Single Ion Impact Research Articles

Masked ion beam lithography with highly charged ions

Masked ion beam lithography using highly charged ions is demonstrated for the first time by producing an array of hundreds of ordered micrometer wide dots using Xe44+ on poly(methylmethacrylate) resist. At low dose, exposure of the resist is incomplete and isolated single-ion impact sites can be seen within the exposed areas. Atomic force microscope images of the single-ion impact sites show craters with a width of 24 nm. At high dose, the exposure is complete and the dot morphology is consistent with limitations from the mask. Scanning electron microscope images indicate that the sidewall slope is steeper than four.

Lattice disorder and density of states change of graphite surface by single ion impact

Lattice disorder and density of states change of graphite surface by single ion impact

Diamond membranes: Applications for single ion detection using secondary electron emission

The assessment of mechanisms for radiation damage in both biological and semiconductor systems is much enhanced by ensuring that each cell or memory unit is hit by one and only one ion. Such “single ion” systems operate by detecting the passage of an ion and using the signal to deflect the beam upstream thus ensuring that only a single ion is injected to the system at a time. For biological investigations of the effects of single ion impact on living cells, the ion needs to be extracted from the vacuum of the accelerator through a thin window into the atmosphere. The requirements on such a window are: (a) that it be thin enough to allow the passage of the ion through it without excessive energy loss; (b) that it possess mechanical strength sufficient to survive the pressure differential; and (c) that upon passage of the ion through it, a sufficiently large signal is generated to ensure a 100% efficient detection of the passage of the ion. This set of demanding requirements can be met by a thin diamond window fabricated by CVD technique, taking advantage of the high secondary electron yield of B-doped diamond. In the present work, we report on the use of B-doped diamond membranes for this purpose.

Craters Produced on Metals by Single Ion Impacts

summaryThe surface is a major perturbation on displacement cascades resulting from ion impacts. Ion impacts on dense materials result in nanometer sized craters on materials with densities greater than 7.3 g/cm3. Although stable when the irradiation is interrupted, single ion impacts can modify or annihilate existing craters. Single ion impacts can extrude material from the specimen. Flow from the melt zone of cascades is responsible for the observed changes.

Scanning tunneling microscopy and atomic force microscopy study of graphite defects produced by bombarding with highly charged ions

The defects produced on a graphite surface by single ion impact using highly charged Ar ions (charge state q⩽8) is investigated by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The defect looks like a protrusion in the STM image, while it is flat in the AFM image. From these two contrasting images, the defects are considered to be due to the increase in the local charge density of state at the surface caused by carbon atom sputtering. The average value for the defect size increases remarkably with the charge state of incident Ar ions. This is explained by the enhancement of potential sputtering due to the Coulomb repulsion between surface holes which are generated by the neutralization of highly charged Ar ions.

Simple model of surface superlattices on graphite

The discrete Laplace equation on a honeycomb lattice, perturbed by a point potential, is considered as a simple model of single-ion impact on a graphite surface. The asymptotic behavior of the perturbation of the local surface density of electron states is analyzed. It is shown that this perturbation is equal to the experimentally observed superlattice. It is then established that the main purpose of the existence of such superlattices is that of unconnectedness of the isoenergetic curve of the Brillouin zone.

Fast-ion-induced sputtering of polymers

Energetic ions in the electronic stopping regime (fast ions) impacting a solid can induce the ejection of intact large organic molecules into the gas-phase. This phenomenon, called electronic sputtering, has been employed in the Time-of-Flight (TOF) mass spectrometry of molecular solids, such as polymers, peptides and proteins. In this paper, results on the electronic sputtering of polymers are briefly reviewed with some comparison with data on sputtering by low-energy (keV) particles. Most of the information derived so far from fast-ion sputtering of polymers originates from low-mass fragment species. Fingerprint fragment ions present in the TOF spectra can be used for material identification and structural analysis of pristine and irradiated polymers. Besides that, studies of the properties of such ions (e.g. velocity distributions, effective ejection cross sections, etc.) can give information on the early physics and chemistry of ion tracks which are difficult to access by means of other experimental techniques. An important dimension in the fast-ion-induced sputtering of polymers is the synthesis and ejection of new and complex molecules. A remarkable example is the emission of large-mass pure carbon clusters from halogenated polymers in response to a single MeV ion impact. The applicability of fast-ion-induced sputtering in the determination of molecular weight distribution of polymers is also discussed in connection with recent results of crater formation by MeV ion impacts on polymer surfaces.

