Ion-induced Secondary Electron Research Articles

Analysis of current-voltage characteristics for Langmuir probes immersed in an ion beam.

Movable electrical probes were used to diagnose the beam flux profile and potential of ion beams since the early 1960s. Experimental measurements of beam plasmas can provide essential data related to the space charge neutralization, but the current-voltage characteristics obtained from such electrical probes are dominated by beam ion impact and ion-induced secondary emission. In this work, we present an analysis of the Langmuir characteristics obtained in a negative ion beam. We identify and discuss separately the contributions to the collected current given by secondary plasma ions and electrons, stripped electrons, beam ions, and ion-induced secondary electron emission. We present the beam plasma parameters obtained at different beam energies in NIO1.

Atmospheric pressure dielectric barrier discharge involving ion-induced secondary electron emission controlled by dielectric surface charges

Secondary electron emission (SEE) induced by interaction between positive ion and dielectric surface is essential physical process for maintaining sustained discharge and controlling the characteristics of atmospheric pressure dielectric barrier discharge (DBD). In this work, SEE is described based on Auger neutralization theory. Dielectric energy band structure and defect energy level, including hole and electron trap energy levels, are taken into account to build a hypothetical relationship γi = f (σ) between secondary electron emission coefficient γi and surface charge density σ. With the introduction of γi = f (σ) in DBD model, the characteristics of DBD are discussed in both first and steady discharges. It is found that, in steady discharge, γi has higher value during the beginning and development state of discharge pulse of DBD, and γi decreases in extinction stage. The negatively charged dielectric surface enhances SEE before discharge, which benefits the formation of diffuse discharge, compared with DBD of constant γi. Moreover, the increase of energy band gap of dielectric reduces γi, leading to the contractive discharge in DBD. The relationship between γi and σ can be further applied to study the influences of defect energy level parameters on SEE and DBD characteristics.

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.

Angular resolution expected from iCHORD orientation maps through a revisited ion channeling model

Crystalline orientation maps are obtained in a Focused Ion Beam (FIB) microscope using the ion CHanneling ORientation Determination (iCHORD) method, which relies on the channeling phenomenon observed in ion-induced secondary electron images. The current paper focuses on the angular resolution that can be expected from such orientation maps, obtained using a revisited ion channeling model. A specific procedure was developed to evaluate the angular resolution, based on the distribution of orientation errors when evaluating controlled sample disorientation. The main advantage is that no external reference is required. An angular resolution of 1° is obtained on a nickel based sample using standard acquisition conditions. This value fulfills most of the needs in terms of microstructural characterization usually carried out by Electron Back Scattered Diffraction.

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.

Modeling of the Influence of the Thickness of an Insulating Film on a Cathode Surface on its Effective Secondary-Electron Emission Yield in Low-Current Gas Discharge

A model of low-current (Townsend) gas discharge in the presence of a thin insulating film on the surface of a cathode is formulated. It takes into account, along with ion-induced secondary-electron emission from the cathode, also the field emission of electrons from the cathode metal substrate into the film under a strong electric field, which is generated in the insulator when the current flows in the discharge. The emission efficiency of the film and the discharge characteristics are calculated as functions of its thickness. It is shown that the experimentally observed nonmonotonic dependences of the effective secondary-electron emission yield of the cathode and discharge ignition voltage on the film thickness can be explained by the nonuniformity of the electric-field distribution across the film.

The Influence of Secondary Electron Emission and Electron Reflection on a Capacitively Coupled Oxygen Discharge

The one-dimensional object-oriented particle-in-cell Monte Carlo collision code oopd1 is applied to explore the role of secondary electron emission and electron reflection on the properties of the capacitively-coupled oxygen discharge. At low pressure (10 mTorr), drift-ambipolar heating of the electrons dominates within the plasma bulk, while at higher pressure (50 mTorr), stochastic electron heating in the sheath region dominates. Electron reflection has negligible influence on the electron energy probability function and only a slight influence on the electron heating profile and electron density. Including ion-induced secondary electron emission in the discharge model introduces a high energy tail to the electron energy probability function, enhances the electron density, lowers the electronegativity, and increases the effective electron temperature in the plasma bulk.

