Ion bombardment of polyethylene—influence of polymer structure

Polyethylenes of various macromolecular and supermolecular structures were studied from the point of view of their susceptibility to an ion beam treatment. An influence of molecular weight (M_w), molecular weight distribution (M_w/M_n) and the degree of branching were compared within the set of low-density polyethylenes (LDPE) studied. An influence of the length of branches was compared between LDPE, linear low-density (LLDPE) and high-density (HDPE) polyethylenes. An influence of the degree of crystallinity and the morphology of a crystalline phase were compared for HDPE samples solidified under various thermal conditions and ultra-high molecular weight polyethylene (UHMWPE). Plate polymer targets ∼2 mm were bombarded with 100 keV He⁺ or 130 keV Ar⁺ ions (dose of 10¹⁴-10¹⁶ ions/cm²; ion energy stream density <0.1 μA/cm²), micromechanical properties of their surface layer (hardness, mechanical modulus and elastic recovery) determined and compared to the virgin materials.Ar⁺ ion beam bombardment generally lowers micromechanical properties of the polyethylenes, whereas He⁺ ion beam treatment makes them higher. The effect is the stronger the higher the molecular weight of polyethylene. However, a long chain branching adversely affects the modification. The degree of crystallinity facilitates an ion beam bombardment from the point of view of micromechanical properties of the materials, however, also the morphology of a crystalline phase was found to play a role.     

Some effects at ion beam modification of polymethyl methacrylate

Measurements of the sensitivity of polymethyl methacrylate to ion irradiation are presented. The results of these studies show for ion energies (40 keV—1.8 MeV) and for the heavier ions (He⁺, Ar⁺) a higher sensitivity of the resist than in H⁺, electron or X-ray exposures. The measured sensitivity shows a dependence on electronic stopping power. In order to understand this relation an electron excitation spike mechanism is suggested. This mechanism is in accordance with investigations of sputtering rate of PMMA during ion bombardment. The effects of low energy ion induced nuclear reactions in the polymer layer and in masks and wafer are also discussed.     

NEXAFS and XPS study of GaN formation on ion-bombarded GaAs surfaces

Interaction of low-energy nitrogen ions (0.3-2 keV N₂⁺) with GaAs (100) surfaces has been studied by X-ray photoemission spectroscopy (XPS) around N 1s and Ga 3d core-levels and near-edge X-ray absorption fine structure (NEXAFS) around the N K-edge, using synchrotron radiation. At the lowest bombardment energy, nitrogen forms bonds with both Ga and As, while Ga-N bonds form preferentially at higher energies. Thermal annealing at temperatures above 350 °C promotes formation of GaN on the surface, but it is insufficient to remove disorder introduced by ion implantation. We have identified nitrogen interstitials and anti-sites in NEXAFS spectra, while interstitial molecular nitrogen provides a clear signature in both XPS and NEXAFS. The close similarity between NEXAFS spectra from thin GaN films and ion-bombarded GaAs samples supports our proposition about formation of thin GaN films on ion-bombarded GaAs.     

He+ backscattering and Auger electron spectroscopy study of the ion-induced modification of copper films deposited on single-crystal silicon

The sputtering yields of copper due to argon ion milling were measured by mega-electronvolt He⁺ Rutherford backscattering and Auger electron spectroscopy for thin copper films deposited on 〈111〉-oriented silicon substrates. We found that recrystallization occurs in the copper films during the argon ion bombardment. The copper profile broadens at the Si-Cu interface and the spread increases with the beam energy during ion milling.     

Surface erosion of TiC coatings under 4He+ and D+ ion irradiation

The surface damage and erosion of chemically vapor-deposited TiC coatings irradiated by 20, 40 and 60 keV D⁺ and ⁴He⁺ were studied for as-deposited and polished surfaces. Scanning electron micrographs of irradiated TiC reveal surface damage and erosion due to blistering and surface exfoliation. The erosion yield increases with increasing ion energy for both D⁺ and He⁺ irradiations, and it is generally larger for D⁺ than for He⁺ irradiations at a given energy. The erosion yield is larger for polished than for as-deposited surfaces for both D⁺ and He⁺ irradiations. The relationship between the blister diameter and the skin thickness agrees well with a relationship derived from a gas pressure model but disagrees with a relationship derived from a model of integrated lateral stresses as a prime mechanism for blister formation.     