Single ion impacts on an In 0.22Ga 0.78As/GaAs(100) surface observed by atomic force microscopy

Very flat In 0.22Ga 0.78As layers grown by metalorganic vapour phase epitaxy (MOVPE) on GaAs(100) surfaces were bombarded with a low dose (10 11 ions cm −2) of arsenic and boron ions at 35 keV. Atomic force microscopy (AFM) observations show craters with average diameters of 14.6 nm and 11.4 nm for As + and B + respectively. It is found that there is a 1:1 relationship between the areal density of craters and ion dose. This implies that each crater is correlated with a single ion impact. The mechanism of the formation of the craters is discussed. We suggest that the craters arise from the combined effects of sputtering, surface mass transport around the ion track and atomic displacements in the cascade.

Ion-beam assisted deposition of thin molybdenum films studied by molecular dynamics simulation

We report results obtained by molecular dynamics simulation of low energy argon-ion assisted growth of thin molybdenum films (≈ 20 Å). The effects of a single ion impact are discussed, but more particularly we consider film growth from a manufacturing viewpoint and examine the properties of the completed films. Results for ion-beam assisted deposition are compared with those for unassisted growth (i.e. physical vapor deposition). Surface orientation, atomic displacements, surface roughness, sputtering, point defects, and the influence of off-normal atom incidence are discussed.

Collisions of fast clusters with solids and related phenomena

Fast ions, impinging on solid targets at a few MeV per atom, deposit energy in the material through electronic excitation processes. The relaxation of this energy induces emissions of photons, electrons, ions and neutral species from the target. The detailed study of these ejecta can give insight on the energy deposition and relaxation steps of the ion-solid interaction. Fast polyatomic projectiles allow to deposit very high energy densities (at the surface and in the bulk) well beyond that being possible with any single ion impact. As a result, new and unexpected phenomena are oberved which will be discussed: large non linear emission of ions and cluster ions, very high sputtering yields, production of giant tracks and craters in various irradiated materials including metals. When entering the solid, the atomic constituents of the projectile remain in a close proximity for a certain distance, and the energy density deposited in the overlapping trajectories region is high enough to induce collective effects. Secondary ion emission experiments, performed with various carbon cluster beams delivered by the Tandem accelerator in Orsay, aim to determine this depth of spatial correlation by probing these collective effects inside the solid. The influence of the proximity of the cluster constituents on some of their properties such as their charge state distributions inside the solid has been studied and it has been shown that the mean charge state of the constituents is significantly smaller than when the atoms penetrate the target independently.

Size Dependence of Bombardment Characteristics Produced by Cluster Ion Beams

ABSTRACTCluster ion beam processes provide new surface modification techniques, such as surface smoothing, high rate sputtering and very shallow implantation, because of the unique interactions between cluster and surface atoms. To understand interactions with cluster and surface, Scanning Tunneling Microscope (STM) observations have been done for single impact traces.Highly Oriented Pyrolitic Graphite (HOPG) surfaces were bombarded by carbon cluster ions (Va≤300kV), and large ridges and craters have been observed as a result of single cluster ion impact. The impact site diameters are proportional to the cluster size up to 10 atoms, and increase drastically for cluster sizes above 10. This indicates that non-linear multiple collisions occur only when a local area is bombarded by more than 10 atoms at the same time.

