Electron-induced Secondary Electron Research Articles

On the importance of excited state species in low pressure capacitively coupled plasma argon discharges

In the past three decades, first principles-based fully kinetic particle-in-cell Monte Carlo collision (PIC/MCC) simulations have been proven to be an important tool for the understanding of the physics of low pressure capacitive discharges. However, there is a long-standing issue that the plasma density determined by PIC/MCC simulations shows quantitative deviations from experimental measurements, even in argon discharges, indicating that certain physics may be missing in previous modeling of the low pressure radio frequency (rf) driven capacitive discharges. In this work, we report that the energetic electron-induced secondary electron emission (SEE) and excited state atoms play an important role in low pressure rf capacitive argon plasma discharges. The ion-induced secondary electrons are accelerated by the high sheath field to strike the opposite electrode and produce a considerable number of secondary electrons that lead to additional ionizing impacts and further increase of the plasma density. Importantly, the presence of excited state species even further enhances the plasma density via excited state neutral and resonant state photon-induced SEE on the electrode surface. The PIC/MCC simulation results show good agreement with the recent experimental measurements in the low pressure range (1–10 Pa) that is commonly used for etching in the semiconductor industry. At the highest pressure (20 Pa) and driving voltage amplitudes 250 and 350 V explored here, the plasma densities from PIC/MCC simulations considering excited state neutrals and resonant photon-induced SEE are quantitatively higher than observed in the experiments, requiring further investigation on high pressure discharges.

Electron-induced secondary electron emission of aluminum alloy processed via the combination of alodine treatment and carbon film deposition

• A porous chromate film is formed on the Al alloy via alodine treatment. • Secondary electron emission (SEE) of Al alloy is suppressed via alodine treatment. • SEE of alodine-treated Al alloy is further suppressed via carbon film deposition. • This SEE suppression is beneficial from the porous structure and sp 2 carbons. Relatively strong electron-induced secondary electron emission (SEE) of ordinary aluminum (Al) alloy limits its applications in high-power microwave devices. In this work, a combination strategy of alodine treatment and amorphous carbon (a-C) film deposition for Al alloy was proposed to achieve a goal of SEE suppression. According to the experimental results, it can be found both alodine treatment and a-C film deposition are advantageous to reducing secondary electron yield (SEY) of Al alloy. Moreover, the combination of these two surface treatments can further suppress the SEE of Al alloy due to the superposition effect of nanoscale porous structure and larger numbers of sp 2 carbon bonds. Compared with those of the untreated one, the SEY peak value, first crossover energy and second crossover energy of the Al alloy processed via the combined surface treatment change from 1.7, 25 eV and 2900 eV to 1.08, 150 eV and 650 eV, respectively.

Vlasov simulation of the emissive plasma sheath with energy-dependent secondary emission coefficient and improved modeling for dielectric charging effects

A one-dimensional Vlasov–Poisson simulation code is employed to investigate the plasma sheath considering electron-induced secondary electron emission (SEE) and backscattering. The SEE coefficient is commonly treated as constant in a range of plasma simulations; here, an improved SEE model of a charged dielectric wall is constructed, which includes the wall charging effect on the SEE coefficient and the energy dependency of the SEE coefficient. Pertinent algorithms to implement the previously mentioned SEE model in plasma simulation are studied in detail. It is found that the SEE coefficient increases with the amount of negative wall charges, which in turn reduces the emissive sheath potential. With an energy-dependent SEE coefficient, the sheath potential is a nonlinear function of the plasma electron temperature, as opposed to the linear relation predicted by the classic emissive sheath theory. Simulation combining both wall-charging effect and SEE coefficient’ energy dependency suggests that the space-charged limited sheath is formed at high plasma electron temperature levels, where both sheath potential and surface charging saturate. Additionally, different algorithms to implement the backscattering in the kinetic simulation are tested and compared. Converting backscattered electrons to secondary electrons via an effective SEE coefficient barely affects the sheath properties. The simulation results are shown to be commensurate with the upgraded sheath theory predictions.

