Low-energy Bombardment Research Articles

Effects of low-energy ion beam bombardment on metal oxides

This paper describes a study of Ar ion bombardment damage in metal oxides. In the energy range 1 to 5 keV, preferential oxygen removal and reduction of the oxides was found to depend on ion current density, but to be independent of beam energy.

Low-energy fast atom bombardment tandem mass spectrometry of monohydroxy substituted unsaturated fatty acids.

The low-energy collision-induced dissociation (CID) of the carboxylate anions generated by fast atom bombardment ionization of monohydroxy unsaturated fatty acids derived from oleic, linoleic, linolenic and arachidonic acids were studied in a tandem quadrupole mass spectrometer. The collisional activation spectra revealed structurally informative ions as to the position of the hydroxyl substituent in relationship to the sites of unsaturation. Five mechanisms are proposed for the fragmentation of hydroxyl substituted unsaturated fatty acids and are dependent upon the presence of alpha- or beta-unsaturation sites. These mechanisms include charge-remote allylic fragmentation, charge-remote vinylic fragmentation, charge-driven allylic fragmentation, charge-driven vinylic fragmentation, and homolytic fragmentation by an oxy-Cope rearrangement process. The assignment of specific fragmentation pathways was supported in many instances with deuterium-labeled analogs. Although no single fragmentation mechanism appears to predominate, a rational approach to the interpretation of these CID spectra is proposed. The CID spectra of unknown compounds could be used to establish the hydroxyl substituent position in relationship to certain sites of unsaturation but would not be indicative of all double bond locations. The oxy-Cope rearrangement is specific for a structural unit, namely the 3-hydroxy-1,5-diene moiety.

Formation of localized states and electronic excitation in solids during low-energy particle bombardment

The excitation of deep holes (2s, 2p, in Al) in collision cascades is known to be responsible for emission of high-energy secondary electrons from solids bombarded by fast particles. The excitation occurs above certain impact energy threshold ( ≈ 800 eV in the case of Al) which is given by the minimum interatomic distance in the binary atomic collision needed for inner shell-valence band energy crossing ( ≈ 0.5 Å in Al). Experiments indicate however, that electrons may be emitted from solids far below the deep-hole excitation threshold. The classical valence electron-particle binary collision mechanism which is responsible for most of the particle electronic energy losses yields only low energy electrons which cannot overcome the work function of solids. We have found out that in collision cascades the deformation of valence electron clouds in more complex collision can lead to localized levels below the bottom of the conduction band of a metal. When these levels loose electrons during dynamical particle-particle interactions their subsequent filling by Auger processes could lead to electron emission. Using a cluster of 123 Al atoms with 3s and 3p overlapping Gaussian orbitals and using the pseudopotential of solid Al we have calculated the localization and energies of these new time-dependent electronic states for some collision configurations. We have also estimated the probability of the hole occupation of such states induced by the dynamics of particle cascades.

Influence of low-energy bombardment of an RF magnetron sputtering discharge on texture formation and stress in ZnO films

In an oxygen planar RF magnetron sputtering discharge, the time-averaged flux and energy of positive ions drifting out of the plasma and striking the substrate surface have been determined as a function of RF discharge power over a range of 100 to 1000 W, and as a function of chamber pressure from 0.2 to 6 Pa by measurement of ion-current density and time-averaged plasma sheath potential at the substrate. These data were related to the resulting crystal structure of the deposited ZnO films which had been studied in detail using well-known methods of X-ray diffraction. The impact energy of the positive ions bombarding the growing film varies from some 10 eV to close 50 eV depending on magnetron RF discharge power and oxygen pressure, respectively. The incident ion flux was found to be below 1× 10 15 cm −2 s −1 up to 1 × 10 16 cm −2 s −1, a value of the same order of magnitude as that for the condensing rate of sputtered ZnO species. The structural results obtained show that both the ion energy and the ion flux in the range mentioned above cause significant changes in the degree of crystallinity, preferred orientation and texture sharpness of the deposited ZnO films. Furthermore, positive ion bombardment during film growth has been found to alter the ZnO unit cell dimension up to 2% relative to the equilibrium bulk or powder value which is responsible for the formation of strong compressive residual stress of up to several GPa within the ZnO film. Following these results, one of the criterions for preparing highly c-axis oriented ZnO films with columnar grain structure is to decrease both the energy and the flux of the positive ion bombardment without decreasing the deposition rate of ZnO species. At a such slight-bombardment RF magnetron deposition the compressive residual stress of the ZnO film can be reduced towards zero.