Defect production during ion-assisted deposition of molybdenum films studied by molecular dynamics

ABSTRACTLow energy argon-ion assisted growth of thin molybdenum films (∼ 60 Å) has been studied by molecular dynamics simulations. The effects of a single ion impact are discussed, but more particularly we consider film growth from a manufacturing viewpoint and examine the properties of the completed films. Results for ion-beam assisted deposition are compared with those for unassisted growth (i.e. physical vapor deposition). Surface morphology, defect generation, argon incorporation, and the various responsible atomic mechanisms are discussed.

Total yield and polar-angle distributions of biomolecules sputtered by fast heavy ions.

Direct measurements of the number of peptide molecules ejected per fast heavy ion impact (total yield) and data on polar-angle distributions of the total amount of sputtered bioorganic material are presented. These data allow direct comparisons with sputtering models and provide information complementary to that obtained by conventional experiments on the fraction of sputtered material ejected in the form of molecular ions. Solid targets composed of the peptide tri-leucine ($m=357$ u) were irradiated by 55-MeV $^{127}\mathrm{I}$ ions incident at an angle of 51\ifmmode^\circ\else\textdegree\fi{} with respect to the target surface normal. The sputtered material was collected on silicon plates placed in various configurations in front of the target. The collectors were subsequently analyzed by two different methods: time-of-flight mass spectrometry and amino acid analysis. A total yield of (3.5\ifmmode\pm\else\textpm\fi{}0.7)\ifmmode\times\else\texttimes\fi{}${10}^{3}$ tri-leucine molecules per incident ion was measured, which corresponds to a volume of about 2\ifmmode\times\else\texttimes\fi{}${10}^{3}$ ${\mathrm{nm}}^{3}$ of target material removed in a single ion impact. In the plane of ion incidence, i.e., the plane encompassing the ion incidence direction and the surface normal, the polar-angle distribution was asymmetric with respect to the surface normal and peaked away from the direction of the incoming ions. In the plane containing the surface normal and perpendicular to the plane of incidence, the ejected material displayed a symmetric and normally peaked polar-angle distribution. Comparisons of data from the two methods of collector analysis suggest that the total sputtering yield is dominated by clusters of molecules. Asymmetric angular distribution and extensive cluster ejection support hydrodynamic-type sputtering models which assume impulsive radial expansion of the solid around the incident ion path.

Surface and sub-surface defects on graphite after single ion impact studied with STM

Utilizing scanning tunneling microscopy we have investigated the defects induced by single ion impacts on the (0001) graphite surface. The ion species ranged from 20Ne + to 208Pb + and their energy was fixed at 50 keV (also 150 keV in case of 132Xe +). For normal incidence we found two types of well-separated hillock-like defects which we attribute to isolated interstitials and interstitial clusters underneath the surface. Each defect originates from a single ion impact. For 75° tilted incidence we found a strongly increased number of hillocks per incident ion which were aligned in chains along the projection of the incident beam direction. The typical length of these chains is close to the projected ion range, and the number of defects can be explained by the recoil distribution in a single displacement cascade.

Thermal spike effects in low-energy single-ion impacts on graphite.

Monte Carlo simulations have been used to obtain three-dimensional distributions of cascade defects and energy deposition due to single-ion impacts in graphite. This energy deposition profile serves as the starting point for the formation and evolution of the thermal spike. In this case the minimum deposited energy density per target atom for the spike formation is the order of the atomic binding energy. An effective spike created by a single-ion impact in the near-surface region is introduced to account for the bump formation on the highly oriented pyrolytic graphite surface. A linear relationship between the bump volume and the effective spike energy is obtained based on the thermal spike model, which agrees well with the scanning tunnel microscope results. It is suggested that the one-dimensional energy deposition rate {ital dE}/{ital dx} is not suitable to describe spike effects. On the other hand, a three-dimensional parameter corresponding to the nature of the spike formation should be used. {copyright} {ital 1996 The American Physical Society.}

