Low Energy Ion Bombardment Research Articles

Nanoparticle formation induced by low-energy ion bombardment of island thin films

In order to explore whether low-energy ion bombardment of island thin films can be applied for preparation of nanoparticles supported on solid surfaces, morphological and compositional modifications of discontinuous silver layers deposited onto amorphous carbon or native oxide covered silicon substrates were followed by transmission electron microscopy (TEM) and electron spectroscopy during sputter etching with Ar+ ions. TEM micrographs confirmed the formation of isolated nanoparticles, while electron spectroscopic measurements supported the lack of chemical interaction and appreciable intermixing, although the latter is considered to be a major drawback of the method. Comparative studies carried out on the gold/silicon system with a strong tendency towards spontaneous alloying revealed that the unavoidable mixing due to ballistic effects can be compensated by chemically guided diffusion processes governed by the thermodynamic properties of the film/substrate pair. According to the theoretical description of ion-induced mixing, such a mechanism is feasible if the components of the film/substrate pair tend to segregate in equilibrium, which gives the necessary condition for the success of ion etching induced nanoparticle preparation.

Comparison of surface morphologies of SiO 2 films prepared by ion-beam induced chemical vapor deposition and ion-beam assisted deposition

The surface topographies of silicon dioxide films prepared by low-energy ion-beam induced chemical vapor deposition (IBICVD) and ion-beam assisted deposition (IBAD) were studied using an atomic force microscopy (AFM). In the case of IBICVD, when prepared using bubbled hexamethyldisiloxane (HMDSO) and assisted O 2 gas under irradiation of 150 eV Ar ions, the root mean square (RMS) of the surface roughness of the film was 2.0 nm. However, when prepared using the bubbled HMDSO under irradiation of 150 eV oxygen ions, the RMS deduced to 0.25 nm. In the case of IBAD, when deposited using evaporated silicon monoxide under irradiation of 150 eV oxygen ions, the RMS was 0.20 nm. As a reference, the RMS of fused silica was changed from original 0.70–0.40 nm after irradiated of 150 eV oxygen ions. It is concluded that low-energy oxygen ion bombardment not only promotes chemical reaction and dissociation of precursor but also leads to a smooth surface due to reactive ion beam etching on the film surface.

Ion beam-induced nanostructuring of A IIIB V semiconductor surfaces studied with dynamic force microscopy and Kelvin probe force spectroscopy

Evolution and structuring of InSb(0 0 1) surface due to ion bombardment has been studied with dynamic force microscopy (DFM) and Kelvin probe force spectroscopy (KPFS) in UHV. Clean surface of InSb(0 0 1) composed of large, atomically flat terraces was obtained by repetitive cycles of low energy ion bombardment and annealing. DFM images revealed that the surface was composed of dimer rows arranged along the 〈1 1 0〉 crystallographic direction. The InSb surface prepared as above was subsequently irradiated, at room temperature with 4 keV Ar + ions at oblique incidence. As a result, wire-like structures were produced which are parallel to the projection of the ion beam on the irradiated surface. The wires with the width of 50–60 nm and the length up to few microns were incorporated into a flat amorphous substrate. KPFS was used to determine the local surface potential on the substrate and along the wires. The surface potential was determined with respect to the work function of the polycrystalline Au film grown on mica.

On the stability of electronic perturbations on a graphite surface produced by low-energy ion bombardment

The stability of the electronic perturbations (EPs) on a graphite surface produced by low-energy argon ion bombardment was investigated. The ion bombardment was performed using an ion irradiation system consisting of a compact microwave ion source directed to the observation site for scanning tunneling microscopy. We measured the change in the number of the EPs upon repetitive ion bombardment and found that the numbers saturated with the ion dose. Similar experiments were made for a carbon ion-bombarded sample, but the number increased in proportion to the ion dose. Long-time observations of the surface after ion bombardment revealed that the number of EPs gradually decreased for the case of argon ion-bombarded sample. The experimental results suggest that desorption of argon may contribute to the decrease of the EP.

