Short-range stationary patterns and long-range disorder in an evolution equation for one-dimensional interfaces

Nanoscale pattern formation produced by ion bombardment of a rotating target: The decisive role of the ion energy

We study the nanoscale patterns that form on the surface of a rotating sample of an elemental material that is bombarded with a broad noble gas ion beam for angles of incidence θ just above the critical angle for pattern formation θ_{c}. The pattern formation depends crucially on the ion energy E. In simulations carried out in the low-energy regime in which sputtering is negligible, we find disordered arrays of nanoscale mounds (nanodots) that coarsen in time. Disordered arrays of nanodots also form in the high-energy regime in which there is substantial sputtering, but no coarsening occurs close to the threshold angle. Finally, for values of E just above the sputter yield threshold, nanodot arrays with an extraordinary degree of hexagonal order emerge for a range of parameter values, even though there is a broad band of linearly unstable wavelengths. This finding might prove to be useful in applications in which highly ordered nanoscale patterns are needed.

Nonlinear Effects of Pulsed Ion Beam in Ultra-High Resolution Material Removal

Ion beam sputtering is widely utilized in the area of ultra-high precision fabrication, coating, and discovering the microworld. A pulsed ion beam (PIB) can achieve higher material removal resolution while maintaining traditional ion beam removal performance and macro removal efficiency. In this paper, a 0.01 s pulse width beam is used to sputter atom layer deposition (ALD) coated samples. The nano-scale phenomenon is observed by high-resolution TEM. The results show that when the cumulative sputtering time is less than 1.7 s, the sputtering removal of solid by ion beam is accompanied by a nonlinear effect. Furthermore, the shortest time (0.05 s) and lowest thickness (0.35 nm) necessary to remove a uniform layer of material were established. The definition of its nonlinear effect under a very small removal amount guides industrial ultra-high precision machining. It reveals that PIB not only has high removal resolution on nanoscale, but can also realize high volume removal efficiency and large processing diameter at the same time. These features make PIB promising in the manufacturing of high power/energy laser optics, lithography objective lens, MEMS, and other ultra-high precision elements.

Surface nanopatterning by ion beam irradiation: compositional effects

Surface nanopatterning induced by ion beam irradiation (IBI) has emerged as an effective nanostructuring technique since it induces patterns on large areas of a wide variety of materials, in short time, and at low cost. Nowadays, two main subfields can be distinguished within IBI nanopatterning depending on the irrelevant or relevant role played by the surface composition. In this review, we give an up-dated account of the progress reached when surface composition plays a relevant role, with a main focus on IBI surface patterning with simultaneous co-deposition of foreign atoms. In addition, we also review the advances in IBI of compound surfaces as well as IBI systems where the ion employed is not a noble gas species. In particular, for the IBI with concurrent metal co-deposition, we detail the chronological evolution of these studies because it helps us to clarify some contradictory early reports. We describe the main patterns obtained with this technique as a function of the foreign atom deposition pathway, also focusing in those systematic studies that have contributed to identify the main mechanisms leading to the surface pattern formation and development. Likewise, we explain the main theoretical models aimed at describing these nanopattern formation processes. Finally, we address two main special features of the patterns induced by this technique, namely, the enhanced pattern ordering and the possibility to produce both morphological and chemical patterns.

Spatially Engraving Morphological Structure on a Polymeric Surface by Ion Beam Milling

Polymer surface patterning and modification at the micro/nano scale has been discovered with great impact in applications such as microfluidics and biomedical technologies. We propose a highly efficient fabricating strategy, to achieve a functional polymer surface, which has control over the surface roughness. The key development in this fabrication method is the polymer positive diffusion effect (PDE) for an ion-bombarded polymeric hybrid surface through focused ion beam (FIB) technology. The PDE is theoretically explored by introducing a positive diffusion term into the classic theory. The conductivity-induced PDE constant is discussed as functions of substrates conductivity, ion energy and flux. The theoretical results agree well with the experiential results on the conductivity-induced PDE, and thus yield good control over roughness and patterning milling depth on the fabricated surface. Moreover, we demonstrate a controllable surface wettability in hydrophobic and superhydrophobic surfaces (contact angles (CA) range from 108.3° to 150.8°) with different CA hysteresis values ranging from 31.4° to 8.3°.