Ion-induced secondary electron emission from metal surfaces

Using a helium ion hitting various metal surfaces as a model system, we describe a general quantum-kinetic approach for calculating ion-induced secondary electron emission spectra at impact energies where the emission is driven by the internal potential energy of the ion. It is based on an effective model of the Anderson−Newns-type for the subset of electronic states of the ion−surface system most strongly affected by the collision. Central to our approach is a pseudo-particle representation for the electronic configurations of the projectile which enables us, by combining it with two additional auxiliary bosons, to describe in a single Hamiltonian emission channels involving electronic configurations with different internal potential energies. It is thus possible to treat Auger neutralization of the ion on an equal footing with Auger de-excitation of temporarily formed radicals and/or negative ions. From the Dyson equations for the projectile propagators and an approximate evaluation of the self-energies, rate equations are obtained for the probabilities with which the projectile configurations occur and an electron is emitted in the course of the collision. Encouraging numerical results, especially for the helium−tungsten system, indicate the potential of our approach.

Moderate pressure plasma source of nonthermal electrons

Plasma sources of electrons offer control of gas and surface chemistry without the need for complex vacuum systems. The plasma electron source presented here is based on a cold cathode glow discharge (GD) operating in a dc steady state mode in a moderate pressure range of 2–10 torr. Ion-induced secondary electron emission is the source of electrons accelerated to high energies in the cathode sheath potential. The source geometry is a key to the availability and the extraction of the nonthermal portion of the electron population. The source consists of a flat and a cylindrical electrode, 1 mm apart. Our estimates show that the length of the cathode sheath in the plasma source is commensurate (~0.5–1 mm) with the inter-electrode distance so the GD operates in an obstructed regime without a positive column. Estimations of the electron energy relaxation confirm the non-local nature of this GD, hence the nonthermal portion of the electron population is available for extraction outside of the source. The use of a cylindrical anode presents a simple and promising method of extracting the high energy portion of the electron population. Langmuir probe measurements and optical emission spectroscopy confirm the presence of electrons with energies ~15 eV outside of the source. These electrons become available for surface modification and radical production outside of the source. The extraction of the electrons of specific energies by varying the anode geometry opens exciting opportunities for future exploration.

A comparison of He and Ne FIB imaging of cracks in microindented silicon nitride

Helium ion microscopy (HIM) offers potential as a high spatial resolution technique for imaging insulating samples that are susceptible to charging artifacts. In this study helium and neon ion microscopy are used to image cracking in microindented samples of the non-conductive ceramic silicon nitride. The crack morphology of radial cracks emanating from the microindentations has been characterized for two different compositions of silicon nitride, with and without conductive coatings. Gold coating enhances crack edge contrast, but masks grain contrast for both He and Ne ion-induced secondary electron (ISE) imaging. Carbon coating enables the crystalline and glassy phases to be distinguished, more clearly with Ne-ISE, and the cracking pathway is found to be primarily intergranular. Zones of <100 nm diameter depleted ion-induced secondary electron emission along the crack paths are identified, consistent with charging ‘hotspots’.

Maximizing stoichiometry control in reactive sputter deposition of TiO2

Thin films of amorphous TiO2 are grown by direct current (DC) reactive magnetron sputtering. Using modern models of DC reactive sputtering, conditions were established to maximize control of the O:Ti ratio by indirectly monitoring the change in ion-induced secondary electron emission of the Ti target. The Ti resistivity was continuously varied through over 12 orders of magnitude, and changes in stoichiometry were observed by Rutherford backscattering. Combining observed changes in stoichiometry with a predictive model of the composition, a percolative transition could be observed exhibiting universal and nonuniversal scaling parameters.

