Double Langmuir Probe Measurements Research Articles

Charge decay in the spatial afterglow of plasmas and its impact on diffusion regimes

The spatial afterglow is a region at the boundary of a non-equilibrium plasma where charged species relax into ambient equilibrium. In many applications, the spatial afterglow is the part of the plasma that interacts with surfaces, such as suspended particles or a material substrate. However, compared to the bulk plasma, there has been little effort devoted to studying the properties of the spatial afterglow, and a fundamental analysis has not yet been developed. Here, we apply double Langmuir probe measurements and develop an advection-diffusion-recombination model to provide a detailed description of charged species in the spatial afterglow over a wide range of pressures, temperatures, plasma dimensions, and flow rates. We find that the density of charged species in the spatial afterglow decays by orders of magnitude, which leads to a transition from ambipolar to free diffusion. These insights can be used to explain or predict experimental observations of phenomena, such as the charging of dust grains and the dose of charged species to a biomaterial.

Capacitively coupled nonthermal plasma synthesis of aluminum nanocrystals for enhanced yield and size control

Uniform-size, non-native oxide-passivated metallic aluminum nanoparticles (Al NPs) have desirable properties for fuel applications, battery components, plasmonics, and hydrogen catalysis. Nonthermal plasma-assisted synthesis of Al NPs was previously achieved with an inductively coupled plasma (ICP) reactor, but the low production rate and limited tunability of particle size were key barriers to the applications of this material. This work focuses on the application of capacitively coupled plasma (CCP) to achieve improved control over Al NP size and a ten-fold increase in yield. In contrast with many other materials, where NP size is controlled via the gas residence time in the reactor, the Al NP size appeared to depend on the power input to the CCP system. The results indicate that the CCP reactor assembly, with a hydrogen-rich argon/hydrogen plasma, was able to produce Al NPs with diameters that were tunable between 8 and 21 nm at a rate up ∼ 100 mg h−1. X-ray diffraction indicates that a hydrogen-rich environment results in crystalline metal Al particles. The improved synthesis control of the CCP system compared to the ICP system is interpreted in terms of the CCP’s lower plasma density, as determined by double Langmuir probe measurements, leading to reduced NP heating in the CCP that is more amenable to NP nucleation and growth.

Two-dimensional electron temperature and density profiles of Hall thruster plume plasmas using tomographically reconstructed optical emission spectroscopy

Two-dimensional electron temperature and density profiles in the plume region of 300 W-class Hall thruster Ar plasmas were obtained using tomographically reconstructed optical emission intensity profiles combined with a collisional-radiative (CR) model. A total of 1242 lines of sight were used by rotating the thruster to apply inverse Radon transform-based tomographic reconstructions and Abel inversion. The reconstruction accuracy of the developed diagnostic system was evaluated using a priori images derived from plasma pictures, and the reconstruction error was less than 1% in the region of interest, exhibiting higher accuracy than the Abel inversion. From the Ar Hall thruster plasma, more than 12 different two-dimensional profiles of Ar I emission intensity within a spectral range of 600–1000 nm were obtained 6 mm from the exit plane of the thruster. A CR model using 31 allowed transitions at 15 different states from the ground state to the 2p i states was incorporated with the tomographically reconstructed emission intensity sets. Consequently, two-dimensional electron temperature and density profiles in the range of 5–18 eV and 2.0 × 1016–4.7 × 1017 m−3 were obtained, respectively, exhibiting reasonable agreement with the double Langmuir probe measurements.

Etching Characteristics of Low-k Dielectric Using C5H2F10 Liquefied Gas Plasma for Mitigating of Global Warming Potential

In this work, we studied the etch characteristics and dielectric constant change of SiOC thin films by plasma etching for the fabrication of nanoscale devices to evaluate the C5H2F10 as alternative etching gas. We performed plasma etching of SiOC films with inductively coupled plasma using the CF4+X+O2 mixed gas, where X = CHF3 and C5H2F10. Plasma diagnosis such as optical emission spectroscopy and double Langmuir probe measurements were carried. We analyzed the chemical compositions of residues on the etched SiOC film surface using X-ray photoelectron spectroscopy. After the process, contact resistance was measured using the transmission line method to analyze the degree of polymer on the surface of the silicon. Ellipsometry were used to evaluate the change in the dielectric constant of the thin film due to plasma exposure. It was confirmed that the etched profile was more vertical than that of the CHF3 gas plasma, and the increase in the dielectric constant of the SiOC thin film by C5H2F10 gas plasma is less than that of CHF3 gas plasma. These results confirmed that C5H2F10 gas was a powerful alternative to CHF3 gas in semiconductor processing for the fabrication of nanoscale devices.

