Ion beam sputtered ultra-thin and nanostructured Ag films for surface plasmon applications

The growth of ultra-thin (10−17 nm) Ag films by ion beam sputtering with controlled morphology suitable for surface plasmon applications is demonstrated. A growth rate of 0.02 nm/s, by employing Ar ion energies as low as 150 eV, is achieved. These conditions result in a surface morphology that consists of oblate particles of size 10-30 nm, depending on the ion energy, with surface roughness of 1-3 nm. The aspect ratio of the grains decreases from 77 to 20 with increase in incident ion energy. The morphology-induced changes are manifested in the shift of the surface plasmon resonance peak from 440 to 480 nm.     

Be radioactive beam production at CIRCE and its utilization in basic and applied physics

A pure ⁷Be beam with an energy E = 1-8 MeV is available for nuclear and applied physics at the 3 MV Pelletron tandem accelerator CIRCE in Caserta. The beam is produced using an offline technique. Typical analyzed beam intensities are about 2 ppA, using cathodes with an activity of the order of 200 MBq. The ⁷Be implantation has been used for both fundamental nuclear physics and applied physics. In particular, different metals have been implanted with ⁷Be in order to study the influence of the chemical composition and of the number of quasi-free electrons of the host material on the ⁷Be half-life. In the field of applied physics, the ⁷Be implantation turns out to be very interesting for wear measurement. In fact, in this case ⁷Be is used as a depth-sensitive tracer. The continuous detection of the sample activity during the wear allows a high sensitivity measurement of wearing speed. The ⁷Be beam production at CIRCE, the implantation procedure and the results obtained from the ⁷Be half-life measurements and the wear characterization of implanted steel samples are described.     

Effect of ion beam irradiation on magnetic and structural properties of Pt/Cr/Co multilayers

This paper discusses the effect of ion beam irradiation on the magnetic and structural properties of Pt/Cr/Co multilayers. We observe Co-Cr-Pt ternary alloy phase formation in 1 MeV N⁺ ion irradiated [Pt (2.5 nm)/Cr (0.8 nm)/Co (3.0 nm)]_×6/Si multilayers for a fluence of 1 × 10¹⁶ ions cm⁻² and beyond. The observed phase formation is accompanied by an enhancement in the average grain size, surface roughness and coercivity. Monte Carlo simulation has been performed to study ion-induced defect evolution and atomic displacements to correlate the above observed effects.     

Cesium/xenon co-sputtering at different energies during ToF-SIMS depth profiling

In this paper, ToF-SIMS dual beam depth profiles of H-terminated silicon wafers were performed with cesium primary ions and for different beam energies. The aim of this study was to investigate the influence of the cesium beam energy on the secondary ion yields during ToF-SIMS dual beam depth profiling. For this purpose, both the cesium beam energy and the cesium surface concentration were varied but the analysis conditions were kept identical for all depth profiles (i.e. Ga⁺ at 25 keV, 45°). For each sputter beam energy (i.e. 250 eV, 750 eV and 2000 eV), the cesium surface concentration was varied by diluting the cesium sputtering beam by xenon ions. This technique allows performing ToF-SIMS depth profiles with cesium surface concentration varying from zero (for pure xenon beam) to a maximum value (for pure Cs beam), depending on the bombardment conditions. For all the beam energies, the Si⁺ signals were found to decrease with the increasing cesium coverage and the lower the energy, the faster the decrease. The Cs⁺, the SiCs⁺ and the signals were found to exhibit a maximum for well defined Cs/Xe mixtures, which were found to depend on the secondary ion species and on the beam energy. Moreover, the maxima were found to shift to higher Cs beam content with the increasing energy. This effect is due to the variation of the cesium surface concentration with the varying beam energy. XPS analysis of the Cs/Xe craters and DYNTRIM computer simulations allowed us to convert the cesium beam scale to a cesium surface concentration scale and to interpret our results.     

Surface amorphization, sputter rate, and intrinsic stresses of silicon during low energy Ga focused-ion beam milling

Transmission electron microscopy (TEM) is a standard technique to characterize microelectronic device structures. As structures shrink to the nanoscale, surface damage produced by focused ion beam (FIB) sample preparation destroying the region of interest and degrading the resolution of TEM images becomes increasingly a problem. The thickness of the damaged layer at the sidewalls of a prepared cross section is around 20-30 nm for silicon at typical beam energies of 30 keV. In order to reduce these artifacts to a minimum low beam energies have been proposed for FIB polishing. We use a combination of molecular dynamics simulations and experiments to assess the influence of the focused ion beam on the surface structure of silicon for beam energies ranging from 1-5 keV and a grazing angle of 10° typically used in low voltage FIB polishing. Under these conditions, the thickness of the amorphous layer depends linearly on the beam energy. Intrinsic surface stresses introduced by FIB are always tensile and of a magnitude of around 1 GPa.     

