Surface Recombination Coefficient Research Articles

Impact of plasma operating conditions on the ion energy and angular distributions in dual-frequency capacitively coupled plasma reactors using CF4 chemistry

A two-dimensional hybrid model is used to simulate an industrial dual-frequency capacitively coupled plasma reactor working at closely spaced frequencies (13.56–40.68 MHz) in pure CF4 chemistry. The goal is to understand how plasma operating conditions (pressure, low-frequency and high-frequency RF powers, and chamber wall conditions) influence critical etching parameters such as the ion energy and angular distribution (IEAD) and the ion flux at the wafer. In base case conditions, the ionic and radical composition at the center of the plasma is analyzed, revealing CF3+ and F− as the primary ions, and F, CF, CF3, CF2, and F2 as the predominant radicals (by decreasing density). The impact of the surface recombination coefficient of F radicals into F2 at the reactor walls, γ(rec,F > F2), is then explored; it is found that increasing γ(rec,F > F2) has a strong impact on the final plasma composition, decreasing CF and F densities while increasing CF3, CF2, and F densities, which highlights the importance of properly considering wall conditions in CF-based plasmas simulation. The IEAD at the wafer is then characterized, showing that the total IEAD shape is affected by the plasma ion composition: heavy ions such as CF3+ (69 amu) form the core of the distribution while lighter species such as F+ (19 amu) form the wing of the distribution due to their lower mass. The low frequency (LF) power (100–900 W) is shown to substantially modify the ion energy distribution function (IEDF) owing to sheath voltage changes, but to also marginally increase the ion flux at the wafer. Conversely, the high-frequency (HF) power (100–1500 W) strongly impacts the ion flux at the wafer due to HF voltage fluctuations, while the IEDF remains mostly unaffected. This study also reveals some coupling between the effects of the LF (13.56 MHz) and HF (40.68 MHz) power, a phenomenon attributable to their proximity in frequency which should not be neglected. Finally, increasing the pressure from 30 to 200 mTorr is found to increase the electronegativity by a factor 4 and to strongly impact the plasma structure, primarily due to variations in ion mobility; it also widens the ion angular spread, potentially influencing etch uniformity. Notably, higher pressures exceeding 100 mTorr result in a decrease in the average ion density and the emergence of a low-energy peak in the ion energy distribution, attributed to charge exchange collisions.

Gas- and plasma-driven hydrogen permeation behavior of stagnant eutectic-solid GaInSn/Fe double-layer structure

Gas-driven permeation (GDP) and plasma-driven permeation (PDP) of hydrogen gas through GaInSn/Fe are systematically investigated in this work. The permeation parameters of hydrogen through GaInSn/Fe, including diffusivity, Sieverts’ constant, permeability, and surface recombination coefficient are obtained. The permeation flux of hydrogen through GaInSn/Fe shows great dependence on external conditions such as temperature, hydrogen pressure, and thickness of liquid GaInSn. Furthermore, the hydrogen permeation behavior through GaInSn/Fe is well consistent with the multi-layer permeation theory. In PDP and GDP experiments, hydrogen through GaInSn/Fe satisfies the diffusion-limited regime. In addition, the permeation flux of PDP is greater than that of GDP. The increase of hydrogen plasma density hardly causes the hydrogen PDP flux to change within the test scope of this work, which is due to the dissolution saturation. These findings provide guidance for a comprehensive and systematic understanding of hydrogen isotope recycling, permeation, and retention in plasma-facing components under actual conditions.

The influence of electron recombination rate on the plasma characteristics in rf argon capacitive discharge

This work investigates the effect of the electron surface recombination coefficient (Ks) on radio-frequency discharge characteristics. This investigation is performed by a fluid model with the drift–diffusion approximation. The simulation results show that when the electron recombination rate is larger, the plasma density is lower in the plasma bulk region. It is found that the cycle-averaged electron and ion fluxes decrease adjacent to the electrode, while the electric potential increases in the bulk plasma region as Ks increases from 0.60 to 2.38. Moreover, the reduction in the electron density in the bulk region leads to decrease in the electron power dissipation in this region. The results also show that the electron ohmic heating and pressure heating, which play a crucial role in sustaining the discharge, can be both reduced in the sheath regions with increasing the electrons recombination rate. Finally, it is observed that the plasma characteristics can significantly be changed depending on the electrode surface properties.

