Precipitate dissolution by high energy collision cascades

The evolution of the microstructure during irradiation is now widely recognized due to: (1) radiation altered kinetic phenomena; (2) collisional processes by energetic cascades. In this paper, we investigate the specific nature of the collisional interaction between energetic cascades and precipitates. A new Monte Carlo-based computer program, TRIPOS, has been developed for the TRansport of Ions in POlyatomic Solids. The computer code utilizes standard nuclear and electronic energy loss formulas, and compares well with experimental data on particle reflection, penetration and sputtering. One of the unique features of the code is its applicability to problems involving multispecie media in multilayers of 3-dimensional configurations. The interaction of neutron-initiated high energy collision cascades is demonstrated to result in the partial dissolution of precipitates. However, the maximum precipitate size that may be completely destroyed by a high energy collision cascade is only a small fraction of the cascade size. Matrix atom implantation inside precipitates as well as preferential sputtering of light atoms from the surface of precipitates into the matrix is demonstrated to lead to changes in precipitate stoichiometry.     

Theoretical electronic state and angular resolved kinetic energy distributions of sputtered diatomic molecules

Electronic state selected translational kinetic energy and polar angular distributions of several homonuclear and heteronuclear diatomic molecules, corresponding to direct ejection and recombination mechanisms of sputtering, are presented. The initial atom separation on, and axis orientation to, the surface are only weakly reflected in the distributions, in contrast to the atomic cascade properties and ejection mechanism. Many observed features of kinetic energy distributions can be reproduced by the simulation, including additional peaks previously interpreted as due to thermal or other processes. Measurements of polar angular distributions are shown to contain important information on atom-pair momentum correlations in collision cascades.     

HCI potential energy sputtering measured with magnetic tunnel junctions

In this paper, the use of tunnel junctions as sensors for estimating the potential energy sputtering yield of highly charged ions (HCIs) incident on ultra-thin insulating films is experimentally explored. HCIs are known to strongly ablate insulating materials upon collision by destabilizing the solid’s chemical structure in the region of impact and producing a mass loss, referred to as potential energy sputtering. In previous work, increased conductance of tunnel junctions due to highly charged ion irradiation was shown while demonstrating that tunnelling remains the dominant transport mechanism. These studies indicate that the conductance increases linearly with the number of HCIs and so, for any given tunnel barrier recipe and HCI charge state, the conductance per HCI can be found. Since the conductance of a tunnel junction is exponentially sensitive to its thickness, the increase in conductance can be used to evaluate an effective tunnel junction thickness reduction, which, in turn, can be explored as a metric for the potential energy sputtering of the tunnel barrier. These tunnel junction measurements provide new information about the nano-features formed on insulators by HCIs, and this paper considers the possibility of using these data to estimate potential energy sputtering. A method for relating the increased conductance to a mass loss is considered and compared against other potential energy sputtering measurements.     

Low-energy ion scattering and sputtering at grazing ion bombardment of clean and oxygen covered Ag(1 1 0) surface

The ion scattering and sputtering processes at low energy grazing N⁺ and Ne⁺ ion bombardment of clean and oxygen covered Ag(1 1 0) surface have been investigated by computer simulation in the binary collision approximation.

The spatial, angular and energy distributions of scattered, sputtered particles and desorbed molecules of oxygen as well as their yields versus the angle of incidence have been calculated. In these distributions the some characteristic peaks were observed and analysed. It was found that an adsorption layer plays a role of the additional surface barrier, i.e. it reflects leaving target atoms back to crystal. The azimuth angular dependencies of Ag sputtering yield and non-dissociative O₂ desorption yield at grazing incidence have been calculated. It was shown that these dependencies correlate the crystal orientation.     

Studies of Li sputtering and particle recycling in glow discharges and in TJ-II

Lithium sputtering is studied at TJ-II with lithiated, boron coated walls. As reported previously, the sputtering yield measured in H plasmas was found to be significantly lower than expected, based on laboratory experiments and TRIM code calculations. The edge temperature dependence of the yield could be consistent with either an energy threshold far exceeding the corresponding pure lithium threshold, or with a strong perturbation of the impinging ion energies. The material mixing effect, one of the candidates for explaining the observed behavior, was studied by depositing boron on the Li layer. Also, particle retention and release was investigated in H/He plasmas in this very low recycling scenario. The release of either species in the opposite plasma was studied in different plasma conditions as well as in glow discharge (GD) plasmas. In He GD plasmas, the I/V characteristics of lithiated stainless steel electrodes were found to be anomalous, in spite of the fact that the dependence of sputtering on the incident particle energy agreed reasonably with expectations. The possible implications of these phenomena for the interaction of reactor plasmas with lithium elements are addressed.     

Molecular simulations of cluster ejection by CO2 cluster impact on carbon-based surfaces

Single (CO₂)_N (N = 1-20) cluster impact on three different carbon-based surfaces of fullerite (1 1 1), graphite and diamond (1 0 0) has been investigated by MD simulations with the cluster collision energy from 5 to 14 keV/cluster as a first step toward the general modeling of the reactive sputtering by cluster impact of a solid surface. A crater permanently remained on the fullerite and graphite surfaces while it was quickly replenished with fluidized carbon material on the diamond surface. In spite of the smaller crater size as well as the crater recovery resulting in the reduction of the surface area, the sputtering yields were the highest on diamond. The effective energy deposition near the surface contributes to the temperature rise and consequent sputtering seemed highly reduced due to the collision cascades especially on the fullerite target.     

