High Secondary Electron Emission Coefficient Research Articles

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.

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.

Effects of secondary electron emission on plasma density and electron excitation dynamics in dual-frequency asymmetric capacitively coupled argon plasmas

Effects of secondary electron emission (SEE) on the plasma density and electron excitation dynamics in dual-frequency (2 MHz and 14 MHz) capacitively coupled Ar plasmas are investigated. The plasma density np is measured with a hairpin probe, and the spatio-temporal distribution of electron excitation rate (ground state into Ar 2p1 state) is determined by phase resolved optical emission spectroscopy. It is generally found that as the low-frequency (LF) voltage ϕL increases, np first decreases at low ϕL, due to compressed the plasma bulk length by the LF source, and then increases slightly with ϕL, suggesting that the plasma is dominated by the α mode. When ϕL exceeds some critical value, np increases dramatically with ϕL, due to significantly enhanced ionization by secondary electrons, indicating a α–γ mode transition. An excitation pattern caused by SEE at the edge of the completely expanded LF sheath is observed at relatively high ϕL. Under various conditions, including the high-frequency voltage ϕH, the pressure p, the electrode gap L, and the electrode material, different dependences of np on ϕL are also discussed. It is found that the discharge turns into the γ mode at a lower ϕL when ϕH is higher. As p increases, the density peak moves axially towards the powered electrode, due to reduced sheath thickness, while its distance to the powered electrode is almost independent of other external conditions (ϕH, L, electrode material, etc). A higher p or higher L is favorable for the enhancement of np in γ mode, because secondary electrons can contribute more to the ionization. Due to the higher SEE coefficient of the aluminum electrode, a more significant increase in np in the γ mode can be seen than that with a stainless steel or copper electrode. Meanwhile, the spatio-temporal distributions of the electron excitation rate under the same conditions are analyzed to further understand the SEE effects.

Effect of Aluminum Nitride on Discharge Mode Transition in Atmospheric Pressure He/O2 DBD

In this paper, the effects of aluminum nitride and alumina ceramics on the discharge mode of He/O2 mixed gas are studied and compared. It is found that when the O2 volume fraction is in the range of 0.2%–1%, dielectric barrier discharge (DBD) with Al2O3 is always in the unstable filamentary mode, and homogeneous and filamentary modes are observed in DBD with AlN. Under the homogeneous discharge mode, there is only one current pulse each half-voltage cycle, and the intensified charge-coupled device discharge image shows that it is glow discharge. The relative permittivity and surface charge decay characteristics of AlN and Al2O3 are measured. The experimental results show that the relative permittivity of AlN is lower than that of Al2O3, but its surface charge decay is obviously slower than that of Al2O3. The derived electron trap density from the surface charge decay of AlN ceramic is higher than that of Al2O3, which shows AlN has a strong charge capacity and can emit more electrons in the second half cycle of the discharge. The higher secondary electron emission coefficient can make the discharge ignition voltage lower and the formation of stable homogeneous discharge easier. This paper provides a new control method for expanding homogeneous discharge in He/O2 mixed gas.

Preparation of MgO/Au Multilayer Composite Films and Related Studies on Secondary Electron Emission Effect

MgO thin film has a high secondary electron emission coefficient. However, the accumulation of electric charge on the surface of the thin film will inhibit it. This paper mainly discusses this phenomenon with the method of reactive magnetron sputtering. We prepared a MgO/Au buffer layer by co-sputtering Au in the inner layer of MgO. The experiments show that Au distributes mainly in the gap of the MgO grains. Adding appropriate Au will promote the growth of MgO grains in the longitudinal direction, increase the conductivity of the material, suppress the accumulation of electric charge on the surface, and increase the secondary electron emission coefficient of the thin film. NiO was also used as the transition layer of MgO and substrate in the experiments, so as to increase the adhesion of the MgO films and improve the quality of the thin film. According to the experiment results, the maximum secondary electron emission coefficient of MgOAu multilayer composite film increased by 28.9% compared with that of the pure MgO film. Under the continuous irradiation of 200 eV incoming energy, the decay rate decreased from 35.2% to 30.6% in 1 h.

