Ion-induced Secondary Electron Emission Coefficient Research Articles

Nonlinear change of ion-induced secondary electron emission in the κ-Al2O3 surface charging from first-principle modelling

Abstract Secondary electron emission (SEE) induced by the positive ion is an essential physical process to influence the dynamics of gas discharge which relies on the specific surface material. Surface charging has a significant impact on the material properties, thereby affecting the SEE in the plasma-surface interactions. However, it does not attract enough attention in the previous studies. In this paper, SEE dependent on the charged surface of specific materials is described with the computational method combining a density functional theory (DFT) model from the first-principle theory and the theory of Auger neutralization. The effect of κ-Al2O3 surface charge, as an example, on the ion-induced secondary electron emission coefficient (SEEC) is investigated by analysing the defect energy level and band structure on the charged surface. Simulation results indicate that, with the surface charge from negative to positive, the SEEC of a part of low ionization energy ions (such as Ei = 12.6 eV) increases first and then decreases, exhibiting a nonlinear changing trend. This is quite different from the monotonic decreasing tendency observed in the previous model which simplifies the electronic structure. This irregular increase of the SEEC can be attributed to the lower escaped probability of orbital energy. The results further illustrate that the excessive charge could cause the bottom of the conduction band close to the valence band, thus leading to the decrease of the orbital energy occupied by the excited electrons. The nonlinear change of SEEC demonstrates a more realistic situation of how the electronic structure of material surface influences the SEE process. This work provides an accurate method of calculating SEEC from specific materials, which is urgent in widespread physical scenarios sensitive to surface materials, such as increasingly growing practical applications concerning plasma-surface interactions.

Application of optical emission spectroscopy to predict the composition of films during reactive magnetron sputtering of Ti-Al composite targets

The processes of reactive magnetron sputtering of Ti-Al composite targets with different Al/Ti ratios have been studied. The dependences of the deposition rate, discharge voltage, elemental composition, and intensity of the control lines of optical plasma radiation on the oxygen concentration in the Ar/O2 gas mixture are established. It is shown that during reactive sputtering of Ti-Al composite targets, the discharge voltage is determined by the effective ion-induced secondary electron emission coefficient (SEEC) which depends on the area occupied by metals on the target, the degree of their oxidation, and the SEEC of these metals and their oxides. The rate of the deposition of TixAl1-xOy films in the metallic and transition sputtering modes increases proportional to the concentration of Al in the target, but the relative concentration of metals in the deposited films depends on the oxygen concentration in the Ar/O2 gas mixture and is determined by the reactivity of the target materials. It has been shown by optical emission spectroscopy (OES) that the ratio of the atomic concentrations of Al and Ti in deposited TixAl1-xOy films unambiguously depends on the ratio of the intensity of the control optical lines of aluminum AlI and titanium TiI in plasma. This makes it possible to use the OES method to predict the content of metals in a film during reactive magnetron sputtering of Ti-Al targets.

Ion-induced secondary electron emission of oxidized nickel and copper studied in beam experiments

Ion-induced secondary electron emission at a target surface is an essential mechanism for laboratory plasmas, i.e. magnetron sputtering discharges. Electron emission, however, is strongly affected by the target condition itself such as oxidation. Data of oxidized targets, however, are very sparse and prone to significant systematic errors, because they were often determined by modeling the complex behavior of the plasma. Thus, it is difficult to isolate the process of ion-induced electron emission from all other plasma-surface-interactions. By utilizing ion beams, the complex plasma environment is avoided and electron yields are determined with higher accuracy. In this study, ion-induced secondary electron emission coefficients (SEECs) of clean, untreated (air-exposed), and intentionally oxidized copper and nickel surfaces were investigated in such a particle beam experiment. Pristine and oxidized metal foils were exposed to beams of singly charged argon ions with energies of 0.2 keV - 10 keV. After the ion beam treatment, the surface conditions were analyzed by ex-situ x-ray photoelectron spectroscopy measurements. Further, a model for the electron emission of a partly oxidized surface is presented, which is in agreement with the experimental data. It was found, that oxidized and untreated/air-exposed surfaces do not show the same SEEC: for intentionally oxidized targets, the electron yields were smaller by a factor of 2 than for untreated/air-exposed surfaces. SEECs of oxides were found to be between the values for clean and for untreated metal surfaces. Further, the SEEC was at maximum for untreated/air-exposed surfaces and at minimum for clean surfaces; the electron yields of untreated/air-exposed and clean surfaces were in agreement with values reported in the literature.

