Neutralization of keV Ne and Ar ions during grazing scattering from an Al(1 1 1) surface

Charge fractions after scattering of Ne⁺ ions, Ne⁰ atoms and Ar⁺ ions with keV energies under a grazing angle of incidence from an atomically clean and flat Al(1 1 1) surface are studied. For incoming Ne⁺ ions we observe defined ion fractions in the scattered beams, whereas for incident Ne⁰ atoms ion fractions are more than one order of magnitude smaller. This experimental result provides clear evidence for a survival of Ne⁺ ions over the whole scattering event. From the dependence of ion fractions on the perpendicular energy component we derive neutralization rates as function of distance from the surface. These rates compare well with recent theoretical calculations for the system He⁺–Al(1 1 1). For incident Ar⁺ ions no survival of ions is found and upper limits for the survival probability and lower limits for the neutralization rate are determined.     

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.     

Temperature dependence of the chemical sputtering of amorphous hydrogenated carbon films by hydrogen

The temperature dependence of chemical erosion and chemical sputtering of amorphous hydrogenated carbon films due to exposure to hydrogen atoms (H⁰) alone and combined exposure to argon ions and H⁰ was measured in the temperature range from 110 to 950 K. The chemical erosion yield for H⁰ alone is below the detection limit for temperatures below about 340 K. It increases strongly with increasing temperature, goes through a maximum around 650–700 K and decreases again for higher temperatures. Combined exposure to Ar⁺ and H⁰ results in substantial chemical sputtering yields in the temperature range below 340 K. In this range the yield does not depend on temperature, but it increases with energy from about 1 (eroded carbon atoms per impinging Ar⁺ ion) to about 4 if the ion energy is increased from 50 to 800 eV. For temperatures above 340 K the measured erosion rates show the same temperature dependence as for the H⁰-only case, but they are higher than for H⁰-only. The difference between the Ar⁺ and H⁰ and the H⁰-only cases increases monotonically with increasing ion energy.     

Surface morphology of the Ar ion-irradiated Ag/Si(1 1 1) system

In the present study, a 500 Å thin Ag film was deposited by thermal evaporation on 5% HF etched Si(1 1 1) substrate at a chamber pressure of 8×10⁻⁶ mbar. The films were irradiated with 100 keV Ar⁺ ions at room temperature (RT) and at elevated temperatures to a fluence of 1×10¹⁶ cm⁻² at a flux of 5.55×10¹² ions/cm²/s. Surface morphology of the Ar ion-irradiated Ag/Si(1 1 1) system was investigated using scanning electron microscopy (SEM). A percolation network pattern was observed when the film was irradiated at 200°C and 400°C. The fractal dimension of the percolated pattern was higher in the sample irradiated at 400°C compared to the one irradiated at 200°C. The percolation network is still observed in the film thermally annealed at 600°C with and without prior ion irradiation. The fractal dimension of the percolated pattern in the sample annealed at 600°C was lower than in the sample post-annealed (irradiated and then annealed) at 600°C. All these observations are explained in terms of self-diffusion of Ag atoms on the Si(1 1 1) substrate, inter-diffusion of Ag and Si and phase formations in Ag and Si due to Ar ion irradiation.     

Autoionising state excitation in collisions of inert gas projectiles with an Al surface and effects of initial stages of oxidation

Results of an angle resolved electron spectroscopy study of low keV He⁺, Ne and Ne⁺ collisions with an Al surface are presented. Production of projectile autoionising states is demonstrated to exist not only for the previously known Ne⁺ case but also for Ne and He⁺. The characteristics of autoionising peak line shapes suggest differences in angular distributions of the different Ne^** states. Initial stages of the oxidation of Al are studied. Saturation coverages of oxygen are reached for exposures greater than 100 L. An initial decrease in the work function is observed amounting to 160 meV for a 30 L exposure. Exposure to oxygen results in a strong decrease of autoionising state production. State-selective attenuation is observed and reflects changes in electron transfer rates.     

