Ion-surface Interactions Research Articles

Bombardment of a Si pitch grating by C+ ions at an inclined incident angle parallel to the structure

In this work, the bombardment of a nano-scale Si pitch grating with 6kV C+ ions at an angle of incidence of 42° parallel to the structure is investigated both experimentally and by simulations with the SDTrimSP-2D code. The study focuses on the relation between the nano- and macro-scale parameters of ion–surface interactions. The macro-scale parameters are the sputter yields of Si and C atoms and the areal density of the C atoms deposited on the surface; the nano-scale parameter is the 2D profile of the structure. The nano-scale surface profile was obtained experimentally by scanning electron microscopy of specimen cross-sections. The comparison between experiment and simulation reveals good agreement on both macro- and nano-scales.

The physics program at the Kielce EBIS-A facility

The status of the new EBIS-A irradiation facility constructed by the Dreebit GmbH company, Dresden, for the new laboratory at the Jan Kochanowski University (JKU) in Kielce designed for the studies of ion-matter interactions is presented. The facility will be used for the interdisciplinary studies involving atoms, molecules, plasmas, surfaces and cells by taking an advantage of low-energy highly charged ions (HCI) and x-ray spectroscopy techniques. The EBIS-A facility, consisting of an EBIT-type ion source, beam guiding and diagnostic system, double focusing analyzing magnet and multipurpose target chamber, offers new opportunities for performing the experiments on slow, highly charged ions colliding with matter, including such different phenomena as formation of nanostructures in ion-surface interactions, recombination processes in plasma, fragmentation of biomolecules by ion and electron impact and the radiobiological effects in living cells. In this paper a concept of the EBIT/X-ray laboratory, construction details of the EBIS-A facility, the physics program, based on our former experimental activities in the atomic and molecular collisions, and application of x-ray spectroscopy in materials science are discussed.

Etching Characteristics and Mechanism of ZnO and Ga-Doped ZnO Thin Films in Inductively Coupled HBr/Ar/CHF3 Plasma

The etching characteristics and mechanisms of ZnO and Ga-doped ZnO (Ga-ZnO) thin films in a HBr/Ar/CHF3 inductively coupled plasma were investigated. The etching rate of ZnO was measured as a function of the CHF3 mixing ratio in the range of 0–15% in a HBr:Ar = 5:2 plasma at a fixed gas pressure (6 mTorr), input power (700 W), bias power (200 W), and total gas flow rate (50 sccm). The plasma chemistry was analyzed by a combination of the global (zero-dimensional) plasma model, Langmuir probe diagnostics (LP) and quadrupole mass spectrometer (QMS) analysis. It was found that the densities of both HBr and Br are significantly affected by the reactions with the CHF3 dissociation products, while both the ZnO and Ga-ZnO etching rates follow the behavior of the Br atom density and flux. This suggests that the ZnO and Ga-ZnO etching processes are not limited by the ion-surface interaction kinetics and appear in the reaction-rate-limited etching regime.

Ionized physical vapor deposited Al2O3films: Does subplantation favor formation of α‐Al2O3?

AbstractThe broad energy distributions of the condensing particles typically encountered in ion assisted vapor deposition techniques are often a drawback when attempting to understand the effect of the energetic bombardment on the film properties. In the current study, a monoenergetic Al+beam generated by a filtered cathodic arc discharge is employed for the deposition of alumina (Al2O3) films at well defined Al+ion energies between 4 eV and 200 eV at a substrate temperature of 720 °C. Structural analysis shows that Al+energies of 40 eV or larger favor the formation of the thermodynamically stable α‐Al2O3phase at the expense of other metastable Al2O3polymorphs. The well defined ion energies are used as input for Monte‐Carlo based simulations of the ion–surface interactions. The results of these simulations reveal that the increase of the Al+ion energy leads to an increase in the fraction of ions subplanted into the growing film. These findings underline the previously not considered role of subsurface processes on the phase formation of ionized physical vapor deposited Al2O3films. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evolution of the 2D surface structure of a silicon pitch grating under argon ion bombardment: Experiment and modeling