Dose Rate Effects During Damage Accumulation in Silicon

AbstractWe combine molecular dynamics and Monte Carlo simulations to study damage accumulation and dose rate effects during irradiation of Silicon. We obtain the initial stage of the damage produced by heavy and light ions using classical molecular dynamics simulations. While heavy ions like As or Pt induce amorphization by single ion impact, light ions like B only produce point defects or small clusters of defects. The amorphous pockets generated by heavy ions are stable below room temperature and recrystallize at temperatures below the threshold for recrystallization of a planar amorphous-crystalline interface. The damage accumulation during light ion irradiation is simulated using a Monte Carlo model for defect diffusion. In this approach, we study the damage in the lattice as a function of dose and dose rate. A strong reduction in the total number of defects left in the lattice is observed for lower dose rates.

Dose Rate Effects During Damage Accumulation in Silicon

AbstractWe combine molecular dynamics and Monte Carlo simulations to study damage accumulation and dose rate effects during irradiation of Silicon. We obtain the initial stage of the damage produced by heavy and light ions using classical molecular dynamics simulations. While heavy ions like As or Pt induce amorphization by single ion impact, light ions like B only produce point defects or small clusters of defects. The amorphous pockets generated by heavy ions are stable below room temperature and recrystallize at temperatures below the threshold for recrystallization of a planar amorphous-crystalline interface. The damage accumulation during light ion irradiation is simulated using a Monte Carlo model for defect diffusion. In this approach, we study the damage in the lattice as a function of dose and dose rate. A strong reduction in the total number of defects left in the lattice is observed for lower dose rates.

Tunneling Current Change of Graphite Surface by Single Ion Irradiation

AbstractWe discuss changes in electronic structure and the topography of a graphite surface undergoing by a single ion impact. Protrusion-like regions (PLRs) found in a scanning tunneling microscope image disappeared in the same view of a noncontact atomic force microscope image. We measured tunneling current versus voltage characteristics to determine the density-of-states change in PLRs. We found that the density of states at the Fermi level of PLRs was greater than that of the intact surface. We therefore concluded that the PLRs were not actual topographical changes, but originated from electronic structural changes in semimetal to metal transition.

AFM studies of a new type of radiation defect on mica surfaces caused by highly charged ion impact

Radiation induced defects on mica caused by the impact of slow very highly charged ions (SVHCI) have been investigated with an atomic force microscope (AFM). Freshly cleaved surfaces of different types of muscovite were irradiated with SVHCI extracted from the LLNL electron beam ion trap (EBIT) at velocities of about 2 keV/amu. Atomic force microscopy of the surface reveals the formation of blisterlike defects associated with single ion impact. The determined defect volume which appears to increase linearly with the incident charge state and exhibits a threshold incident charge state has been determined using the AFM. These results indicate that target atoms are subjected to mutual electrostatic repulsion due to ionization through potential electron emission upon approach of the ion. If the repulsion leads to permanent atomic displacement, surface defects are formed.

Friction Change Induced by Single Mev Ion Impact Measured by Scanning Probe Microscope

AbstractIn order to evaluate the damage caused by a single MeV ion impact, we prepared highly oriented pyrolytic graphite specimens, each implanted with 3.1 MeV Au, Ag, Cu and Si ions at a dose of 2.3×1011 cm-2 (2300 μm-2). We then observed specimens by using friction force microscope, and found round regions caused by single-ion impacts on the implanted surface. The frictional force between the surface and the silicon-nitride tip increased in disordered regions. The number density of the regions differed at individual ion-implanted surface, and was about 1000 μm-2 at the Au-implanted surface, 880 μm-2 at the Ag-implanted surface, 372 μm-2 at the Cu-implanted surface, and 44 μπτ2 at the Si-implanted surface. We calculated the electronic stopping power and the probability of knock-on atom generation by the nuclear collision to determine which process affected the density. We found that the difference in the number density was closely related to that of nuclear collisions.