Energy Distribution Of Secondary Ions Research Articles

Space Charge Compensation Study of High Intensity Ion Beams

The space charge effect is one of the key factors affecting ion beam transport and ion beam quality, especially for low-energy high-intensity ion beams. It can be partially compensated by the secondary electrons produced from the ionization of residual gas molecules in the beam pipe, and we are interested in the accurate value of this compensation degree. In this paper, we use a Three-grid Energy Analyzer (TEA) to measure the secondary ion energy distribution of the beam, which corresponds to the compensated beam potential distribution. To have accurate evaluation of the compensation degree, it is essential to adopt a proper transverse distribution of the ion beam, other than using the Gaussian theorem for uniform beams. The results of a space charge compensation degree study for singly charged ion beams with a 2.45GHz microwave ion source and for multiply charged ion beams with a high charge state ECR ion source are presented.

New SIMS method to characterize hydrogen in polysilicon films

A new analysis protocol for profiling the hydrogen (H) concentration depth distributions in polysilicon (poly-Si) thin films on Si oxide was developed by using secondary ion mass spectrometry (SIMS). Traditional SIMS determination of H concentrations in poly-Si films is to monitor and collect the atomic H−, but this analysis method presents some challenges for providing stable and repeatable H depth profiles, especially, for shallow profiling in thinner poly-Si films on thicker Si oxide films. In this paper, we present a thorough study of the SIMS characterization of H levels in poly-Si thin films on Si oxide. We describe an approach for the use of cluster SiH− instead of atomic H− to determine the H concentrations. The utilization of the SiH− cluster ion minimizes both H adsorption effects on the film surface regions and sample surface charging effects arising from the Si oxide layer underneath, thus enhancing the secondary ion signal stability in poly-Si films, as discussed with secondary ion energy distribution on the samples. This method differs from the conventional SIMS analysis of H in thin films and significantly improves the data quality and accuracy. Fourier transform infrared spectroscopy analysis was used to study the Si–H chemical bonding information for investigating the nature of the SIMS collected SiH− cluster ions. Both the H implant sample with no significant Si–H bonding and the poly-Si samples with high levels of H and the weak Si–H bonding exhibited strong SiH− secondary cluster ion intensities in our studies. This indicates the cluster ion signals arise from either the combination of Si and H ions through the physics formation process or weak chemically bonded Si and H.

A multi-detector experimental setup for the study of space radiation shielding materials: Measurement of secondary radiation behind thick shielding and assessment of its radiobiological effect

Space agencies have recognized the risks of astronauts’ exposure to space radiation and are developing complex model-based risk mitigation strategies. In the foundation of these models, there are still significant gaps of knowledge concerning nuclear fragmentation reactions which need to be addressed by ground-based experiments. There is a lack of data on neutron and light ion production by heavy ions, which are an important component of galactic cosmic radiation (GCR). A research collaboration has been set up to characterize the secondary radiation field produced by GCR-like radiation provided by a particle accelerator in thick shielding. The aim is to develop a novel method for producing high-quality experimental data on neutron and light ion production in shielding materials relevant for space radiation protection. Four complementary detector systems are used to determine the energy and angular distributions of high-energy secondary neutrons and light ions. In addition to the physical measurement approach, the biological effectiveness of the secondary radiation field is determined by measuring chromosome aberrations in human peripheral lymphocytes placed behind the shielding. The experiments are performed at the heavy ion