Plasma Etching of SiON Films Using Liquefied C7F14 Gas as Perfluorocarbon Alternative

In this study, we evaluated the possibility of replacing existing perfluorocarbon gas with C7F14, which can be recovered in its liquid state from room-temperature air. We performed plasma etching of SiON films using the CF4 + X + O2 mixed gas, where X = CHF3, C4F8, or C7F14, and examined the etching characteristics of the films (e.g., etching rate, etching profile, and selectivity over Si). Using contact angle goniometry, atomic force microscopy, and X-ray photoelectron spectroscopy, we analyzed the physicochemical changes in the etched SiON film surface. Moreover, optical emission spectroscopy and double Langmuir probe measurements were carried out for plasma diagnosis. Compared with the conventional CHF3 and C4F8 mixed plasma, the C7F14 mixed plasma exhibited a more perpendicular etching profile with higher SiON/Si selectivity and a smoother surface.

Concentric Split-Ring Resonator Microwave Microplasma Generation at Off-Resonant Frequencies

The driving frequency of a microstrip concentric split-ring resonator (CSRR) was varied to determine how the generated microplasma changes as a function of frequency, specifically at off-resonant frequencies. The microplasmas were generated in argon at 1 torr, using a forward microwave power level of approximately 10 W. The resonant frequency was experimentally found to be 796 MHz, and measurements were obtained at this frequency and ±10 MHz from resonance. Double Langmuir probe measurements were performed on a 2-D grid, parallel to the surface of the device, to obtain plasma density and electron temperature maps. The results show multiple peaks in plasma properties due to electromagnetic coupling with the nested ring. The electron temperature above the primary discharge gap is 8.5 eV at 786 MHz, 9.5 eV at 796 MHz, and 7.6 eV at 806 MHz. A secondary discharge is also observed above the secondary nested ring gap, and the plasma in this region is generally of lower temperature. The off-resonant operation resulted in lower temperatures and densities, with larger decreases in the 806 MHz case. The electromagnetic response of the device was modeled to understand the electric field in the discharge gap. The modeling revealed that the electric field vectors at the bounds of the discharge gap point in the opposite directions for the 786 and 796 MHz cases, increasing the electric field in the gap, but these vectors point in the same direction for the 806 MHz case, decreasing the electric field.

Diagnostics of microwave assisted electron cyclotron resonance plasma source for surface modification of nylon 6

Looking at the increasing scope of plasma processing of materials surface, here we present the development and diagnostics of a microwave assisted Electron Cyclotron Resonance (ECR) plasma system suitable for surface modification of polymers. Prior to the surface-treatment, a detailed diagnostic mapping of the plasma parameters throughout the reactor chamber was carried out by using single and double Langmuir probe measurements in Ar plasma. Conventional analysis of I-V curves as well as the elucidation form of the Electron Energy Distribution Function (EEDF) has become the source of calibration of plasma parameters in the reaction chamber. The high energy tail in the EEDF of electron temperature is seen to extend beyond 60 eV, at much larger distances from the ECR zone. This proves the suitability of the rector for plasma processing, since the electron energy is much beyond the threshold energy of bond breaking in most of the polymers. Nylon 6 is used as a representative candidate for surface processing in the presence of Ar, H2 + N2, and O2 plasma, treated at different locations inside the plasma chamber. In a typical case, the work of adhesion is seen to almost get doubled when treated with oxygen plasma. Morphology of the plasma treated surface and its hydrophilicity are discussed in view of the variation in electron density and electron temperature at these locations. Nano-protrusions arising from plasma treatment are set to be responsible for the hydrophobicity. Chemical sputtering and physical sputtering are seen to influence the surface morphology on account of sufficient electron energies and increased plasma potential.