Molecular dynamics and quasidynamics simulations of low-energy particle bombardment effects during vapor-phase crystal growth: Production and annihilation of defects due to 50 eV Si incident on (2×1)-terminated Si(001)

Molecular dynamics and quasidynamics simulations, utilizing the Tersoff many-body potential, were used to investigate projectile incorporation and defect production as well as lattice relaxation, diffusion, and annihilation of defects resulting from 50 eV Si irradiation of (2×1)-terminated Si(001). A unity trapping probability, in sites distributed between the epitaxial overlayer and the fourth lattice layer (l=4), was obtained for Si projectiles irradiating an array of high- and low-symmetry points in the primitive surface unit cell. Exchange epitaxy events were observed in which a lattice atom came to rest at an epitaxial (1×1) bridge site while the projectile stopped in a substitutional lattice site. In addition, several collision sequences resulted in the opening of additional dimers, up to four per irradiation event, thus providing (1×1) sites for migrating adatoms during ion-assisted crystal growth. The primary residual lattice defects produced were split and hexagonal interstitials, although tetragonal, bond-centered, and pentagonal interstitials were also produced in layers l=2 through l=14 with the average interstitial layer depth <l≳=2.3. Calculated interstitial migration activation energies Em decreased toward the surface with minimum energy paths generally involving tetragonal sites. Ion-irradiation-induced interstitials can thus be easily annealed out over time periods corresponding to the deposition of less than one monolayer under typical Si molecular-beam-epitaxy conditions. Fewer vacancies were produced, although they have a higher migration activation energy, and they were distributed over a shallower depth, l≤7. Complete annealing of ion-irradiation-induced vacancies requires interaction with deeper interstitials, moving toward the surface, and incident Si projectiles.

Molecular dynamics simulation of sputtering from Au(100) with low-energy Xe ions and Xe dimers

Low-energy Xe dimer bombardment of a Au(100) surface has been studied by molecular dynamics simulations as a function of dimer orientation. Dimer impact at energies of 250 and 500 eV leads to collisional mechanisms that are similar to those found for single ion impact. A characteristic dependency of the sputter yield on the orientation of the Xe dimer is found. Dimers aligned parallel to the surface are more efficient in sputtering as compared to dimers aligned normal to the surface, and dimers oriented along the 〈001〉 directions are more efficient in sputtering than those aligned in the 〈011〉 directions. The sputter yield is found to be linear at bombardment energies of 250 and 500 eV. The spot patterns obtained from a Au(100) surface by simulation of low-energy Xe single ion and Xe dimer bombardment are found to be similar.

The dimer sputtering mechanism of Cu(001) at low bombarding energy

The emission of dimers during bombardment of a Cu(001) surface by Cu atoms of 300 and 1000eV energy is studied. A molecular dynamics simulation method based on many-body potentials is employed. At 300eV bombarding energy, around 81% of the sputtered dimers originate from second-nearest neighbor sites. 58% of these are ejected in a collision sequence correlated by the projectile. These dimers lead to a second maximum in the kinetic energy distribution of emitted dimers at around 8eV, besides a maximum at 4eV. Only the latter is found at 1000eV bombarding energy. As in this case mostly next-neighbor surface atoms are sputtered, the specific emission mechanism found at 300eV is irrelevant. Finally, we show that the direction of the angular momentum of sputtered dimers is correlated with the original surface site of the dimers.

Calibration method in isotopic carbon analysis

Unlike neutrons or gamma-rays, charged particles readily lose energy when they penetrate matter; thus, the main difficulty in its calculation is, perhaps, the complication introduced in its mathematical treatment. In our experiments, determination of 13C in breath test samples by proton-induced gamma-ray emission, interesting regularities were found that allowed drastic simplification in the theoretical treatment of the problem, and led to the derivation from first principles of a simple, sufficiently accurate basic equation which proved reliable for the determination of the isotopic abundance ratio 13C 12C by low-energy proton bombardment of biological samples with simultaneous prompt gamma spectrometry.