Polarity dependence of electrical breakdown in argon under a non-uniform electric field

A considerable difference in the breakdown potential of a uniform pressure gas (specifically argon) under an asymmetric non-uniform electric field is observed depending on the polarity of the field. The degree of variation in the breakdown potential between the two polarities can be explained if one assumes a strong variation of the ion-induced secondary electron emission coefficient with the magnitude of the reduced electric field at the cathode. The magnitude of the breakdown potential can be predicted by evaluating the breakdown criterion along multiple field lines if one includes a numerical factor to account for diffusion of electrons out of the preferred breakdown channel.

Unveiling the role of dielectric trap states on capacitively coupled radio-frequency plasma discharge: dynamic charging behaviors

The influence of charge trap states in the dielectric boundary material on capacitively coupled radio-frequency (RF) plasma discharge is investigated with theory and particle-in-cell/Monte Carlo collision simulation. It is found that the trap states of the wall material manipulated discharge properties mainly through the varying ion-induced secondary electron emission (SEE) coefficient in response to dynamic surface charges accumulated within the solid boundary. A comprehensive SEE model considering surface charging is established first, which incorporates the valence band electron distribution, electron trap density, and charge trapping through Auger neutralization and de-excitation. Theoretical analysis is carried out to reveal the effects of trap states on sheath solution, stability, plasma density and temperature, particle and power balance, etc. The theoretical work is supported by simulation results, showing the reduction of the mean RF sheath potential as charging-dependent emission coefficient increases. As the gas pressure increases, a shift of the maximum ionization rate from the bulk plasma center to the plasma-sheath interface is observed, which is also influenced by the trap states of the electrode material where the shift happens at a lower pressure with traps considered. In addition, charge traps are proven to be helpful for creating asymmetric plasma discharges with geometrically symmetric structures; such an effect is more pronounced in γ-mode discharges.

Heavy-particle induced secondary electrons in capacitive radio frequency discharges driven by tailored voltage waveforms

Particle-in-Cell/Monte Carlo Collision simulations are performed to investigate the effects of heavy-particle induced secondary electrons (SEs) on the ionization dynamics and on the control of ion properties at the electrodes in geometrically symmetric capacitively coupled argon discharges driven by tailored voltage waveforms. The driving voltage waveform is composed of a maximum of four (1≤N≤4) consecutive harmonics of the fundamental frequency of 13.56 MHz and is tailored by adjusting the identical phases of the even harmonics, θ. The simulations are carried out at neutral gas pressures of 3 Pa (nearly collisionless low-pressure regime) and 100 Pa (collisional high-pressure regime). Different approaches are used in the simulations to describe the secondary electron emission (SEE) at the electrodes: we adopt (i) constant ion-induced secondary electron emission coefficients (SEECs), γ, and (ii) realistic, energy-dependent SE yields for ions and fast neutrals. The mean ion energy at the electrodes, ⟨Ei⟩, can be controlled by θ at both pressures, for both approaches adopted to describe the SEE in the simulations. At a low pressure of 3 Pa, we obtain largely different dependencies of the ion flux at the electrodes, Γi, on θ, depending on the value of the γ-coefficient. For γ=0.2, Γi remains nearly constant as a function of θ, independently of the choice of N, i.e., the mean ion energy can be controlled separately from the ion flux by adjusting θ. However, for values of γ different from 0.2, the quality of the separate control of the ion properties changes significantly. At a high pressure of 100 Pa, independently of the choice of γ, for a given N≥2, the ion flux varies as a function of θ. At both pressures, the surface conditions affect the plasma parameters and the quality of the separate control of ion properties at the electrodes. Adopting realistic, energy-dependent SE yields for heavy particles in the simulations can lead to significantly different results compared to those obtained by assuming constant SEECs.