Pattern-wavelength coarsening from topological dynamics in silicon nanofoams

We report the experimental observation of a submicron cellular structure on the surface of silicon targets eroded by an ion plasma. Analysis by atomic force microscopy allows us to assess the time evolution and show that the system can be described quantitatively by the convective Cahn-Hilliard equation, found in the study of domain coarsening for a large class of driven systems. The space-filling trait of the ensuing pattern relates it to evolving foams. Through this connection, we are actually able to derive the coarsening law for the pattern wavelength from the nontrivial topological dynamics of the cellular structure. Thus, the study of the topological properties of patterns in nonvariational spatially extended systems emerges as complementary to morphological approaches to their challenging coarsening properties.

Self-organized chains of nanodots induced by an off-normal incident beam

We propose a model to show that under off-normal bombardment of an incident ion beam, a solid surface may spontaneously form nanoscale dots lining up into chains perpendicular to the incident beam direction. These dots demonstrate a highly ordered hexagonal pattern. We attribute the self-organization behavior to surface instability under concurrent surface kinetics and to a shadow effect that causes the self-alignment of dots. The fundamental mechanism may be applicable to diverse systems, suggesting an effective approach for nanofabrication.

Ion-Induced Nanoscale Ripple Patterns on Si Surfaces: Theory and Experiment

Nanopatterning of solid surfaces by low-energy ion bombardment has received considerable interest in recent years. This interest was partially motivated by promising applications of nanopatterned substrates in the production of functional surfaces. Especially nanoscale ripple patterns on Si surfaces have attracted attention both from a fundamental and an application related point of view. This paper summarizes the theoretical basics of ion-induced pattern formation and compares the predictions of various continuum models to experimental observations with special emphasis on the morphology development of Si surfaces during sub-keV ion sputtering.

Observation and modeling of interrupted pattern coarsening: surface nanostructuring by ion erosion

We report the experimental observation of interrupted coarsening for surface self-organized nanostructuring by ion erosion. Analysis of the target surface by atomic force microscopy allows us to describe quantitatively this intriguing type of pattern dynamics through a continuum equation put forward in different contexts across a wide range of length scales. The ensuing predictions can thus be consistently extended to other experimental conditions in our system. Our results illustrate the occurrence of nonequilibrium systems in which pattern formation, coarsening, and kinetic roughening appear, each of these behaviors being associated with its own spatiotemporal range.

The morphology of amorphous SiO(2) surfaces during low energy ion sputtering

The morphology of different amorphous or amorphized SiO(2) surfaces, including thermally grown films, fused silica, and single crystalline quartz, during low energy ion sputtering has been investigated by means of atomic force microscopy. For all three materials, the formation of periodic ripple patterns oriented normal to the direction of the ion beam is observed at intermediate incident angles. At near-normal incidence, the SiO(2) surfaces remain flat, whereas a rotation of the ripple patterns is observed at grazing incidence. At intermediate angles, the patterns on the different surfaces exhibit wavelength coarsening of different strengths, which can be attributed to different amounts of near-surface mass transport by the surface-confined ion-enhanced viscous flow. In the framework of the recent hydrodynamic model of ion erosion, the observed differences in ripple coarsening are consistent with this interpretation and indicate that the surface energies of thermally grown SiO(2) and amorphized quartz are lower and higher than that of fused silica, respectively.

Coupling of morphology to surface transport in ion-beam-irradiated surfaces: normal incidence and rotating targets

Continuum models have proved their applicability to describe nanopatterns produced by ion-beam sputtering of amorphous or amorphizable targets at low and medium energies. Here we pursue the recently introduced 'hydrodynamic approach' in the cases of bombardment at normal incidence, or of oblique incidence onto rotating targets, known to lead to self-organized arrangements of nanodots. Our approach stresses the dynamical roles of material (defect) transport at the target surface and of local redeposition. By applying results previously derived for arbitrary angles of incidence, we derive effective evolution equations for these geometries of incidence, which are then numerically studied. Moreover, we show that within our model these equations are identical (albeit with different coefficients) in both cases, provided surface tension is isotropic in the target. We thus account for the common dynamics for both types of incidence conditions, namely formation of dots with short-range order and long-wavelength disorder, and an intermediate coarsening of dot features that improves the local order of the patterns. We provide for the first time approximate analytical predictions for the dependence of stationary dot features (amplitude and wavelength) on phenomenological parameters, that improve upon previous linear estimates. Finally, our theoretical results are discussed in terms of experimental data.