties, it can be concluded that the change in the gas ion to metal ion flux ratio results in a competitive crystal growth of the film, which results in changing coating properties.

Properties of ultra-thin Cu films grown by high power pulsed magnetron sputtering

Because of the superior properties of copper, it has been of great interest as a conducting material to replace Al in device manufacturing and Ag in multilayer low-emission coatings. In this study, we investigated the influence of the pulsing frequency and the ion-to-atom ratio at direct-current (DC) and high power impulse magnetron sputtering on the structural, optical and electrical properties of Cu films of thickness less than 25nm. The ratio of ion flux to deposited atom flux at the substrate was varied by changing the average discharge current density during the pulse from 26 to 1220mA/cm2 and pulse repetition rate from 0.5 to 5kHz. Properties of nanometer-thick Cu films were found to be very sensitive to the ion-to-atom ratio. The Cu films were deposited with island-type growth. For the experimental conditions employed in the present study low-resistivity ultra-thin Cu films were obtained at moderate average discharge current density during the pulse (340mA/cm2), pulse frequency of 3kHz and ion-to-atom ratio of 1.5. We also determined the critical thickness at which Cu films exhibit continuous growth as 5–6nm. At this thickness films deposited under optimum conditions have resistivity of about 8μΩ·cm, which is 8 times smaller than for films deposited by DC magnetron sputtering. This difference is due to the fact that films grown in DC regime have twice the concentration of oxygen atoms.

Role of Low-Energy Ion Impact in Plasma Deposition of Cubic Boron Nitride Films

Cubic boron nitride (cBN) is potentially expected as the semiconductor for high temperature condition due to the large band gap, high breakdown field, and high resistance to oxidation. cBN films have been synthesized from vapor phase using a variety of physical vapor deposition such as sputtering and ion beam deposition rather than plasma-enhanced chemical vapor deposition. However, high-energy (>50 eV) ion bombardment for cBN formation results in low-quality cBN films and thus obstructs their electrical applications. In our previous studies, high-quality cBN films were deposited under low-energy ion bombardment (<40 eV) by plasma-enhanced chemical vapor deposition using the chemistry of fluorine. The mean ion energy for cBN formation was reduced greatly, then the resulting films consisted of micron-sized grains with crystallographic morphology. In this study, the deposition mechanism of our cBN films is examined in terms of ion energy, ion flux, and ion to depositing boron flux ratio onto the substrate. The ion energy and the ion to boron flux ratio are determined from the sheath potential and the ratio of incident ion flux to net deposited boron flux, respectively. For negative substrate biases where sp2-bonded BN phase only or no deposit is formed, both the ion energy and the ion to boron flux ratio are high. For positive substrate biases where cBN phase is formed, the ion energy and the ion to boron flux ratio are estimated in the range of a few eV to 35 eV and 100 to 130, respectively. The impact of negative ions is presumed to be negligible due to their low kinetic energy. The impact of positive ions with high ion to boron flux ratios is primarily responsible for reduction of the ion energy for cBN film deposition. [1] K. Teii and S. Matsumoto, Thin Solid Films 576, 50-54 (2015).

Roughness generation during Si etching in Cl2 pulsed plasma

Pulsed plasmas are promising candidates to go beyond limitations of continuous waves' plasma. However, their interaction with surfaces remains poorly understood. The authors investigated the silicon etching mechanism in inductively coupled plasma (ICP) Cl2 operated either in an ICP-pulsed mode or in a bias-pulsed mode (in which only the bias power is pulsed). The authors observed systematically the development of an important surface roughness at a low duty cycle. By using plasma diagnostics, they show that the roughness is correlated to an anomalously large (Cl atoms flux)/(energetic ion flux) ratio in the pulsed mode. The rational is that the Cl atom flux is not modulated on the timescale of the plasma pulses although the ion fluxes and energy are modulated. As a result, a very strong surface chlorination occurs during the OFF period when the surface is not exposed to energetic ions. Therefore, each energetic ion in the ON period will bombard a heavily chlorinated silicon surface, leading to anomalously high etching yield. In the ICP pulsed mode (in which the ion energy is high), the authors report yields as high as 40, which mean that each individual ion impacts will generate a “crater” of about 2 nm depth at the surface. Since the ion flux is very small in the pulsed ICP mode, this process is stochastic and is responsible for the roughness initiation. The roughness expansion can then be attributed partly to the ion channeling effect and is probably enhanced by the formation of a SiClx reactive layer with nonhomogeneous thickness over the topography of the surface. This phenomenon could be a serious limitation of pulsed plasma processes.

A comparative study of capacitively coupled HBr/He, HBr/Ar plasmas for etching applications: Numerical investigation by fluid model

Fluid model has been applied to perform a comparative study of hydrogen bromide (HBr)/He and HBr/Ar capacitively coupled plasma discharges that are being used for anisotropic etching process. This model has been used to identify the most dominant species in HBr based plasmas. Our simulation results show that the neutral species like H and Br, which are the key player in chemical etching, have bell shape distribution, while ions like HBr+, Br+, which play a dominant rule in the physical etching, have double humped distribution and show peaks near electrodes. It was found that the dilution of HBr by Ar and/or He results in an increase in electron density and electron temperature, which results in more ionization and dissociation and hence higher densities of neutral and charged species can be achieved. The ratio of positive ion flux to the neutral flux increases with an increase in additive gas fraction. Compare to HBr/He plasma, the HBr/Ar plasma shows a maximum change in the ion density and flux and hence the etching rate can be considered in the ion-assisted and in the ion-flux etch regime in HBr/Ar discharge. The densities of electron and other dominant species in HBr/Ar plasma are higher than those of HBr/He plasma. The densities and fluxes of the active neutrals and positive ions for etching and subsequently chemical etching versus physical sputtering in HBr/Ar and HBr/He plasmas discharge can be controlled by tuning gas mixture ratio and the desire etching can be achieved.

Impact of low-energy ions on plasma deposition of cubic boron nitride

Plasma deposition of cubic boron nitride (cBN) films under low-energy (a few eV to ~40eV) ion impact with the chemistry of fluorine is studied in terms of ion energy, ion flux, and ion to boron flux ratio onto the substrate. The ion energy and the ion to boron flux ratio are determined from the sheath potential and the ratio of incident ion flux to net deposited boron flux, respectively. For negative substrate biases where a mixture of turbostratic and amorphous BN phases only or no deposit is formed, both the ion energy and the ion to boron flux ratio are high. For positive substrate biases where cBN phase is formed, the ion energy and the ion to boron flux ratio are in the range of a few eV to 35eV and 100 to 130, respectively. The results indicate that the impact of positive ions with high ion to boron flux ratios makes a substantial contribution to the formation of cBN phase, while that of negative ions and electrons makes only a minor contribution.

Mechanisms for plasma etching of RRAM SiO2 with diblock copolymer selectivity

To minimize the critical dimension of resistive switching random access memory (RRAM), good anisotropy and selectivity with diblock copolymer are required for silicon dioxide etching. Inductively coupled plasma (ICP) etcher using CHF 3/ H 2 mixture is used for effective etching of SiO 2. In this paper, a commercial software CFD-ACE+ was used to simulate reactor scale and feature scale model of SiO 2, diblock copolymer and Pt . Etch properties of SiO 2 at different chamber conditions were discussed. It was found that etch rate increased at the expense of selectivity as ICP power increased, which was the opposite trend for pressure. Selectivity and anisotropy are achieved at neutral to ion flux ratio 100:1. Moreover, the appropriate overetch time for SiO 2 layer to Pt layer was discussed.