This work aims at validation of the newly developed SDTrimSP-2D code by comparing experimental measurements and modeling of the evolution of a 2D surface structure under argon ion bombardment. SDTrimSP-2D allows for the simulation of ion-surface interactions with 2D micro-structured surfaces, where the first dimension is the depth and the second is parallel to the surface. The main advantage of the approach implemented in SDTrimSP-2D is that the interdependency of surface morphology and sputtering is naturally taken into account. The code follows the evolution of surface morphology and provides a detailed nanoscale description of processes characterizing the ion-surface interactions. In this work, a Si pitch grating with typical dimensions of 200–250 nm, provides a well-characterized 2D system. This is bombarded with 6 keV Ar + ions at normal incidence angle as well as at an angle of 42° both parallel and perpendicular to the pitch grating structure. Cross-sections of the bombarded Si pitch grating were obtained by SEM and compared to the calculated surface profile revealing good agreement between experiment and simulation. The calculations also provide improved insight into the mechanisms of grating erosion and material transport by redeposition.

Modernized setup for studying the interaction of ions with energies to 40 keV with the surface

A modernized setup for studying the ion-surface interaction is briefly described. The main ion channel implemented on the “Large MEPhI mass monochromator” with an ion energy range of 1–40 keV makes it possible to obtain ion beams with (EM/Z) ≤ 4000 keV × a.m.u. Operation with intermediate-energy hydrogen ions allows obtaining information on the thickness of layers of atoms with masses differing from substrate atoms, by analyzing energy spectra of protons scattered from two- or three-layer targets. The use of new digital power units and relevant control programs also allows automation of the ion beam control and makes it possible to perform ion-beam experiments using preliminarily developed irradiation programs.

Lithium research as a plasma facing component material at the University of Illinois

Plasma facing component (PFC) materials are critical to the development of future fusion reactor. While several different PFC materials have been considered in the past, there is an increased interest in the scientific community on the use of lithium as a future PFC material, after its initial success demonstrated by the TFTR “supershots” and more recent work performed on the CDX-U and NSTX spherical tori. The worldwide usage of lithium in tokamaks has grown rapidly over the past few years because of these results. The Center for Plasma-Material Interactions (CPMI) at the University of Illinois at Urbana-Champaign (UIUC) has been actively involved in lithium research since its foundation and currently, there are three experimental facilities dedicated to studying lithium interactions relevant to fusion conditions namely: 1. Ion surface InterAction eXperiment (IIAX); 2. Divertor Erosion and Vapor Shielding eXperiment (DEVeX); and 3. Solid/Liquid Lithium Divertor Experiment (SLiDE). One of the significant advantages of these experimental facilities is the ability to study the basic physics of the phenomenon taking place. In this review paper, a brief description of the experimental facilities and their capabilities is provided and the interested reader is advised to look in future publications for the recent results.

Ion-Specific Excluded-Volume Correlations and Solvation Forces

Realistic ion-ion and ion-surface potentials from explicit-water simulations are used in implicit-solvent Monte Carlo simulations to study the ionic structure and double-layer forces in a nanometer slab confinement. The highly salt-specific results can be reproduced and rationalized by a simple nonlocal Poisson-Boltzmann theory of a nonadditive primitive model, in which effective hard-sphere radii are obtained from the short-ranged part of the pair potentials. Steric corrections to solvation forces are mainly repulsive and strongly coupled to the ion-surface interactions.

Towards hot electron mediated charge exchange in hyperthermal energy ion–surfaceinteractions

We have made Na + and He + ions incident on the surface of solid state tunnel junctions and measured the energy lossdue to atomic displacement and electronic excitations. Each tunnel junction consists of anultrathin film metal–oxide–semiconductor device which can be biased to create a band ofhot electrons useful for driving chemical reactions at surfaces. Using the binary collisionapproximation and a nonadiabatic model that takes into account the time-varying natureof the ion–surface interaction, the energy loss of the ions is reproduced. The energy loss forNa + ions incident on the devices shows that the primary energy loss mechanism is the atomicdisplacement of Au atoms in the thin film of the metal–oxide–semiconductor device. Wepropose that neutral particle detection of the scattered flux from a biased device could be aroute to hot electron mediated charge exchange.