The influence of sputtering on FeSi

The effect of different (0.5, 2 and 4 keV) Ar⁺ energy ions on (a) the composition of an FeSi surface, (b) the oxidation of the FeSi surface after bombardment, and (c) the segregation of silicon after bombardment, has been monitored by Auger electron spectroscopy. Silicon was found to be preferentially sputtered by the Ar⁺ ions at all the different energies during bombardment. This effect was more pronounced at the 0.5 keV ion energy bombardment. There was a slight increase in the oxidation rate from the higher to the lower Ar⁺ ion energy at which the sample was sputtered before oxidation. The rate of silicon diffusion to the surface at 593 K after the sample had been sputtered, was lower when the sample had been sputtered by 0.5 keV ions than by 2 keV ions.     

In Situ Mitigation of Subsurface and Peripheral Focused Ion Beam Damage via Simultaneous Pulsed Laser Heating

Focused helium and neon ion (He⁺/Ne⁺) beam processing has recently been used to push resolution limits of direct‐write nanoscale synthesis. The ubiquitous insertion of focused He⁺/Ne⁺ beams as the next‐generation nanofabrication tool‐of‐choice is currently limited by deleterious subsurface and peripheral damage induced by the energetic ions in the underlying substrate. The in situ mitigation of subsurface damage induced by He⁺/Ne⁺ ion exposures in silicon via a synchronized infrared pulsed laser‐assisted process is demonstrated. The pulsed laser assist provides highly localized in situ photothermal energy which reduces the implantation and defect concentration by greater than 90%. The laser‐assisted exposure process is also shown to reduce peripheral defects in He⁺ patterned graphene, which makes this process an attractive candidate for direct‐write patterning of 2D materials. These results offer a necessary solution for the applicability of high‐resolution direct‐write nanoscale material processing via focused ion beams.     

Diffusion of Pb in (100) Si under electron beam annealing following dual ion implantations of Pb/Ne, Pb/O and Pb/N

(100) Si was dual-implanted with the ions Pb⁺/²²Ne⁺ (7 and 30 keV), Pb⁺/¹⁶O⁺ (7 and 26 keV) and Pb⁺/¹⁴N⁺ (7 and 24 keV) to peak concentrations of typically 10 at.%. The implanted samples were then electron beam annealed at 900 °C for 30 s with a temperature gradient of 5 °C s⁻¹ under high vacuum conditions. Channelled RBS measurements performed with 1.5 MeV ⁴He⁺ ions showed that annealing of the Pb/Ne and Pb/O samples resulted in an almost complete recrystallisation of the amorphous layers caused by the ion implantations and a total loss of the implanted Pb. For the Pb/Ne samples the Ne diffused out to leave a rough surface sprinkled with deep craters; for the Pb/O samples some SiO₂ formed below the surface. In contrast, for the Pb/N samples most of the amorphous layer survives annealing and almost all the Pb is retained. A striking feature is that annealing causes the Pb to diffuse away from the surface to be trapped in a deep diffusion sink provided by the implanted N. XRD analyses exhibited Pb (111) and Pb (220) reflections suggesting that Pb nanoclusters have grown in the understoichiometric silicon nitride layer. These structures offer an interesting opportunity for controlled carbon nanotube (CNT) growth on silicon nitride.     

Ion bombardment-induced enhancement of the properties of indium tin oxide films prepared by plasma-assisted reactive magnetron sputtering

Research on tin doped indium oxide (ITO) has for many years been stimulated by the need to simultaneously optimize the electrical, optical and mechanical properties, and by new challenges related to the deposition of transparent conducting oxides on flexible plastic substrates. In the present work, we investigate the growth and optical, electrical, and mechanical (hardness, elastic modulus and stress) properties of ITO films deposited by plasma assisted reactive magnetron sputtering (PARMS) from an indium-tin alloy target. PARMS achieves an effective control of bombardment by reactive species (e.g., O₂⁺, O⁺) on the surface of the growing film by varying the bias voltage, V_B, induced by a radiofrequency power applied to the substrate. Stress-free films possessing high transparency (> 80% — film on glass) and low resistivity (4 × 10^− 4 Ω cm) can be deposited by PARMS under conditions of intense ion bombardment (≤ 600 eV).     