Transport modeling of convection dominated helicon discharges in Proto-MPEX with the B2.5-Eirene code

Data-constrained interpretative analyses of plasma transport in convection dominated helicon discharges in the Proto-MPEX linear device, and predictive calculations with additional Electron Cyclotron Heating/Electron Bernstein Wave (ECH/EBW) heating, are reported. The B2.5-Eirene code, in which the multi-fluid plasma code B2.5 is coupled to the kinetic Monte Carlo neutrals code Eirene, is used to fit double Langmuir probe measurements and fast camera data in front of a stainless-steel target. The absorbed helicon and ECH power (11 kW) and spatially constant anomalous transport coefficients that are deduced from fitting of the probe and optical data are additionally used for predictive simulations of complete axial distributions of the densities, temperatures, plasma flow velocities, particle and energy fluxes, and possible effects of alternate fueling and pumping scenarios. The somewhat hollow electron density and temperature radial profiles from the probe data suggest that Trivelpiece-Gould wave absorption is the dominant helicon electron heating source in the discharges analyzed here. There is no external ion heating, but the corresponding calculated ion temperature radial profile is not hollow. Rather it reflects ion heating by the electron-ion equilibration terms in the energy balance equations and ion radial transport resulting from the hollow density profile. With the absorbed power and the transport model deduced from fitting the sheath limited discharge data, calculated conduction limited higher recycling conditions were produced by reducing the pumping and increasing the gas fueling rate, resulting in an approximate doubling of the target ion flux and reduction of the target heat flux.

B2.5-Eirene modeling of radial transport in the MAGPIE linear plasma device

Radial transport in helicon heated hydrogen plasmas in the MAGnetized Plasma Interaction Experiment (MAGPIE) is studied with the B2.5-Eirene (SOLPS5.0) code. Radial distributions of plasma density, temperature and ambipolar potential are computed for several magnetic field configurations and compared to double Langmuir probe measurements. Evidence for an unmagnetized ion population is seen in the requirement for a convective pinch term in the continuity equation in order to fit the centrally peaked density profile data. The measured slightly hollow electron temperature profiles are reproduced with combinations of on-axis and edge heating which can be interpreted as helicon and Trivelpiece–Gould wave absorption, respectively. Pressure gradient driven radial charged particle diffusion is chosen to describe the diffusive particle flux since the hollowness of the temperature profiles assists the establishment of on-axis density peaking.

A hybrid electron cyclotron resonance metal ion source with integrated sputter magnetron for the production of an intense Al⁺ ion beam.

A metal ion source prototype has been developed: a combination of magnetron sputter technology with 2.45 GHz electron cyclotron resonance (ECR) ion source technology-a so called magnetron ECR ion source (MECRIS). An integrated ring-shaped sputter magnetron with an Al target is acting as a powerful metal atom supply in order to produce an intense current of singly charged metal ions. Preliminary experiments show that an Al(+) ion current with a density of 167 μA/cm(2) is extracted from the source at an acceleration voltage of 27 kV. Spatially resolved double Langmuir probe measurements and optical emission spectroscopy were used to study the plasma states of the ion source: sputter magnetron, ECR, and MECRIS plasma. Electron density and temperature as well as Al atom density were determined as a function of microwave and sputter magnetron power. The effect of ECR heating is strongly pronounced in the center of the source. There the electron density is increased by one order of magnitude from 6 × 10(9) cm(-3) to 6 × 10(10) cm(-3) and the electron temperature is enhanced from about 5 eV to 12 eV, when the ECR plasma is ignited to the magnetron plasma. Operating the magnetron at constant power, it was observed that its discharge current is raised from 1.8 A to 4.8 A, when the ECR discharge was superimposed with a microwave power of 2 kW. At the same time, the discharge voltage decreased from about 560 V to 210 V, clearly indicating a higher plasma density of the MECRIS mode. The optical emission spectrum of the MECRIS plasma is dominated by lines of excited Al atoms and shows a significant contribution of lines arising from singly ionized Al. Plasma emission photography with a CCD camera was used to prove probe measurements and to identify separated plasma emission zones originating from the ECR and magnetron discharge.

RF compensation of double Langmuir probes: modelling and experiment

An analytical model describing the physics of driven floating probes has been developed to model the RF compensation of the double langmuir probe (DLP) technique. The model is based on the theory of the RF self-bias effect as described in Braithwaite’s work [], which we extend to include time-resolved behaviour. The main contribution of this work is to allow quantitative determination of the intrinsic RF compensation of a DLP in a given RF discharge. Using these ideas, we discuss the design of RF compensated DLPs. Experimental validation for these ideas is presented and the effects of RF rectification on DLP measurements are discussed. Experimental results using RF rectified DLPs indicate that (1) whenever sheath thickness effects are important overestimation of the ion density is proportional to the level of RF rectification and suggest that (2) the electron temperature measurement is only weakly affected.