Adsorption of cobalt (II) octaethylporphyrin and 2H-octaethylporphyrin on Ag(111): new insight into the surface coordinative bond

The adsorption of cobalt (II) octaethylporphyrin (CoOEP) and 2H-octaethylporphyrin (2HOEP) on Ag(111) was investigated with scanning tunneling microscopy (STM) and photoelectron spectroscopy (XPS/UPS), in order to achieve a detailed mechanistic understanding of the surface chemical bond of coordinated metal ions. Previous studies of related systems, especially cobalt (II) tetraphenylporphyrin (CoTPP) on Ag(111), have revealed adsorption-induced changes of the oxidation state of the Co ion and the appearance of a new valence state. These effects were attributed to a covalent interaction of the Co ion with the silver substrate. However, recent studies show that the porphyrin ligand of adsorbed CoTPP undergoes a pronounced saddle-shape distortion, which could alter the electronic structure and thus provide an alternative explanation for the new valence state previously attributed to the formation of a surface coordinative bond. With the octaethylporphyrins investigated here, which were found to adsorb in a flat, undistorted conformation on Ag(111), the effects of geometric distortion can be separated from those of the electronic interaction with the substrate. The CoOEP monolayer gives rise to an adsorption-induced shift of the Co 2p signal (−1.9 eV relative to the multilayer), a new valence state at 0.6 eV below the Fermi energy, and a work-function shift of −0.84 eV (2HOEP: −0.44 eV) relative to the clean surface. Comparison with data for the distorted CoTPP confirms the existence of a covalent ion–surface interaction that is insensitive to the conformation of the ligand.

Etching characteristics and mechanism of indium tin oxide films in an inductively coupled HBr∕Ar plasma

The investigations of etch characteristics and mechanisms for indium tin oxide (In2O3)0.9:(SnO2)0.1 (ITO) thin films using HBr∕Ar inductively coupled plasma were carried out. The ITO etch rate was measured in the range of 0%–100% Ar in the HBr∕Ar mixture at fixed gas pressure (6mTorr), input power (700W), and bias power (200W). Plasma parameters and composition were examined with a combination of plasma diagnostics by double Langmuir probe and global (zero-dimensional) plasma model. It was found that the ITO etch rate follows the behavior of Br atom flux but contradicts with that for H atoms and positive ions. This suggests that the ITO etch process is not limited by the ion-surface interaction kinetics and appears in the reaction-rate-limited etch regime with the Br atoms as the main chemically active species.

Why Direct or Reversed Hofmeister Series? Interplay of Hydration, Non-electrostatic Potentials, and Ion Size

A modified Poisson-Boltzmann analysis is made of the double layer interaction between two silica surfaces and two alumina surfaces in chloride electrolyte. The analysis incorporates nonelectrostatic ion-surface dispersion interactions based on ab initio ionic excess polarizabilities with finite ion sizes. A hydration model for the tightly held hydration shell of kosmotropic ions is introduced. A direct Hofmeister series (K > Na > Li) is found at the silica surface while the reversed series (Li > Na > K) is found at alumina, bringing theory in line with experiment for the first time. Calculations with unhydrated ions also suggest that surface-induced dehydration may be occurring at the alumina surface.

Ion Specific Surface Charge Density of SBA-15 Mesoporous Silica

Potentiometric titrations were used to estimate the surface charge density of SBA-15 mesoporous silica in different salt solutions. It was found that surface charge depends both on cation type, following a Hofmeister series (Cs(+) < Guanidinium(+) < K(+) < Na(+) < Li(+)), and on salt concentration (in the range 0.05-1 M). The surface charge series is reproduced by theoretical calculations performed using a modified Poisson-Boltzmann equation that includes ionic dispersion forces with ab initio ion polarizabilities and hydrated ions. The hydration model assigns an explicit hydration shell to kosmotropic (strong hydrated) ions only. The Hofmeister series appears to be due to the combination of ion-surface dispersion interactions and ion hydration.