Properties of secondary ions in ion beam sputtering of Ga2O3

The energy distributions of secondary ions for the ion beam sputtering of a Ga2O3 target using O2+ and Ar+ ions are measured in dependence on various process parameters using energy-selective mass spectrometry. The process parameters include sputtering geometry (ion incidence angle α, polar emission angle β, scattering angle γ), the energy of incident ions Eion, and the background pressure of O2. The main secondary ion species are identified to be Ga+, O+, O2+, and, when argon is used as a process gas, Ar+. The changes in the sputtering geometry and the primary ion energy have the most impact on the energy distributions of secondary Ga+ and O+ ions, giving control over the high-energy tail, which is attributed to anisotropy effects in sputtering. The formation of O2+ ions is attributed to collisions with background gas molecules, as their energy distributions are not influenced by the sputtering geometry or the primary ion energy. The increase of the O2 pressure leads to a minor decrease of the energy of Ga+ ions due to collisions with the background gas particles. The use of primary Ar+ ions with O2 background pressure does not show any specific effect on energy distributions of Ga+, O+, and O2+ ions except for the case without additional O2 background. In the latter case, much fewer O+ and O2+ ions are produced indicative of oxygen depletion of the surface due to preferential sputtering of oxygen. At all considered O2 pressures, the energy distributions of Ar+ ions have a high-energy peak, attributed to direct scattering events. The trends in experimental data show qualitative agreement to simulations using the Monte Carlo code SDTrimSP.

A reassessment of the quasi-simultaneous arrival effect in secondary ion mass spectrometry

Quasi-simultaneous arrival (QSA) effects in secondary ion mass spectrometry can create mass-independent inaccuracies in isotope measurements when using electron multiplier detectors (EMs). The simple Poisson statistical model of QSA does not explain most experimental data. We present pulse-height distributions (PHDs) and time-series measurements to better study QSA. Our data show that PHDs and the distribution of multiple arrivals on the EM are not consistent with the Poisson model. Multiple arrivals are over-dispersed compared to Poisson and are closer to a negative binomial distribution. Through an emission–transmission–detection model we show that the QSA correction depends on the non-Poisson emission of multiple secondary ions, the secondary ion energy distribution, and other factors, making an analytical correction impractical. A standards-based correction for QSA is the best approach, and we show the proper way to calculate standards-normalized δ values to minimize the effect of QSA.

Energy distributions of secondary ions for the Ar ion beam sputtering of indium tin oxide

The energy distributions of secondary ions for the Ar ion beam sputtering of indium tin oxide were measured in dependence on geometric parameters (ion incidence angle, polar emission angle, scattering angle), ion energy, and O2 background pressure using energy-selective mass spectrometry. The most prevalent ion species were identified to be O+, O2+, Ar+, In+, and Sn+. The energy distributions of O+, In+, and Sn+ ions show a low-energy maximum between 10 and 20 eV, followed by a power-law decay if the scattering angle is γ>90°. If γ<90°, an additional high-energy structure evolves, which is assigned to anisotropy effects, namely, directly sputtered particles. The energy distributions of the Ar+ ions show a low-energy maximum and, in dependence on the scattering angle, up to two additional high-energy structures, which are also assigned to anisotropy effects. Here it is related to direct scattering events. All additional structures show systematic correlations with scattering angle and ion energy. The energy distributions of the O2+ ions exhibit a low-energy maximum followed by a sudden signal drop. There is almost no variation with scattering angle or ion energy. In general, increasing the O2 background pressure results in a decrease of the particle energy due to an energy loss upon interaction with background gas particles. The experimental results are compared and discussed with calculations based on elastic two-particle collision theory and using srim, and Monte Carlo simulations using SDTrimSP.

Secondary particle properties for the ion beam sputtering of TiO2 in a reactive oxygen atmosphere