Materials surface modification by reactive gas-ion bombardment: low-energy irradiation

Several problems of materials modification by low-energy reactive gas-ion bombardment are considered. On single and multi-component systems and on single and multi-phase materials the possibility of deep penetration of gas atoms into metals forming dispersively spread secondary phase precipitates at high ion fluences and the possibility of surface relief change due to chemical compound formation and preferable or selective sputtering effects are shown. The equipment for low-energy ion-beam treatment and for reactive ion mixing and several examples of application of the methods are briefly described.

Microstructure and some properties of boron modified graphite USB-15

Boronized graphites, in particular USB-15, have some unique properties which make them applicable in thermonuclear confinement experiments. The optimization of USB-15 manufacture technology demands more detailed knowledge of its properties and, first of all, its microstructure. In this connection microstructure of USB-15 was studied in detail by means of X-ray diffraction, microanalysis, analytical TEM, SEM and other methods, both in original state and after annealing at 2300, 2700 and 3100 K. The erosion resistance to low energy bombardment from deuterium plasma was investigated between 350 and 900 K. In the course of 3 keV D+-ion implantation up to a dose ~ 5 × 1020 D+ m−2 deuterium is mainly trapped in vacancy complexes, which were studied by means of TDS and deuterium profiling using SIMS and RGA. The ratio of CD4 to D2 molecules emitted from USB-15 during post-implantation thermal desorption is considerably less than that from other types of graphites, indicating that the recombination desorption of deuterium is greatly enhanced by B in solid solution. Summary is given on property evolution of USB-15 (before and after thermal treatment) as a result of irradiation up to a fast neutron fluence (E > 0.18 MeV) of 2.5 × 1025 n m−2 in a temperature range 360–1100 K. Thermal annealing of USB-15 in the range 2200–2400 K is recommended for improvement of its thermal strength and radiation stability.

Isomeric yield ratios and excitation functions in alpha -induced reactions on 107,109Ag.

Isomeric yield ratios for the reactions $^{107}\mathrm{Ag}$(\ensuremath{\alpha},3n${)}^{108}$In, $^{107}\mathrm{Ag}$(\ensuremath{\alpha},\ensuremath{\alpha}3n${)}^{104}$Ag, $^{109}\mathrm{Ag}$(\ensuremath{\alpha},2n${)}^{111}$In, and $^{109}\mathrm{Ag}$(\ensuremath{\alpha},3n${)}^{110}$In are determined in the energy range of 20--63 MeV \ensuremath{\alpha} particles. Excitation functions for the above reactions as well as for the $^{107}\mathrm{Ag}$(\ensuremath{\alpha},2n${)}^{109}$In, $^{107}\mathrm{Ag}$(\ensuremath{\alpha},\ensuremath{\alpha}2n${)}^{105}$Ag, $^{109}\mathrm{Ag}$(\ensuremath{\alpha},4n${)}^{109}$In, $^{109}\mathrm{Ag}$(\ensuremath{\alpha},5n${)}^{108}$In, and $^{109}\mathrm{Ag}$(\ensuremath{\alpha},\ensuremath{\alpha}4n${)}^{105}$Ag reactions are also presented. Experimental excitation functions are compared with statistical model calculations taking into account precompound particle emission. Isomeric yield ratios are found to depend strongly on the root mean square orbital angular momentum in the entrance channel. A semiempirical method for the prediction of isomeric yield ratios failed to reproduce experimental data even for compoundlike reactions. Isomeric yield ratios were also calculated in the frame of a statistical model under consideration of angular momentum effects in the preequilibrium and the equilibrium stage. Overall agreement between the theory and the experiment for isomeric yield ratios was found to be satisfactory especially at low bombarding energy when compound nucleus reaction channel is dominant. The discrepancy observed at higher bombarding energies needs to be theoretically investigated in greater detail.