Subsurface excitations in a metal

We investigate internal hot carrier excitations in a Au thin film bombarded by hyperthermal and low energy alkali and noble gas ions. Excitations within the thin film of a metal-oxide-semiconductor device are measured revealing that ions whose velocities fall below the classical threshold given by the free-electron model of a metal still excite hot carriers. Excellent agreement between these results and a nonadiabatic model that accounts for the time-varying ion-surface interaction indicates that the measured excitations are due to semilocalized electrons near the metal surface.

The science and technology of low-energy ion-surface interactions

Low-energy ion-surface interactions have played a significant role in both the analysis and modification of the near-surface layers of solid materials. From ion scattering spectrometry to eV implantation, the technology of the ion beam or plasma system has played a key role in determining the effectiveness of the processes involved. The development and understanding of the technology required for low-energy ion implantation, ion-beam deposition, ion plating and reactive ion etching has relied heavily on the availability of analytical systems capable of characterising the nature of both the ion beams or plasmas and the surface region of the solid being treated. The present review follows the development of ion beam systems from the low-current analytical beams used for low and medium ion scattering and secondary ion and photon emission through to the high-current machines used for ion implantation and ion beam deposition. The transport concepts used in the latter machines formed the basis of high current, low-energy systems used commercially to produce the ultra-shallow junctions required for the current generation of MOS devices. These developments will be described in the context of the growth in understanding of the beam and plasma physics and the atomic collision processes involved. All the technological developments at Salford were carried out in the course of detailed studies of the atomic collision processes associated with ion trapping and release, ion scattering and sputtering. The results of some of these studies and the ways in which they have contributed to our current understanding of the physics of ion-surface interactions will also be reviewed.

Nonelectrostatic interactions between ions with anisotropic ab initio dynamic polarisabilities

Ion specific effects are common in colloidal and biological systems. Bubble lifetime before coalescence depends on the electrolyte. The strength of different salts at precipating proteins leads to Hofmeister series. Theories of ion and colloid interactions based on electrostatics alone, including the Debye–Hückel theory of electrolytes or the DLVO theory of colloids, are unable to predict these ion specific effects. Rather, the theories need to be modified to account for quantum mechanically derived nonelectrostatic dispersion interactions [1]. The dynamic polarisability (i) of an ion lies at the heart of its dispersion interactions. In general it may be written as a sum over many quantum modes, each with frequency n. Approximations in the past have reduced it to a single mode with modal frequency derived from the ionisation potential (IP) of the ion [2]. We have calculated the exact dynamic polarisabilities of a wide range of ions [3] using ab initio quantum mechanics and present here comparisons against the single-mode IP approximation. The error in calculated dispersion energies due to the single-mode IP approximation averages around 40%, and reaches as high as 86% error for halide ions. Ionic self-energies, and ion–ion and ion–surface interaction energies are calculated. Applications to activity coefficients in the bulk electrolyte are presented. Hofmeister series in the activity coefficients of alkali halides consistent with experiment are found. We also discuss further development of the theory of dispersion interactions with the aim of obtaining a quantitatively, not merely qualitatively, predictive theory. The steps include (i) taking into account the nonspherical anisotropic features of the ions and (ii) using a nonlocal description of the solvent to include solvent spatial structure. Step (i) should be crucial in resolving the adsorption of anisotropic ions such OH to interfaces, which is currently subject of debate between theory and experiment [4,5].

Density functional theories of surface interactions in salt solutions

Most current density functional theories rely upon the presence of hard cores, also between ions of like charge. These hard cores should in principle reflect exchange repulsion. However, by the way in which these theories are formulated, the hard cores effectively determine the range of ion-ion correlations. This is because the mutual repulsion between like-charged ions is truncated below the corresponding hard sphere diameter. In most relevant applications, at least those related to ion correlations and surface forces in colloidal dispersions, exchange repulsion between like-charged ions is unimportant. This can easily be demonstrated by simulations. Unfortunately, the hard cores can in practice serve as fitting parameters in calculations with traditional density functional theory. In this work, we present alternative density functional theories to describe aqueous salt solutions. In these approaches, an approximation of the relevant "Coulomb hole" that results from correlations between like-charged ions is calculated for the system under study. Hence, our theories are completely free from fitting parameters, and the results are appropriately insensitive to the exchange repulsion acting between ions of like charge. The theories are evaluated by comparing predictions with simulation data, with an emphasis on ion correlations and surface interactions.