Evolution of sputtering target surface composition in reactive high power impulse magnetron sputtering

The interaction between pulsed plasmas and surfaces undergoing chemical changes complicates physics of reactive High Power Impulse Magnetron Sputtering (HiPIMS). In this study, we determine the dynamics of formation and removal of a compound on a titanium surface from the evolution of discharge characteristics in an argon atmosphere with nitrogen and oxygen. We show that the time response of a reactive process is dominated by surface processes. The thickness of the compound layer is several nm and its removal by sputtering requires ion fluence in the order of 1016 cm−2, much larger than the ion fluence in a single HiPIMS pulse. Formation of the nitride or oxide layer is significantly slower in HiPIMS than in dc sputtering under identical conditions. Further, we explain very high discharge currents in HiPIMS by the formation of a truly stoichiometric compound during the discharge off-time. The compound has a very high secondary electron emission coefficient and leads to a large increase in the discharge current upon target poisoning.

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.

A Comparison of Continuum and Kinetic Simulations of Moderate pd Microplasmas Integrated With High Secondary Yield Cathodes

The computational techniques commonly used for low‐temperature plasma simulations are compared in the context of modeling microplasmas driven by cathodes with high secondary electron emission coefficient. Simulations of 100 µm argon microplasmas operating at pressures of 100 Torr and secondary electron emission coefficient of 0.1 are performed using particle‐in‐cell with Monte Carlo collisions (PIC‐MCC), and fluid model using the full‐momentum equations for both electrons and ions. Results obtained for plasma density, potential, electric field, and electron temperature using continuum simulations are compared with the PIC‐MCC simulations as benchmark. The comparison demonstrates significant discrepancies and a need to calibrate continuum simulation parameters based on kinetic simulations.

A feedback model of magnetron sputtering plasmas in HIPIMS

We present a 1D feedback model that captures the essential elements of plasma pulse initiation and is useful for control and diagnostics of sputtering plasmas. Our model falls into the class of single-species population models with recruitment and time delay, which show no oscillatory behaviour. The model can reproduce essential features of published time–current traces from plasma discharges and is useful to determine the key parameters affecting the evolution of the discharge. We include the external circuit and we focus on the time evolution of the current as a function of the applied voltage and the plasma parameters. We find the necessity of a nonlinear loss term in the time-dependent plasma ion population to ensure a stable discharge, and we show that a higher secondary electron emission coefficient reduces the time delay for current initiation. We report that I–V characteristics in the plateau region, where it exists, fit a power curve of the form I = kVn, where n is influenced most strongly by the nonlinear loss term.

Electron heating and control of ion properties in capacitive discharges driven by customized voltage waveforms

We investigate the electron heating dynamics in capacitively coupled radio frequency plasmas driven by customized voltage waveforms and study the effects of modifying this waveform and the secondary electron emission coefficient of the electrodes on the spatio-temporal ionization dynamics by particle-in-cell simulations. We demonstrate that changes in the electron heating dynamics induced by voltage waveform tailoring strongly affect the dc self-bias, the ion flux, Γi, and the mean ion energy, 〈Ei〉, at the electrodes. The driving voltage waveform is customized by adding N consecutive harmonics (N ⩽ 4) of 13.56 MHz with specific harmonics' amplitudes and phases. The total voltage amplitude is kept constant, while modifying the number of harmonics and their phases. In an argon plasma, we find a dc self-bias, η, to be generated via the electrical asymmetry effect for N ⩾ 2. η can be controlled by adjusting the harmonics' phases and is enhanced by adding more consecutive harmonics. At a low pressure of 3 Pa, the discharge is operated in the α-mode and 〈Ei〉 can be controlled by adjusting the phases at constant Γi. The ion flux can be increased by adding more harmonics due to the enhanced electron-sheath heating. 〈Ei〉 does not remain constant as a function of N at both electrodes due to a change in η. These findings verify previous results of Lafleur et al. At a high pressure of 100 Pa and using a high secondary electron emission coefficient of γ = 0.4, the discharge is operated in the γ-mode and mode transitions are induced by changing the driving voltage waveform. Due to these mode transitions and the specific ionization dynamics in the γ-mode, Γi is no longer constant as a function of the harmonics' phases and decreases with increasing N.