Note: Ion-induced secondary electron emission from oxidized metal surfaces measured in a particle beam reactor

The secondary electron emission of metals induced by slow ions is characterized in a beam chamber by means of two coaxial semi-cylindrical electrodes with different apertures. The voltages of the outer electrode (screening), inner electrode (collector), and sample holder (target) were set independently in order to measure the effective yield of potential and kinetic electron emissions during ion bombardment. Aluminum samples were exposed to quantified beams of argon ions up to 2000 eV and to oxygen atoms and molecules in order to mimic the plasma-surface interactions on metallic targets during reactive sputtering. The variation of electron emission yield was correlated to the ion energy and to the oxidation state of Al surfaces. This system provides reliable measurements of the electron yields in real time and is of great utility to explore the fundamental surface processes during target poisoning occurring in reactive magnetron sputtering applications.

The potential link between high angle grain boundary morphology and grain boundary deformation in a nickel-based superalloy

Focused ion beam (FIB) based serial sectioning was utilized to characterize the morphology of two high angle grain boundaries (HAGB) in a nickel based superalloy, one that experienced grain boundary sliding (GBS) and the other experienced strain accumulation, during elevated temperature constant stress loading conditions. A custom script was utilized to serial section and collect ion-induced secondary electron images from the FIB-SEM system. The MATLAB based MIPARTM software was utilized to align, segment and reconstruct 3D volumes from the sectioned images. Analysis of the 3D data indicates that the HAGB that exhibited GBS had microscale curvature that was planar in nature, and local serrations on the order of ±150nm. In contrast, the HAGB that exhibited strain accumulation was not planar and had local serrations an order of magnitude greater than the other grain boundary. It is hypothesized that the serrations and the local grain boundary network are key factors in determining which grain boundaries experience GBS during creep deformation.

Ion-induced secondary electron emission, optical and hydration resistant behavior of MgO, Mg–Mo–O and Mg–Ce–O thin films

Optical transmittance, hydration resistance and secondary electron emission characteristics of e-beam evaporated pure and Mo- or Ce-containing MgO thin films have been investigated. While the increased grain size and pyramidal columnar morphology following incorporation of molybdenum and cerium in MgO are responsible for the excellent discharge characteristics, emergence of neutral {100} and {110} MgO surfaces preferentially give rise to high optical transmittance (~92–100%) and stability against hydration. Further, addition of Mo (or Ce) in MgO causes significant increase in defect density which, in turn, enhances the photoluminescence (PL) emission from 5-, 4- and 3-coordination sites. The changes lead to lowering of the breakdown voltage and hence improvement in the secondary electron emission (SEE) efficiency. These facts have been supported by ion-induced SEE yield (γ) deduced from the a.c. breakdown voltage observed, taking neon as a discharge gas, and determined semi-empirically as well with Hagstrum's theory based on Auger neutralization process using (i) band offset parameters and surface band gap data derived from X-ray photoelectron spectroscopy signal and (ii) information of defect energy levels obtained from photoluminescence (PL) measurements. The experimental values of neon ion-induced SEE yield (γ) are found to be 0.35, 0.42 and 0.39 for MgO, Mg–Mo–O (x=0.035) and Mg–Ce–O (x=0.01) thin films, respectively.

Surface behavior based on ion-induced secondary electron emission from semi-insulating materials in breakdown evolution

This study focuses on analyses of secondary electron emission (SEE) at semiconductor surfaces when the sufficient conditions of space–time distribution occur. Experimental measurements and calculations with the approach of Townsend coefficients, which include the evaluations of ionization coefficient (α) and SEE coefficient (γ) were performed in high-ohmic InP, GaAs, and Si semiconductor cathodes with argon and air environments in a wide range of E/N (300–10 000 Td). The direct calculations of γ were carried out to determine the behavior of cold-semiconductor cathode current in a wide range of microgaps (45–525 μm). Paschen curves are interpreted in the dependence of large pd range on breakdown voltage through γ and α/N. Ion-induced secondary electrons exhibit the direct behaviors affecting the timescale of breakdown evolution in the vicinity of the Paschen minimum during the natural bombardment process with ions of semiconductor cathodes. Also, when α/N rapidly drops and the excitations of gas atoms densely occupy the gas volume, we determined that the photoelectric effect provides a growth for electron emission from semiconductor surfaces at the breakdown stage at the reduced values of E/N. At all pressures, the emission magnitudes of electrons liberated by semiconductor cathodes into vacuum are found as γInP > γGaAs > γSi in breakdown evolution.