This work deals with the properties of secondary particles, i.e., sputtered target and scattered primary particles, for the ion beam sputtering of TiO2 in a reactive oxygen atmosphere. The angular and energy distributions in dependence on ion beam parameters (ion energy, ion species, or ion mass) and geometrical parameters (ion incidence angle, polar emission angle, or scattering angle) were investigated. The angular distributions of sputtered Ti particles were determined by the collection method, i.e., by growing TiO2 thin films and measuring the thickness, composition, and mass density. The distributions can be described by the superposition of an isotropic and an anisotropic part. The anisotropic part relative to the isotropic part gets more pronounced with decreasing ion energy, increasing ion incidence angle or changing the process gas from Ar to Xe. An energy-selective mass spectrometer was used to measure mass and energy distributions of secondary ions. The most prevalent species were identified to be O+, O2+, Ti+, TiO+, Ar+, and Xe+ ions. The energy distributions of Ti+ and TiO+ ions show a low-energy maximum between 10 eV and 30 eV, followed by an exponential decay. When decreasing the scattering angle, the high-energy tail extends to higher energies. O+, O2+, Ar+, and Xe+ ions show a low-energy maximum between 5 eV and 20 eV, which is followed by a sudden signal drop for scattering angles larger than 90°. For scattering angles smaller than 90°, additional peak(s) at higher energy appear, which are assigned to direct scattering and/or direct sputtering events. The experimental results are compared to calculations based on elastic collision theory and to simulations done with the Monte Carlo code SDTrimSP. The results are also discussed in relation to secondary particle properties for the reactive ion beam sputtering of Ti and to properties of TiO2 thin films grown by reactive ion beam sputter deposition with the same setup and the same parameters. Among others, it was found that sputtering of the TiO2 target exhibits less anisotropy contributions than sputtering of the Ti target.

Internal Energy Distribution of Secondary Ions Under Argon and Bismuth Cluster Bombardments: "Soft" Versus "Hard" Desorption-Ionization Process.

The emission/ionization process under massive argon cluster bombardment was investigated by measuring the internal energy distributions of a series of benzylpyridinium ions. Argon clusters with kinetic energies between 10 and 20keV and cluster sizes ranging from 500 to 10,000 were used to establish the influence of their size, energy, and velocity on the internal energy distribution of the secondary ions. It is shown that the internal energy distribution of secondary ions principally depends on the energy per atom or the velocity of the cluster ion beam (E/n ∝ v2). Under low energy per atom (E/n˂10eV), the mean internal energy and fragmentation yield increase rapidly with the incident energy of individual constituents. Beyond 10eV/atom impact (up to 40eV/atom), the internal energy reaches a plateau and remains constant. Results were compared with those generated from bismuth cluster impacts for which the mean internal energies correspond well to the plateau values for argon clusters. However, a significant difference was found between argon and bismuth clusters concerning the damage or disappearance cross section. A 20 times smaller disappearance cross section was measured under 20keV Ar2000+ impact compared to 25keV Bi5+ bombardment, thus quantitatively showing the low damage effect of large argon clusters for almost the same molecular ion yield. Graphical Abstract ᅟ.

Reactive ion beam sputtering of Ti: Influence of process parameters on angular and energy distribution of sputtered and backscattered particles

This work focuses on the angular and energy distribution of secondary particles during the ion beam sputtering of a Ti target in a reactive oxygen atmosphere. The influence of ion species, ion energy, and scattering geometry (ion incidence angle and polar emission angle) was investigated systematically. Ion energies of 0.5, 1.0, and 1.5 keV and an ion current of roughly 10 mA were used to sputter deposit TiO2 films from a pure Ti target at an operating pressure of 5.0 × 10−5 and 6.5 × 10−5 mbar and an oxygen partial pressure of 1.0 × 10−5 and 2.5 × 10−5 mbar for sputtering with Ar and Xe ions, respectively. The angular distribution of the flux of sputtered Ti particles was determined by measuring the thickness of TiO2 films. The flux of sputtered Ti particles is described by the superposition of an isotropic and anisotropic part. The isotropic part increases when increasing ion energy, decreasing the incidence angle, or changing the sputtering gas from Xe to Ar. An energy-selective mass spectrometer was used to measure the mass and energy distribution of secondary ions. Several species of sputtered particles were detected, e.g., Ti, TiO, TiO2, Ti2, O, and O2 ions. The most prevalent species are Ti, TiO, and O ions. The energy distribution of the sputtered Ti and TiO ions shows a maximum at an energy between 10 and 30 eV followed by a high-energetic tail proportional to exp(−n·E). Decreasing the scattering angle leads to a decrease in the slope of the high-energetic tail, i.e., it extends to higher energies. The backscattered primary ions (Ar and Xe) and the sputtered atomic O ions show a maximum between 5 and 20 eV, which is followed by a sudden signal drop. For scattering angles less than 90°, additional peaks at higher energy can appear in the energy distribution of Ar, Xe, and O ions. These peaks are related to direct scattering and sputtering events. The experimental results are compared with calculations based on a simple elastic two-particle-interaction theory and with simulations carried out using the Monte Carlo code SDTrimSP.