Secondary electron emission yield measurements of dielectrics based on a novel collector-only method

The total electron-induced Secondary Electron Yield (SEY) of alkali-free glass has been determined by a novel technique which is solely based on the use of a collector electrode and of two interchangeable dielectric samples. The current from a charge-saturated sample provides a direct measure of the primary beam current. The SEY from the (uncharged) sample under investigation can then be determined simply as the ratio between the collector currents measured at the same electron energy from both samples. We further apply a low-current, short-pulse procedure to limit sample charging, needed to mitigate distortion of the SEY by charging effects. The potential build-up during the experiment could be reconstructed based on the primary electron currents and the measured SEY. This strategy permitted us to measure a 6-point SEY curve, without the need for intermediate discharging of the sample, as the total induced surface potential rise could be kept below +2.55 V.

An electron-induced secondary electron model for photoelectric sensitivity and quantum efficiency of metal surfaces

This paper studies an electron-induced secondary electron model ESEM for photoelectric sensitivity PS and quantum efficiency QE of metal surfaces near threshold frequency γ0, which was developed from Sommerfeld’s theory and the theories of transport and escape of electron-induced secondary electrons. The ESEM includes the effects of γ and T, properties of metals (Fermi energy and work function φ including Schottky effect) and applied field on metal surface, where T is Kelvin temperature, γ is the frequency of incident light. The PS curves show that for γ near γ0 there is a marked increase in photoemission with T, for γ farther away there is no change of photoemission with T, and for γ still father away there is a slight decrease in photoemission with T; these characteristics of PS curves were explained, respectively. Two methods of determining φ by using formulas deduced here for PS or QE curves of metal surfaces near γ0 to fit corresponding experimental data were presented, respectively. The ESEM is compared with existing widely used classical models, i. e., the three-step model and the Fowler-DuBridge model. Even though with different assumptions and settings, it is found that PS and QE curves of metal surfaces near γ0 in ESEM agree well with corresponding curves in other models, and that the ESEM has several advantages over other models.

Strongly temperature-dependent, anomalous secondary electron emission of liquid lithium surfaces exposed to a plasma

• First time reported SEE of a CPS wetted with liquid lithium exposed to a plasma. • Formation and dissolution of a strongly temperature dependent surface oxide film. • SEE values from ~0.5 for clean surfaces to ~2 for heavily oxidized samples. • Exposure to 3 × 10 −7 Torr during one hour increases the SEE to values of about 2. • Annealing at increasing temperatures decreases the SEE to lower values. Electron-induced secondary electron emission (SEE) on a liquid lithium surface confined in a capillary porous system (CPS) and exposed to a plasma is reported for the first time. The liquid lithium surface is exposed to the bombardment of a suprathermal electron flux with energies up 150 eV created in a low pressure He Dc-Glow discharge. Various methods of surface oxidation have been used to reproduce realistic conditions in fusion plasma experiments. Exposure to very low residual gas pressures (around 3 × 10 −7 Torr) is enough to increase the maximum of the SEE to values of about 2. The formation and dissolution of a surface oxide film is strongly temperature dependent and plays an important role in the resulting SEE yield and its time evolution: values have been found to range from ~0.8 for clean surfaces to ~2 for samples that have been heavily oxidized. In the case of O 2 molecule exposure, a clear difference with temperature was observed. The molecular exposure at 330 °C had a much stronger effect on the increase in SEE than the molecular exposure at 220 °C. These results have a direct impact on the development of lithium-based divertor targets in fusion as well as in the understanding of the SEE characteristics of contaminated liquid surfaces, which has never been reported before.

Substrate Temperature Dependent Microstructure and Electron-Induced Secondary Electron Emission Properties of Magnetron Sputter-Deposited Amorphous Carbon Films.

For special instruments or equipments including particle accelerators, space microwave devices and spacecrafts, the suppression for electron-induced secondary electron emission (SEE) occurring on the component surfaces is of great significance due to a negative influence caused by SEE on their normal operations. In this paper, amorphous carbon (a-C) films were prepared on stainless-steel substrates by radio frequency magnetron sputtering, and the effects of substrate temperature (Ts) and continuous electron bombardment on the microstructure and secondary electron emission yield (SEY) of a-C film were investigated in order to achieve a better inhibition for SEE. The experimental results show that a rise of Ts during the a-C film preparation is conducive to a SEY reduction and an increase of multipactor threshold due to the increases of surface roughness and sp2 bond content. In addition, although the SEY of a-C film has a slight increase with the rise of electron bombardment time, the a-C film sample with a lower SEY keeps its lower SEY all the time during continuous electron bombardment. The a-C film prepared at Ts of 500 °C has the lowest SEY peak value of 1.09 with a reduction of 30.6% in comparison with the stainless-steel substrate.

