Assessing electronic stopping cross sections of light ions at low ion energies: The impact of crystallinity and surface orientation

ABSTRACTWe have explored the origin of the ion-induced conductivity in polymer films and the distinction between low energy ion-implantation and high-energy ion irradiation. In experiments involving irradiation of polymer and carbon films with ions of energy from 200 keV to 25 MeV we have established that with both low and high energy ions the polymers undergo carbonization. However, the saturation resistivity obtained with low energy implantation is four to six orders of magnitude larger than those obtained by high energy ion irradiation. In experiments on irradiation of carbon films low energy ions caused a two orders of magnitude increase in the resistivity while high energy ion caused a two orders of magnitude decrease. This implies that the structure of the carbonized polymer is different for the low and the high energy ion irradiation. While in the former case there may be no crystalline order in the films; in the latter case, a microcrystalline graphitic structure is obtained (with four orders of magnitude larger conductivity than in the former case). The formation of graphitic crystalline order with increasing high energy ion dose was verified by electron energy loss spectroscopy. This is an interesting example of crystallization induced by electronic excitation alone with no macroscopic thermal effect.

A new technique for the excitation of inorganic phosphors by low kinetic energy ions (<40 eV) and low kinetic energy electrons (<20 eV) has been demonstrated and characterized. The characteristic light output of the phosphor has been used as a probe of the excitation mechanism. An empirical relationship between the light output and the ion and electron currents has been determined and used to ascertain the change in light output as a function of ion and electron kinetic energy and ionization potential. The excitation mechanism starts with the formation of holes by the ion beam. Although the holes are mobile in the lattice, some of the holes get trapped. In a second step electrons interact with these trapped holes to create an excited state (s) of the phosphor. Energy is transferred to the luminescent centers resulting in the characteristic emission of the phosphor. The slow or rate‐limiting step in the production of light by low energy ions and electrons is the emission of light. Of the ten phosphors examined for low energy ion‐electron excitation, six showed detectable levels of light output and, hence, we conclude that this excitation technique is quite general. The efficiency of light production observed for is ∼1 photon per 40 ions and no effort has been made to increase this efficiency. We refer to the light produced by this excitation technique as low energy ion‐electron luminescence (LEIEL).

A coupled rate theory-Monte Carlo model of helium bubble evolution in plasma-facing micro-engineered tungsten

Defect production efficiency in Fe 60Co 40 thin films irradiated with swift heavy ions

The formation of metallic nanostructures by exposure of molybdenum and tungsten surfaces to high fluxes of low energy helium ions is studied as a function of the ion energy, plasma exposure time, and surface temperature. Helium plasma exposure leads to the formation of nanoscopic filaments on the surface of both metals. The size of the helium-induced nanostructure increases with increasing surface temperature while the thickness of the modified layer increases with time. In addition, the growth rate of the nanostructured layer also depends on the surface temperature. The size of the nanostructure appears linked with the size of the near-surface voids induced by the low energy ions. The results presented here thus demonstrate that surface processing by low-energy helium ions provides an efficient route for the formation of porous metallic nanostructures.

Low-energy reactive ion etching is desirable as a method to fabricate devices, because it does not damage the sample surface. However, anisotropical properties are difficult to obtain with low energy ions. Thus, anisotropical properties with an electron cyclotron resonance (ECR) plasma etcher, with which it is easy to obtain a directional low energy (≤20 eV) ion flux, were examined. The conditions under which a clear enough anisotropical cross section figure was produced, were found and applied to sub-half-micron patterns. An ECR plasma etcher is used here with a standing wave cavity TE113 storing 2.45 GHz microwaves and with a sample chamber separate from the cavity. This kind of etcher does not permit control of an ion’s energy apart from the radicals. However, it is possible to control ionic performance by reducing the gas pressure to ∼1.0 mTorr and obtaining a satisfactory anisotropical etching mode. However, ion and molecule collision appear in the ion flux at that gas pressure. The collision makes disanisotropy; the collisional angle changed from about 15–25 deg for gas pressures from 0.2 mTorr to 1.0 mTorr. According to a simple theoretical calculation, the angle is about 30 deg on the assumption of a one time collision, about twice the value of the experimental result. At last, we obtained the high aspect ratio up to 10 at a gas pressure of 0.1–0.2 mTorr. It follows that it was possible to etch the sample well anisotropically with a low-energy ion flux without causing damage by using an Ion-Flux Type ECR etcher and controlling the gas pressure, and further without spoiling its undamaged nature.