Anisotropic highly selective electron cyclotron resonance plasma etching of polysilicon

High polysilicon etch rates have been obtained in combination with high etch selectivities with respect to SiO2 and photoresist in a microwave electron cyclotron resonance plasma etch reactor. The etch system is vacuum load locked, used Cl2–O2 chemistry, and employs backside helium substrate cooling with 150 mm wafer handling. Etch rates in excess of 4000 Å/min are obtained in undoped polysilicon with zero applied substrate bias, using Cl2 with 0.6% O2 at 3 mTorr total pressure and 700 W of 2.45 GHz input microwave power. The corresponding polysilicon-oxide selectivities are greater than 150, and the polysilicon-resist selectivity is 15–20. Half-micron polysilicon gates etched with zero applied substrate bias have vertical sidewalls and residue-free SiO2 floors. Electron and ion energy distribution measurements show that a high ion flux (≳10 mA/cm2) and simultaneous low ion bombardment energy are controlling parameters for high rate selective etching.

Recent advances in the deposition of ferroelectric thin films

Abstract Recent developments in ferroelectric thin film deposition involving plasma based approaches, are described, which include a) multi-magnetron sputter deposition, b) Multi-ion-beam reactive sputter (MIBERS) deposition, c) Pulsed excimer laser ablation and d) ECR (Electron cyclotron resonance) plasma assisted deposition. These methods commonly prevailed intrinsic low energy ion bombardment during the growth process, which may be used for the control over composition, crystallization temperature and microstructure. A low energy (60–75 eV) ion bombardment of the ferroelectric Pb(Zr, Ti)O3 thin films indicated a reduction in the phase formation/crystallization temperature, improved the electrical properties, microstructure and the surface smoothness. Discussion is presented exphasizing the effects of low energy bombardment in different deposition processes. Recent findings using rapid thermal annealing process are also described.

Molecular-dynamics simulations of bulk and surface damage production in low-energy Cu→Cu bombardment

Molecular-dynamics simulations are employed to study in detail the effects of low-energy (≤100 eV) bombardment of a Cu (001) surface by Cu atoms. By following the simulation up to 4 ps in real time, the end configuration of defects in the target can be observed. We present results on the vacancy and interstitial distribution in the target, the spontaneous defect recombination, the number of surface vacancies and adatoms produced, and the mixing of target atoms induced by the bombardment. Furthermore, the fate of the projectile atom−backscattering and implantation−and the sputtering behavior are investigated. The relevance of the results on the modelling of ion-beam (assisted) deposition is discussed.

Trapping of low-energy xenon ions in surfaces of transition metals

Trapping states of 5 keV xenon ions in surfaces have been investigated for nineteen kinds of transition-metal targets by means of X-ray photoelectron spectroscopy (XPS). The quasi-saturated values of the Xe/Me (Me = metal) ratio in the surface layer were found to be appreciably different among the metal targets. It ranges from nearly 0 for the gold target to 4.1 × 10 −2 for scandium and titanium targets. Since the rate of sputtering of the implanted layer becomes comparable to that of the implantation for such low energy bombardment, the real trapping efficiency in the surface detectable by XPS was obtained by correcting the effect of sputtering during the implantation. As a result of this correction, it was found that a relationship exists between the trapping efficiency and the position in the periodic table of the target metal. This finding is discussed in relation to the effect of the potential of metal d-electrons on the trapping states of implanted xenon.

Defect generation and morphology of (001) Si surfaces during low-energy Ar-ion bombardment.

Low-energy Ar{sup +}-ion bombardment of atomically rough (001) Si surfaces has been investigated using atomistic simulations. The simulations suggest that ions with energy less than 20 eV selectively displace surface atoms without causing bulk damage and further that the displacement yield is surface site specific in this energy range. Interpreted in the context of recent experimental kinetic data for Si-surface self-diffusion, the simulations imply that above room temperature the most important effect of ion bombardment on surface self-diffusion and surface morphology is an increase in the formation rate of single adatoms rather than enhancement of the migration component of surface self-diffusion.