Continuous polymer films deposited on top of porous substrates using plasma-enhanced atomic layer deposition and molecular layer deposition

Molecular layer deposition (MLD) is able to produce ultrathin polymer films with control over thickness, cross-linking, and chemical composition. With these capabilities, MLD should be useful in the fabrication of novel polymer membranes on porous supports. However, confining a continuous MLD film to the surface of porous substrates is difficult because of MLD film growth in the pores. The deposition in the pores lowers the conductance of the porous support. This paper presents a method to deposit continuous polymer films on top of porous substrates. In this method, Al2O3 plasma-enhanced atomic layer deposition (PE-ALD) using trimethylaluminum and oxygen plasma as the reactants was first used to cap the pores of the substrate. Subsequently, a polyamide MLD film was deposited on the Al2O3 PE-ALD capping layer using m-phenylenediamine and trimesoyl chloride as the reactants. The Al2O3 pore caps were then removed from the porous substrate by etching from the backside using a timed exposure to a dilute sodium hydroxide solution. This method was demonstrated using anodic aluminum oxide (AAO) and polyethersulfone (PES) porous substrates. Al2O3 PE-ALD film growth was limited to the top of the porous substrate, resulting in rapid surface recombination or high sticking coefficients for the reactive plasma species within the pores. Gas permeance measurements confirmed the pore capping of the AAO substrates. The reopening of the pores by dissolving the Al2O3 pore caps with a sodium hydroxide solution was monitored using gas permeance versus etch time. The removal of the Al2O3 pore caps from the PES substrates could also dissolve the Al2O3 layer underneath the MLD film. The loss of this Al2O3 layer led to the detachment of the MLD film from the PES substrate. However, the MLD film could be anchored to the PES support at fractures located in the Al2O3 film prior to the MLD. The Al2O3 film fracture allowed the MLD film to anchor firmly to the PES substrate by MLD in the pores of the PES porous substrate. The distance between the anchor points was a function of fracture density. This distance could be controlled by applying a tensile stress to the Al2O3 PE-ALD film to fracture the film through sample bending. This method produced firmly anchored polymer MLD films on top of the PES porous substrates.

Transport parameters and permeation behavior of hydrogen isotopes in the first wall materials of future fusion reactors

Abstract The first wall of a future magnetic fusion device is essentially defined as the plasma-facing surface of the breeding blankets, which is supposed to be subjected to bi-directional hydrogen isotopes permeation: in one direction by edge plasma-driven permeation (PDP) of deuterium as well as tritium into blankets, and in the other direction by breed tritium gas-driven permeation (GDP) into the edge plasma. Deuterium and tritium PDP will complicate the recovery of tritium from the blanket, while tritium GDP will lead to an unwanted increase of particle recycling in the first wall region, which could even affect core confinement performance. Reduced activation ferritic/martensitic (RAFM) steels are widely proposed as candidate structural materials for the blanket of a DEMO reactor, the surface coatings made of tungsten are necessary to protect the plasma-facing wall from sputtering under high-energy particle bombardment. Therefore, the characterization of hydrogen isotopes transport through a multi-layer W + RAFM wall is of crucial importance to evaluate major reactor design issues including tritium retention, particle recycling and breeding feasibility, etc. This paper is intended to provide a review over the transport parameters of hydrogen isotopes in RAFM steels, including permeability, diffusivity, solubility and surface recombination coefficient. In addition, the present research status of hydrogen isotopes permeation and retention behavior of tungsten coated RAFM steels is briefly introduced.

Modeling the sharp deuterium release from beryllium co-deposits

Many studies have shown a sharp deuterium desorption from beryllium at around 450 K. In an attempt to model this sharp release, a new model was developed, describing the formation and decomposition of beryllium deuteride. Such a model was found to reproduce the sharp desorption peak in the thermal desorption spectrum. Moreover, in contrast to other models, the new model was able to reproduce the constant deuterium desorption from beryllium held at a constant elevated temperature. Fitting the modeling results to experimental data allowed us to determine the deuterium solubility limit in beryllium, which indicated an endothermic formation of beryllium deuteride. New values of the deuterium surface recombination coefficient were also obtained, differing from the ones found in the literature.

