Recoiling Target Atoms Research Articles

A generic algorithm for Monte Carlo simulation of proton transport

A mixed (class II) algorithm for Monte Carlo simulation of the transport of protons, and other heavy charged particles, in matter is presented. The emphasis is on the electromagnetic interactions (elastic and inelastic collisions) which are simulated using strategies similar to those employed in the electron–photon code penelope. Elastic collisions are described in terms of numerical differential cross sections (DCSs) in the center-of-mass frame, calculated from the eikonal approximation with the Dirac–Hartree–Fock–Slater atomic potential. The polar scattering angle is sampled by employing an adaptive numerical algorithm which allows control of interpolation errors. The energy transferred to the recoiling target atoms (nuclear stopping) is consistently described by transformation to the laboratory frame. Inelastic collisions are simulated from DCSs based on the plane–wave Born approximation (PWBA), making use of the Sternheimer–Liljequist model of the generalized oscillator strength, with parameters adjusted to reproduce (1) the electronic stopping power read from the input file, and (2) the total cross sections for impact ionization of inner subshells. The latter were calculated from the PWBA including screening and Coulomb corrections. This approach provides quite a realistic description of the energy-loss distribution in single collisions, and of the emission of X-rays induced by proton impact. The simulation algorithm can be readily modified to include nuclear reactions, when the corresponding cross sections and emission probabilities are available, and bremsstrahlung emission.

Monte Carlo study of the ion-induced electron current tunneling through a metal–insulator–metal junction

A Monte Carlo program is developed to investigate the kinetically excited electrons passing through a realistic Ag–Al2O3–Al junction when Ar+ ions impact on the top Ag layer. The program includes excitation of the target electrons (by projectile ions, recoiling target atoms and fast primary electrons) and subsequent transport of these excited electrons from Ag to bottom Al layer of the metal–insulator–metal (MIM) junction. The calculated tunneling electron yield is consistent with the recently reported experimental results. The simulation, however, enables the calculation of partial tunneling electron yields of the electrons excited by the projectile ions, recoil atoms and cascade electrons, the depth distribution of the electron excitation points in the MIM junction and energy distribution of the tunneling electrons. Our calculation showed that the electrons excited by fast cascade electrons are the major contributor to the tunneling electron yield while the direct contribution of projectile ions to tunneling electron yield is evident only at the projectile energies greater than 10keV. The tunneling electrons have their origin close to the bottom end of the Ag layer and bulk of the tunneling electrons have energies around 2eV.

Secondary electron emission in scanning Ga ion, He ion and electron microscopes

The secondary electron (SE) emission for Ga ion, He ion and electron impact have been calculated using Monte-Carlo simulations, in which the trajectories of all the collision partners (i.e., primary ions, recoiled target atoms, and excited electrons (electron cascade) along which SEs are excited) have been simulated. The SE yields for Ga ion impact show a gradual decrease with increasing Z 2 which is opposite to that found for electron impact. The main reasons for the decrease in SE yield for Ga are the decrease in number and energy of electrons excited by the primary ions and the decrease in contribution of the recoiled atoms to the SE yield with increasing Z 2. For electron impact, both primary electrons and backscattered electrons (BSEs) excite the SEs. The additional SE excitation created by the electron cascade by BSEs is enhanced for high- Z 2 metals especially at E > a few keV. For He ion impact, the Z 2-dependence is between that for the Ga ions and the electrons and is weak because the He ion is light but still much heavier than an electron. As to the lateral resolution, the electron excitations by trapped He ions dominate the SE yield, so that the SE excitation volume is narrower than for electron and Ga impacts. This small contribution of BS He ions to the SE yield does not increase the information depth determined by the trapped He ions, in contrast with the large contribution of BSEs to the SE yields for SEM imaging. The simulated incident-angle dependence in SE yields shows that the topography contrast for He-SIM imaging is clearer than that obtained by SEM and Ga-SIM imaging.

