Low Energy Ion Beam Sputtering Research Articles

Molybdenum assisted self-organized pattern formation by low energy ion beam sputtering

Molybdenum assisted self-organized pattern formation by low energy ion beam sputtering

Effect of medium energy He+, Ne+ and Ar+ ion irradiation on the Hf-In-C thin film composites

Thin films of Hf-In-C ternary compounds were synthesized by a 2-step method consisting of a low-energy ion beam sputtering and thermal annealing. The radiation tolerance of the composites and the effects of irradiation by medium energy light and heavy ions (100 keV He+, 100 keV Ne+, and 200 keV Ar+) with an extreme fluence (1017 ions/cm2) were analyzed by several methods: composition and profiling of elements by Rutherford Backscattering Spectroscopy and Nuclear Resonance Analysis, microstructure and surface morphology by High-Resolution Transmission Electron Microscopy and Atomic Force Microscopy, and mechanical properties (elastic modulus and hardness) by nanoindentation.The study showed that the as-prepared Hf-In-C thin films form a mixture of different binary and ternary phases, including nanostructured Hf2InC, and oxides of metallic building elements. The irradiation with light ions (He+) had only a mild effect on the structure, composition, and mechanical properties of the composites. However, irradiation with heavy ions (Ne+, Ar+) led to a significant change in all monitored parameters and an overall collapse of the sample structure (especially for the Ar+ ions). It turned out that although thin Hf-In-C composites show to be highly tolerant to light ions, they have very limited resistivity to medium energy heavy ions.

Recent developments in ion beam-induced nanostructures on AIII-BV compound semiconductors

Ion-beam sputtering of two-component substrates constitutes an alternative route for the nanofabrication of 3D (three-dimensional) structures, such as quantum dots or nanowires with unique properties like a high degree of local ordering. To allow for feasibility in precision manufacturing, control and optimization it is necessary to completely understand all the phenomena behind the evolution of nanostructures. The formation and development during the ion irradiation of similar features has been extensively studied for almost a half of century, but only over the last few years have new results appeared, ones stimulating real progress within this field. In this paper we report on the growth of such 3D nanostructures after low energy ion-beam sputtering on specific materials belonging to the group of AIII-BV binary compound crystals. Special emphasis is given to the role of sample temperature (during irradiation or post-annealing) on the evolution of nanostructure patterns and their ordering. The formation of such systems will be explained as seen from a phenomenological perspective.

Surface morphology of molybdenum silicide films upon low-energy ion beam sputtering

The surface morphology of molybdenum silicide (MoxSi1−x) films has been studied after low-energy Ar+ ion beam sputtering (IBS) to explore eventual pattern formation on compound targets and, simultaneously, gather information about the mechanisms behind silicide-assisted nanopatterning of silicon surfaces by IBS. For this purpose, MoxSi1−x films with compositions below, equal and above the MoSi2 stoichiometry (x = 0.33) have been produced by magnetron sputtering, as assessed by Rutherford backscattering spectrometry (RBS). The surface morphology of silicon and silicide films before and after IBS has been imaged by atomic force microscopy (AFM), comprising conditions where typical nanodot or ripple patterns emerge on the former. In the case of irradiated MoxSi1−x surfaces, AFM shows a marked surface smoothing at normal incidence with and without additional Mo incorporation (the former results in nanodot patterns on Si). The morphological analysis also provides no evidence of ion-induced phase separation in irradiated MoxSi1−x. Contrary to silicon, MoxSi1−x surfaces also do not display ripple formation for (impurity free) oblique irradiations, except at grazing incidence conditions where parallel ripples emerge in a more evident fashion than in the Si counterpart. By means of RBS, irradiated MoxSi1−x films with 1 keV Ar+ at normal incidence have also been used to measure experimentally the (absolute) sputtering yield and rate of Si and MoxSi1−x materials. The analysis reveals that, under the present working conditions, the erosion rate of silicides is larger than for silicon, supporting simulations from the TRIDYN code. This finding questions the shielding effect from silicide regions as roughening mechanism in metal-assisted nanopatterning of silicon. On the contrary, the results highlight the relevance of in situ silicide formation. Ripple formation on MoxSi1−x under grazing incidence is also attributed to the dominance of sputtering effects under this geometry. In conclusion, our work provides some insights into the complex morphological evolution of compound surfaces and solid experimental evidences regarding the mechanisms behind silicide-assisted nanopatterning.