An inverted cylindrical sputter magnetron as metal vapor supply for electron cyclotron resonance ion sources.

An inverted cylindrical sputter magnetron device has been developed. The magnetron is acting as a metal vapor supply for an electron cyclotron resonance (ECR) ion source. FEM simulation of magnetic flux density was used to ensure that there is no critical interaction between both magnetic fields of magnetron and ECR ion source. Spatially resolved double Langmuir probe and optical emission spectroscopy measurements show an increase in electron density by one order of magnitude from 1 × 10(10) cm(-3) to 1 × 10(11) cm(-3), when the magnetron plasma is exposed to the magnetic mirror field of the ECR ion source. Electron density enhancement is also indicated by magnetron plasma emission photography with a CCD camera. Furthermore, photographs visualize the formation of a localized loss-cone - area, when the magnetron is operated at magnetic mirror field conditions. The inverted cylindrical magnetron supplies a metal atom load rate of R > 1 × 10(18) atoms/s for aluminum, which meets the demand for the production of a milliampere Al(+) ion beam.

Examination of an ICP magnesium plasma by spectroscopy and double Langmuir probe measurements

Optical emission spectroscopy and double Langmuir probe measurements are performed on a magnesium plasma from an inductively coupled plasma source. These measurements are performed in order to examine their feasibility to characterize the plasma, its degree of ionization and its deviation from thermal equilibrium.

Modeling of an inductively-coupled Cl2/Ar plasma using neural network

In this study, a neural network model for an inductively-coupled Cl2/Ar plasma (ICP) process has been proposed. Plasma experiments were performed in a planar inductively coupled plasma reactor. The input parameters considered for plasma modeling were the gas mixing ratio, and the source and bias powers, which were varied in the ranges 0–100% Cl2/Ar, 500–800W, and 50–300W, respectively. Plasma diagnostics were performed by double Langmuir probe measurements. Analysis of voltage–current curves in order to obtain electron temperature and total positive ion density was carried out. A back propagation neural network model with a pre-processor was constructed. The prediction errors for the proposed model were shown to be very small compared to those of a conventional neural network model. In the experiments conducted, it was found that an increase in Ar mixing ratio or input source power resulted in an increase in the total positive ion density. An increase in the input bias power, however, corresponded to a decrease in the total positive ion density. The electron temperature increased as the Ar fraction, input source power, or input bias power increased.

Investigation of Tin and Chromium Metal Plasmas by Langmuir Probe Measurements

Double Langmuir probe measurements are performed during the deposition of tin and chromium coatings in order to examine the degree of ionisation and the deviation from thermal equilibrium of these metal plasmas.

A differentially pumped argon plasma in the linear plasma generator Magnum-PSI: gas flow and dynamics of the ionized fraction

Magnum-PSI is a linear plasma generator designed to reach the plasma–surface interaction (PSI) regime of ITER and nuclear fusion reactors beyond ITER. To reach this regime, the influx of cold neutrals from the source must be significantly lower than the plasma flux reaching the target. This is achieved by a differential pumping scheme, where the vacuum vessel is divided by skimmers into separate chambers which are individually pumped. The non-magnetized expansion of 5 Pa m3 s−1 (3 slm) argon in a low background pressure was studied in the differentially pumped vacuum vessel fitted with non-cooled flat skimmers. The behavior of the neutral component was studied with direct simulation Monte Carlo simulations and Rayleigh scattering measurements. Thomson scattering and double Langmuir probe measurements were performed on the ionized fraction. It was found that the electrons and neutral particles are not completely coupled in the shock front. The neutral fraction shows clear signs of invasion from hotter background gas, causing the average temperature and density to increase before the shock. This is also shown in the ionization ratio, which has been determined in front of and behind the first skimmer. This study helps us to understand the behavior of the gas flow in the machine and validates our modeling.

Etching characteristics and mechanism of ZnO thin films in inductively coupled HBr/Ar plasma

The etching characteristics and mechanisms of ZnO thin films in an HBr/Ar inductively coupled plasma were investigated. The etch rate of ZnO was measured as a function of the HBr/Ar mixing ratio in the range of 0–100% Ar at a fixed gas pressure (6 mTorr), input power (900 W) and bias power (200 W). The plasma diagnostics were performed by double Langmuir probe measurements and quadrupole mass spectrometry. A global (0-dimensional) plasma model was used to obtain the data on the densities and fluxes of the active species. It was found that the etch rate of ZnO is proportional to the flux of Br atoms, but inversely proportional to those of the H atoms and positive ions. This suggests that the ZnO etch process is not limited by the ion-surface interaction kinetics and that the Br atoms are the main chemically-active species.