Etching characteristics and mechanism of ZnO thin films in inductively coupled HBr/Ar plasma

The etching characteristics and mechanisms of ZnO thin films in an HBr/Ar inductively coupled plasma were investigated. The etch rate of ZnO was measured as a function of the HBr/Ar mixing ratio in the range of 0–100% Ar at a fixed gas pressure (6 mTorr), input power (900 W) and bias power (200 W). The plasma diagnostics were performed by double Langmuir probe measurements and quadrupole mass spectrometry. A global (0-dimensional) plasma model was used to obtain the data on the densities and fluxes of the active species. It was found that the etch rate of ZnO is proportional to the flux of Br atoms, but inversely proportional to those of the H atoms and positive ions. This suggests that the ZnO etch process is not limited by the ion-surface interaction kinetics and that the Br atoms are the main chemically-active species.

Ground-state energy shift of He close to a surface and its relation with the scattering potential: A confinement model

The relation between scattering potential and ground-state energy shift of a helium atom (ion) close to a surface is investigated through use of a model of spatial limitation whereby the surface is represented by an infinitely rigid planar boundary. The model of an atom confined in a semi-infinite space with a plane boundary allows the variational evaluation of the ground-state energies and wave functions for He and ${\mathrm{He}}^{+}$ at different positions from the surface. The respective Born-Oppenheimer energy curves serve to model the ground-state energy shift for the elastic scattering channel in atom (ion) surface interactions. Independent calculations for the He-graphite and He-Al (111), (110), and (100) continuous planar potentials are carried out using high-quality ab initio calculations reported in the literature for the lowest He-C and He-Al binary interaction potentials. It is shown in this case that the He ground-state energy shift obtained within this model corresponds to an upper limit to the usual continuous planar potential. A discussion on the physical origin of this agreement is presented in terms of the static nature for the surface considered in both the hard-wall model and the atomic binary interactions used to construct the planar potentials, i.e., no account of the dynamic surface response is allowed as the projectile approaches. This is done by taking a reference pilot calculations based on electron nuclear dynamics for $100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ He $({\mathrm{He}}^{+})$-benzene $({\mathrm{C}}_{6}{\mathrm{H}}_{6})$ interactions by considering the benzene molecule as a rough approximation to a local graphitic surface sector. It is found that the static planar potential provides a reasonable average representation of the interaction for neutral He, and supporting evidence for the use of the static O'Connor-Biersack potential is given. Finally, the effective scattering potential for $\mathrm{He}∕{\mathrm{He}}^{+}\text{\ensuremath{-}}\mathrm{Al}$ (111) is constructed through the use of the static planar potential for He-Al (111) considering the energy shift due to the classical image interaction for ${\mathrm{He}}^{+}$ approaching a perfectly Al (111) conducting plane before charge neutralization takes place. It is concluded that this scattering potential is directly related with the ground-state energy shift of the emerging already neutralized He atoms in $\mathrm{He}∕{\mathrm{He}}^{+}\text{\ensuremath{-}}\mathrm{Al}(111)$ grazing scattering experiments.

Face-dependent Auger neutralization and ground-state energy shift for He in front of Al surfaces

He atoms and ions with keV energies are scattered under grazing angles of incidence from Al(111), Al(100), and Al(110) surfaces. Fractions of surviving ions and normal energy gains of ${\text{He}}^{+}$ ions prior to neutralization, derived from shifts of angular distributions for incident atoms and ions, are compared to results from three-dimensional Monte Carlo simulations based on theoretically calculated Auger neutralization rates and He ground-state energy shifts. From the good agreement of experimental data with simulations, we conclude a detailed microscopic understanding for a model system of ion-surface interactions. Our work provides further evidence for the recently reported surface Miller index dependence for the neutralization of ${\text{He}}^{+}$ ions at metal surfaces. The study is extended to the face dependence of the He ground-state energy shift.