Nanostructure formation and regulation during low-energy ion beam sputtering of fused silica surfaces

Ion beam sputtering (IBS) possesses strong surface nanostructuring behaviors, where dual microscopic phenomenon can be aroused to induce the formation of ultrasmooth surfaces or regular nanostructures. Low-energy IBS of fused silica surfaces is investigated to discuss the formation mechanism and the regulation of the IBS-induced nanostructures. The research results indicate that these microscopic phenomena can be attributed to the interaction of the IBS-induced surface roughening and smoothing effects, and the interaction process strongly depends on the sputtering conditions. Alternatively, ultrasmooth surface or regular nanostructure can be selectively generated through the regulation of the nanostructuring process, and the features of the generated nanostructures, such as amplitude and period, also can be regulated. Consequently, two different technology aims of nanofabrication, including nanometer-scale and nanometer-precision fabrication, can be realized, respectively. These dual microscopic mechanisms distinguish IBS as a promising nanometer manufacturing technology for the optical surfaces.

Formation of nanoripples on amorphous alumina thin films during low-energy ion-beam sputtering: Experiments and simulations

The formation of nanopatterns induced by low-energy (0.5--1.5 keV) ${\mathrm{Xe}}^{+}$ ion-beam sputtering of amorphous alumina thin films is investigated by atomic force microscopy and grazing incidence small-angle x-ray scattering. The observed dependence of the surface morphology on ion incidence angle, temperature, ion energy, and fluence is compared with the predictions of linear and nonlinear continuum theoretical models. The results show that ion-induced mass redistribution stabilizes the surface at near-normal and very grazing incidence angles, while curvature-dependent erosion governs the formation of periodic nanoripples in the range of incidence angles between ${50}^{\ensuremath{\circ}}$ and ${65}^{\ensuremath{\circ}}$. Surface-confined ion-induced viscous flow is shown to be the dominant relaxation mechanism during erosion. Moreover, pattern evolution with ion fluence (pattern ordering and asymmetry of the ripple profile, in particular) suggests that nonlinear effects that are ignored by the Sigmund's collision cascade theory of sputtering contribute strongly to the observed dynamics of ripple formation.

Temperature and high fluence induced ripple rotation on Si(100) surface

The topography evolution of Si(100) surface due to oblique incidence low energy ion beam sputtering (IBS) is investigated. Experiments were carried out at different elevated temperatures from 20 °C–450 °C and at each temperature, the ion fluence is systematically varied in a wide range from 1 × 1018 cm−2 to 1 × 1020 cm−2. The ion sputtered surface morphologies are characterized by atomic force microscopy and high-resolution cross-sectional transmission electron microscopy. At room temperature, the ion sputtered surfaces show periodic ripple nanopatterns where their wave-vector remains parallel to ion beam projection for the entire fluence range. With an increase of substrate temperature, these patterns tend to demolish and reduce into randomly ordered mound-like structures around 350 °C. A further rise in temperature above 400 °C leads orthogonally rotated ripples beyond fluence 5 × 1019 cm−2. All the results are discussed combining the theoretical framework of linear, non-linear and recently developed mass redistribution continuum models of pattern formation by IBS. These results have technological importance regarding the control over ion-induced pattern formation, as well providing useful information for further progress in the theoretical field.