Separate cobalt-copper interface smoothening under the action of low-energy argon ion bombardment

Multilayer film structures of the types “buffer gold layer (20 nm)-copper (x nm)-cobalt (y nm)-protective gold layer (10 nm)” and “buffer gold layer (20 nm)-cobalt (y nm)-copper (x nm)-protective gold layer (10 nm)” with x+y≈6 nm obtained by ion beam deposition in vacuum were studied. It was established that the smoothest surface is obtained for a structure of the second type in which the cobalt layer prior to copper deposition was additionally irradiated with a beam of argon ions at an energy below the threshold energy of sputtering in the common vacuum deposition cycle. Additional irradiation with a mixture of argon and helium ions under analogous conditions leads to the formation of a less smooth structure. It is concluded that the cobalt-copper interface is smoothened as a result of multiple collisions of the low-energy argon ion with cobalt atoms during the former ion stopping within two to three of uppermost atomic layers of the target.

Stability of subatomic carbon films on metal surfaces against low-energy gas ion bombardment

The earlier developed original experimental technique for measuring and analyzing the parameters of low-frequency fluctuations of the field-emission current in metal film systems is used to measure the sputtering yield Yf of carbon films (with a coverage Θ ranging from 1 to 4) applied on Fe, Nb, Ta, and U substrates. The value of Yf is calculated by an expression derived within a theoretical model developed. The sputtering ratios were measured for the case when the carbon films are sputtered by H+ and He+ ions with an energy Ei between 2 and 10 keV. With Θ fixed, the energy dependences of Yf are obtained for each of the ions. In addition, for each of the ions, the Θ dependences of Yf are found for several values of Ei. In all the cases, the measured values of Yf far exceed those for pure carbon. With another original technique that combines field-ion microscopy (FIM) and precise measurement of current and/or luminous properties of local regions in FIM images, the energy thresholds Eth of sputtering carbon films applied on the metal surfaces are found. The energy distributions of Yf in the near-threshold energy range for various Θ are obtained.

Polarity dependent carbon enrichment on 6H–SiC{ [formula omitted]} due to low energy ion bombardment

Ion sputtering induced surface composition changes by applying various ions (He +, Ne +, Ar +, Xe +) in the energy range of 0.2–1.5 keV, was measured by AES on the polar faces of 6H–SiC{0 0 0 1}. Carbon enrichment was observed on both faces, and it was different if Ne +, Ar +, Xe + ion sputtering was applied with ion energy lower than 0.4–0.8 keV (depending on projectile); no different enrichment was found for He + ion bombardment at any energy (in this range). Thus C/Si ratio measured by AES after low energy ion (e.g. Xe +) bombardment can be used to identify polarity of the surface.

Electrical properties of CdxHg1−x Te and ZnxCdyHg1−x−y Te modified by low-energy ion bombardment

The phenomenon of conversion of the conductivity type in p-type samples of the CdxHg1−xTe (0.28≤x≤0.55) and ZnxCdyHg1−x−yTe solid solutions bombarded by low-energy argon ions was studied. It is shown that a necessary condition for the conversion effect in CdxHg1−xTe with 0.28≤x≤0.39 is ion neutralization in the bombarding beam. The dependence of the conversion layer thickness in CdxHg1−xTe on the solid solution composition agrees with that predicted by the diffusion model of ion-bombardment-induced conversion of the conductivity type.

Sputtering from ion-beam-roughened Cu surfaces

A comprehensive theoretical and experimental study of sputtering from copper surfaces roughened by low-energy ${\mathrm{Ar}}^{+}$ ion bombardment is reported. The total sputtering yields of thermally deposited Cu samples bombarded by 400-eV and 800-eV ions at 0\ifmmode^\circ\else\textdegree\fi{}--70\ifmmode^\circ\else\textdegree\fi{} angles of incidence have been measured and compared with a numerical model we have developed. To compute sputtering yields from rough surfaces, an original approach has been introduced, which accounts for sputtering anisotropy and shadowing of material emitted at grazing angles. The approach is flexible with respect to surface morphology and can be applied with any submicron structures. To specify the morphology that develops on the Cu surface under low-energy ion bombardment, the surface of bombarded Cu samples has been investigated by scanning electron microscopy. The morphology has been found highly unstable, appearing with random roughening, inclined conelike structures, ripples, or almost flat surfaces, depending on the bombardment conditions. For the samples considered it is found that the angular dependency of the total sputtering yield is strongly affected by surface morphology, which varies with the angle of ion incidence and bombardment energy. Approximations for accounting for the surface roughness required to describe sputtering at particular energy and angular regimes are discussed.

