Kinetic energy distributions of neutral In and In2 sputtered by polyatomic ion bombardment

Kinetic energy distributions of neutral In monomers and In₂ dimers sputtered from a polycrystalline indium surface under bombardment with 5 keV/atom Au₁⁻ and Au₂⁻ projectiles have been investigated by means of laser postionization time-of-flight mass spectrometry. Results show that 5 keV Au₁ bombardment leads to results in full compliance with linear cascade sputtering theory. For polyatomic ion bombardment, we find a clear transition to a collisional spike dominated emission process. The spike contribution appears as a low-energy part in the sputtered flux which increases with increasing projectile nuclearity and energy. We show that, the velocity spectrum associated with the low-energy contribution is virtually identical for sputtered monomers and dimers. This finding has important implications with respect to the particle emission mechanism under polyatomic projectile bombardment.     

Determination of energy dependent ionization probabilities of sputtered particles

We present a novel method to determine the spectral ionization probability of sputtered species as a function of their emission velocity or energy. The technique is based on detection of neutral and ionic species in a reflectron time-of-flight mass spectrometer under otherwise identical experimental conditions. Using a pulsed ion extraction scheme in combination with sufficiently short primary ion pulses, the spectral ionization probability α⁺(v) can be determined without knowledge of possible energy discrimination effects in instrument transmission. Comparing the measured ionization probability with theoretical predictions, we find that none of the prevailing ionization models is capable of describing the experimental data over the whole velocity range studied.     

Modeling the dissociation and ionization of a sputtered organic molecule

The evolution of an organic molecule after sputtering from a gold surface has been analyzed by classical molecular dynamics and ab initio calculations to gain insight into the ionization and fragmentation processes occurring in SIMS. The calculated ionization potential (6.2 eV) of the tetraphenylnaphthalene (TPN) molecule has been found to be close to the unimolecular dissociation energy (5.4 eV) of the most favorable reaction channel involving the loss of a phenyl ring. On the other hand, our calculations show that the internal energies of sputtered TPN molecules can be significantly larger than 5-6 eV. Therefore, it appears energetically possible to relax such excited molecules via both fragmentation and ionization.We propose to virtually decompose the TPN molecule into its basic fragments. The rationale is that, if the molecule is very excited, then separate parts (e.g. pendant phenyl rings) can interact with each other almost independently. The analysis of the molecular motion after emission shows that the oscillations along the phenyl-naphthalene bond direction, expected to induce the molecule fragmentation by the loss of a phenyl ring, are relatively small (they store only about 0.2 eV). On the other hand, the relative energy stored in the inter-phenyl interactions, modulated by their bending and responsible for ionization according to our hypothesis, oscillates over a range of 6-7 eV and favors ionization.     

Optical emission spectroscopy by Ar ion sputtering of Ti surface under O2 environment

Visible light emission from atoms and ions sputtered on a polycrystalline Ti surface was observed under irradiation of 30 keV Ar³⁺ ions. A number of atomic lines of Ti I and II were observed in the wavelength of 250-850 nm. The intensity of Ti II emission increased 1.3-5.6 times by introducing oxygen molecules at a pressure of 5.8 × 10⁻⁵ Pa, whereas that of Ti I decreased 0.5-0.8 times. Factors enhancing or reducing photon intensities were plotted as a function of energy of the corresponding electrons in the excited states for Ti atoms and Ti⁺ ions.     

Angular distribution of sputtered matter under Cs bombardment with oblique incidence

Silicon, germanium and indium phosphide targets are sputtered with a cesium ion beam. The energy of impact is changed from 2 keV to 10 keV and the incidence angle of bombardment is modified from 30° to 60°. Emitted matter is collected on a semi-cylindrical copper foil. Subsequently, spatially resolved thicknesses and elemental compositions of the deposit are determined by means of SIMS depth profiles. These distributions across the deposit allow us to deduce the angular distribution of emitted matter. Our experimental data show that the preferential direction as well as the spreading around this direction can be altered, with more or less efficiency, by the variation of the bombardment parameters. For the indium phosphide, we also study the elemental composition of the deposit in function of the emission angle. It shows an increasing deviation from stoichiometry with increasing emission angle.     