20—80keV Ar~+离子对稀释Si(Co,Ta)合金的溅射

荷能离子对单元素的溅射已有较好的理论解释,但是对多元合金或化合物溅射的研究才开始不久,目前尚无一个能够成功地解释实验现象的理论。 溅射后合金样品近表面组成的改变,是一种有趣的实验现象。实验中发现三元合金的现象比二元合金更为复杂,例如,元素的近表面改变与溅射离子的能量有关,这是用现有的择优溅射理论无法解释的。为研究这种现象,我们选取了Si(Co,Ta)三元合金,在分别用20到     

Investigation of the projectile atomic number influence to the total sputtering yield in the keV energy region

The non-monotonous dependence of the total sputtering yield on the projectile atomic number, which is unexpected in the frame of the Sigmund linear cascade theory, is investigated using Monte Carlo simulations (program SRIM 2003). This effect is studied on the example of aluminum sputtering by six different projectiles (N, Ne, Al, Ar, Kr and Xe) at normal incidence. The incident projectile energy is 2 keV. Investigation consists of the analyses of ASI distributions of sputtered atoms as well as of nuclear energy loss depth distributions of projectiles with fixed number of ejected atoms. The results show that the non-monotonous behavior of Y(Z) is due to the ability of projectiles somewhat lighter than aluminum to efficiently eject large number of atoms by formation of collision cascades in the subsurface region which are directed towards the surface. On the other hand, ions that are heavier or significantly lighter than aluminum cannot form this type of cascades - the heavier ions cannot transfer a lot of energy to recoils in a primary knock-on collision that will move towards the surface, while significantly lighter ions transfer the energy too deep into the target.     

Fluctuations in collision cascades with a realistic scattering model

Stochastic quantities of a collision cascade are calculated, such as variance of the sputtering yield and the defect number, as well as two-point correlation functions (in the depth variable). All calculations were made by a binary collision Monte Carlo algorithm. A realistic scattering model is used, i.e. a power law scattering function with an energy dependent exponent, and electronic stopping is also accounted for. Both infinite media (defects only) and half spaces (defects and sputtering) are considered. It is found that the variance of the defects is over-Poisson and the reason is the energy degradation process whereas the effect of the free surface is negligible. Quantitative values of various parameters, both mean values, average depth distributions, variances and two-point correlation functions are given and discussed.     

Photon emission produced by particle-surface collisions

Visible, ultraviolet and infrared optical emission results from low-energy (20 eV–10 keV) particle-surface collisions. Several distinct kinds of collision induced optical radiation are discussed which provide fundamental information on particle-solid collision processes. Line radiation arises from excited states of sputtered surface constituents and backscattered beam particles. This radiation uniquely identifies the quantum state of sputtered or reflected particles, provides a method for identifying neutral atoms sputtered from the surface and serves as the basis for a sensitive surface analysis technique. Broadband radiation from the bulk of the solid is attributed to the transfer of projectile energy to the electrons in the solid. Continuum emission observed well in front of transition metal targets is believed to arise from excited atom clusters (diatomic, triatomic etc.) ejected from the solid in the sputtering process. Application of sputtered atom optical radiation for surface and depth profile analysis is demonstrated for the case of submonolayer quantities of chromium on silicon and aluminium implanted in SiO₂.     

Sputtering induced by cluster impact on metal targets: Influenceof electronic stopping

The effect of electronic stopping on the sputtering of metals by cluster impact is discussed. We focus on the specific case of Au₁₃ impact on a Au surface. Using molecular-dynamics simulation, we study several strategies to include electronic stopping. Electronic stopping influences both the magnitude of the sputter yield and the duration of the sputter process. In the usual procedure, electronic stopping only affects sufficiently fast atoms with kinetic energies above a threshold energy, which is of the order of the target cohesive energy. When assuming that electronic stopping holds down to thermal energies <1 eV, or even to 0 eV, the collision spike is rapidly quenched and the sputter yields become unrealistically small. Furthermore, we implement a scheme to include electronic stopping based on local (electron) density information readily available in a simulation.     

W fuzz layers: very high resistance to sputtering under fusion-relevant He+ irradiations

In this study, we have modeled the sputtering process of energetic He⁺ ions colliding with W nano-fuzz materials, based on the physical processes, such as the collision and diffusion of energetic particles, sputtering and redeposition. Our modeling shows that the fuzzy nanomaterials with a large surface-to-volume ratio exhibit very high resistance to sputtering under fusion-relevant He⁺ irradiations, and their sputtering yields are mainly determined by the thickness of fuzzy nano-materials, the reflection coefficients and mean free paths of energetic particles, surface sputtering yields of a flat base material, and the geometry of nano-fuzz. Our measurements have confirmed that the surface sputtering yield of a W nano-fuzz layer with the columnar geometry of nano-fuzz in cross-section is about one magnitude of order lower than the one of smooth W substrates. This work provides a complete model for energetic particles colliding with the nano-fuzz layer and clarifies the fundamental sputtering process occurring in the nano-fuzz layer.     