Argon irradiation damage at a Cu/Al2O3 interface for different alumina structures

The evolution of damages at a Cu/Al₂O₃ device interface after Ar⁺ irradiation, depending on alumina structure, and the effect of surface roughness on sputtering have been studied. A polycrystalline Cu/Al₂O₃ bilayer and polycrystalline Cu on amorphous alumina were irradiated with 400 keV Ar⁺ ion beam at doses ranging from 5 × 10¹⁶ to 10¹⁷ Ar⁺/cm² at room temperature. The copper layer thicknesses were between 100 and 200 nm. RBS analysis was used to characterize the interface modification and to deduce the sputtering yield of copper. The SEM technique was used to control the surface topography. A RBS computer simulation program was used to reproduce experimental spectra and to follow the concentration profile evolutions of different elements before and after ion irradiation. A modified TRIM calculation program which takes into account the sputtering yield evolution as well as the concentration variation versus dose gives a satisfactory reproduction of the experimental argon distribution. The surface roughness effect on sputtering and the alumina structure influence at the interface on mixing mechanisms are discussed.     

Ion beam enhanced etching of LiNbO3

Single crystals of z- and x-cut LiNbO₃ were irradiated at room temperature and 15 K using He⁺- and Ar⁺-ions with energies of 40 and 350 keV and ion fluences between 5 × 10¹² and 5 × 10¹⁶ cm⁻². The damage formation investigated with Rutherford backscattering spectrometry (RBS) channeling analysis depends on the irradiation temperature as well as the ion species. For instance, He⁺-irradiation of z-cut material at 300 K provokes complete amorphization at 2.0 dpa (displacements per target atom). In contrast, 0.4 dpa is sufficient to amorphize the LiNbO₃ in the case of Ar⁺-irradiation. Irradiation at 15 K reduces the number of displacements per atom necessary for amorphization. To study the etching behavior, 400 nm thick amorphous layers were generated via multiple irradiation with He⁺- and Ar⁺-ions of different energies and fluences. Etching was performed in a 3.6% hydrofluoric (HF) solution at 40 °C. Although the etching rate of the perfect crystal is negligible, that of the amorphized regions amounts to 80 nm min⁻¹. The influence of the ion species, the fluence, the irradiation temperature and subsequent thermal treatment on damage and etching of LiNbO₃ are discussed.     

Inelastic processes in Ne collisions with a Mg surface; charge fractions and surface roughness

Results of a study of inelastic scattering in Ne⁺ collisions with a polycrystalline Mg target, performed by measurements of charge fractions, are reported. In agreement with previous studies, ion production is found to be due to binary collisions with surface atoms and the charge fractions can be well described by a simple exponential dependence on the perpendicular velocity in the outgoing trajectory. This is also shown to be the case for Ar⁺ collisions. Charge fractions and angular distributions of scattered particles are strongly affected by surface roughness effects.     

Neutralization rate for slow Ar ions in front of KCl(0 0 1)

Charge state distributions of reflected ions are measured when 5 keV Ar^q+(q = 0−2) ions are incident on a clean KCl(0 0 1) surface at grazing angle, θ_i. Although the charge state distribution does not depend on the incident charge state at larger θ_i, significant dependence of the charge state distribution on incident charge state is observed at smaller θ_i. The ionization of Ar⁰ is completely suppressed at θ_i < 20 mrad, while large neutralization probability is observed for Ar⁺ incidence. These features allow us to derive the position-dependent neutralization rate of Ar⁺ in front of KCl(0 0 1). The obtained neutralization rate decreases exponentially with distance from the surface as it is usually assumed.     

Sputtering from spherical Au clusters by energetic atom bombardment

Using molecular-dynamics simulation, we study the effect of 100 keV Au atom bombardment of spherical Au clusters (radius R=40 Å), containing 15,784 atoms. Results range from projectile transmission with only few atoms sputtered to more or less complete cluster disintegration. During disintegration, besides major fragments of the original cluster, monatomics and a large number of clusters with sizes up to 100 atoms, and even beyond, are created. Angular and energy spectra of sputtered atoms show features of both collisional sputtering and evaporation: particle emission is isotropic with an additional contribution of preferential emission along [1 1 0] directions. Energy spectra show the high-energy E⁻² fall-off typical of linear-cascade sputtering plus an additional low-energy thermal component.     