Material dependent modeling of secondary electron emission coefficients and its effects on PIC/MCC simulation results of capacitive RF plasmas

The ion-induced secondary electron emission coefficient (γ) is a vital parameter in the modeling of low temperature RF plasmas. Often, the value of γ drastically affects the electron power absorption dynamics, the plasma parameters and the quality of the separate control of ion flux and mean ion energy at the electrodes. Experimental results for γ under plasma exposure are difficult to obtain. Therefore, γ is either assumed to be a constant chosen with some uncertainty, or is approximated as a quantity that is a function of the ion energy and cleanliness of the electrode surface. It is hypothesized that these assumptions are not valid for all materials and plasma conditions. In this work, Hagstrum’s theory on Auger emission is suggested as a robust, ab initio model for accurately predicting γ for metal surfaces with a wide range of surface conditions and for a variety of ion species. To demonstrate the effect of the choice of γ on modeling results, we carry out particle-in-cell/Monte Carlo collision simulations of 13.56 MHz, single-frequency argon and helium capacitive discharges. Simulations are run assuming that: (i) γ is a constant, (ii) γ is an energy and surface condition dependent quantity that is independent of the electrode material, and (iii) γ is obtained from the ab initio model for different clean metals. The energy distribution of the emitted electrons resulting from Hagstrum’s theory is also implemented as a uniform, metal dependent distribution with physically accurate energy domain. It is found that this is important for some metals in both helium and argon. Lastly, it is observed that, depending on the assumed surface conditions, the plasma properties change dramatically. Based on these results we conclude that a realistic, material dependent implementation of γ is required to obtain realistic simulation results and that Hagstrum’s model suits this purpose.

Study on the effect of target on plasma parameters of magnetron sputtering discharge plasma

In this study, the effect of magnetron target on different plasma parameters of Argon/Hydrogen (Ar - H2) direct current (DC) magnetron discharge is examined. Here, Copper (Cu) and Chromium (Cr) are used as magnetron targets. The value of plasma parameters such as electron temperature (kTe), electron density (Ne), ion density (Ni), degree of ionization of Ar, and degree of dissociation of H2 for both the target are studied as a function of input power and hydrogen content in the discharge. The plasma parameters are determined by using Langmuir probe and Optical emission spectroscopy. On the basis of the different reactions in the gas phase, the variation of plasma parameters and sputtering rate are explained. The obtained results show that electron and ion density decline with gradual addition of Hydrogen in the discharge and increase with rising input power. It brings significant changes on the degree of ionization of Ar and dissociation of H2. The enhanced value of electron density (Ne), ion density (Ni), degree of Ionization of Ar, and degree of dissociation of H2 for Cr compared to Cu target is explained on the basis of it's higher Ion Induced Secondary Electron Emission Coefficient (ISEE) value.

Sputtering Voltage in the Growth of Vanadium Oxide Thin Films

Vanadium oxide thin films have been deposited by reactive DC magnetron sputtering. The results showed that the sputtering voltage has a special change behavior in Ar condition and Ar/O2 mixed conditions. With O2 gas flow input instantaneously, the sputtering voltage climbs quickly, and then tends to be stable shortly. Also, the sputtering voltage evolution appears a hysteretic loop characteristics with O2 gas supplied increasing and decreasing. The sputtering voltage behavior observed is related to the ion induced secondary electron emission (ISEE) coefficient of the target material. The ISEE coefficient of the oxidized vanadium on target surface is lower than the ISEE coefficient of the pure vanadium metal, which is not in agreement with the generally accepted idea that the ISEE coefficient of oxides is much larger than for metals.

Discharge diagnosis of high Xe concentration and high‐γ protective layer in PDPs

Abstract— The high‐Xe‐concentration and high‐γ (ion‐induced secondary‐electron emission coefficient) protective layer have been diagnosed from both experimentation and simulation. The experimental results show that there is a great increase in luminance and luminous efficacy, while the breakdown voltage decreases in the high‐Xe and high‐γ discharge. In the high‐Xe discharge, the great increase in VUV radiation mainly results from an increase in excimer VUV emission. The application of high‐Xe concentration can greatly increase the luminous efficacy, while the high‐γ protective layer can promote it further. Considering that the total discharge efficiency can be divided into the electron heating efficiency, the Xe excitation efficiency, and the VUV radiation efficiency, both the electron heating efficiency and Xe excitation efficiency increased for a high‐Xe discharge; while for a high‐γ discharge, the increase in electron heating efficiency contributes to the improvement in discharge efficiency.