Sputtering of thin benzene and polystyrene overlayers by keV Ga and C60 bombardment

The mechanisms of ion-stimulated desorption of thin organic overlayers deposited on metal substrates by mono- and polyatomic projectiles are examined using molecular dynamics (MD) computer simulations. A monolayer of polystyrene tetramers (PS4) physisorbed on Ag{1 1 1} is irradiated by 15 keV Ga and C₆₀ projectiles at normal incidence. The results are compared with the data obtained for a benzene overlayer to investigate the differences in sputtering mechanisms of weakly and strongly bound organic molecules. The results indicate that the sputtering yield decreases with the increase of the binding energy and the average kinetic energy of parent molecules is shifted toward higher kinetic energy. Although the total sputtering yield of organic material is larger for 15 keV C₆₀, the impact of this projectile leads to a significant fragmentation of ejected species. As a result, the yield of the intact molecules is comparable for C₆₀ and Ga projectiles. Our data indicate that chemical analysis of the very thin organic films performed by detection of sputtered neutrals will not benefit from the use of C₆₀ projectiles.     

Anisotropic energy distribution of sputtered atoms induced by low energy heavy ion bombardment

The theory of anisotropic sputtering published in Phys. Rev. B 71(2), 026101 (2005) and Radiat. Effects Defects Solids 159(5), 301 (2004) has been modified and used to calculate the sputtering yield energy distributions for copper, tungsten, and aluminum targets bombarded by low-energy argon ion. As usual, the electronic stopping is ignored in the analysis. The present theory (modified Sigmund’s theory) has been shown to fit the corresponding experimental results of sputtering yield energy distributions well, except for the cases where the larger ion incident angle and larger sputtering emission angles were considered. The larger discrepancy between the present theory and the experimental result in the latter cases is probably due to the influence of direct recoil atoms on the energy spectrum. Compared with Falcone’s analytical theory, the present theory can reproduce much better experimental results of sputtering phenomena. The fact clearly demonstrates the intrinsic relation between the ion–energy dependence of the total sputtering yield and the sputtering yield energy distribution and suggests the great importance of momentum deposited on the target surface in the physical sputtering     

Quantitative study of oxygen enhancement of sputtered ion yields. I. Argon ion bombardment of a silicon surface with O2 flood

A novel quantification approach is applied to determine in situ the amount of surface oxygen within the sputtered particle escape depth during steady-state sputter depth profiling of silicon under simultaneous oxygenation with an oxygen flood gas or with an oxygen primary ion beam. Quantification is achieved by comparing the secondary ion intensities of ¹⁶O that is adsorbed or implanted at the Si surface with the measured peak intensities of a calibrated ¹⁸O ion implant used as a reference standard. Sputtered ion yields can thereby be related to surface oxygen levels. In the present work the dependences of the partial silicon sputter yield Y and of the positive and negative secondary ion useful yields UY(X^±) (X = B, O, Al, Si, P) on the oxygen/silicon ratio, O/Si, in the sputtered flux are studied for ⁴⁰Ar⁺ bombardment of Si with simultaneous O₂ flooding. The silicon sputter yield is found to decrease with increasing flood pressure and O/Si ratio by up to a factor of 3. Both positive and negative secondary ion yields are enhanced by the presence of oxygen at the silicon surface. The useful ion yield of Si⁺ scales non-linearly with the atom fraction of surface oxygen; this behavior is shown to invalidate models that suggest that Si⁺ ion yield enhancement is dominated either by isolated oxygen atoms or by formation of SiO₂ precipitates. In contrast a microscopic statistical model that assumes that local Si⁺ ion formation depends only on the number of oxygen atoms coordinated to the Si atom to be ejected fits the ion yield data quantitatively.     