Purpose/Methods:Monte Carlo (MC) track structure simulations follow the primary as well as all produced secondary particles in an event‐by‐event manner, from starting or ejection energy down to total stopping. They provide useful information on physics and chemistry of the biological response to radiation. They depend on reliable interaction cross sections and transport models of the considered radiation quality with biologically relevant materials. Most transport models focus on sufficiently fast and bare (i.e., fully ionized) ions and cross sections calculated within the (relativistic) first Born or Bethe approximations. These theories consider the projectile as a point particle and rely on proton cross sections and simple charge‐scaling methods; they neglect the atomic nature of the ion and break down at low and intermediate ion energies. Heavier ions are used in particle therapy and slow to intermediate and low energies in the biologically interesting Bragg peak. Lighter and slower fragment ions, including alpha particles, protons, and neutrons are also produced in nuclear and break up reactions of charged particles. Secondary neutrons also produce recoil protons and ions, mainly in the intermediate energy range.Results/Conclusion:This work reviews existing models for track structure simulations and cross section calculations for light and heavy ions focusing on the low and intermediate energy range. It also presents new and updated aspects on cross section calculations and simulation techniques for ions and discusses the need for new models, calculations, and experimental data.

Surface roughness of deposited or etched film strongly depends on ion bombardment. Relationships between ion bombardment variables and surface roughness are too complicated to model analytically. To overcome this, an empirical neural network model was constructed and applied to a deposition process of silicon nitride (SiN) films. The films were deposited by using a pulsed plasma enhanced chemical vapor deposition system in <TEX>$SiH_4$</TEX>-<TEX>$NH_4$</TEX> plasma. Radio frequency source power and duty ratio were varied in the range of 200-800 W and 40-100%. A total of 20 experiments were conducted. A non-invasive ion energy analyzer was used to collect ion energy distribution. The diagnostic variables examined include high (or) low ion energy and high (or low) ion energy flux. Mean surface roughness was measured by using atomic force microscopy. A neural network model relating the diagnostic variables to the surface roughness was constructed and its prediction performance was optimized by using a genetic algorithm. The optimized model yielded an improved performance of about 58% over statistical regression model. The model revealed very interesting features useful for optimization of surface roughness. This includes a reduction in surface roughness either by an increase in ion energy flux at lower ion energy or by an increase in higher ion energy at lower ion energy flux.

Morphology modifications of quantum dots on Si(0 0 1) surface by ion sputtering

Preferential sputtering from isotopic mixtures and alloys of near-neighbor elements

Effects of substrate temperature on the redeposition of bottom-emitted particles to the sidewall surface and the resulting changes in the sidewall properties during SiO2 etching in a CF4 plasma were investigated. A Faraday cage and specially designed, step-shaped substrates located in a plasma etcher allowed us to observe lateral and vertical etch rates, the temperature dependence of redeposition, and resulting changes in the chemical composition of the sidewall surface. We conducted two sets of experiments under different process conditions to observe changes in the temperature effect with ion energy and plasma density. Process (I), which was carried out with a 200 W source power and a −400 V bias voltage, represented a typical reactive ion etching condition for low plasma density and high ion energy, and process (II), with 500 W and −200 V, represented an inductively coupled plasma condition of high plasma density and low ion energy. Lateral etching was more sensitive to substrate temperature than vertical etching. As the substrate temperature was raised, the redep-effect, which was defined as the difference in deposition rates between two sidewalls, either affected by bottom-emitted particles or not, was slightly decreased in process (I) but was significantly increased in process (II). The chemical composition of the sidewall surface was highly dependent on substrate temperature. The carbon content and the F/C ratio of the surface carbon-containing layer formed on the sidewall increased and decreased, respectively, with substrate temperature. The O/Si ratio of the redep-etch combined layer formed beneath the surface carbon-containing layer decreased when the substrate temperature was increased.