Highly-ordered ripple structure induced by normal incidence sputtering on monocrystalline GaAs (001): ion energy and flux dependence

Low energy ion beam sputtering induced nanostructure formation on GaAs (001) surfaces at elevated temperature and at normal ion incidence has been studied for different ion energy and flux of the incident Ar+. Well-ordered nanoripples are found to develop, whose wave-vector is oriented along 〈110〉 crystallographic direction. The ripple structure is found to coarsen with ion energy, while it remains invariant with the ion flux. The evolution of the ripples can be attributed to the biased diffusion of vacancies arising from Ehrlich–Schwoebel barrier.

Ion damage overrides structural disorder in silicon surface nanopatterning by low-energy ion beam sputtering

We investigate the role of the initial structural condition in silicon surface nanopatterning by low-energy ion beam sputtering. Specifically, we address the influence of the target atomic structure in ripple formation under oblique irradiation by 500 eV ions. To this end, we compare results obtained on single-crystal, amorphous, and pre-implanted silicon targets. In spite of the differences in terms of structural order, and in contrast to previous results for medium energies, surface dynamics are found to be quantitatively similar in all these systems. We explain our results through molecular dynamics simulations of the initial irradiation stages, with the conclusion that the damage induced by low-energy ion bombardment overrides the initial atomic state of the silicon target, irrespectively of its preparation method and allows silicon re-using for nanostructuring.

Super-roughening scaling behaviour of Si surface morphology at grazing incidence low energy ion beam sputtering

The morphological evolution on Si (100) surface for grazing incidence (75°) and low energy (500eV) Ar+ sputtering is studied at various ion fluences. The ion induced surface topography was characterized by atomic force microscopy. Initially ion sputtering generates irregular dot-like structure on the surface. These dots begin to elongate along the ion beam direction with the increase of sputtering time and then transform to needle shaped structure at the late stage of sputtering. The morphological data are quantitatively analyzed within the framework of the dynamic scaling theory. Estimated scaling exponents do not follow the conventional Family–Vicsek relation rather exhibit typical anomalous scaling. A crossover in scaling behaviour of local surface width is observed at fluence 1.5×1018cm−2, above which the eroded surface shows super-rough scaling behaviour along the axis of the conical structures with global exponents αs=α=1.6±0.1, β=0.99±0.03 and local exponents αlocal=1andβlocal=0.63±0.05. The importance of nonlocal ion reflection effects for the development of super-rough scaling is discussed.

Nanopatterning of optical surfaces during low-energy ion beam sputtering

Ion beam figuring (IBF) provides a highly deterministic method for high-precision optical surface fabrication, whereas ion-induced microscopic morphology evolution would occur on surfaces. Consequently, the fabrication specification for surface smoothness must be seriously considered during the IBF process. In this work, low-energy ion nanopatterning of our frequently used optical material surfaces is investigated to discuss the manufacturability of an ultrasmooth surface. The research results indicate that ion beam sputtering (IBS) can directly smooth some amorphous or amorphizable material surfaces, such as fused silica, Si, and ULE® under appropriate processing conditions. However, for IBS of a Zerodur® surface, preferential sputtering together with curvature-dependent sputtering overcome ion-induced smoothing mechanisms, leading to the granular nanopatterns’ formation and the coarsening of the surface. Furthermore, the material property difference at microscopic scales and the continuous impurity incorporation would affect the ion beam smoothing of optical surfaces. Overall, IBS can be used as a promising technique for ultrasmooth surface fabrication, which strongly depends on processing conditions and material characters.

The structural transition from epitaxial Fe/Pt multilayers to an ordered FePt film using low energy ion beam sputtering deposition with no buffer layer

An epitaxial L10 FePt thin film grown from an [Fe(10Å)/Pt(10Å)]15 multilayer with the orientation of (001) was prepared by an ion beam sputtering deposition method without buffer layer. From the measurement data of X-ray diffraction and X-ray reflectivity, the multilayer structure was totally disappeared and a uniform FePt alloy thin film was formed at temperatures higher than 600°C. For the as-deposited thin film grown at 100°C, the multilayer already possesses an epitaxial structure. The epitaxial relation is FePt(001)[100]//MgO(001)[100] and this epitaxial relation persists after sequential high temperature annealing. An epitaxial L10 ordered FePt(001) film with order parameter of 0.95 was obtained when the annealing temperature reached 650°C. The ordered FePt(001) thin film has a perpendicular magnetic anisotropy with a squareness of 0.95±0.03 on the magnetic hysteresis loop. This experiment demonstrates that the low energy ion beam sputtering deposition will preserve the epitaxial relation with no buffer layer between multilayer and substrate.