Formation of Sim and SimCn clusters by C60 sputtering of Si

The secondary ion mass spectrum of silicon sputtered by high energy C₆₀⁺ ions in sputter equilibrium is found to be dominated by Si clusters and we report the relative yields of Si_m⁺ (1 ≤ m ≤ 15) and various Si_mC_n⁺ clusters (1 ≤ m ≤ 11 for n = 1; 1 ≤ m ≤ 6 for n = 2; 1 ≤ m ≤ 4 for n = 3). The yields of Si_m⁺ clusters up to Si₇⁺ are significant (between 0.1 and 0.6 of the Si⁺ yield) with even numbered clusters Si₄⁺ and Si₆⁺ having the highest probability of formation. The abundances of cluster ions between Si₈⁺ and Si₁₁⁺ are still significant (>1% relative to Si⁺) but drop by a factor of ∼100 between Si₁₁⁺ and Si₁₃⁺. The probability of formation of clusters Si₁₃⁺-Si₁₅⁺ is approximately constant at ∼5 × 10⁻⁴ relative to Si⁺ and rising a little for Si₁₅⁺, but clusters beyond Si₁₅ are not detected (Si_m≥16⁺/Si⁺ < 1 × 10⁻⁴). The probability of formation of Si_m⁺ and Si_mC_n⁺ clusters depends only very weakly on the C₆₀⁺ primary ion energy between 13.5 keV and 37.5 keV. The behaviour of Si_m⁺ and Si_mC_n⁺ cluster ions was also investigated for impacts onto a fresh Si surface to study the effects that saturation of the surface with C₆₀⁺ in reaching sputter equilibrium may have had on the measured abundances. By comparison, there are very minor amounts of pure Si_m⁺ clusters produced during C₆₀⁺ sputtering of silica (SiO₂) and various silicate minerals. The abundances for clusters heavier than Si₂⁺ are very small compared to the case where Si is the target.The data reported here suggest that Si_m⁺ and Si_mC_n⁺ cluster abundances may be consistent in a qualitative way with theoretical modelling by others which predicts each carbon atom to bind with 3-4 Si atoms in the sample. This experimental data may now be used to improve theoretical modelling.     

Energy distributions of atomic and molecular ions sputtered by C60 projectiles

In the process of investigating the interaction of fullerene projectiles with adsorbed organic layers, we measured the kinetic energy distributions (KEDs) of fragment and parent ions sputtered from an overlayer of polystyrene (PS) oligomers cast on silver under 15 keV C₆₀⁺ bombardment. These measurements have been conducted using our TRIFT™ spectrometer, recently equipped with the C₆₀⁺ source developed by Ionoptika, Ltd. For atomic ions, the intensity corresponding to the high energy tail decreases in the following order: C⁺(E^−0.4) > H⁺(E^−1.5) > Ag⁺(E^−3.5). In particular, the distribution of Ag⁺ is not broader than those of Ag₂⁺ and Ag₃⁺ clusters, in sharp contrast with 15 keV Ga⁺ bombardment. On the other hand, molecular ions (fragments and parent-like species) exhibit a significantly wider distribution using C₆₀⁺ instead of Ga⁺ as primary ions. For instance, the KED of Ag-cationized PS oligomers resembles that of Ag⁺ and Ag_n⁺ clusters. A specific feature of fullerene projectiles is that they induce the direct desorption of positively charged oligomers, without the need of a cationizing metal atom. The energy spectrum of these PS⁺ ions is significantly narrower then that of Ag-cationized oligomers. For characteristic fragments of PS, such as C₇H₇⁺ and C₁₅H₁₃⁺ and polycyclic fragments, such as C₉H₇⁺ and C₁₄H₁₀⁺, the high energy decay is steep (E⁻⁴ − E⁻⁸). In addition, reorganized ions generally show more pronounced high energy tails than characteristic ions, similar to the case of monoatomic ion bombardment. This observation is consistent with the higher excitation energy needed for their formation. Finally, the fraction of hydrocarbon ions formed in the gas phase via unimolecular dissociation of larger species is slightly larger with gallium than with fullerene projectiles.     