Discharge Characteristics of Plasma Display Panels with SrCaO Protective Layer Manufactured Using “All-in-Vacuum” Process

Plasma display panels (PDPs) with a SrCaO protective layer of high secondary electron emission coefficient were manufactured using the “all-in-vacuum” process, whereby the protective layer deposition and panel sealing were performed continuously in high vacuum in order to keep the surface of the protective layer as clean as possible. The PDP with the SrCaO protective layer manufactured using the “all-in-vacuum” process (“all-in-vacuum” SrCaO-PDP) for Ne­20 vol% Xe exhibited a luminance 2.1 times larger, a luminous efficacy 1.7 times larger, and the same discharge time lag, compared with the PDP with the MgO protective layer manufactured using the conventional process (conventional MgO-PDP) for Ne­4 vol% Xe, which had almost the same discharge voltage. [doi:10.2320/matertrans.M2011181]

Mechanism of high luminous efficacy in plasma display panel with high secondary electron emission coefficient cathode material analyzed through three-dimensional fluid model simulation

The mechanism to realize high luminous efficacy in a plasma display panel fabricated with a cathode material possessing a high secondary electron emission coefficient (γ) for Ne and Xe ions was studied via three-dimensional numerical simulation. When a high γ cathode material is used, the increased electron heating efficacy is responsible for increasing vacuum ultraviolet (VUV) efficacy with 10% Xe content gas. However, the continued availability of sufficient secondary electrons during the dynamic moving phase of the cathode sheath in which the electric field remains weakened causes increasing VUV efficacy with 30% Xe content gas. It was found that the improvement of the luminous efficacy of the plasma display panel with a high γ cathode material is maximized under the condition of high Xe content gas because of the simultaneous increase of the electron heating efficacy and Xe excitation efficacy.

Discharge Characteristics of MgO Nano-Powdered Functional Layer in AC-PDP

We have studied discharge characteristics of MgO nano-powdered functional layer in accordance with their particle sizes in AC-PDP. The MgO nano-powder's size was controlled and confirmed that the different particle sizes were distributed according to the sintering times throughout the particle size analyzer (PSA) measurement. We made the different 6 inch test panels, in which the different particle size's functional layers were applied, for measurements of the firing voltage, secondary electron emission coefficient and cathodoluminescence (CL) spectrum. It is shown that the functional layer makes the firing voltage and sustain voltage low, and the particles with large size have the lowest firing voltage and sustain voltage, which are related to the high secondary electron emission coefficient.

Numerical Simulation for the Effect of Wall Material on Near Wall Conductivity in Hall Thrusters

The wall material plays an important role for the electron current due to near wall conductivity in Hall Thrusters. A Monte Carlo method combined with a one dimensional steady sheath model is presented and is applied to simulate the electron conductive current due to near wall conductivity for the different channel wall materials of Hall thruster. The simulation results show that the higher the secondary electron emission (SEE) coefficient of the channel wall material is, the greater the electron conductive current is. Based on the simulation, a physical explanation is given from the viewpoint of near wall conductivity. For the channel wall material with low SEE coefficient, the secondary electrons taking part in the near wall conductivity becomes less. In addition, the absolute potential drop in the sheath near the wall increases, which means that the sheath can stop more electrons from colliding with the channel wall. And consequently the electron conductive current due to near wall conductivity is much less. The situation is vice verse for the channel wall material with high SEE coefficient. The simulation results are qualitatively in accordance with the experiments. The results can help to choose and design the wall material of the Hall Thrusters with a high performance.

Experimental Observation of Discharge Characteristics Under Variable Ambient Temperature in AC Plasma Display Panel

The effect of an ambient temperature ranging from -5degC to +65degC on the discharge characteristics was examined. The change in the firing voltage according to the ambient temperature was much higher under MgO cathode conditions with a high secondary electron emission coefficient than under phosphor cathode conditions with a low secondary electron emission coefficient. Through the weak reset discharge, the wall voltages were increased with an increase in the ambient temperature, as the wall charges were less erased at a higher ambient temperature during the ramp-down period. An increase in the ambient temperature was also found to have a more significant influence on the statistical delay time than on the formative delay time. Since the statistical delay time is strongly related to the priming condition, a higher ambient temperature may contribute to a better priming condition for reducing the statistical delay time during the address period.