A number of new mass spectrometric methods for biomolecules have been introduced in recent years by which it has been possible to considerably extend the mass region towards higher masses. Among many new techniques introduced,those involving neutral or charged particle bombardment of a sample seem most promising. Fast (MeV) ions, like fission fragments, are used in 252Cf-PDMS introduced by R.D. MACFARLANE and collaborators [1–3] in 1974. The use of low energy (keV) ions as in SIMS was pioneered by BENNINGHOVEN and co-workers [4]. CHAIT and STANDING [5] have used a pulsed ion source and time-of-flight technique in connection with low-energy ions. Fast (MeV) ions interact primarily with the electrons in a medium while slow (keV) ions interact with the atoms and cause atomic recoil-cascades. In spite of this difference the mass spectra of desorbed ions look very much the same. However recent measurements by our own group indicate that there are differences [6]. Fast ions are more efficient,i.e. give larger yields of molecular ions than low-energy primary ions and this seems to be even more so the larger the molecule is. Recently BARBER et al. [7] have introduced the so called FAB- (Fast Atom Bombardment) technique. This method has proven to be very efficient in most cases studied so far. Although a beam of keV neutral particles is used the basic interaction with the sample is the same as with keV ions. A maybe more significant difference from the other techniques mentioned is the use of a liquid sample “holder”,i.e. glycerol.KeywordsFast Atom BombardmentFission FragmentBovine InsulinAminoacid ResidueMetastable DecayThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Radiobiological studies focused mainly on tracking the effects of photons (X and gamma rays) and high-energy charged particles over low energy ions, which raises interest in studying their biological effects. Therefore, the aim of this study is to investigate the interaction of low-energy hydrogen ions beam with xcortical bovine bone as a biological model. A hydrogen ions beam was used to bombard cortical bone with different fluences; 45, 70 and [Formula: see text][Formula: see text]ions/cm2 at low energy of 5[Formula: see text]keV. The interaction of hydrogen ions with cortical bone was estimated theoretically by the SRIM/TRIM code. The obtained results showed that the average distribution range and the maximum depth for 5[Formula: see text]keV hydrogen ions are 91 and 150[Formula: see text]nm respectively. The energy losses from incident ions are 4.45[Formula: see text]keV for ionization, 170[Formula: see text]eV for phonons and 18[Formula: see text]eV for vacancies. Also, the energy losses from the recoil atoms are 119[Formula: see text]eV for ionization, 234[Formula: see text]eV for phonons and 8[Formula: see text]eV for vacancies. At the same time, the effect of low-energy hydrogen ions bombardment on the structure of bone was studied by X-ray diffraction (XRD) and FTIR.

In the field of radiation physics, understanding the impact of low-energy ions with high linear energy transfer (LET) is crucial for assessing both radiation protection and particle therapy risks. However, predicting their biological effectiveness is challenging, because commonly assumed track-segment conditions, where ions maintain a constant LET and energy, no longer hold at low energies. Additionally, as ion track sizes shrink to the scale of chromatin structures, inhomogeneities within the cell nucleus can be resolved and the assumption of a uniformly sensitive nucleus becomes inadequate. To address these challenges, we present a low-energy adaption (LEA) of the local effect model (LEM IV), which introduces three key modifications: 1. modeling ion deceleration within the cell nucleus by dividing it into discrete slices to account for energy and LET gradients; 2. incorporating a heterogeneous target structure by distinguishing between radiation-sensitive and insensitive chromatin domains; 3. a more accurate prediction of the linear-quadratic parameter βion by introducing a saturation correction for very high LET. Our results demonstrate that the LEA LEM IV notably improves predictive accuracy at low ion energies. With these adaptions, the LEA successfully reflects the reduced inactivation cross sections observed experimentally, which remain below the geometric cross section of the nucleus. The model shows good agreement with three sets of experimental data, including inactivation cross sections for carbon, argon, and uranium ions, as well as αion values for alpha particles. While computationally more intensive, the LEA provides a crucial tool for precise modeling in low-energy scenarios.

An ultrahigh vacuum (UHV) operating scanning tunneling microscope (STM) has been designed and built with the aim of studying surfaces irradiated with low energy ions. Because the low energy ion implanter was sited in a location remote from the STM laboratory, it was also necessary to design and build a portable vacuum system which was capable of interfacing with both the ion implanter and the STM chamber, and thus, effect a sample transfer under UHV. This article presents a description of the STM and a selection of recent research results which demonstrate the capabilities of the system. In particular, the article reports on the development of small-scale structures on graphite, platinum, and copper surfaces after irradiation with low-energy helium ions and on the growth of copper islands formed by ion beam deposition on graphite substrates.