Steady-state surface concentration profiles of primary ion species during secondary ion mass spectrometry measurements

The steady-state surface concentration profiles of Cs ions during secondary ion mass spectrometry (SIMS) measurements were investigated in details for Si using low-energy oxygen primary ion bombardment. Depending on the bombardment conditions of Cs primary ions, there are shoulder regions or transient regions under the sputtered surface where Si secondary ion intensities are lower than those at bulk region. To achieve accurate depth profiling with minimum transient region using Cs primary ions, it is necessary to choose bombardment conditions such that primary ion energy is below 1 keV or large angle of incidence over 80° will be needed for higher primary ion energy.

In situ probing of surface hydrides on hydrogenated amorphous silicon using attenuated total reflection infrared spectroscopy

An in situ method based on attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) is presented for detecting surface silicon hydrides on plasma deposited hydrogenated amorphous silicon (a-Si:H) films and for determining their surface concentrations. Surface silicon hydrides are desorbed by exposing the a-Si:H films to low energy ions from a low density Ar plasma and by comparing the infrared spectrum before and after this low energy ion bombardment, the absorptions by surface hydrides can sensitively be separated from absorptions by bulk hydrides incorporated into the film. An experimental comparison with other methods that utilize isotope exchange of the surface hydrogen with deuterium showed good agreement and the advantages and disadvantages of the different methods are discussed. Furthermore, the determination of the composition of the surface hydrogen bondings on the basis of the literature data on hydrogenated crystalline silicon surfaces is presented, and quantification of the hydrogen surface coverage is discussed.

Twin-layer films of copper phthalocyanine and amorphous titanyl or vanadyl phthalocyanine - an x-ray photoelectron spectroscopic study

Metallophthalocyanine (MPc) twin-layer thin films on glass substrates, consisting of copper phthalocyanine (CuPc) overlayer and amorphous vanadyl phthalocyanine (VOPc) as well as titanyl phthalocyanine (TiOPc) buffers, were found to have different gas sensing characteristics. These twin-layer thin films were studied using x-ray photoemission spectroscopy. Photoelectrons of either V or Ti core levels were found to be present at the surface, indicating the existence of possible molecular migrations. The stability of the twin-layers under thermal annealing up to a maximum temperature of 250°C and low-energy Ar ion bombardment was also examined and compared with that of CuPc directly grown on the substrates. We concluded that the twin-layer structures were thermally fairly stable. Ar ion bombardment, however, caused substantial damage to the Pc ligands and a reduction of the valence state of the central Cu atoms.

Interstitial nitrogen induced by low-energy ion beam nitridation of AIII–BV semiconductor surfaces

The nitridation of GaAs, InAs, and InSb by low-energy N2+ ion bombardment at room temperature was studied by near-edge x-ray absorption fine structure (NEXAFS) and x-ray photoelectron spectroscopy measurements. The formation of thin surface nitride layers, consisting mostly of GaN or InN but also containing minor amounts of mixed nitrides, was observed. Besides the nitride-related features, sharp peaks in the NEXAFS due to π* resonance at 401.0 eV and correlated peaks at 403.8 eV in N 1s core level spectra were detected. Both spectral features could be assigned to the presence of interstitial nitrogen, most likely molecular nitrogen. It was found that the amount of interstitial nitrogen in the surface layer strongly depends on the AIII–BV semiconductor system and may be affected by modification of the conditions during low energy ion bombardment.

Ability of a gridless ion source to functionalize polypropylene surfaces by low-energy (60–100 eV) nitrogen ion bombardment. Effects of ageing in air and in water

Polymer surface treatments are of great importance for various industrial applications that can range from food packaging or car bumper painting to biocompatible implants. For such applications, plasma or corona discharges are well known and used “on-line” in polymer plants to incorporate new chemical functions at polymer surfaces. The aim of this work is to investigate the ability of a simple ion source to functionalize polypropylene surfaces in comparison with these intensively used industrial methods. Estimation of the influence of the nitrogen ion dose (from 1×10 15 to 1×10 16 ions/ cm 2 ) and the ion energy (from 60 to 100 eV) on incorporation is attempted in order to be able to use them in a controlled way. The ageing of these polymer surfaces in air and in water was studied by XPS for several months.

The effect of low-energy ion bombardment on the density and crystal structure of thin films

The effect of low-energy ion bombardment on the growth and properties of thin films deposited by rf plasma sputtering at low substrate temperatures is studied. The dependences of the film thickness, density, crystal structure, and conductivity on the bias voltage applied to the substrate are obtained. At biases ranging from 0 to −30 V, nickel films are polycrystalline; at higher biases, they exhibit axial (111) texture. At the bias −60 V, the density of the Ni films is close to that of the bulk metal and the crystal structure of the films is the most ordered. With a further increase in the bias, the density of the films drops because of gas (argon and residual gases) atoms incorporated into the films. The same bias dependence of the density is observed for amorphous films of binary alloys of d and f metals. In this case, the films deposited at the substrate bias −40 V have the highest density.