Metal-oxide-junction, triple point cathodes in a relativistic magnetron

Triple point, defined as the junction of metal, dielectric, and vacuum, is the location where electron emission is favored in the presence of a sufficiently strong electric field. To exploit triple point emission, metal-oxide-junction (MOJ) cathodes consisting of dielectric "islands" over stainless steel substrates have been fabricated. The two dielectrics used are hafnium oxide (HfO(x)) for its high dielectric constant and magnesium oxide (MgO) for its high secondary electron emission coefficient. The coatings are deposited by ablation-plasma-ion lithography using a KrF laser (0-600 mJ at 248 nm) and fluence ranging from 3 to 40 J/cm(2). Composition and morphology of deposited films are analyzed by scanning electron microscopy coupled with x-ray energy dispersive spectroscopy, as well as x-ray diffraction. Cathodes are tested on the Michigan Electron Long-Beam Accelerator with a relativistic magnetron, at parameters V=-300 kV, I=1-15 kA, and pulse lengths of 0.3-0.5 micros. Six variations of the MOJ cathode are tested, and are compared against five baseline cases. It is found that particulate formed during the ablation process improves the electron emission properties of the cathodes by forming additional triple points. Due to extensive electron back bombardment during magnetron operation, secondary electron emission also may play a significant role. Cathodes exhibit increases in current densities of up to 80 A/cm(2), and up to 15% improvement in current start up time, as compared to polished stainless steel cathodes.

Reactive gas effects in pulsed magnetron sputtering: Time-resolved investigation

A pulsed magnetron deposition discharge in argon with different admixtures of oxygen as reactive component has been studied. The target material used was magnesium and the pulsed discharge was operated in metallic and reactive mode. The discharge evolution during each pulse has been characterised by the time-resolved Langmuir double probe and time-resolved optical emission spectroscopy. Charge carrier density and optical emission exhibit two more or less strong peaks at the beginning of the “on” phase, the second of which being the most dominant. The temporal development of the discharge is clearly different for the two discharge modes: in the reactive mode, the structures in the “on” phase — except of the first maximum — significantly shift to shorter times compared to the metallic mode. This is suggested to be due to the much higher secondary electron emission coefficient ( γ) of MgO in comparison to Mg leading to increased ionisation starting with the second maximum. The first maximum, however, is attributed to residual electrons from the precedent pulse and is thus independent of the γ coefficient. A significant feedback from the discharge to the power supply is observed by changes in the voltage waveform: the more efficient discharge development is accompanied by a sharper and lower peak in the voltage.

Measurement of secondary electron emission coefficient of plane target in plasma immersion ion implantation

AbstractThis paper describes in‐process measurements of secondary electron emission coefficient (SEEC) during plasma immersion ion implantation for a plane target. In the plane target, trajectories of secondary electrons (SEs) emitted from the target strongly depend on the plasma conditions as well as the amplitude of pulse voltages applied to the target because of variation of the curvature radius of the sheath formed around the target. Therefore, absolute measurements of the SE current as well as of the SEEC were difficult to perform. However, calibration of an SE detector using a plane target with very high SEEC enabled the SEEC measurements. The measurements of the SE current revealed that the sheath curvature was affected by the plasma condition and the target shape. This calibration will be useful for SEEC measurements of arbitrary‐shaped target. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 151(3): 1–7, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20096

Efficacy Improvement of Mercury-Free Xe Flat Discharge Fluorescent Lamps by Increasing Xe Partial Pressure

The luminance and luminous flux of external-electrode, mercury-free, Xe flat discharge fluorescent lamp for LCD backlights and lighting equipment are equivalent to those of conventional Hg fluorescent lamps. However its efficacy is lower. In this paper, improvement of efficacy was attempted. First, the electrode width was reduced from 1.0mm to 0.5mm in order to widen the operating pressure margins. This enabled the Xe partial pressure to be increased from 1.2 kPa to 3.0 kPa, which increased efficacy 60%. Then a MgO, which has a high secondary electron emission coefficient, was coated on the dielectric layer, improving efficacy a further 10%. As a results, an efficacy of 35 lm/W with a luminance 10,100 cd/m2 were obtained with a 2.8-inch diagonal lamp.

Ion beam-induced erosion and humidity effect of MgO protective layer prepared by vacuum arc deposition

MgO films have been suggested as the protective layer for alternating current plasma display panels, which play significant roles in preventing the failure of the dielectric layer from sputtering by ions, in lowering the firing and sustain voltages due to a higher secondary electron emission coefficient ( γ), and in allowing higher optical transmittance. To investigate the structural and optical properties of the MgO protective films, MgO thin films were deposited on glass substrates by vacuum arc deposition with a magnesium metal target at various oxygen gas flows. To measure the erosion rate of the MgO protective layers, an acceleration test method was used, namely, the Ar+ ion beam-induced erosion test. The effect of moisture on the optical transmittance of the MgO protective layers was also examined. It was found that the optical transmittance decreased with the relative humidity. X-Ray diffraction and AFM have also been used to study the behaviors of the structure and surface morphology.