Synergism in sputtering of copper nanoclusters on graphite substrate at low energy Cu2 bombardment

Molecular dynamics simulation of Cu cluster sputtering by 50-200 eV/atom Cu₂ dimers and Cu single atoms has been performed. The clusters were located on a (0 0 0 1) graphite surface and consisted of 13-195 atoms. Synergy features were identified in the sputtering yield and energy distributions of sputtered particles calculated for the cases of cluster bombardment with Cu dimers and monomers at the same velocity. The reason for the nonlinear effects in surface cluster sputtering is the overlapping of collision cascades generated by each of the dimer atoms.     

Modelling the erosion of beryllium carbide surfaces

Redeposition of beryllium eroded from main chamber plasma facing components of ITER onto the divertor material carbon creates a mixed material, beryllium carbide Be₂C, whose interaction with the plasma is not well known. In this study, we have investigated the erosion of Be₂C by deuterium using molecular dynamics simulations and ERO impurity modelling. We found that beryllium sputters preferentially over carbon and identified the sputtering mechanism in the ion energy range 10-100 eV to be both physical and swift chemical sputtering. In addition to single atoms, different types of small molecules/clusters were sputtered, the most frequently occurring molecules being BeD, Be₂D, and CD. The sputtering threshold was found to lie between 10 and 15 eV. The MD sputtering yields were used in plasma impurity simulations, serving as a replacement for input data obtained with TRIM. This changes the accumulation rate of impurity Be in the divertor region compared to previous estimates.     

Sputtering of condensed noble gases by keV heavy ions

Bombardment of condensed Kr and Xe by 2–8 keV noble gas ions results in very high sputtering yields. A considerable fraction (10–30%) of the sputtered particles consists of Van der Waals clusters, with Kr₂, Kr₃, Xe₂, XeKr, XeKr₂ and ArKr having been observed. The kinetic energy distributions of the sputtered monomer-species are in agreement with a collision cascade mechanism. However, at the low energy side an excess yield is observed. This is explained by a model which takes into account the large sputtering yields and the damage of the surface during the sputtering process. The energy distributions of the dimers and trimers are satisfactorily explained by a statistical model. It is concluded that the dimer and trimer species are sputtered during an early stage of the cascade.     

Collision cascade temperature

Interaction of a projectile with a solid has been considered in detail. It has been found that any collision cascade generated by a projectile can be characterized by the average kinetic energy of cascade atoms that represents an “instantaneous temperature” of the cascade during its very short lifetime (10⁻¹² s). We refer to this value as the “dynamic temperature” in order to emphasize the fact that cascade atoms are in a dynamic equilibrium and have a definite energy distribution. The dynamic temperature defines the electron distribution in the cascade area and, hence, the ionization probability of sputtered atoms. The energy distribution of cascade atoms and, as a consequence, the dynamic temperature can be found experimentally by measuring the energy distribution of sputtered atoms. The calculated dynamic temperature has been found to be in good agreement with the experimental data on ion formation in the case of cesium and oxygen ion sputtering of silicon. Based on the developed model we suggest an experimental technique for a radical improvement of the existing cascade sputtering models.     

Surface swelling of sapphire induced by MeV and GeV heavy ions

-Al₂O₃ single crystals were bombarded with MeV xenon ions from 10¹⁵ to 10¹⁷ ions cm⁻² and GeV uranium ions from 10¹¹ to 10¹³ ions cm⁻² to study the surface swelling of sapphire at 77 and 300 K due to atomic collision processes (Xe) and electronic energy loss processes in the 20–45 keV/nm regime (U). The induced damage was studied by channeling Rutherford backscattering. Surface swelling was measured with a profilometer. The step height induced by nuclear cascades of MeV xenon increases with the ion fluence and saturates. With GeV uranium, an electronic stopping power threshold for surface swelling was observed and the step height increased with the damage for dE/dx higher than this threshold.     

High-speed processing with Cl2 cluster ion beam

Cluster ion beam processes can produce high rate sputtering with low damage compared with monomer ion beam processes. Cl₂ cluster ion beams with different size distributions were generated with controlling the ionization conditions. Size distributions were measured using the time-of-flight (TOF) method. Si substrates and SiO₂ films were irradiated with the Cl₂ cluster ions at acceleration energies of 10–30 keV and the etching ratio of Si/SiO₂ was investigated. The sputtering yield increased with acceleration energy and was a few thousand times higher than that of Ar monomer ions. The sputtering yield of Cl₂ cluster ions was about 4400 atoms/ion at 30 keV acceleration energy. The etching ratio of Si/SiO₂ was above eight at acceleration energies in the range 10–30 keV. Thus, SiO₂ can be used as a mask for irradiation with Cl₂ cluster ion beam, which is an advantage for semiconductor processing. In order to keep high sputtering yield and high etching ratio, the cluster size needs to be sufficiently large and size control is important.