Angular dependence of electronic sputtering from HOPG

We have studied the angular distribution of 120 MeV Au ion beam induced sputtering yield for three cases: from crystalline highly oriented pyrolytic graphite (HOPG) for (A) normal and (B) 70° incidence and from (C) amorphous carbon sample for normal incidence. An anisotropic distribution of sputtering is observed for HOPG samples studied with a distribution Y = Acosⁿθ + Bexp[−(θ − μ)²σ²]. Though the over-cosine function dependence is observed for all the cases, the anomalous peak observed at 53° for normal incidence for HOPG sample is found to shift to 73° when the sample is tilted by 20°. No peak is observed in the amorphous carbon sample which further confirms that the anisotropy observed is due to the crystal structure and formation of a pressure pulse. The high exponent of over-cosine distribution of sputtering yield (n = 3.2-3.8) signifies formation of intense pressure pulse induced jet like sputtering.     

Study of density effect of large gas cluster impact by molecular dynamics simulations

Large gas cluster impacts cause unique surface modification effects because a large number of target atoms are moved simultaneously due to high-density particle collisions between cluster and surface atoms. Molecular dynamics (MD) simulations of large gas cluster impacts on solid targets were carried out in order to investigate the effect of high-density irradiation with a cluster ion beam from the viewpoint of crater formation and sputtering. An Ar cluster with the size of 2000 was accelerated with 20 keV (10 eV for each constituent atom) and irradiated on a Si(1 0 0) solid target consisting of 2 000 000 atoms. The radius of the Ar cluster was scaled by ranging from 2.3 nm (corresponding to the solid state of Ar) to 9.2 nm (64× lower density than solid state). When the Ar cluster was as dense as solid state, the incident cluster penetrated the target surface and generated crater-like damage. On the other hand, as the cluster radius increased and the irradiation particle density decreased, the depth of crater caused by cluster impact was reduced. MD results also revealed that crater depth was mainly dominated by the horizontal scaling rather than vertical scaling. A high sputtering yield of more than several tens of Si atoms per impact was observed with clusters of 4-20× lower volume density than solid state.     

Sputtering and crystalline structure modification of bismuth thin films deposited onto silicon substrates under the impact of 20-160 keV Ar ions

The sputtering of bismuth thin films induced by 20-160 keV Ar⁺ ions has been studied using Rutherford backscattering spectrometry, scanning electron microscopy and X-ray energy dispersive and diffraction spectroscopy. These techniques revealed increasing modifications of the Bi film surfaces with increasing both ion beam energy and fluence up to their complete deterioration under irradiation conditions E = 160 keV and φ = 1.5 × 10¹⁶ cm⁻², leaving isolated islands of preferred (0 1 2) orientation on the Si substrate. The observed surface morphology and crystalline structure evolutions are likely due to a complex interplay of interaction mechanisms involving both elastic nuclear collisions and inelastic electronic ones. The measured Bi sputtering yields versus Ar⁺ ion fluence for a fixed ion energy exhibit a significant depression at very low φ-values followed by a steady state regime above ∼2.0 × 10¹⁴ cm⁻². Measured sputtering yields versus Ar⁺ ion energy with fixing ion fluence to 1.2 × 10¹⁶ cm⁻² in the upper part of the yield saturation regime are also reported. Their comparison to theoretical model and SRIM 2008 Monte Carlo simulation predictions is discussed.     

Studies of InP nano dots formation after keV Ar irradiation

Temporal evolution of nano dots fabricated, in off-normal geometry but in the absence of rotation, on InP(1 1 1) surfaces by 3 keV Ar ion sputtering is reported here. After 10 min of sputtering, self-assembled nano dots with mean diameter of 24 ± 4 nm display square short range weak ordering. Fully developed square celled arrays of dots with mean diameter of 90 ± 26 nm, are seen beyond the non-linear coarsening regime at the critical time of 40 min. Inverse coarsening of dots in conjunction with surface smoothening, never seen in earlier studies of dot evolution, is observed beyond the critical time.     