Features of the Mocvd Formation of MgO−RuO2 Electron-Emitting Film Structures

Thin-film structures based on magnesium and ruthenium oxides are obtained by metal-organic chemical vapor deposition (MOCVD) on flat silicon substrates from volatile bis-(dipivaloylmethanato)(N, N, N’,N’-tetramethylethylenediamine)magnesium(II) and tris-(acetylacetonato)ruthenium(III) complexes in the presence of oxygen. The samples are studied by SEM, EDS, powder XRD, and differential dissolution; their emission characteristics are also measured. The features of the phase formation and the microstructure in the films formed are shown and different forms of magnesium and ruthenium oxides, including nonstoichiometric ones, are found. The MOCVD experimental parameters to form composite structures with high coefficients of electron-induced secondary electron emission (up to 7.2) are determined. These materials can be used as efficient emitting coatings in modern 3D electron multipliers.

Monte Carlo modeling of low-energy electron-induced secondary electron emission yields in micro-architected boron nitride surfaces

Surface erosion and secondary electron emission (SEE) have been identified as the most critical life-limiting factors in channel walls of Hall-effect thrusters for space propulsion. Recent wall concepts based on micro-architected surfaces have been proposed to mitigate surface erosion and SEE. The idea behind these designs is to take advantage of very-high surface-to-volume ratios to reduce SEE and ion erosion by internal trapping and redeposition. This has resulted in renewed interest to study electron–electron processes in relevant thruster wall materials. In this work, we present calculations of SEE yields in micro-porous hexagonal BN surfaces using stochastic simulations of electron-material interactions in discretized surface geometries. Our model consists of two complementary parts. First we study SEE as a function of primary electron energy and incidence angle in flat surfaces using Monte Carlo simulations of electron multi-scattering processes. The results are then used to represent the response function of discrete surface elements to individual electron rays generated using a ray-tracing Monte Carlo model. We find that micro-porous surfaces result in SEE yield reductions of over 50% in the energy range experienced in Hall thrusters. This points to the suitability of these micro-architected surface concepts to mitigate SEE-related issues in compact electric propulsion devices.

Secondary electron emission enhancement of MgO/Au composite film by adopting a gold buffer layer

An electron-induced secondary electron emission (SEE) enhancement strategy of depositing a gold buffer layer for MgO/Au composite film was proposed. The influence of the gold buffer layer thickness on the microstructure, electrical conductivity and SEE properties of MgO/Au film was investigated. To deposit a gold buffer layer between the substrate and MgO/Au composite film markedly promotes the subsequent MgO grain growth, which leads to an increase in the SEE coefficient. Additionally, the improved electrical conductivity of this composite film caused by the gold buffer layer can effectively suppress the surface charging, which makes the MgO/Au film maintain a higher SEE coefficient for a longer time under continuous electron bombardment. An MgO/Au composite film with a 20-nm-thick gold buffer layer has a SEE coefficient of 5.1 at a primary electron energy of 200 eV with an increase of 13.3% compared with the one without a gold buffer layer.

Analysis of secondary electron emission properties of MgO/Au composite film with an Al-doped MgO surface layer

To improve the electron-induced secondary electron emission (SEE) properties of MgO/Au composite film, the strategy of Al doping in the MgO surface layer is adopted and investigated. For an MgO/Au film, Al doping in its MgO surface layer results in a slight increase of its grain size as well as a reduction of its surface roughness according to microstructure characterizations and also causes the decreases of both the band gap and work function of MgO crystal on the basis of the first-principles calculations, all of which have a close relationship with the improvement of secondary electron yield (SEY) demonstrated by the SEE measurement. An MgO/Au film including an Al-doped MgO surface layer with a doping ratio of 4.4% has an SEY of 4.7 with an increase of 14.6% at a primary electron energy (Ep) of 200 eV and the maximum SEY of 10.4 with an increase of 8.3% at Ep of 1100 eV in comparison with an MgO/Au film including an undoped MgO surface layer.