Measurements of the erosion of stainless steel, carbon, and SiC by hydrogen bombardment in the energy range of 0.5 – 7.5 keV

Total erosion yields by sputtering and blistering for 1 to 15 keV H₂⁺ bombardment at normal incidence have been measured by weight loss of 304 stainless steel, pyrolytic graphite, carbon fibres, glassy carbon and SiC. The erosion yields are in the range of 3 × 10⁻³ to 2.6 × 10⁻² atoms per incident hydrogen atom. Observation in the scanning electron microscope shows that blisters occur in stainless steel and SiC at doses of 5 × 10¹⁸ particles/cm², but disappear at doses of 5 × 10 particles/cm² . The surface roughening observed depends largely on grain orientation. On carbon no blistering could be found. After bombardment the carbon surfaces are generally more smooth than before.     

Desorption of silver atoms from benzene-covered Ag(1 1 1) by energetic Ar bombardment

Experiments have been conducted to gain insight into the processes of desorption of neutral species from surfaces covered with organic molecules due to bombardment with keV particles. The system is comprised of benzene molecules adsorbed onto Ag(1 1 1) and bombarded with 8 keV Ar⁺ ions. Molecular dynamics (MD) simulations of the same system have been performed. Results show that the presence of the benzene alters the yield, the kinetic energy distributions, and the angular distributions of the silver atoms. These changes of the desorption characteristics are the result of collisions between the Ag atoms and the benzene molecules adsorbed to the surface. As more benzene is adsorbed to the surface, the changes to the Ag atom desorption characteristics become more pronounced. The simulations reproduce the modifications to the Ag atom energy and angle distributions.     

Effects of ion irradiation on cobalt nanocomposite

The effects of ion irradiation of 3-dimensional arrays of Co nanoparticles were investigated. Arrays were obtained by electron beam deposition of 15 Co/SiO₂ bilayers 0.5 and 20 nm thick, respectively. The Co layers consist of Co nanoparticles 3.2 nm in diameter with a standard deviation of 16%. Irradiation was carried out using combinations of 150 kV Ar²⁺ and 90 kV Ar⁺ ion bombardments with a Ar⁺:Ar²⁺ fluence ratio of 1:4. The effects of ion irradiation were followed by Rutherford backscattering, measurements of hysteresis loops at 5 K and of temperature-dependent field-cooled (FC) and zero-field-cooled (ZFC) measurements of the magnetic susceptibility. A decrease of the peak temperature in the ZFC curve for displacements per atom (DPA) up to 1.1 was observed. Irradiation also induces progressively lower coercivity values. The Co particles showed a remarkable high resistance to ion irradiation, surviving damage up to 33 DPA.     

Implantation profiles and sputtering studied by detecting the optical radiation from sputtered particles during ion bombardment

A method of sputter-etching and simultaneous registration of the emitted light is used to obtain depth profiles of implanted Li atoms. Measured mean penetration depths (R_p) and straggling values (ΔR_p) for implanted Li into Ag, V, Si and Al are reported. We find e.g. for 10 keV Li⁺ into $\text{V: R}{}_{\mathrm{n}}\text{= 400 ± 30}\text{A}\text{?}$ and $\text{ΔR}{}_{\mathrm{p}}\text{= 250 ± 30}\text{A}\text{?}$. Known implanted profiles are used as a scale of depth to determine sputtering yields of keV He⁺ ions. With this new method the sputtering yield of 40 keV He⁺ ions bombarding Ag was found to be 0.095 ± 0.020. Sputtering through a thin film is also used to determine sputtering yields. A remarkable increase in the light intensity from excited Ag I atoms is observed during sputtering through the interface between an Ag film and the underlying Al substrate. This is found to be due to a change in the excitation mechanism. Continuous features in the observed emission spectrum have in a few cases been identified as originating from deexcitation radiation of molecules formed in the sputtering process.     

Ion implantation doping and electrical conductivity enhancement of C60 films

Pristine C₆₀ films sublimed onto sheet mica were implanted with 20 keV K⁺ ions and I⁺ ions at doses of 1.0 × 10¹⁶/cm², 3.0 × 10¹⁶/cm² and 5.0 × 10¹⁶/cm², and with 20 keV Ar⁺ ions at a dose of 5.0 × 10¹⁶/cm². The distributions of dopants were studied using Rutherford backscattering spectrometry (RBS). The temperature dependence of sheet resistivity of the films was investigated applying a four-probe system. It was proposed that the conductivity enhancement of K⁺ implanted C₆₀ films was due to the implanted ions in the films, while for I⁺ implanted C₆₀ films, both implanted I⁺ ions and irradiation effects of the ions contributed to the enhancement of conductivity.     