Plasma Density, Electron Temperature, Excited Xenon Density and Ion-Induced Secondary Electron Emission Coefficient for the Vacuum Ultraviolet Luminous Efficiency in an Alternating Current Plasma Display Panel

Plasma Density, Electron Temperature, Excited Xenon Density and Ion-Induced Secondary Electron Emission Coefficient for the Vacuum Ultraviolet Luminous Efficiency in an Alternating Current Plasma Display Panel

Excited xenon density and ion-induced secondary electron emission coefficient for vacuum ultraviolet luminous efficiency in alternating current plasma display panel

It is shown for Xe content ratio of 10% that the excited xenon atom density for the 1s 5 metastable state is 3.0 × 10 12 cm − 3 with a gap distance of 50 μm under a fixed gas pressure of 5.3 × 10 4 Pa in alternating current plasma display panels. It is observed that the VUV emission of 173 nm continues after its peak and lasts much longer than 147 nm emission. We also found that the time of VUV peak emission intensity for higher xenon mole fraction is longer. It is also concluded that the luminous efficiency is strongly dependent on the ion-induced secondary electron emission coefficient.

Study on breakdown characteristics of a low-pressure dielectric barrier discharge

AbstractThe breakdown characteristics of a low-frequency dielectric barrier discharge (DBD) at low pressure are investigated experimentally in He, Ne and Ar. The current waveform of this DBD, a series of pulses, is caused by the results of the electron avalanches under the action of the applied field and the quenching effect of the wall-charge field on the avalanches. Taking into consideration the diffusion loss of the charged particles in the breakdown processes, the experimental investigation and the theoretical analysis of its breakdown characteristics indicate that its breakdown voltage is higher than the calculation by the Paschen law and depends on the filled-gas pressure and the distance between the electrodes, instead of their product. The ion-induced secondary electron emission coefficient and the mean electron energy at the moment of breakdown can also be determined approximately by measuring the breakdown characteristics of the discharge tubes with different distances between the electrodes and combining the theoretical deductions. The experimental results and the theoretical analysis of the breakdown characteristics of this DBD are discussed.

Correlation between electron and negative O− ion emission during reactive sputtering of oxides

The energy distribution of negative O− ions has been measured during the reactive magnetron sputtering of 13 different target materials by the means of energy resolved mass spectrometry. For the same series of target materials the ion-induced secondary electron emission coefficient was determined in an earlier published research. A correlation between this ion-induced secondary electron emission coefficient and the emission of the high energetic negative O− ions was observed. This correlation should be taken into consideration in the selection of oxides for their high electron emission coefficients as these materials will emit at the same time negative O− ions.

36.3: Secondary Electron Emission Coefficient of Pure and Cs-doped MgO for Low Energy Noble Gas Ions

The ion-induced secondary electron emission coefficient (γ) of MgO was accurately measured in-situ for noble gas ions in the energy range of 30–250 eV under negligible surface-charging conditions by using pulsed ion beams of very low duty factor. The results can be qualitatively interpreted in terms of potential emission(PE) and energy-dependent kinetic emission(KE). The γ vs. E curves can be well fitted to a quadratic function of the ion energy. Upon mixing MgO with 5% Cs2O KE is enhanced to result in an increase in γ by 30–80% for Ne+. Even larger KE enhancement as well as a threshold energy shift from 73 to 32 eV are observed for Xe+. However PE still does not occur.

High-rate deposition of MgO by reactive ac pulsed magnetron sputtering in the transition mode

A reactive ac pulsed dual magnetron sputtering process for MgO thin-film deposition was equipped with a closed-loop control of the oxygen flow rate (FO2) using the 285nm magnesium radiation as input. Owing to this control, most of the unstable part of the partial pressure versus flowrate curve became accessible. The process worked steadily and reproducible without arcing. A dynamic deposition rate of up to 35nmm∕min could be achieved, which was higher than in the oxide mode by about a factor of 18. Both process characteristics and film properties were investigated in this work in dependence on the oxygen flow, i.e., in dependence on the particular point within the transition region where the process is operated. The films had very low extinction coefficients (<5×10−5) and refractive indices close to the bulk value. They were nearly stoichiometric with a slight oxygen surplus (Mg∕O=48∕52) which was independent of the oxygen flow. X-ray diffraction revealed a prevailing (111) orientation. Provided that appropriate rf plasma etching was performed prior to deposition, no other than the (111) peak could be detected. The intensity of this peak increased with increasing FO2, indicating an even more pronounced (111) texture. The ion-induced secondary electron emission coefficient (iSEEC) was distinctly correlated with the markedness of the (111) preferential orientation. Both refractive index and (111) preferred orientation (which determines the iSEEC) were found to be improved in comparison with the MgO growth in the fully oxide mode. Consequently, working in the transition mode is superior to the oxide mode not only with respect to the growth rate, but also to most important film properties.