Multi-ion-beam reactive sputter deposition of ferroelectric Pb(Zr,Ti)O3 thin films

A multi-ion-beam reactive sputter (MIBERS) deposition technique was devised to grow ferroelectric lead zirconate titanate (PZT) thin films of different compositions (Zr/Ti ratios of 50/50 and 56/44) from individual metal targets of Pb, Zr, and Ti. This technique offers a highly controllable deposition process allowing excellent uniformity in composition and thickness over a large area (7.5 cm diameter) on a reproducible basis. The PZT films were deposited on a variety of unheated substrates and annealed by two different techniques, rapid thermal annealing and conventional furnace annealing. Both techniques induced a perovskite phase with good morphology. The effect of the excess Pb content was observed in terms of the crystallization and morphology. It was seen that the presence of excess Pb tends to enhance perovskite phase formation but degrades the morphology. The effect of the substrates was observed in terms of crystallization and orientation. A low-energy oxygen ion beam was employed to modify the film growth. Secondary-ion bombardment seems to be a promising approach to optimize the film quality as it showed a variety of effects such as enhancing crystallization, inducing preferred orientation, and improving the film morphology. The present MIBERS-grown PZT films showed fairly high dielectric constants, about 850 for PZT (50/50) and about 1150 for PZT (56/44), and low dielectric losses. Ferroelectricity of these films was established with the values of the remnant polarization and the coercive field for PZT (50/50) of 23 μC/cm2 and 80 kV/cm and for PZT (56/44) of 20 μC/cm2 and 60 kV/cm, respectively. The low-energy oxygen ion-beam bombardment during the growth of PZT (50/50) films caused an increase in the dielectric constant to about 1000, reduction in the dissipation factor to 0.02, increase in the remnant polarization to about 26 μC/cm2, and decrease in the coercive field to about 60 kV/cm. The C-V characteristics of PZT (50/50) films in a metal-ferroelectric-metal configuration also indicated a clear dielectric polarization hysteresis.

Substrate Trenching Mechanism during Plasma and Magnetically Enhanced Polysilicon Etching

Plasma and magnetically enhanced reactive ion etching processes with chlorine gas have been developed for subhalf micron polysilicon gate electrode etching (1). In this paper, gate oxide trenching during polysilicon gate etching using chlorine‐based reactive ion etching (RIE) and magnetically enhanced reactive ion etching (MERIE) processes is studied in detail. The trenching mechanism was found to be a direct result of off‐angle ion bombardment and reflection from the partially etched polysilicon sidewall during plasma etching. The off‐angle ion bombardment is caused by the elastic collision between ions and neutrals as ions move across the plasma sheath. Monte Carlo simulation confirms the angular distribution of ions under most plasma processing conditions. A two‐step MERIE process, first with high etch rate and high anisotropy and second with high selectivity to the gate oxide, with low ion energy bombardment is also developed to minimize the trenching effect during polysilicon‐gate etching.

Sputtering of TiC coating by low-energy and high flux plasma bombardment in PISCES

Chemical vapour TiC coating deposited on AISI 316 SS for plasma-facing component applications were investigated in conditions simulating the edge plasma of a tokamak. Samples were exposed to continuous plasma bombardment under the following conditions; n e from 5 × 10 11 to 5 × 10 12 cm −3, Te from 3 to 10 eV, flux on target from 5 × 10 17 to 2 × 10 18 ions cm −2 s −1 and fluence on the target up to 10 22 ions cm −2. Surface modification and chemical composition change were examined by AES and XPS analysis, whereas morphological changes were studied by SEM techniques.

Trends in fragment heating in the damped reaction 165Ho+56Fe at 7.2 MeV/nucleon.

Neutron folds in two high-efficiency neutron multiplicity meters (NMM) were measured in coincidence with projectilelike fragments from the $^{165}\mathrm{Ho}$${+}^{56}$Fe reaction at a low bombarding energy of ${\mathit{E}}_{\mathrm{lab}}$=7.2 MeV/nucleon. Utilizing a complex Monte Carlo procedure to simulate both the fragment decay cascades and the response of the NMM's, the average excitation energies of the fragments were deduced as functions of the total-kinetic-energy loss. The rather slow evolution of the experimental excitation energy partition from an equipartition at the initial stages of the collision towards its thermal equilibrium limit is found to be in quantitative agreement with the predictions of the one-body nucleon-exchange model.