Influence of type of bonds in compounds on the mechanism of the sputtered excited particles formation under ion bombardment

A review of studies of the radiation of the excited particles ejected from metals and its chemical compounds under ion bombardment are presented. The observed features of ion-photon emission (IPE) are analyzed taking into account the physical-chemical parameters of solids and spectroscopic parameters of ejected excited particles. Different mechanisms of the ejected excited particles formation for the explanation of the experimental data are considered.     

Energy and size effects in sputtering of surface metal nanoclusters under low energy ion bombardment

Molecular dynamics simulations of sputtering of copper clusters, which consisted of 13, 27, 39, 75 and 195 Cu atoms on a (0 0 0 1) graphite surface by 100-400 eV Ar and Xe ions have been performed. Some of the results have been published previously [Nucl. Instru. Meth. B 227 (2005) 261 and Nucl. Instru. Meth. B 228 (2005) 41], but are again discussed here together with additional results. Energy and size effects of ion backscattering from and sputtering of isolated surface clusters are discussed.     

A comparative study of secondary ion yield from model biological membranes using Aun and C60 primary ion sources

Au_n⁺ and C₆₀⁺ primary ion sources have been used to acquire spectra from phospholipids, symmetric liposomes and asymmetric liposomes. We demonstrate that when using different ion beams different chemical information can be obtained. Symmetric and asymmetric liposomes, with 95% asymmetry, were produced and analysed with Au⁺, Au₃⁺ and C₆₀⁺ primary ion beams. C₆₀⁺ gave the greatest yield from the symmetric liposome but after correcting for the yield effects on the data obtained from the asymmetric liposome it has been shown that C₆₀⁺ is the most surface sensitive, providing the least information from the inner leaflet of the liposome. Au_n⁺ provides the greatest amount of information from the inner leaflet. The results present the possibility of designing ToF-SIMS experiments that selectively probe specific regions of a (bio)molecular surface.     

Secondary ion emission from polymer layers by atomic and molecular ion bombardment: Data evaluation based on linear-cascade sputtering theory

This paper constitutes an attempt to rationalise impact-energy dependent yields of molecular secondary ions emitted from polymer samples under bombardment with atomic and molecular primary ions. The evaluation was based upon a comparison with sputtering yields calculated from linear-cascade sputtering theory, including threshold effects. To explore general trends, sputtering yields for carbon, silicon and silver were calculated under impact of normally incident C, F, S, Ga, Xe, Au, SF₅, C_11, C₆₀ and Au₅. The yields of carbon, for example, bombarded with C₆₀ are larger than for Ga by factors of ∼5 and ∼10 at 10 and 100 keV, respectively. However, owing to the fact that the effective threshold energy for sputtering increases with the number of constituents of the projectile, the yields for molecular ion impact start to exceed the yields for atomic ions only at energies between 0.5 and 5 keV. The analysed experimental results relate to molecular ion emission from one monolayer (1 ML) and 9 ML films of polymethacrylate on silver bombarded with Xe and SF₅ ions at energies E between 0.5 and 10 keV. Comparison of (initial) secondary ion yields S⁺ (m/z 143) with calculated sputtering yields suggests that S⁺ constitutes the sum of two contributions. The first, labelled , prevails at low energies and appears to reflect molecule ejection due to the mean effect of nuclear energy deposition (“ordinary” linear-cascade sputtering). The second contribution, , dominates at E > 1.5 keV, increases with a high power of E, but does not correlate with the calculated sputtering yield. It is suggested that is a measure of those impact events which occasionally generate a very high energy density at the surface, thus providing optimum conditions for very efficient ejection of molecules that are located at the rim of the agitated area. The SF₅/Xe secondary ion yield ratios are distinctly different for the two contributions, only about 0.3 for but ∼4 (1 ML) and ∼20 (9 ML) for . The pronounced secondary ion “yield enhancement” frequently reported in the literature for molecular versus atomic ion bombardment appears to be due to an enlargement of the contribution, more so the higher the impact energy and the more massive the projectile. The total (integrated) secondary ion yields, estimated by making use of the reported damage cross sections, were found to be the same for SF₅ bombardment of the 1-ML and the 9-ML samples. This finding calls for more attention towards obtaining high secondary ion yields at minimum sample consumption.     