Monte Carlo study of ion-induced secondary electron emission from SiO2

Here we describe a recently developed direct Monte Carlo program to study kinetic electron emission from SiO2 target. The program includes excitation of the target electrons (by projectile ions, recoiling target atoms and fast primary electrons), subsequent transport and escape of these electrons from the target surface. The program can be used to calculate the electron yields, distribution of electron excitation points in the target and other physical parameters of the emitted electrons. In order to demonstrate the capabilities of this program, we report a study on the kinetic electron emission from SiO2 induced by fast (1–10keV) rare gas ions. The calculated kinetic electron yield for various ion energies and masses is in good agreement with the predictions of most frequently applied theoretical model. In addition, the effects of projectile energy, mass and impact angle on the depth distribution of electron excitation points and average escape depth of the outgoing electrons were investigated. It is important to mention that the existing experimental techniques are not capable to measure these parameters.

Ion-induced secondary electron emission from MgO and Y 2O 3 thin films

We report a detailed study of the electron emission from MgO and Y 2O 3 induced by the impact of 0.1–1 keV Ar + ions. The mechanisms of ion-induced secondary electron emission from oxides are far less understood because charging of the target surface during ion irradiation prohibits the precise measurement of electron yield. For this study, targets were prepared by depositing 20 nm thick films of MgO and Y 2O 3 on the semi-conducting SnO 2 substrate, which helps in charge neutralization. Additionally, a pulsed ion beam was used to further reduce the surface charging. It was found that the electron yield of both targets increases with energy of the ion. However, at a given ion energy the electron yield of Y 2O 3 was larger than MgO. Another important result of this study is that the electron emission from these large band gap insulators did not show any threshold effect, in contrast to the metal targets. It may be due to local reduction of the band gap through electron promotion processes. In addition, a Monte Carlo program was used to calculate the yield of secondary electrons excited by projectile ions, recoiling target atoms and electron cascades, and average escape depth of the secondary electrons emitted from the MgO and Y 2O 3 thin films.

Monte Carlo simulation of ion-induced secondary electron emission from MgO film

Kinetic electron emission from MgO film under the impact of 50–300 eV rare-gas ions has been investigated by Monte Carlo simulation. The program includes excitation of the target electrons (by projectile ions, recoiling target atoms and fast primary electrons), subsequent transport and escape of these electrons from the target surface. Potential electron yields due to Auger neutralization of various ions are obtained by comparing the calculated kinetic electron yield with the experimental data. The program generates partial yields of the secondary electrons excited by projectile ions, recoiling target atoms and electron cascades. The partial yields of the electrons excited by projectile ions and electron cascades fluctuate with mass of the projectile ion like the electronic stopping. Moreover, lateral and depth distribution of the electrons excited by three excitation processes were quite different. For the range of investigated ion energies, the average electron escape depth was independent of the mass of the incident ion, which indicates that electron escape depth is a material property.

Monte Carlo study of ion-induced backward and forward secondary electron emission from thin Al foil

A direct Monte Carlo program has been developed to calculate the backward (γb) and forward (γf) electron emission yields from 20nm thick Al foil for impact of C+, Al+, Ar+, Cu+ and Kr+ ions having energies in the range of 0.1–10keV/amu. The program incorporates the excitation of target electrons by projectile ions, recoiling target atoms and fast primary electrons. The program can be used to calculate the electron yields, distribution of electron excitation points in the target and other physical parameters of the emitted electrons. The calculated backward electron emission yield and the Meckbach factor R=γf/γb are compared with the available experimental data, and a good agreement is found. In addition, the effect of projectile energy and mass on the longitudinal and lateral distribution of the excitation points of the electrons emitted from front and back of Al target has been investigated.

Simulation Study on Image Contrast and Spatial Resolution in Helium Ion Microscope

We performed Monte Carlo simulation of helium (He) ion induced secondary electron (SE) emission in order to compare the secondary electron image characteristics between He and gallium (Ga) scanning ion microscopes (SIM) and scanning electron microscope (SEM). For 10-50 keV He ion bombardment SE yield increases gradually with increasing the atomic number, Z2, of the target, as well as for the electron bombardment. However, for 30 keV Ga ion bombardment, SE yield shows an opposite Z2 dependence. The calculated SE yield is much larger than that for both electron and Ga ion bombardment. The incident angle dependence of the SE yield approximately obeys the inverse cosine law even at high angles of 85 degrees and more. On the other hand, for electron bombardment, the incident angle dependences are much weaker for low energy and high Z2. These indicate that the image contrast on He-SIM is clearer than those of SEM. Among the electron excitations by incident He ions, recoiled target atoms and excited electrons, the first one having narrow excitation volume dominates the SE yield, so that the spatial image resolution in SIM using zero-diameter He beams with the energies of 10-50 keV is prospected to be smaller or better (<0.1 nm) than for 30 keV Ga ion and 1 keV electron beams.