Au Doping Effect on the Secondary Electron Emission Performance of MgO Films.

Au-doped MgO films were prepared by reactive sputtering of individual Mg and Au targets, and the Au doping effect on the electron-induced secondary electron emission (SEE) performance was explored by means of surface analysis, first-principle calculation, and electrical characteristic measurement. The results show that the size enlargement of MgO grains and the reduction of surface work functions induced by Au doping are the main reasons for the increase of the SEE coefficient (δ). Additionally, the superior SEE degradation property of the Au-doped MgO film under continuous electron bombardment results from the improvement of electrical conductivity. Through the optimization of Au doping concentration (x), Au-doped MgO film with an x value of 3.0% was found to have the best SEE performance due to its highest SEE coefficient and longest duration of maintaining a relatively high SEE coefficient; its maximum δ value reached 11.5—an increase of 32.2% in comparison with the undoped one.

Electron-induced secondary electron emission of Zn-doped MgO/Au composite film

An MgO/Au composite film with a 1.8 at% Zn-doped MgO surface layer prepared by reactive magnetron sputtering exhibited a superior electron-induced secondary electron emission (SEE) performance, and it had a SEE coefficient of 5.4 with an increase of 9.3% and a similar SEE degradation rate in comparison with an undoped composite film under primary electron bombardment of 200 eV. The mechanism for this SEE coefficient increase is tightly related to the reduced work function of Zn-doped MgO crystal analyzed by first-principles calculations and the enlarged MgO grains as well as the decreased surface roughness of Zn-doped MgO/Au film observed by morphological characterizations. In addition, for the Zn-doped MgO/Au film its improved electrical conductivity induced by Zn doping results in a similar SEE degradation rate even though it emits secondary electrons with the higher coefficient all the time under continuous electron bombardment compared with the undoped film.

Influence of the substrate temperature on the microstructure and electron-induced secondary electron emission properties of MgO/Au composite film

The influence of the substrate temperature (Ts) on the microstructure and electron-induced secondary electron emission (SEE) properties of MgO/Au composite film prepared by reactive magnetron sputtering was investigated in order to improve the SEE performance of this composite film. During the deposition of MgO/Au film, a relatively high Ts is conducive to the growth of MgO grains and causes the formation of some large aggregated gold particles on which MgO grains cover in the composite film. In terms of the SEE properties of MgO/Au film, the increase of Ts brings about a significant improvement of SEE coefficient (δ), while it also leads to a slightly accelerated SEE degradation under the continuous electron bombardment due to a faster surface charging. The MgO/Au film prepared at Ts of 773 K has a δ value of 4.9 at a primary electron energy of 200 eV and the maximum δ value of 11.5.

Electron-Induced Secondary Electron Emission Properties of MgO/Au Composite Thin Film Prepared by Magnetron Sputtering

As a type of electron-induced secondary electron emitter, MgO/Au composite thin film was prepared by reactive magnetron sputtering of individual Mg target and Au target, and the effects of key process parameters on its surface morphology and secondary electron emission (SEE) properties were investigated. It is found that to deposit a NiO buffer layer on the substrate is conducive to the subsequent growth of MgO grains owing to the lattice matching. The gold addition can raise the electrical conductivity of MgO film and further suppress the surface charging. However, the gold deposition would interfere with the MgO crystallization and increase the surface roughness of MgO/Au film. Therefore, MgO/Au composite thin film with a NiO buffer layer and proper deposition times of MgO and Au can achieve superior SEE properties due to good MgO crystallization, low surface roughness and reasonable electrical conductivity. The optimized MgO/Au composite thin film has a higher SEE coefficient and a lower 1-h SEE degradation rate under electron beam bombardment in comparison with MgO film.