Ionization of atomic particles sputtered from TiO2(100) surfaces

We report a new experimental method which gives information on ionization probabilities of particles sputtered from solid state surfaces. The method consists of simultaneous measurements of the surface work-functions and the positive and negative sputtered ion currents for different specific surface conditions. Experiments were carried out on stoichiometric, reduced and Cs-covered TiO₂ (100) surfaces. A model of the ionization process is discussed which explains the reported experimental data.     

Characteristics of sputtered TaBx thin films as diffusion barriers between copper and silicon

Thin films of TaB_x interposed between Cu and Si are examined here as diffusion barriers for Cu metallization. In order to investigate the performance of Cu/TaB_x/Si contact systems, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), sheet resistance measurement, scanning electron microscopy (SEM), cross-sectional transmission electron microscopy (XTEM), and Auger electron spectroscopy (AES) depth profile were used. Results of this study indicate that the barrier characteristics are significantly affected by the B/Ta ratio. In addition, the failure mechanism for the Cu/TaB_x/Si contact systems is also discussed herein.     

Caesium/xenon dual beam depth profiling: Velocity of the sputtered atom and ionization probability

In this work, a caesium/xenon co-sputtering gun was used to perform depth profiles of a RhSi layer with varying caesium beam concentration. The positive ion yields were monitored with respect to the varying work function of the solid and the intensities of the ions were plotted with respect to the caesium surface concentration. As expected by the tunneling model, all the M⁺ signals decrease exponentially with the increasing caesium beam concentration. Moreover, the heaviest ion yields decrease faster than the lighter ion ones. This phenomenon can be explained by the different velocities of the departing atoms, which has an important impact on the ionization processes. We then studied the variations of the MCs⁺ yields with respect to the caesium surface concentration and with respect to the nature of the departing atom. Finally, we applied models based on the tunneling model in order to fit our results.     

A reversion of atomic segregation under ion bombardment of Nix Pdy alloys

The angular distribution of atoms sputtered from Ni_xPd_y alloys (x, y=1, 5) under 3 and 10 keV Ar⁺ ion bombardment has been studied experimentally and using computer simulations. A collector technique combined with RBS to analyze the distribution of collected material was used. It was found that the Pd/Ni yield ratio increases with the polar ejection angle θ for all targets excluding NiPd₅. This peculiarity of sputtering was explained by a reversion of atomic segregation at high initial concentrations of Pd atoms in the target.     

Magnetic characterization of Bi(Fe1 − xMnx)O3

Bi(Fe_1 − xMn_x)O₃ ceramics (x up to 0.3) were prepared by rapid sintering. Weak ferromagnetism with two magnetic anomalies at low temperatures was observed for Bi(Fe_0.95Mn_0.05)O₃ and Bi(Fe_0.9Mn_0.1)O₃. From temperature-dependent magnetic relaxation measurements, the anomalies at 20 K and 100 K are related to the freezing of cluster spin glass.     

Effect of buffer layer on VOx film fabrication by reactive RF sputtering

Vanadium oxide VO_x films were fabricated by RF magnetron sputtering on various metal buffer layers or silica glass substrates at a substrate temperature of 400 °C. V₂O₅ film was fabricated on a silica glass substrate, and VO₂ films were fabricated on V, W, Fe, Ni, Ti, and Pt metal buffer layers. The transition temperature of the sample on the V buffer layer was 68 °C and that on the W buffer layer was 53 °C. The VO₂ film was also fabricated on the V buffer layer by non-reactive sputtering using a V₂O₅ target at a substrate temperature of 400 °C.