Synthesis of metal nitrides by low-energy ion assisted film growth

In this short report the influence of the low-energy ion bombardment of growing films on the structural properties is demonstrated and discussed. The preferred orientation and texture are measured in dependence on the ion energy, angle of incidence and film thickness. The preferred orientation is changed from a {111} alignment of the TiN crystallites to the {100} orientation with increase of ion energy and ion current density. The film formation at ion bombardment under a specifically selected angle results in a totally fixed orientation or biaxial texture of the crystallites. The measurements also show that the {100} biaxial texture is changed to a {111} biaxial texture with increase of the film thickness. The crystalline structure of epitaxial GaN films on c-plane sapphire depends strongly on the ion beam parameter. To obtain high quality films low ion energies are required. Pole figure measurements show that even with unadvantageous growth parameters the GaN films remain epitaxial.

Formation of supported nanoparticles from island thin films during ion etching

Nanosized particles can be prepared either by deposition of material onto a substrate or by removing atoms from an already existing structure. While the former approach is well established and widely used, the latter is hardly explored, although it can be realized in such a simple way as reducing the particle sizes of an island thin film by low energy ion bombardment. In this work, the process of nanoparticle formation has been investigated during Ar + ion etching of silver layers deposited on native silicon oxide and amorphous carbon. TEM morphological studies have revealed gradual decrease in the size of the islands due to increasing ion dose. Electron spectroscopic measurements have supported the lack of chemical interaction and appreciable intermixing, although the latter is considered to be a major drawback of the method. On the contrary, more reactive systems such as gold on silicon have shown significant mixing induced by the ion beam, which suggests that chemical inertness of the island–substrate couple is a prerequisite for the success of nanoparticle preparation during ion etching.

Dual-electrode biasing for controlling ion-to-adatom flux ratio during ion-assisted deposition of diamond

A dual-bias method using a grid mesh inserted into the front of a substrate has been employed to control the ion-to-adatom flux ratio in an inductively coupled plasma for depositing crystalline materials preferring low-energy ion bombardment. The Langmuir probe measurements revealed that the ion flux toward the substrate decreased with increasing a positive substrate bias with the grid grounded, while it increased with increasing a positive grid bias with the substrate grounded. Ion energy analyses along the diffusing plasma stream by using a probe and a mass spectrometer revealed the contribution of a high-energy tail in the ion-energy distribution into the bombarding ion flux. The ion-assisted deposition of diamond at a pressure of 10 mTorr was performed at a bombarding ion energy as low as the drifting energy (∼several eV). The results indicate the need for optimizing the ion-to-adatom flux ratio for efficient migration and clustering of precursor adatoms yielding a high nucleation density over 109 cm−2.

Dynamics of pattern formation during low-energy ion bombardment of Si(0 0 1)

Sputtering of surfaces by collimated, low-energy ion beams results in spontaneous pattern formation in many systems. In order to explore the mechanisms that control the pattern formation, we have used in situ light scattering to measure the evolution of sputtered Si(0 0 1) surfaces. The results are interpreted within a linear instability model originally proposed by R.M. Bradley and J.M.E. Harper [J. Vac. Sci. Technol. A 6 (1988) 2390] that includes the dependence of the sputter yield on the local surface morphology.

Surface modifications in diamond-like carbon films submitted to low-energy nitrogen ion bombardment

Amorphous hydrogenated carbon films (a-C:H) films deposited by plasma enhanced chemical vapor deposition in CH 4 atmosphere were submitted to nitrogen r.f.-plasma treatment. The influence of several parameters was investigated: the chamber pressure, ranging from 0.5 to 10 Pa, the self-bias voltage in the range between −50 and −500 V and the exposure time. The erosion rate was determined by profilometry. It increases as the self-bias increases and tends to saturate for higher pressures. The nitrogen incorporated in the films was measured by nuclear reaction analysis. The results indicate a decrease in the N content as the pressure or the self-bias increase. The chemical bonds were probed by XPS and two chemical environments can be identified, one with N atoms surrounded by C atoms and the other one with N atoms binding to H. Surface roughness and friction coefficient modifications were followed by atomic force and lateral force microscopies. The surface roughness is independent on the value of the self-bias voltage, while the friction coefficient decreases.