Electron emission and surface etching by slow and medium highly charged ions on HOPG surface

Highly charged (129Xeq+, q=10–30) ion-induced secondary electron emission and formation of nanostructure on the surface of highly oriented pyrolytic graphite (HOPG) have been studied on the 320kV ECR platform for Highly Charged Ion Beam at IMP-CAS, Lanzhou. The experimental data of the total secondary electron yields are used to separate contributions of kinetic and potential electron yields. The estimated kinetic electron yields are about 17 and 35 electron/ion by ions with kinetic energies of 600keV and 5MeV, respectively. The potential electron yield increases linearly with increasing potential energy of the incident ion. The total electron yield, however, does not depend significantly on kinetic energy. AFM analysis of the HOPG surface bombarded with Xe30+ ions revealed regular hillock like nanostructure. The relationship between hillock size (height and width) and ion’s energy (including potential energy and kinetic energy) shows the same trend with the total electron yield which increases with potential energy and is independent on incident kinetic energy.

Understanding image contrast to optimize procedures for focused ion beam contact level circuit editing

Contact level focused ion beam (FIB) circuit editing (CE) is a powerful methodology to realize full thickness back side CEs with high success rates. Tungsten contacts present themselves as dark circles surrounded by bright rings and extended halos in the ion-induced secondary electron (iSE) images from the FIB. Similarly, polysilicide gates appear as gray bars surrounded by a brighter perimeter. The iSE images are compared to electron-induced secondary electron images from a scanning electron microscope. Elemental data from energy dispersive spectroscopy and topographical data from atomic force microscopy (AFM) evince that topography rather than conductivity plays a determinant role in the contrast in the iSE images. A model emerges whereby the Ga+ ion beam erodes the dielectrics more quickly than the W contacts and polysilicide gate liners, leaving the W contacts and polysilicide gate liners with rounded sidewalls protruding from the floor of the FIB trench. The high yield of secondary electrons from the sidewalls of protruding structures dominates image formation and follows the secant law with a modified exponent. Topographical data from the AFM combined with the secant law suffice to simulate the main features of iSE images although a complete interpretation of the polysilicide gates mandates considering the conductivity of the materials as well.

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.

On Explanation of the Double-Valued Paschen-Like Curve for RF Breakdown in Argon

This paper represents an investigation of the dependence of the breakdown voltage on the gas pressure in radio-frequency argon discharges under conditions when ion-induced secondary electron production is negligible. Calculations were performed by using a Monte Carlo collision code including electrons only. Our simulation results clearly show a region, occurring at low pressure, where multiple values of the breakdown voltage exist at a given pressure, in agreement with previous experimental observations. The two different regimes of operation, each satisfying the breakdown condition, may be best analyzed in contour plots of electron density, ionization rate, and mean energy.

Ion-induced secondary electron emission from ZnS thin films deposited by closed-spaced sublimation

ZnS thin films were deposited on soda lime glass and aluminum substrates by close-spaced sublimation technique. The change in composition, structural and optical properties of the films was investigated as a function of the substrate temperature. The deposited films were stoichiometric and crystalline in nature having cubic structure oriented only along (1 1 1) plane. The energy band gap of the films deposited at the substrate temperature of 150, 250 and 350 °C was 3.52, 3.58 and 3.63 eV respectively. These films were then bombarded with 2–10 keV energy pulsed Ar + beam and their electron yield was determined from impinging ion and emitted electron currents. The electron yield of ZnS films was much high as compared to the metals. The electron yield of ZnS films increased with energy of the incident ion and got saturated at about 8 keV. The most important result of this study was that the electron yield of ZnS films having same composition was different. Monte Carlo simulations performed to interpret these experimental findings showed that the dissimilar electron yields of ZnS films is due to the combined effect of energy band gap, surface barrier potential and density of the films.