Hydrogen bombardment of the oxide layer on beryllium

Beryllium is being considered as a plasma-side material of low atomic number Z for use in fusion reactors. Untreated beryllium surfaces are usually covered by a thin oxide layer which forms during air exposure or from residual gas adsorption in vacuum. Secondary ion mass spectrometry measurements have been made to investigate the durability of this oxide layer during hydrogen ion bombardment at an energy of 1 keV, which is similar to that expected at the plasma edge in a fusion reactor. The surface condition of machined beryllium samples undergoing hydrogen bombardment was monitored by observing secondary ion emission of O^-, OH^- and BeO^-. On the basis of sputtering yield data for BeO, the oxide layer appears to be up to 10² monolayers in thickness. At the metal interface it was possible to determine the cross section for the removal of adsorbed oxygen. The measured value is 1.5 × 10^-17 cm², which is in agreement with bulk oxide sputtering data. This suggests that adsorbed oxygen forms a surface oxide with a binding energy that is similar to that of the bulk oxide. These results are used to predict the conditions in which an oxide layer will exist on exposed beryllium surfaces during fusion reactor operation.     

Preparation and characterization of superhard TiC/Mo multilayers

Nanometer scale TiC/Mo multilayers have been prepared by ion beam assisted deposition (IBAD) at nearly ambient temperature. The modulation wavelength was in the range of 2 nm to 14 nm and the individual layer thickness was maintained to be equal. Two series (series A and series B) were prepared which were bombarded with different bombarding energy (0 eV and 50 eV) in order to investigate the bombardment effect on the nano-hardness of the multilayers. Low angle X-ray diffraction (LXRD) was used to analyzed the layered structure of multilayers. Mechanical properties of these multilayers were thoroughly studied using a nanoindentation facility. The nanohardness showed a strong dependence on the sharpness of the interlayer and the modulation wavelength. It was found that the multilayer hardness was greater than the volume weighted mean of the component hardness. With the modulation wavelength adjusted, the multilayer can be even harder than its hard component (TiC). A maximum hardness of 47.62 GPa, about 1.5 times larger than that of the TiC values, was found at = 8 nm multilayer deposited without ion beam bombardment. It was also found that the films without ion bombardment were much harder than those bombarded by 50 eV Ar⁺ ion beam.     

Effect of ion implantation on thin hard coatings

The surface mechanical properties of thin hard coatings of carbides, nitrides and borides deposited by r.f. sputtering were improved after deposition by ion implantation. The thickness and the stoichiometry of the films were measured by Rutherford backscattering spectrometry and nuclear reaction analysis before and after ion bombardment. The post ion bombardment was achieved with heavy inert ions such as Kr⁺ and Xe⁺ with an energy sufficient to penetrate the film and to reach the substrate. Both the film adhesion and the microhardness were consistently improved. In order to achieve a more detailed understanding, Rb⁺ and Ni⁺ ions were also used as projectiles, and it was found that these ions were more effective than the inert gas ions.     

Effect of the target surface temperature on the distribution of nanoparticles formed by ion implantation

Metal-nanoparticle-containing composite layers were synthesized by implantation of 60 keV Ag⁺ ions into a soda-lime silicate glass to a dose of 3×10¹⁶ ion/cm² at an ion beam current density of 3 μA/cm² and a substrate temperature of 35°C. The results of implantation depend on the temperature effects developed in the glass targets of various thicknesses. Data on the silver distribution profile and the nucleation and growth of metal nanoparticles in depth of the implanted layers were obtained from the optical reflection spectra. It is demonstrated that even small variations in the surface temperature of the ion-bombarded glass substrate lead to significant changes in the conditions of nanoparticle formation in the sample.     

Trapping and re-emission of deuterium from molybdenum following bombardment at 77 K

The re-emission of deuterium from molybdenum during 7 keV D⁺ bombardment at 77 K has been measured and this data compared with 20 keV D⁺ bombardment. Thermal desorption spectra show an increase in the number of peaks at lower temperatures as the incident ion dose is increased. The results may be interpreted by an increase in the population of trapping (damage) sites created during previous irradiations, in addition to a broadening of the range distribution to intersect the surface. This leads to a diffusion peak much wider than that normally considered for diffusive release.     

Atomic force microscope study of crater formation in ion bombarded polymer

Polyimide Kapton films with a thickness of 62 m were bombarded by Ar+, N+, He+ and D+ ions at energies from 10–50 keV. After bombardment at room temperature, the surface topographic changes of the polymer were investigated using an atomic force microscope (AFM). The most common feature of the ion-bombarded Kapton surface is the formation of craters which often have circular shape and rims. The crater sizes suggest they are unlikely to have been caused by a single ion but by the collective effects including diffusion and trapping of gas atoms and gas molecules in ion-bombarded polymer. A model for the formation of these craters is proposed.     