Electronic and atomic structure modifications of copper nitride films by ion impact and phase separation

We have studied electronic and atomic structure modifications of Cu₃N films under 100 keV Ne and 100 MeV Xe ion impact. Cu₃N films were prepared on R(11-2 surface)-cut-Al₂O₃ substrates at 250 °C by using a RF-magnetron sputter deposition method. X-ray diffraction (XRD) shows that unirradiated films are polycrystalline with (1 0 0) orientation of cubic structure. We find that the electrical resistivity (∼10 Ω cm before ion impact) decreases by more than two orders of magnitude after the Ne impact at a fluence of ∼10¹³ cm⁻², where no Cu phase separation is observed. For further ion impact (larger than ∼10¹⁵ cm⁻²), XRD shows Cu diffraction peak (Cu phase separation), and the resistivity decreases further (three orders of magnitude). Decomposition and phase separation are discussed based on these results, as well as temperature dependence of the resistivity and optical absorption. The results of 100 MeV Xe ion impact are compared with those of Ne ion impact.     

Nano-scale modification of 2D surface structures exposed to 6 keV carbon ions: Experiment and modeling

In this work, a Si pitch grating with typical lateral dimensions of 200-250 nm was exposed to 6 keV C⁺ ions at normal incidence and at an angle of 42° both parallel and perpendicular to the grating structure. In contrast to volatile and recycling ions (like Ar⁺ or H⁺), non-recycling ions are able to modify the surface not only due to sputtering, but also due to implantation of incident ions and the re-deposition of projectile atoms following sputtering or reflection. The target-projectile combination used in this work is an example of such a system forming a mixed Si-C surface. The interaction between the ion beam and the surface has been studied both experimentally and numerically with the focus on validation of the numerical model of the newly developed SDTrimSP-2D code. SDTrimSP-2D is capable of following the evolution of the Si-C system including ion-surface interactions with 2D micro- and nano-structured surfaces. The SDTrimSP-2D code takes the interdependency of surface morphology, sputtering and implantation into account. The simulated surface morphology has been compared to cross-sections of bombarded Si pitch grating obtained by SEM, revealing good agreement between experiment and simulation. The calculations also provide improved insight into the mechanisms of surface modification by sputtering, implantation and material transport by redeposition.     

Ion beam induced modifications in electron beam evaporated aluminum oxide thin films

Al₂O₃ thin films find wide applications in optoelectronics, sensors, tribology etc. In the present work, Al₂O₃ films prepared by electron beam evaporation technique are irradiated with 100 MeV swift Si⁷⁺ ions for the fluence in the range 1 × 10¹² to 1 × 10¹³ ions cm⁻² and the structural properties are studied by glancing angle X-ray diffraction. It shows a single diffraction peak at 38.2° which indicates the γ-phase of Al₂O₃. Further, it is observed that as the fluence increases up to 1 × 10¹³ ions cm⁻² the diffraction peak intensity decreases indicating amorphization. Surface morphology studies by atomic force microscopy show mean surface roughness of 34.73 nm and it decreases with increase in ion fluence. A strong photoluminescence (PL) emission with peak at 442 nm along with shoulder at 420 nm is observed when the samples are excited with 326 nm light. The PL emission is found to increase with increase in ion fluence and the results are discussed in detail.     

Change in magnetic properties of MgB2 thin films after irradiation by 200 MeV Au ion beam

The SHI irradiation induced effects on magnetic properties of MgB₂ thin films are reported. The films having thickness 300-400 nm, prepared by hybrid physical chemical vapor deposition (HPCVD) were irradiated by 200 MeV Au ion beam (S_e ∼ 23 keV/nm) at the fluence 1 × 10¹² ion/cm². Interestingly, increase in the transition temperature T_c from 35.1 K to 36 K resulted after irradiation. Substantial enhancement of critical current density after irradiation was also observed because of the pinning provided by the defects created due to irradiation. The change in surface morphology due to irradiation is also studied.     

Radiation tolerance of Cu/W multilayered nanocomposites

Magnetron sputtered Cu/W multilayer samples with individual layer thicknesses from 2.5 to 50 nm were irradiated by 50 keV He⁺ ions at ion fluences from 7 × 10²⁰ to 6 × 10²¹ m⁻² at room temperature. Evolution of the interfacial structure during irradiation is monitored by X-ray diffraction and cross-sectional transmission electron microscopy. Moreover, radiation responses on the individual layer thickness and He⁺ ion irradiation fluence are revealed. The highly morphological stability of the multilayered structure suggests that the interfacial structure and grain boundary can serve as sinks for radiation-induced defects.     

Effect of different ion beam energy on properties of amorphous carbon film fabricated by ion beam sputtering deposition (IBSD)

Amorphous carbon (a-C) films were fabricated by ion beam sputtering technique. The influence of sputtering ion beam energy on bonding structure, morphologic, mechanical properties, tribological properties and corrosion resistance of a-C films are investigated systematically. Morphology study shows that lowest surface roughness exists for mid-ion beam energy. Improved adhesion is observed for the films that are prepared under high ion beam energy, attributed to film graphitization, low residual stress and mixed interface. Relatively, a-C films prepared with ion beam energy of 2 keV exhibits optimum sp³ bond content, mechanical properties and corrosion resistance. It is found that the wear rate of DLC films decrease with increased ion beam energy in general, consistent with the varied trend of the H/E value which has been regarded as a suitable parameter for predicting wear resistance of the coatings. The correlation of the sp³ bond fraction in the films estimated from Raman spectroscopy with residual stress, nanohardness and corrosion resistance has been established.     