Preliminary Measurement of Space Charge Neutralization Level for Proton Beam

For high-intensity low-energy ion beams, space charge effect is a main cause of beam divergence and emittance growth. Fortunately, residual gas molecules in the drift space tend to be ionized and neutralize the beam space charge spontaneously. The level of Space Charge Neutralization (SCN) is measured through the detection of created secondary ion energy distribution in the beam region. A so-called non-interceptive Three-grid Energy Analyzer (TEA) has been designed and manufactured at Institute of Modern Physics, Chinese Academy of Sciences (IMP). This paper will present the details of the TEA detector and the application to diagnose proton beam SCN level in the Low Energy Beam Transport (LEBT) line. As a preliminary result, for an 18.5 mA proton beam a best compensating point appears at the vacuum pressure of 1.510*3 Pa. And the neutralization level is advanced with the growth of beam current in a constant vacuum pressure.

The lunar surface-exosphere connection: Measurement of secondary-ions from Apollo soils

Low energy secondary ions ejected by the solar wind are an important component of tenuous exospheres surrounding airless bodies, since these ions carry information on the planetary surface composition. In this work we examine the dependence of secondary-ion abundance, as a function of energy and mass, on surface composition. The surface compositions of two Apollo soils (10084 and 62231) and a synthetic Corning glass lunar simulant were measured with X-ray photoelectron spectroscopy and correlated with the spectra of secondary-ions ejected from the same soils by 4keV He ions. XPS spectra for lunar soils show that the surface compositions are similar to the bulk, but enriched in Fe and O, while depleted in Mg and Ca. 4keV He irradiation on the lunar soils and a glass simulant preferentially removes O and Si, enriching the surface in Al, Ti, Mg, and Ca. Secondary-ion species ejected from the Apollo soils by 4keV He include: Na+, Mg+, Al+, Si+, Ca+, Ca++, Ti+, Fe+, and molecular species: NaO+, MgO+ and SiO+. Secondary ion energy distributions for lunar soil 10084 and 62231 rise rapidly, reach a maxima at ∼5eV for molecular ions and Na+, ∼7.5eV for Fe+, and ∼10eV for Mg+, Al+, Si+, Ca+ and Ti+, then decrease slowly with energy. We present species-dependent relative conversion factors for the derivation of atomic surface composition from secondary-ion count rates for 4keV He irradiation of lunar soils 10084 and 62231, as well as the Corning glass lunar simulant.

Secondary particle emission from semiconductor crystals

Sputtering and secondary ion emission (SIE) from semiconductors with different values of the band gap have been studied experimentally and by means of computer simulation. The particular oscillations of secondary ion energy distribution for graphite, sapphire, and silicon single crystals were observed under irradiation by 1 and 10 keV Ar+ ions. This result cannot be interpreted using existing theories of SIE. We have explained them by the interplay of the charge-exchange processes between emitted and incident particles. Comparing the evidence from our experiments and computer simulation, we have drawn the conclusion that secondary ions emerge as a result of charge exchange with Ar+ ions not only when exiting the target, but also in the bulk. This is very different from what happens in metals, and needs to be taken into account when describing the processes in SIE, as well as in practical applications in mass spectrometry of secondary ions.