The role of Al-Auger electrons in kinetic electron emission from Al surfaces by slow Ne+ and Na+ ions

This work reports measurements of energy distributions and yields of electrons emitted in the interaction of slow singly charged neon and sodium ions with Al surfaces. The measurements show the strong interplay between different emission mechanisms, such as projectile and target Auger electron emission, decay of bulk plasmons and electronic collision cascade. These results indicate that the Auger decay of Al-2p excitations occurring in binary atomic collision between recoiling target atoms, is the dominant primary excitation mechanism. Measurements performed using sodium projectile show that a significant contribution to the total electron emission yield comes also from a vacancy transfer process that produce Al-2p excitation by a vacancy transfer process in asymmetric collisions between target atoms and incoming ions that have survived neutralization at the surface.

The role of atomic collisions in kinetic electron emission from Al surfaces by slow ions

We measured energy distributions of electrons emitted in the interaction of slow Kr+ and Na+ with Al surfaces. The data allow to correlate emission intensities with spectral signatures of electron excitation processes. Our results indicate that electron promotion processes leading to the excitation of Al target atoms plays the dominating role in kinetic electron emission from Al surfaces by slow ions. In the case of Kr+ ions, electron promotion occurs in close atomic collisions between recoiling target atoms. For Na+ projectiles, a significant contribution to Al excitation comes also from a vacancy transfer process in asymmetric collisions involving ions that have survived neutralization in the interaction with the surface.

Comparison in spatial spreads of secondary electron information between scanning ion and scanning electron microscopy.

Monte Carlo simulations have been carried out to compare the spatial spreads of secondary electron (SE) information in scanning ion microscopy (SIM) with scanning electron microscopy (SEM). Under Ga ion impacts, the SEs are excited by three kinds of collision-partners, that is, projectile ion, recoiled target atom, and target electron. The latter two partners dominantly contribute to the total SE yield gamma for the materials of low atomic number Z2. For the materials of high Z2, on the other hand, the projectile ions dominantly contribute to gamma. These Z2 dependencies generally cause the gamma yield to decrease with an increasing Z2, in contrast with the SE yield delta under electron impacts. Most of the SEs are produced in the surface layer of about 5lambda in depth (lambda: the mean free path of SEs), as they are independent of the incident probe. Under 30 keV Ga ion impacts, the spatial spread of SE information is roughly as small as 10 nm, decreasing with an increasing Z2. Under 10 keV electron impacts, the SEI excited by the primary electrons has a small spatial spread of about 5lambda, but the SEII excited by the backscattered electrons has a large one of several 10 to several 100 nanometers, decreasing with an increasing Z2. The main cause of a small spread of SE information at ion impact is the short ranges of the projectile ions returning to the surface to escape as backscattered ions, the recoiled target atoms, and the target electrons in collision cascade. The 30 keV Ga-SIM imaging is better than the 10 keV SEM imaging in spatial resolution for the structure/material measurements. Here, zero-size probes are assumed.

Comparative study of target atomic number dependence of ion induced and electron induced secondary electron emission

Target atomic number, Z 2, dependence of ion-induced electron emission is investigated by means of a Monte Carlo simulation for 17 species ( Z 2=4–79) of solids bombarded with light (He, Ne) and heavy (Kr, Xe) ions with the energy of 50 keV. The simulation derives the contributions of electron excitation by a projectile ion, recoiled target atoms and electron cascades from the total electron yield. With increasing Z 2 the electron yield decreases while showing a detailed structure. The Z 2 dependence is different from that for electron bombardment, where the secondary electron yield increases with increasing Z 2. The decreasing electron yield for heavy ions with increasing Z 2 is due to the decrease in the number and energy of electrons excited by the projectile ion, resulting from a large energy transfer from the projectile ion to the recoiled atoms. Furthermore, the decreasing component from the recoiled atoms contributes it as well.