Temperature dependence of liquid Sn sputtering by low-energy He and D bombardment

Absolute sputtering yields of liquid tin from 240 to 420 °C due to irradiation by low-energy helium and deuterium have been measured. For ion energies ranging from 300 to 1000 eV, temperature enhancement of liquid tin sputtering was noted. These measurements were obtained by IIAX (the Ion-surface InterAction eXperiment) using a velocity-filtered ion beam at 45° incidence to sputter material from a liquid tin target onto deposition monitors. Sputtering yields from 500 eV ion bombardment at 45° incidence increase from 0.1 ± 0.03 and 0.019 ± 0.008 Sn particles/ion at room temperature, for He⁺ and D⁺ ions respectively, to 0.30 ± 0.12 and 0.125 ± 0.05 Sn particles/ion for 380 °C. Temperature enhanced sputtering has been seen in other liquid metals (namely lithium, tin-lithium, and gallium) using both ion beam and plasma irradiation.     

Growth and surface morphology of ion-beam sputtered Ti-Ni thin films

Titanium-nickel thin films have been deposited on float glass substrates by ion beam sputtering in 100% pure argon atmosphere. Sputtering is predominant at energy region of incident ions, 1000 eV to 100 keV. The as-deposited films were investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). In this paper we attempted to study the surface morphology and elemental composition through AFM and XPS, respectively. Core level as well as valence band spectra of ion-beam sputtered Ti-Ni thin films at various Ar gas rates (5, 7 and 12 sccm) show that the thin film deposited at 3 sccm possess two distinct peaks at binding energies 458.55 eV and 464.36 eV mainly due to TiO₂. Upon increasing Ar rate oxidation of Ti-Ni is reduced and the Ti-2p peaks begin approaching those of pure elemental Ti. Here Ti-2p peaks are observed at binding energy positions of 454.7 eV and 460.5 eV. AFM results show that the average grain size and roughness decrease, upon increasing Ar gas rate, from 2.90 μm to 0.096 μm and from 16.285 nm to 1.169 nm, respectively.     

Angular distribution of GaAs sputtered under oblique Cs bombardment

The angular distribution of Ga and As sputtered from Gallium Arsenide (1 0 0) by a Cs⁺ ion beam was experimentally measured through a collector technique allowing modifications of the energy and incidence angle of the ion beam. The impact energy was varied in the range of 2-10 keV and the angle of incidence from 30° to 60°.The angular distributions of emitted matter are determined by means of SIMS depth profiles. Our series of experiments show an evolution of the preferential direction of emission as well as the spreading around this direction in function of the characteristics of the ion beam.The second objective is the study of the evolution of the stoichiometry of the deposit in function of the emission angle. A decrease of the As/Ga ratio around the preferential direction of emission and an increase of this ratio for oblique emission are observed for different conditions of primary bombardment. Considering that the angular distribution depends on the depth of origin, our results suggest that the Cs⁺ bombardment changes the stoichiometry of the near-surface layers of the sample with an enrichment of As in the outmost layers while the sub-surface region is impoverished in As due to preferential sputtering.     

Fundamentals of surfactant sputtering

We introduce a new sputter technique, utilizing the steady-state coverage of a substrate surface with up to 10¹⁶ cm⁻² of foreign atoms simultaneously during sputter erosion by combined ion irradiation and atom deposition. These atoms strongly modify the substrate sputter yield on atomic to macroscopic length scales and therefore act as surfactant atoms (a blend of “surface active agent”). Depending on the surfactant-substrate combination, the novel technique allows enhanced surface smoothing, generation of novel surface patterns, shaping of surfaces and formation of ultra-thin films. Sputter yield attenuation is demonstrated for sputtering of Si and Fe substrates and different surfactant species using 5 keV Xe ions at different incidence angles and fluences up to 10¹⁷ cm⁻². Analytical approaches and Monte Carlo simulations are used to predict the sputtering yield attenuation as function of surfactant coverage. For sputtering of Si with Au surfactants we observe high sputter yields despite a steady-state surfactant coverage, which can be explained by strong ion-induced interdiffusion of substrate and surfactant atoms and the formation of a buried Au_xSi surfactant layer in dynamic equilibrium.