MCsn+-SIMS: An Innovative Approach for Direct Compositional Analysis of Materials without Standards

This review primarily deals with the compensation of ‘matrix effect’ in secondary ion mass spectrometry (SIMS) for direct quantitative analysis of materials using MCs+-SIMS approach. Emphasis has been given on exploring the formation mechanisms of MCsn+ (n = 1, 2,..) molecular ions (M denotes the element to be analyzed and Cs+ is the bombarding ion) emitted in the SIMS process. Following a brief introduction on SIMS, a study on MCsn+ molecular ions emitted from various metal targets under Cs+ primary bombardment has been discussed. These studies essentially concentrate on the secondary ion energy distributions of the emitted species under different bombarding energies and impact angles. We have discussed a systematic study on the effect of reactive species like oxygen and cesium on the emission of MCsn+ molecular ions. In all cases we have estimated the changes in local surface work function from the leading parts of the secondary ion energy distributions. The observations are explained in the light of surface work function changes at the sputtering sites. Formation mechanisms of MCsn+ molecular ions are explained in the framework of sputter-ion emission models. Suppression of matrix effects in the compositional analysis of molecular beam epitaxy (MBE) - grown SiGe alloy structures has been discussed in detail. We have essentially applied the MCs+-SIMS method for the minimization of ‘matrix effect’, which is routinely encountered in conventional SIMS analysis hindering quantification of materials. Under the impact of energetic Cs+ ions, both SiCs+ and GeCs+ ions have been monitored for SiGe alloy samples with Ge concentration between 0 and 72 at.%. Following optimization of various experimental parameters, we have shown that the matrix effects are practically suppressed for Ge content up to 72 at.% in SiGe samples. A procedure for the accurate quantification of Ge concentration in Si1−xGex alloys using this method has been presented. This novel methodology has successfully been applied in the depth profiling of Si/Ge multilayer structures grown by MBE technique.

Secondary emission of carbon ions from graphite nanocrystallites

Emission of secondary C+ ions from the surface of well-oriented graphite nanocrystallites irradiated with 10 keV Ar+ ions has been studied. The energy distribution of emitted secondary ions has been measured. It has been found that the form of energy distribution and the energy corresponding to the maximum of distribution Emax depend on the polar angle θ of C+ ion emission. The linear dependence of energy in the maximum of energy spectrum of single-charged secondar y carbon ions on the polar angle of their emission Emax(θ) has been obtained. The mechanism responsible for the observed features of the energy spectra is discussed.

Ionization probability of sputtered particles as a function of their energy: Part II. Positive Si + ions

In this paper we represent the experimental ionization probability of sputtered silicon atoms as a function of their energy, which has been obtained for positive Si + ions sputtered from silicon by O 2 + ion beam. To explain the experimental data, we have considered ionization of an outgoing atom at a critical distance from the surface, which occurs due to the electron transition between this atom and the surface, and suggested the formation of a local surface charge with the polarity opposite to that of the outgoing ion that has just been formed. Then we have considered the interaction between those two charges, outgoing ion, and surface charge as a process of the particle passage through a spherical potential barrier; as a result, we have obtained the theoretical energy distribution of secondary ions. Together with the well-known Sigmund–Thompson energy distribution of sputtered atoms, the obtained ion energy distribution allowed us to derive the equation for the secondary ion yield versus the sputtered particle energy. Both equations derived have exhibited a quite good correlation with our experimental results and also with a large number of published experimental data.

Electron work function decrease in SIMS analysis induced by neutral cesium deposition

The negative secondary ion yields in secondary ion mass spectrometry (SIMS) increase when electropositive elements, especially alkali metals, are used as primary ions. In previous papers by the same authors, useful yield variations of several elements have been studied experimentally with respect to the neutral cesium deposition conditions. Besides, the Cs surface concentrations have been simulated using the TRIDYN code. The determined secondary ion sensitivities have been discussed with respect to the experimental conditions and they have been compared to the electron-tunneling model describing ion emission from metallic and semi-conducting samples. In this paper, the variations of the electron work function of the sample will be studied with respect to the experimental conditions used in the previous papers. The energy distributions of negative secondary ions will be recorded for the different experimental conditions. The electron work function shift, on which is based the electron-tunneling model and which thus gives evidence for the influence of cesium on ion emission, is extracted from these distributions. The variations of the electron work function are discussed with respect to the experimental conditions as well as the simulated cesium surface concentration. Besides, the secondary ion sensitivities are plotted with respect to the electron work function, giving a direct comparison with the electron-tunneling model.