Plasma Immersion Ion Implantation With Lithium Atoms

A new method was developed to produce lithium plasma for plasma immersion ion implantation. Initially, an argon glow discharge with operation pressure ranging from 2 times 10-1 to 1 mbar is generated by negatively polarizing an electrode from -400 to -1500 V. Small pieces of metallic lithium that are 99.9% pure fill the top of a conic crucible, with a depth of 2 cm, in electric contact with the electrode. Argon ions from the plasma are used to bombard this target, where heat is created by the momentum transfer from the impacting ions to the crucible. By controlling the operation pressure and the electrode voltage polarization, it is possible to easily heat the crucible to temperatures above the lithium melting point (180degC), causing its evaporation. Lithium atoms are then ionized, mainly due to collisions, with argon ions moving toward the crucible. Double Langmuir probe measurements indicated variation in the density of the discharge from 4 times 109 cm-3 to 1010 cm-3 after lithium evaporation. Silicon wafer pieces immersed in this mixed plasma were submitted to repetitive negative high-voltage pulses (3 kV/6 mus/2.5 kHz) to accelerate plasma ions. High strain in the treated layers was measured by high-resolution X-ray diffraction. Photoluminescence intensity increased after annealing. X-ray photoelectron spectrometry measurement revealed lithium implantation in silicon with an atomic concentration of 78% on the top surface and a penetration depth of about 75 nm.

Properties of thin films deposited from HMDSO/O 2 induced remote plasma: Effect of oxygen fraction

Thin films deposited from Hexamethyledisiloxane (HMDSO)/O 2 mixture excited in a radio-frequency hollow cathode discharge system have been investigated for their structural, optical and corrosive properties as a function of oxygen fraction χ O 2 ( χ O 2 = 0 , 0.38 , 0.61 , 0.76 and 0.90 ). It is found that the effect of oxygen fraction on films properties is related to O 2 dissociation degree ( α d) behavior in pure oxygen plasma. α d has been investigated by actinometry optical emission spectroscopy (AOES) combined with double Langmuir probe measurements, a maximum of O 2 dissociation degree of 15% has been obtained for 50 sccm flow rate of O 2 ( χ O 2 = 0.61 in HMDSO/O 2 plasma). Fourier transform infrared spectroscopy (FTIR) and optical measurements showed that the behavior of both identified IR group densities and deposition rate as a function of oxygen fraction is similar to that of O 2 dissociation degree. The inorganic nature of the films depends significantly on oxygen fraction, the best inorganic structure of deposited films has been obtained for 62% HMDSO content in the mixture HMDSO/O 2 ( χ O 2 = 0.38 ). The refractive index for deposited films from pure HMDSO ( χ O 2 = 0 ) has been found to be higher than that of films deposited from HMDSO/O 2 mixture. In HMDSO/O 2 plasma, it has a behavior similar to that of deposition rate, and it is comparable to that of quartz. The effect of oxygen fraction on the corrosive properties of thin films deposited on steel has been investigated. It is found that the measured corrosion current density in 0.1 M KCl solution decreases with the addition of O 2 to HMDSO plasma, and it is minimum for χ O 2 = 0.38 .

Diagnostic study of low-pressure Ar– [formula omitted] remote plasma generated in HCD-L 300 system: Relative density of O atom

The relative density of O atom of Ar– O 2 remote plasma excited in a low-pressure 13.56 MHz hollow cathode discharge system has been investigated. The measurements were carried out at a total pressure of 0.05 mbar, radiofrequency (RF) power of 200 W and at three different axial distances in the plasma chamber below the outlet of the discharge source. Using optical emission spectroscopy (OES), the relative density of O ground state was determined from intensity ratio of O(844.6 nm) and Ar(750.4 nm) lines. The electron temperature and O 2 + densities have been measured using double langmuir probe measurements. The kinetic study of Ar– O 2 plasma, combined with both spectroscopy and langmuir probe measurements, revealed that the main production mechanism of the excited O ( 3 p P 3 ) is direct excitation by electron impact. A maximum of O ground state relative density and correspondingly a minimum of O 2 + density are obtained for the ratio O 2 / Ar :60/40. The maximum O density in the remote zone is found to be 4.5 times higher than at the outlet of source.