Optimization of growth and ordering of Ag nanoparticle arrays on ripple patterned alumina surfaces for strong plasmonic coupling

Low-energy ion beam sputtering of alumina thin films followed by growth of metallic nanoparticles by glancing angle deposition is optimized in order to produce arrays of silver nanoparticle chains with a strong plasmonic dichroism. A systematic study is undertaken in order to establish the influence of the angle of silver deposition and the ordering of the pre-patterned rippled surface on the morphology and organization of the nanoparticles, and on their associated optical properties. High ion fluence for ripple formation and low glancing angle for metal deposition favor the formation of aligned and elongated particles with sub-nanometer gaps. Numerical simulations show that these nanoparticle arrays generate high electric field enhancements for an excitation parallel to the particle chains, and therefore can be used for surface enhanced spectroscopies.

Size-dependent optical properties of TiO2 nanostructures

The size-dependent optical properties of the nanostructures created on the TiO2(110) surfaces, via low-energy ion-beam sputtering technique, have been investigated here. The crystalline nanostructures have been produced in off-normal geometry. A significant enhancement in UV and visible light absorption has been observed for TiO2 surfaces patterned with nanostructures. Moreover, this enhancement depends on the sizes of the nanostructures. Preferential sputtering of oxygen atoms, during ion beam irradiation, leads to the presence of excess Ti on the surface. Ti-rich zones thus formed can promote nucleation of self-assembled nanostructures on the TiO2(110) surface. These results have been observed in the absence of any dopant. The formation of crystalline TiO2 nanostructures and the development of Ti-rich zones on the surface, after sputtering, are responsible for the enhancement in visible absorbance seen in the present study. Although small-sized (∼10 nm) nanostructures display increased absorbance and a higher bandgap, compared to bulk TiO2, due to quantum effects, much higher absorbance with decreased bandgap is observed from larger-(∼50 nm) sized nanostructures. This enhancement in absorbance is due to the presence of well-developed (200 and 310) crystalline faces in bigger nanostructures.

Nanopatterning of mica surface under low energy ion beam sputtering

Irradiation of crystalline muscovite mica samples by 500 eV Ar+ ions at different incident angles can induce significant surface morphological variations. A periodic ripple pattern of nano-dimensions forms in the angle window 47°-70°. On the other hand, tilted conical protrusions develop on the surface at grazing incidence angles around 80°. From the derivative of the topographic images the distribution of the side-facet slopes in the ion incidence plane are measured, which is found to be strongly related to the pattern morphology. Additionally, it has been shown that, for the ripple structures, the base angles can be tuned by changing the ion fluence. An asymmetric sawtooth profile of the ripples obtained at low fluence is transformed to a symmetrical triangular profile at high fluence. As the slopes are found to be small, the pattern formation is not provoked by the gradient-dependent erosion mechanism rather it is the general effect of the curvature-dependent sputtering phenomena.

Topography evolution mechanism on fused silica during low-energy ion beam sputtering

In this study, the topography evolution of fused silica surfaces during low-energy ion beam erosion has been investigated depending on the ion incidence angle and with focus on the importance of the initial surface topography. Ripple prepattern, also prepared by ion beam erosion, that exhibits an anisotropic surface with adjustable surface amplitudes and gradients was utilized. Based on experimental results that confirm smoothing and patterning behavior, gradient-dependent sputtering is identified being the dominant topography evolution mechanism.