Deuterium Transport and Retention in Liquid Ga and Li Under Steady-State Plasma Bombardment

The experiments on deuterium plasma-driven permeation through liquid Ga and Li were conducted in a linear plasma device VEHICLE-1. Deuterium transport parameters such as diffusivity and surface recombination coefficient for Ga and Li were measured from the experiments. By using the measured transport parameters, deuterium dynamic retentions in Ga and Li under steady-state plasma bombardment were evaluated with the DIFFUSE code. It has been found that the deuterium dynamic retention in Ga is ~five orders of magnitude lower than that in Li under present experimental conditions. After exposed to the steady-state deuterium plasma at temperatures from 78 °C to 360 °C, deuterium static retentions in liquid Ga are measured by the thermal desorption spectroscopy. It has been found that the deuterium static retentions in Ga are close to the dynamic retentions when the exposure temperatures are below 250 °C.

Bi-directional hydrogen isotopes permeation through a reduced activation ferritic steel F82H

The first wall of a magnetic fusion reactor serves to separate the edge plasma from breeding blankets, which will be subjected to bi-directional hydrogen isotopes permeation: in one direction by plasma-driven permeation, and in the other direction by gas-driven permeation. In this work, hydrogen isotopes transport through a reduced activation ferritic steel F82H has been investigated in the temperature range of 150–500°C. The transport parameters including permeability, diffusivity, solubility and surface recombination coefficient have been measured and the isotopic mass effect has been discussed. Bi-directional hydrogen isotopes gas- and plasma-driven permeation has been demonstrated for the first time under controlled experimental conditions.

Modeling of Point Defects Annihilation in Multilayered Cu/Nb Composites under Irradiation

This work focuses on a mathematical modeling of the response to irradiation of a multilayer composite material. Nonstationary balance equations are utilized to account for production, recombination, transport, and annihilation, or removal, of vacancies and interstitials at interfaces. Although the model developed has general validity, Cu/Nb multilayers are used as case study. Layer thickness, temperature, radiation intensity, and surface recombination coefficients were varied systematically to investigate their effect on point defect annihilation processes at interfaces. It is shown that point defect annihilation at interfaces mostly depends on point defect diffusion. The ability of interfaces to remove point defects can be described by a simple map constructed using only two dimensionless parameters, which provides a general tool to estimate the efficiency of vacancy and interstitial removal in multilayer composite materials.

The glow discharge inception and post-discharge relaxation of charged and neutral active particles in synthetic air at low pressure

The study of dc glow discharge inception and post-discharge relaxation of charged and neutral active particles in synthetic air at low pressure is presented. The breakdown time delay dependence as a function of relaxation time (the memory curve) is measured and modelled from milliseconds to the saturation region determined by the cosmic rays and natural radioactivity level. Due to fast conversion , relaxation of dc discharge in synthetic air in the time interval from one to about ninety milliseconds is dominated by the diffusion decay of molecular oxygen ions. The change of regimes, from ambipolar to the free diffusion limit, is investigated and the variation of effective diffusion coefficients is determined. The late relaxation is explained by the kinetics of nitrogen atoms, recombining on the surfaces of gas discharge tube and stainless steel electrodes and relevant surface recombination coefficients are determined.

Gas- and plasma-driven hydrogen permeation through a reduced activation ferritic steel alloy F82H

The first wall of a magnetic fusion power reactor will be subjected to hydrogen isotope permeation by the two mechanisms: one is gas-driven and the other is plasma-driven. Hydrogen transport through a reduced activation ferritic steel alloy F82H has been investigated using a steady-state laboratory-scale plasma device. Permeation parameters including permeability, solubility and diffusivity have been measured in the temperature range from 150 to 520°C. The surface recombination coefficient for hydrogen has also been estimated by a one-dimensional steady-state permeation model with the input data taken from experiments. Using these parameters, the hydrogen plasma-driven permeation flux and inventory for a 0.5cm thick first wall around 500°C are estimated to be ∼1.0×1013atomcm−2s−1 and ∼2×1016atomcm−3, respectively. Also, the implications of all these data on reactor operation are discussed.

The influence of nitrogen on deuterium permeation through tungsten

Mixed deuterium–nitrogen ion-driven permeation experiments through tungsten were performed as a function of temperature (600 < T < 1000 K). The influence of nitrogen resulted in a strong increase in the permeation flux at T < 900 K with decreasing temperature. An order of magnitude increase was observed at T = 650 K. A simple transport model was used to determine order of magnitude estimates of the nitrogen-induced changes to deuterium surface recombination coefficient and diffusivity.