Electron emission induced by H and H incident on thin carbon foils: influence of charge changing processes

The forward and backward electron emission yields γ_F and γ_B have been calculated by Monte Carlo simulation for protons (H⁺) and hydrogen atoms (H⁰) (with energies between 25 keV and 5 MeV) incident on thin amorphous carbon foils. Direct electron excitations by the incident projectiles as well as electron excitations resulting from charge exchange processes undergone by H⁺ or H⁰ have been taken into account. For the latter, Auger and Shell processes have been considered. Subsequent electron transport has been considered in order to calculate the forward and backward electron emission yields γ_F and γ_B.     

Material dependence of total electron emission yields following slow highly charged ion impact

The total electron emission yields following the interaction of “Slow (2 keV/a.u.) Highly Charged Ions” (SHCI) (O³⁺⁷⁺, Xe¹²⁺⁵²⁺, Au⁵⁴⁺⁶⁹⁺) with different target surfaces (highly-oriented pyrolytic graphite (HOPG), Au and SiO2) have been measured. The emission yields increase with charge state, and is found to be highest for carbon, the HOPG target, and lowest for the SiO₂ target. An empirical formula for the electron emission is including recent results from investigations of plasmon excitation following SHCI impact are used to interpret the results.     

Resonant ion stimulated desorption of O from the TiO2(1 1 0) surface: Effects of oxygenation and ion bombardment

The O⁺ desorption from reduced, oxygenated, and ion-bombarded TiO₂(1 1 0) surfaces has been investigated during He⁺ irradiation. The O⁺ desorption is initiated by creation of an antibonding O 2s core hole state via quasi-resonant charge exchange with the He⁺ 1s state, followed by the intra-atomic Auger decay of the O 2s hole. Upon oxygenation of the reduced TiO₂(1 1 0) surface, the O⁺ yield increases by one order of magnitude. The O₂ molecule is dissociated at the vacancy site of bridging oxygen and the oxygen atoms either fill a vacancy site or chemisorb at a fivefold-coordinated Ti⁴⁺ site as an adatom. The latter is detected with much higher efficiency than the former. The O⁺ yield is increased during He⁺ bombardment of the reduced TiO₂(1 1 0) surface due to formation of lower coordinated oxygen atoms. The oxygen species thus formed by ion bombardment or oxygenation are unstable on the surface and tend to diffuse into bulk vacancy sites or higher coordination surface sites even at room temperature.     

Radiation effects on MgAl2O4–yttria stabilized ZrO2 composite material irradiated with Ne ions at high temperatures

Spinel (MgAl₂O₄) and yttria stabilized ZrO₂ (YSZ) are candidates for fuel materials for use in nuclear reactors and the optical and insulating materials for fusion reactors. In our previous studies, the amorphization of spinel under 60 keV Xe ion irradiation at RT was observed. On the other hand, amorphization could not be confirmed in YSZ single crystals under the same irradiation conditions. In the present study, the damage evolution process of polycrystalline spinel–YSZ composite materials has been studied by in situ TEM observation during ion irradiation. The irradiation was performed with 30 keV Ne⁺ ions at a flux of 5 × 10¹³ ions cm⁻² s⁻¹ at 923 K and 1473 K, respectively. The observed results revealed a clear difference in morphology of damage depending on irradiation temperature and crystal grains. In the irradiation at 923 K, defect clusters and bubbles were formed homogeneously in YSZ grains. On the other hand, at 1473 K, only bubble formation was observed. The bubbles grew remarkably with increasing ion fluence in both grains. Even though the growth of the bubbles was observed in both grains, the average diameter of grown bubbles in spinel grains was larger than those in YSZ ones. The bubbles tended to form along the grain boundary at both temperatures.     

PECVD Si nitride and Si oxide layers — Hydrogen analysis and etching after ion implantation

lon-implantation-induced selective etching of dielectric materials is considerably diminished with increasing hydrogen content. Making use of the ¹H(¹⁵N,γ)¹²C resonance reaction, low-temperature PECVD Si oxide and Si nitride layers were observed to contain 12 and 23 at.% H, respectively. For different reagents etch rates were measured regarding the virgin and ion—implanted-He⁺ Ne⁺ at 60, 100 keV — PECVD films.