Target material dependence of secondary electron images induced by focused ion beams

Monte Carlo calculation of ion-induced kinetic secondary electron emission was performed to study the target material dependence of secondary electron images in scanning ion microscopy (SIM) by using focused ion beams, which is different from that in scanning electron microscopy (SEM). In the calculation, the electron excitation by projectile ions is treated using the partial wave scattering cross-section of conduction electrons by the ions, and the cascade multiplication process of the excited electrons is simulated as well as the elastic collision cascade of recoiling target atoms. The calculated secondary electron yields of Al ( Z 2=13), Cu ( Z 2=29) and Au ( Z 2=79) for 30 and 10 keV Ga + ion impacts decrease with increasing atomic number, Z 2, of the materials, whereas those for 1 keV H + ions and 10 keV electrons increase with Z 2. This is consistent with the observed opposite trend between the SIM and SEM in the Z 2 dependence of secondary electron images. Simultaneous calculation of individual elastic and inelastic collisions of the projectile ion, the recoiling target atoms and the excited electrons suggests that the Z 2 dependence of the secondary electron yield for heavy-ion impact is related to large elastic energy loss of the projectile ion and low energy of the excited electrons in the solid.

Monte Carlo simulation of heavy ion induced kinetic electron emission from an Al surface

A Monte Carlo simulation is performed in order to study heavy ion induced kinetic electron emission from an Al surface. In the simulation, excitation of conduction band electrons by the projectile ion and recoiling target atoms is treated on the basis of the partial wave expansion method, and the cascade multiplication process of the excited electrons is simulated as well as collision cascade of the recoiling target atoms. Experimental electron yields near conventional threshold energies of heavy ions are simulated by an assumption of a lowering in the apparent surface barrier for the electrons. The present calculation derives components for electron excitations by the projectile ion, the recoiling target atoms and the electron cascades, from the calculated total electron yield. The component from the recoiling target atoms increases with increasing projectile mass, whereas the component from the electron cascade decreases. Although the components from the projectile ion and the electron cascade increase with increasing impact energy, the component from the recoiling target atoms is less influenced by the energy in the energy range of tens of keV. The escape depth of the excited electrons is independent of the excitation processes; however, lateral distributions of the excited electrons are clearly different between the excitations by the projectile ion, the recoiling target atoms and the electron cascades.

Efficiency enhancements to Monte Carlo simulation of heavy ion elastic recoil detection analysis spectra

Monte Carlo (MC) simulation can be used to simulate heavy ion elastic recoil detection analysis spectra, including the broadening and tailing effects of multiple and plural scattering, although it is very costly in terms of computer time. In this work, kinematic relationships and experimental parameters are exploited to implement efficiency improvements in the MC modeling process. For thin films, incident ions that pass through the sample without undergoing a significant scattering event need not be tracked. Ions that might generate a detectable scattered or recoiled ion are predicted by generating, in advance, the impact parameters which will define its path. Light recoiled target atoms may be dealt with in the same way. For heavy atoms, however, the probability of large angle scattering events is so high that the paths of most recoil atoms are dominated by several scattering events with large angular deflections.

Elastic recoil detection analysis for large recoil angles (LA-ERDA)

In this paper, elastic recoil detection (ERD) measurements at recoil angle of 60° using ion-induced electron emission (IEE) for particle identification are presented. In our IEE system for particle identification, recoiled target atoms and scattered projectiles penetrate a set of thin carbon foils before their energy is analyzed in a solid state detector. Particle identification is based on the fact that the total number of electrons emitted from the foils depends on the particle nuclear charge. This method is characterized by its low minimum detectable energy, which stimulated us to study ERDA at 60°. Due to collision kinematics and due to the angular dependence of the scattering cross-sections, it is expected that the sensitivity can be significantly improved. In this work, the detection efficiency of the IEE particle identification system for H recoils at energies below 1 MeV was determined. LA-ERDA measurements were performed with 4He and 12C projectiles using two different types of samples with a well-known amount and depth distribution of H atoms near the surface. Sample 1 consisted of a 50 μg/cm 2 melamine layer evaporated on a flat Si substrate, sample 2 was a Si wafer with implanted H. Sensitivity and depth resolution were measured using LA-ERDA with a recoil angle of 60° and ERDA with recoil angles of 30° and 45°. The results for different recoil geometries and projectiles are discussed and compared with theoretical predictions.