Characterization and global modelling of low-pressure hydrogen-based RF plasmas suitable for surface cleaning processes

In this paper we present results of measurements and global modelling of low-pressure inductively coupled H2 plasma which is suitable for surface cleaning applications. The plasma is ignited at 1 Pa in a helicon-type reactor and is characterized using optical emission measurements (optical actinometry) and electrical measurements, namely Langmuir and catalytic probe. By comparing catalytic probe data obtained at the centre of the chamber with optical actinometry results, an approximate calibration of the actinometry method as a semi-quantititative measure of H density was achieved. Coefficients for conversion of actinometric ratios to H densities are tabulated and provided. The approximate validity region of the simple actinometry formula for low-pressure H2 plasma is discussed in the online supplementary data (stacks.iop.org/JPhysD/46/475206/mmedia). Best agreement with catalytic probe results was obtained for (Hβ, Ar750) and (Hβ, Ar811) actinometric line pairs. Additionally, concentrations of electrons and ions as well as plasma potential, electron temperature and ion fluxes were measured in the chamber centre at different plasma powers using a Langmuir probe. Moreover, a global model of an inductively coupled plasma was formulated using a compiled reaction set for H2/Ar gas mixture. The model results compared reasonably well with the results on H atom and charge particle densities and a sensitivity analysis of important input parameters was conducted. The influence of the surface recombination, ionization, and dissociation coefficients, and the ion–neutral collision cross-section on model results was demonstrated.

Study on monatomic fraction improvement with alumina layer on metal electrode in hydrogen plasma ion source

A high monatomic beam fraction is an important factor in a hydrogen ion source to increase the application efficiency. The monatomic fraction of hydrogen plasmas with different plasma electrode materials is measured in a helicon plasma ion source, and aluminum shows the highest value compared to that with the other metals such as copper and molybdenum. Formation of an aluminum oxide layer on the aluminum electrode is determined by XPS analysis, and the alumina layer is verified as the high monatomic fraction. Both experiments and numerical simulations conclude that a low surface recombination coefficient of the alumina layer on the plasma electrode is one of the most important parameters for increasing the monatomic fraction in hydrogen plasma ion sources.

Atomic scale study of InP etching by Cl2-Ar ICP plasma discharge

A gas phase kinetic model combined to a 3D atomic etching model have been developed to study the etching process of InP under Cl 2 -Ar ICP plasma discharge. A gas phase global kinetic model is used to calculate the reactive particle fluxes implied in the etching mechanisms. The 3D atomic InP etching model is based on the Monte Carlo kinetic approach where the plasma surface interactions are described in the probability way. The coupling between the plasma chemistry model and the surface etching model is an interesting approach to predict the etched surface properties in terms of the etch rate, the surface roughness and surface steochiometry as a function of the operating conditions. A satisfactory agreement is obtained by comparing the experimental and the simulation results concerning the evolution of the main plasma discharge parameters such as the electron density and temperature versus the ICP source power for a surface recombination coefficient of atomic chlorine fixed at γ Cl = 0.04. On the other hand, simulation results show the effect of the operating conditions on the etched surface roughness and the etch rate evolutions with time in the early stage. Moreover, the simulation results show the correlation between the decrease of the ion to chlorine flux ratio and the decrease of the RRMS as a function of the pressure.

Surface loss rates of H and Cl radicals in an inductively coupled plasma etcher derived from time-resolved electron density and optical emission measurements

A study is undertaken of the loss kinetics of H and Cl atoms in an inductively coupled plasma (ICP) reactor used for the etching of III-V semiconductor materials. A time-resolved optical emission spectroscopy technique, also referred to as pulsed induced fluorescence (PIF), has been combined with time-resolved microwave hairpin probe measurements of the electron density in a pulsed Cl2/H2-based discharge for this purpose. The surface loss rate of H, kwH, was measured in H2 plasma and was found to lie in the 125–500 s−1 range (γH surface recombination coefficient of ∼0.006–0.023), depending on the reactor walls conditioning. The PIF technique was then evaluated for the derivation of kwCl, and γCl in Cl2-based plasmas. In contrast to H2 plasma, significant variations in the electron density may occur over the millisecond time scale corresponding to Cl2 dissociation at the rising edge of the plasma pulse. By comparing the temporal evolution of the electron density and the Ar-line intensity curves with 10% of Ar added in the discharge, the authors show that a time-resolved actinometry procedure using Ar as an actinometer is valid at low to moderate ICP powers to estimate the Cl loss rate. They measured a Cl loss rate of ∼125–200 s−1 (0.03≤γCl≤0.06) at 150 W ICP power for a reactor state close to etching conditions. The Cl surface loss rate was also estimated for high ICP power (800 W) following the same procedure, giving a value of ∼130–150 s−1 (γCl∼0.04), which is close to that measured at 150 W ICP power.