Self-organized patterning on Si(001) by ion sputtering with simultaneous metal incorporation

This report focuses on the effect of simultaneous Fe incorporation in the self-organized pattern formation on Si(001) by low-energy ion-beam sputtering. Experimental observations giving evidence for the correlation between different ion-beam parameters, Fe concentration on the substrate, and the resulting topography are shown. It was observed that the incorporation of Fe affects the evolution of the topography and it is a requisite for the formation of ripples at near-normal incidence. It is shown also that Fe is not homogeneously distributed on the surface, but there is a higher concentration at the crests of the ripples than at the valleys. For the experimental setup used for this study, the Fe flux that reaches the surface is determined mainly by the acceleration voltage (U acc), while the ion energy (E ion) and ion-beam incidence angle (α) control the concentration of Fe in the steady state. The adjustment of these operational parameters of the ion source enables the fine-tuning of surface patterns.

The preparation of Zn-ferrite epitaxial thin film from epitaxial Fe 3O 4:ZnO multilayers by ion beam sputtering deposition

A new method to grow a well-ordered epitaxial ZnFe 2O 4 thin film on Al 2O 3(0001) substrate is described in this work. The samples were made by annealing the ZnO/Fe 3O 4 multilayer which was grown with low energy ion beam sputtering deposition. Both the Fe 3O 4 and ZnO layers were found grown epitaxially at low temperature and an epitaxial ZnFe 2O 4 thin film was formed after annealing at 1000 °C. X-ray diffraction shows the ZnFe 2O 4 film is grown with an orientation of ZnFe 2O 4(111)//Al 2O 3(0001) and ZnFe 2O 4(1–10)//Al 2O 3(11–20). X-ray absorption spectroscopy studies show that Zn 2+ atoms replace the tetrahedral Fe 2+ atoms in Fe 3O 4 during the annealing. The magnetic properties measured by vibrating sample magnetometer show that the saturation magnetization of ZnFe 2O 4 grown from ZnO/Fe 3O 4 multilayer reaches the bulk value after the annealing process.

Formation and characterization of perpendicular mode Si ripples by glancing angle O2+ sputtering at room temperature

Off-normal low energy ion beam sputtering of solid surfaces often leads to morphological instabilities resulting in the spontaneous formation of ripple structures in nanometer length scales. In the case of Si surfaces at ambient temperature, ripple formation is found to take place normally at lower incident angles with the wave vector parallel to the ion beam direction. The absence of ripple pattern on Si surface at larger angles is due to the dominance of ion beam polishing effect. We have shown that a gentle chemical roughening of the starting surface morphology can initiate ripple pattern under grazing incidence ion beam sputtering (θ>64° with respect to the surface normal), where the ripple wave vector is perpendicular to the ion beam direction. The characteristics of the perpendicular mode ripples are studied as a function of pristine surface roughness (2–30 nm) and projectile fluence (5×1016–1.5×1018 O atoms cm−2). The quality of the morphological structure is assessed from the analysis of ion induced topological defects.

Characterization of nanostructured GaSb: comparison between large-area optical and local direct microscopic techniques

Low energy ion-beam sputtering of GaSb results in self-organized nanostructures with the potential of structuring large surface areas. Characterization of such nanostructures by optical methods is studied and compared to direct (local) microscopic methods. The samples consist of densely packed GaSb cones on bulk GaSb, approximately 30, 50, and 300 nm in height, prepared by sputtering at normal incidence. The optical properties are studied by spectroscopic ellipsometry, in the range 0.6-6.5 eV, and with Mueller matrix ellipsometry in the visible range, 1.46-2.88 eV. The optical measurements are compared to direct topography measurements obtained by scanning electron microscopy, high resolution transmission electron microscopy, and atomic force microscopy. Good agreement is achieved between the two classes of methods when the experimental optical response of the short cones (<55 nm) is inverted with respect to topological surface information, via a graded anisotropic effective medium model. The main topological parameter measured was the average cone height. Optical methods are shown to represent a valuable characterization tool of nanostructured surfaces, in particular when a large coverage area is desirable. Because of the fast and nondestructive properties of optical techniques, they may readily be adapted to in situ configurations.