Backward electron emission from carbon and gold foils under impact of MeV Au+n clusters

Gold cluster beams delivered by the 2.5 MV Van de Graaff accelerator of the Institut de Physique Nucléaire de Lyon, equipped with a liquid metal ion source (LMIS), were used for secondary electron emission measurements. We investigated backward emission from thin carbon and gold targets bombarded by Au+n projectiles (1⩽n⩽9) of energies between 150 and 500 keV/atom in a standard vacuum of 10−7 mbar. For Au+ ions incident on carbon and gold targets it is observed that the electron yield follows the velocity dependence of the energy deposited at the surface of the target in ionization processes induced both by the incident projectile and by the recoiling target atoms. For Au+n clusters of a given velocity the electron yield per incident gold atom is observed to decrease when the cluster size increases. Given the large contribution of the recoil atoms to the electron emission, we studied the dependence of the cluster effect on the target thickness, for thicknesses of the order of the size of the atomic displacement cascades. The comparison of the electron yields of thin carbon targets of various thicknesses at the impact of Au+ and Au+n projectiles of the same velocity has effectively evidenced a thickness effect for cluster induced electron emission.

RBS/ERD simulation problems: Stopping powers, nuclear reactions and detector resolution

A new program has been developed for the graphical analysis of data from Rutherford Back scattering (RBS) and Elastic Recoil Detection (ERD) material analysis experiments. The program can evaluate experiments using any incident ion, at any energy, for any planar target. It incorporates the full stopping power database of the SRIM/TRIM ion beam programs. All scattered ions and recoiling target atoms are followed out of the target and through any filtering absorbers (ERD absorber foils and foils on detector windows). Straggling is included using any of five optional straggling theories. Commonly used nuclear reaction cross-sections are automatically incorporated. By comparison with experimental data, weaknesses in the theoretical basis of RBS/ERD simulations become apparent. These will be discussed with emphasis on stopping powers, detector resolution, target setup and nuclear reaction cross-sections. The complete RBS/ERD software package can be downloaded from the web site: http://www.research.ibm.com/ionbeams. This package includes an installation manual and a full tutorial.

A simplified collisional model of sputtering in the linear cascade regime

Thin film deposition by sputtering is usually performed with projectile-energy-target combinations placing the sputtering mechanism within the linear cascade regime of collisional sputtering. For the purposes of estimating sputter yield values, as well as conveying an understanding of this most important sputtering mechanism, there is a need for an analytical model whose yield expression has a clear and obvious physical meaning, is easy to evaluate with a modest computer, and yet faithfully represents sputtering in this regime. To meet this need, a deliberately simplified model is developed. The yield expression is a product of three terms: the number of recoiling target atoms at the practical endpoint of the cascade, times the probability that they are sufficiently near the surface to escape, and times the probability that they are traveling in the right direction. It is evaluated by calculating (1) the surface binding energy from thermodynamic data, (2) an average energy of the population of target recoils, from their energy distribution at the practical endpoint of a single collision cascade, and (3) an escape probability from the projected ranges of projectile and recoils, based on nuclear energy loss theory. The simplified collisional model reproduces trends in experimental data for the projectile energy-, projectile mass-, and target-dependences of sputter yield.

Influences of ion backscattering and recoil cascade on incident angle dependence of ion-induced kinetic electron emission from solids

Abstract The incident angle dependence of ion-induced kinetic electron emission (KEE) from Cu is calculated in the energy range from 100 eV to 1 MeV, using a Monte Carlo simulation of transport of incident ions (H+ and Xe+) and recoiling target atoms and a semiempirical theory of KEE. The calculated results are in good agreement with the experimental data and reveal dominant effects of backscattering ions and recoiling target atoms on the deviation from the inverse cosine law for light and heavy ion impacts, respectively.