Secondary electron emission from lithium and lithium compounds

In this work, measurements of electron-induced secondary electron emission (SEE) yields of lithium as a function of composition are presented. The results are particularly relevant for magnetic fusion devices such as tokamaks, field-reversed configurations, and stellarators that consider Li as a plasma-facing material for improved plasma confinement. SEE can reduce the sheath potential at the wall and cool electrons at the plasma edge, resulting in large power losses. These effects become significant as the SEE coefficient, γe, approaches one, making it imperative to maintain a low yield surface. This work demonstrates that the yield from Li strongly depends on chemical composition and substantially increases after exposure to oxygen and water vapor. The total yield was measured using a retarding field analyzer in ultrahigh vacuum for primary electron energies of 20–600 eV. The effect of Li composition was determined by introducing controlled amounts of O2 and H2O vapor while monitoring film composition with Auger electron spectroscopy and temperature programmed desorption. The results show that the energy at which γe = 1 decreases with oxygen content and is 145 eV for a Li film that is 17% oxidized and drops to less than 25 eV for a fully oxidized film. This work has important implications for laboratory plasmas operating under realistic vacuum conditions in which oxidation significantly alters the electron emission properties of Li walls.

Electron-induced secondary electron emission coefficient of lithium, tungsten and stainless steel surfaces exposed to low-pressure plasmas

The secondary electron emission (SEE) coefficient by electron impact of Li, W and stainless steel (SS) surfaces exposed to a glow discharge is evaluated and analyzed in the energy range of Ee<200eV. While the values of the SEE coefficient for SS and W show a small increase with respect to their vacuum value, an enhancement of this parameter up to a factor of 6 has been deduced for clean Li surfaces. Experiments with different plasma gas discharges (He, Ar and H2) are undertaken in order to address the possible mechanisms related to such enhancement. No major effect of the bombarding ion mass or incident electron flux is observed. The implications of these findings on the use of Li as a plasma-facing component in fusion devices are addressed.

Note: A simple charge neutralization method for measuring the secondary electron yield of insulators

We report on a simple and effective charge neutralization method for measuring the total electron-induced secondary electron yield of insulators in a measurement system with a single pulsed electron gun. In this method, the secondary electron collector is negatively biased with respect to the sample to force some emitted secondary electrons to return to the sample surface and therefore to neutralize positive charges accumulated in the sample during the previous measurement. The adequate negative bias is determined and the equilibrium state of negative charging is discussed. The efficacy of the method is demonstrated by the measured electron yields in the cases with and without charge neutralization and by comparison with existing electron yield data of polyimide.

Total Electron Emission Yield of Solar Cell Coverglass and Optical Solar Reflector

Electron-induced secondary electron (SE) emission from space insulating materials is a very important factor in understanding spacecraft charging behavior. Because of the injection of primary electrons (PEs) and the emission of SEs in the surface layer of insulating materials, the target surface can be either negatively or positively charged. In the conventional measurement methods, a single short, low-density pulsed e-beam, charge neutralization using low energy flood e-gun and UV light, and charge dissipation by heating is used. In this paper, a pulsed scanning method for the total electron emission yield measurement of insulating, semiconducting or conducting solid materials is proposed. A total electron detector with large hollow space and insulating spacer was designed, and a pulsed yield scanning measurement system was developed. This method can avoid influence from the charged surface of the insulating material. Using the system, the total SE emission yields of 200 nm thick Au coating on glass, and 11 kinds of solar cell coverglasses and optical solar reflectors produced by Qioptiq have been reported, as induced by a 30 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm s}$</tex></formula> pulse of PEs with <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\sim}{\rm 20}~{\rm nA}$</tex></formula> incident beam current and energies up to 2500 eV.

The quantitative effect of thermal treatment on the secondary electron yield from air-exposed silver surface

Air-exposed silver surface is investigated in a multifunctional ultrahigh vacuum system to find out the quantitative relation between thermal treatment and the electron-induced secondary electron yield (SEY). Silver-coated samples are heated in the range of about 100–320 °C for up to 120 min, and thermal desorption is characterized. The SEY is then measured in situ as a function of primary electron energy. The maximum SEY is found to decrease almost linearly with the increase in the logarithmic thermal desorption amount determined by the heating time in a designed process. Reduction of SEY also shows a linear relationship with sample temperature in the observed range. Residual gas analysis demonstrates that the SEY reduction depends strongly on the rapid water desorption. In combination with the results of Ar-ion cleaning and Monte Carlo simulation, this work provides a new insight into the SEY variation due to surface desorption of different gases.