Surface and electronic structure of Ga0.92In0.08N thin film investigated by photoelectron spectroscopy

The surface and electronic structure of Ga_0.92In_0.08N layers grown by metal organic chemical vapor deposition (MOCVD) have been investigated by means of photoemission. Stability of chemical composition of the surface subjected to Ar⁺ ion sputtering was proven by means of X-ray photoemission spectroscopy. The analysis of the relative intensities of In 3d, Ga 3p, and N 1s peaks showed that argon ion bombardment does not change significantly the relative contents of the layer constituents. Simultaneous efficient removal of the main contaminants (O and C) was observed during the sputtering procedure, proving that argon sputtering can be used as a method for preparation of clean Ga_1−xIn_xN surfaces.For a clean (0001)-(1×1) surface prepared by repeated cycles of Ar⁺ ion sputtering and annealing, electronic structure was investigated. The band structure was explored along the Γ-A direction of the Brillouin zone, measuring angle-resolved photoemission spectra along the surface normal. A similar set of data was also acquired for the same surface of GaN layer. Comparison of the collected data revealed an additional feature at the valence band edge, which can be ascribed to the presence of In in the layer.     

Investigation of the surface morphology of ion-bombarded biocompatible materials with a SEM and profilograph

The surface morphology (topography and roughness) is a very important factor which affects the response of biological tissue to an implant material. The effect of an incident ion beam on surface morphology of various biocompatible materials was studied. All materials were bombarded by Ar⁺ ions at an applied voltage of 7 kV at various incident angles from 0 to 1.4 rad (0 to 80°) and at a beam current up to 0.1 mA. The surface topographies of ion-bombarded samples were examined with a Japan Electron Optics Laboratory, model JSM-35, scanning electron microscope. The roughness of the surface was calculated from the shape of a surface profile, which was recorded by a profilograph, the ME10 (supplied by VEB Carl Zeiss, Jena).     

Electron energy loss spectroscopy study of the effect of low-energy Ar-ion bombardment on the surface structure and composition of Pt80Co20 (1 1 1) alloy

Electron energy loss spectroscopy has been employed for investigation of the effect of 600 eV Ar⁺-ion irradiation in the dose range 7×10¹⁶-4×10¹⁷ ions/cm² on the atomic structure and surface composition of alloy Pt₈₀Co₂₀(1 1 1). A method of the layer-by-layer reconstruction of the lattice interplanar distance changes based on the analysis of the plasmon spectra excitation was proposed. The ion bombardment was shown to result in a non-monotonic variation of the lattice interplanar distance due to formation of the stable defects, with the topmost layer being in the state of compression. Using the ionization energy loss spectra, a layer-by-layer concentration profile of the alloy components was reconstructed for different doses of ion irradiation of the surface. The Ar⁺-ion bombardment of the alloy was found to result in the preferential sputtering of Co and in the enrichment of the near-surface region by Pt atoms with formation of an altered layer, which is characterized by a non-monotonic concentration profile dependent on the irradiation dose. The results obtained are discussed in the framework of the models of preferential sputtering and radiation-induced segregation.     

Surface erosion and modification by energetic ions

Surface erosion and modification by energetic highly charged and cluster ions are important in the development of semiconductor devices, TeV accelerators, fission and fusion reactors, and in the development of extreme ultra-violet lithography devices. Gas cluster ion beam (GCIB) surface treatment can significantly mitigate the high-gradient electric vacuum breakdown of rf-cavities. GCIB can also mitigate Q-slope drop in superconducting Nb-cavities. Various mechanisms of the highly charged ion (HCI) energy transfer into the solid target, such as hollow atom formation, charge screening and neutralization, shock wave generation, and sputtering were analyzed. Surface erosions caused by GCIB, HCI bombardments, and by low energy He⁺ and H⁺ ions typical for fission and fusion devices were studied by using molecular dynamics simulation. A He bubble splashing mechanism of liquid Li containing was developed, and surface erosion was simulated. The mechanism of bubble explosion could significantly contribute to the surface erosion at high ion fluxes and explain the existing experimental results.     

Surface morphology of polycrystalline diamond films etched by Ar+ beam bombardment

Polycrystalline diamond films etched by Ar⁺ beam bombardment were investigated by scanning electron microscopy and Raman spectroscopy. In an ion sputtering apparatus, an etching rate of 14 m C^–1 was obtained when 10 kV-accelerated Ar⁺ ions penetrated with an angle of 15–30° from the normal. A number of cavities were created on the surface treated at low incidence angle. In contrast, micro-prominence was seen under the condition of high incidence angle. The degree of surface roughness on etched films was also changed with the incidence angle of the beam. A relatively smooth surface appeared after the treatment with an incidence angle of 15°. Raman spectroscopy revealed that the physical etching of diamond is effective in obtaining high quality surface of polycrystalline diamond films.