Stochastic and Relaxation Processes in Argon by Measurements of Dynamic Breakdown Voltages

AbstractStatistically based measurements of breakdown voltages U b and breakdown delay times td and their variations in transient regimes of establishment and relaxation of discharges are a convenient method to study stochastic processes of electrical breakdown of gases, as well as relaxation kinetics in afterglow. In this paper the measurements and statistical analysis of the dynamic breakdown voltages U b for linearly rising (ramp) pulses in argon at 1.33 mbar and the rates of voltage rise k up to 800 V s –1 are presented. It was found that electrical breakdowns by linearly rising (ramp) pulses is an inhomogeneous Poisson process caused by primary and secondary ionization coefficients α , γ and electron yield Y variations on the voltage (time). The experimental breakdown voltage distributions were fitted by theoretical distributions by applying approximate analytical and numerical models. The afterglow kinetics in argon was studied based on the dependence of the initial electron yield on the relaxation time Y 0 (τ ) derived from fitting of distributions. The space charge decay was explained by the surface recombination of nitrogen atoms present as impurities. The afterglow kinetics and the surface recombination coefficients on the gas tube and cathode were determined from a gas‐phase model. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Recombination coefficients of O and N radicals on stainless steel

Surface recombination coefficients of O and N radicals in pure O2 and N2 plasmas, respectively, have been estimated on the stainless steel walls of a low-pressure inductively coupled plasma reactor. The recombination coefficients are estimated using a steady state plasma model describing the balance between the volume generation of the radicals from electron-impact dissociation of the parent molecules, and the loss of the radicals due to surface recombination. The model uses radical and parent molecule number densities and the electron energy distribution function (EEDF) as input parameters. We have measured the radical number density using appearance potential mass spectrometry. The parent neutral number density is measured using mass spectrometry. The EEDF is measured using a Langmuir probe. The recombination coefficient of O radicals on stainless steel walls at approximately 330 K is estimated to be 0.17±0.02, and agrees well with previous measurements. The recombination coefficient of N radicals is estimated to be 0.07±0.02 on stainless steel at 330 K.

Influence of admixtures on production rate of atomic nitrogen

In this paper, we present the dependencies of atomic nitrogen concentration in the afterglow on the amount of oxygen admixture added to the main gas. It is observed that until a certain threshold, the concentration of atomic nitrogen rises rapidly with the amount of admixture. If even more of the admixture is added, the concentration of nitrogen atoms decreases again. We suggest the explanation of this phenomenon by two antagonistic processes acting together: (i) decrease of the surface recombination coefficient by blocking of active sites and (ii) chemical reaction between N and O2 in the volume.

Investigation of recombination parameters in silicon structures by infrared and microwave transient absorption techniques

Contactless techniques of infrared and microwave absorption by free carriers for the monitoring of silicon structures are described. Theoretical principles of photoconductivity decay analysis and methodology for the determination of recombination parameters are given for both homogeneous and non-homogeneous excess carrier generation. Different approximations (the methods of decay amplitude - asymptotic lifetime analysis, the simulation of the whole decay curve, the variation of effective lifetime with wafer thickness and the asymptotic lifetime measurement for stepwise varying parameters in layered structure) corresponding to real experimental conditions for various structures and treatments of materials, which are important for microelectronics, are discussed. The determined recombination parameters in the range of bulk lifetime , velocity of surface recombination and diffusion coefficient are illustrated for Si wafers obtained by various doping and preparation processes. The necessity to consider carrier trapping effects and nonlinear recombination processes is demonstrated by the analysis of experimental results obtained at different excitation levels for carrier concentrations in the range . The possibility of extracting the parameters of the traps (with activation energy values , and ) from the temperature-dependent asymptotic carrier lifetime measurements is illustrated for neutron transmutation doped wafers.