Secondary ion emission from Ti and Si targets induced by medium energy Ar + ion bombardment – Experiment and computer simulation

The paper presents experimental results of secondary ion energy distributions obtained for Ti and Si targets bombarded by 20–30 keV monoisotope Ar + ion beam. The influence of the extraction voltages between target and a slit of the electrostatic energy analyzer entrance on the energy distributions of secondary ions was investigated. After optimization of the secondary ion extraction system, the mass spectra of secondary ions were also measured. The investigations were done using recently built experimental system. Experimental data are compared with the computer simulation results obtained using TRQR and SATVAL codes.

Resistivity analysis of epitaxially grown, doped semiconductors using energy dependent secondary ion mass spectroscopy

A characterization technique is discussed that allows quantitative optimization of doping in epitaxially grown semiconductors. This technique uses relative changes in the host atom secondary ion (HASI) energy distribution from secondary ion mass spectroscopy (SIMS) to indicate relative changes in conductivity of the material. Since SIMS is a destructive process due to sputtering through a film, a depth profile of the energy distribution of sputtered HASIs in a matrix will contain information on the conductivity of the layers of the film as a function of depth. This process is demonstrated with Mg-doped GaN, with the Mg flux slowly increased through the film. Three distinct regions of conductivity were observed: one with Mg concentration high enough to cause compensation and thus high resistivity, a second with moderate Mg concentration and low resistivity, and a third with little to no Mg doping, causing high resistivity due to the lack of free carriers. During SIMS analysis of the first region, the energy distributions of sputtered Ga HASIs were fairly uniform and unchanging for a Mg flux above the saturation, or compensation, limit. For the second region, the Ga HASI energy distributions shifted and went through a region of inconsistent energy distributions for Mg flux slightly below the critical flux for saturation, or compensation. Finally, for the third region, the Ga HASI energy distributions then settled back into another fairly unchanging, uniform pattern. These three distinct regions were analyzed further through growth of Mg-doped step profiles and bulk growth of material at representative Mg fluxes. The materials grown at the two unchanging, uniform regions of the energy distributions yielded highly resistive material due to too high of Mg concentration and low to no Mg concentration, respectively. However, material grown in the transient energy distribution region with Mg concentration between that of the two highly resistive regions yielded low resistivity (0.59Ωcm) and highly p-type (1.2×1018cm−3 holes) Mg-doped GaN.

Influence of changes in the resistivity of the sample surface on ultra-shallow SIMS profiles for arsenic

High concentration dopant distributions in silicon like those required to form ultra shallow junctions can affect SIMS analyses introducing matrix effects on secondary ions, otherwise not observed in more dilute regimes. In this work the effect of high arsenic concentration on negative secondary ion yield has been investigated when sputtering with a 0.5 keV Cs + primary beam in a magnetic sector instrument. Two effects have been distinguished: the first one is a variation of matrix signals observed on as implanted samples in correspondence with the projected range, probably due to the damage induced by the high dose implant; the second is a step observed for several species and related to the arsenic distribution. The latter seems due to the effect of the active dopant on secondary ion energy distributions. A wide and well-centred energy slit can limit this effect allowing a more accurate quantification of the activated high As concentrations.

Track electrostatic model for describing secondary ion emission of insulators

A model based on the nuclear track potential is described and used to predict trajectories and the energy distribution of secondary ions emitted by insulating targets. In this model, the electric field generated by each track pushes away the secondary ions formed on the target surface. The effects on the potential due to target thickness, track charge density, projectile incidence angle and secondary ion mass are analyzed. Predictions are compared with experimental data existing in the literature. It is found that the proposed model describes partially the behavior observed in the angular distribution of the emitted ions and new processes are proposed to be included in the model.