Discharge voltage measurements during magnetron sputtering

The influence of different parameters on the discharge voltage, measured during planar magnetron sputtering, is studied for three different magnetron configurations. Using twelve different target materials, the influence of the material dependent ion-induced secondary electron emission (ISEE) coefficient is studied. An inversely proportional relation between the ISEE coefficient and the discharge voltage was found. Moreover, the discharge voltage increases with decreasing pressure, a behaviour which can be understood from the increasing recapture of electrons by the target. Furthermore, the influence of the magnetic field (target thickness) was investigated. A stronger magnetic field results in a better confinement of the electrons, resulting in a decrease of the discharge voltage at constant current.The current–voltage (I–V) characteristics of the three magnetron configurations was studied in a wide range of pressures and magnetic fields. The I–V characteristics are fitted to the relation I=β(V−V0)2. The value of β expresses the steepness of the I–V characteristic and its value depends strongly on the ISEE coefficients. Of special interest is the influence of the pressure on β: whereas at low magnetic fields β increases with increasing pressure, it decreases with increasing pressure at strong magnetic fields.

Significant enhancement of ion-induced secondary electron current by photons in plasma immersion ion implantation

This paper describes effects of vacuum ultraviolet (VUV) optical emission on the secondary electron emission coefficient (SEEC) in plasma immersion ion implantation (PIII) processes. To examine this, time-resolved in situ SEEC measurements were carried out for square wave-modulated Ar discharges. The SEEC in the activeglow was enhanced in proportion to the SEEC in the afterglow regardless of the ion-bombarding energy. The SEEC enhancement in the activeglow was caused by photon effects, and was explained by synergistic effects of photon irradiation and ion bombardment. Oxygen addition to the Ar discharge also caused a significant increase in the SEEC, and enhanced both ion-induced SEEC and photon enhancement factor.

Measurement of surface roughness and ion-induced secondary electron emission coefficient of MgO films prepared by high-pressure sputter deposition

Magnesium oxide (MgO) films were prepared by RF sputter deposition technique, and their secondary electron emission (SEE) coefficient was examined in relation to film thickness and surface morphology. The optical constant and film thickness were evaluated from the optical transmission spectroscopy for samples prepared on the glass substrate. The ion-induced SEE coefficient was measured for MgO films deposited on Si substrates of both mirror-polished (root mean square (RMS) roughness <1 nm) and etched-rough (RMS roughness ∼40 nm) faces, in an XPS apparatus using the primary ion beam of Ar+ at 1 keV. The SEE coefficient was as large as 2.5 for 100 nm MgO films, and did not depend strongly on the process pressure nor the roughness of the silicon substrate. Samples of 200 nm MgO thickness showed a current instability in the SEE measurement, probably due to the charge up.

Measurement of surface roughness and ion-induced secondary electron emission coefficient of MgO films prepared by high-pressure sputter deposition

Magnesium oxide (MgO) films were prepared by RF sputter deposition technique, and their secondary electron emission (SEE) coefficient was examined in relation to film thickness and surface morphology. The optical constant and film thickness were evaluated from the optical transmission spectroscopy for samples prepared on the glass substrate. The ion-induced SEE coefficient was measured for MgO films deposited on Si substrates of both mirror-polished (root mean square (RMS) roughness <1 nm) and etched-rough (RMS roughness ∼40 nm) faces, in an XPS apparatus using the primary ion beam of Ar + at 1 keV. The SEE coefficient was as large as 2.5 for 100 nm MgO films, and did not depend strongly on the process pressure nor the roughness of the silicon substrate. Samples of 200 nm MgO thickness showed a current instability in the SEE measurement, probably due to the charge up.