Can One Series of Self-Organized Nanoripples Guide Another Series of Self-Organized Nanoripples during Ion Bombardment: From the Perspective of Power Spectral Density Entropy?

Ion bombardment (IB) is a promising nanofabrication tool for self-organized nanostructures. When ions bombard a nominally flat solid surface, self-organized nanoripples can be induced on the irradiated target surface, which are called intrinsic nanoripples of the target material. The degree of ordering of nanoripples is an outstanding issue to be overcome, similar to other self-organization methods. In this study, the IB-induced nanoripples on bilayer systems with enhanced quality are revisited from the perspective of guided self-organization. First, power spectral density (PSD) entropy is introduced to evaluate the degree of ordering of the irradiated nanoripples, which is calculated based on the PSD curve of an atomic force microscopy image (i.e., the Fourier transform of the surface height. The PSD entropy can characterize the degree of ordering of nanoripples). The lower the PSD entropy of the nanoripples is, the higher the degree of ordering of the nanoripples. Second, to deepen the understanding of the enhanced quality of nanoripples on bilayer systems, the temporal evolution of the nanoripples on the photoresist (PR)/antireflection coating (ARC) and Au/ARC bilayer systems are compared with those of single PR and ARC layers. Finally, we demonstrate that a series of intrinsic IB-induced nanoripples on the top layer may act as a kind of self-organized template to guide the development of another series of latent IB-induced nanoripples on the underlying layer, aiming at improving the ripple ordering. The template with a self-organized nanostructure may alleviate the critical requirement for periodic templates with a small period of ~100 nm. The work may also provide inspiration for guided self-organization in other fields.

Enhancing the quality of self-organized nanoripples by Ar-ion bombardment of a bilayer system.

Ion bombardment (IB) is a promising nanofabrication technique for producing nanoripples. A critical issue that restricts the application of IB is the limited quality of IB-induced nanoripples. Photoresist (PR) and antireflection coating (ARC) are of technological relevance for lithographic exposure processes. Moreover, to improve the quality of IB-induced self-organized nanoripples, in this study, a PR/ARC bilayer was bombarded at an incidence angle of 50°. The surface normalized defect density and power spectral density, obtained via scanning atomic force microscopy, indicate the superiority of the PR/ARC bilayer nanoripples over those of single PR or ARC layers. The growth mechanism of the improved nanoripples, deciphered via the temporal evolution of the morphology, involves the following processes: (i) formation of a well-grown IB-induced nanoripple prepattern on the PR, (ii) transfer of nanoripples from the PR to the ARC, forming an initial ARC nanoripple morphology for subsequent IB, and (iii) conversion of the initial nonuniform ARC nanoripples into uniform nanoripples. In this unique method, the angle of ion-incidence should be chosen so that ripples form on both PR and ARC films. Overall, this method facilitates nanoripple improvement, including prepattern fabrication for guiding nanoripple growth and sustainable nanoripple development via a single IB. Thus, the unique method presented in this study can aid in advancing academic research and also has potential applications in the field of IB-induced nanoripples.

Optical anisotropy of self-organized gold quasi-blazed nanostructures based on a broad ion beam.

To circumvent elaborate conventional lithographic methods for realizing metallic nanostructures, it is necessary to develop self-organized nanofabrication methods for suitable template structures and their optical characterization. We demonstrate the potential of ion bombardment with impurity co-deposition to fabricate terraced or quasi-blazed nanostructure templates. Self-organized terraced nanostructures on fused silica were fabricated using Ar+ ion bombardment with iron impurity co-deposition and subsequent Au shadow deposition. The aspect ratios are enhanced threefold, and the range of nanostructure period variation is significantly increased with respect to that of conventional nanostructures realized by pure ion bombardment. We reveal the key features of the method via atomic force microscopy and optical characterization. Variable-profile quasiperiodic nanostructures with periods of 100-450 nm, heights of 25-180 nm, and blaze angles of 10°-25° were fabricated over an area of 20×40mm2, and these exhibited tunable and broadening optical anisotropy across the nanostructured area. Thus, the proposed method is a viable technique for rapid, cost-effective, and deterministic fabrication of variable nanostructure templates for potential optical applications.

Ion beam planarization of diamond turned surfaces with various roughness profiles.

The effectiveness of ion beam planarization (IBP) to reduce surface roughness of diamond turned NiP surfaces was investigated. The surfaces with various spatial wavelengths and depths of turning marks were spray-coated and planarized with broad ion beam. The ion beam planarization was performed at a special angle where the etching rate of photoresist is closely similar to NiP. It is found that the combined process of spray-coating and ion-beam-planarization can effectively reduce the surface roughness of diamond turned NiP. The spatial wavelength and depth of turning marks have limited influence on surface roughness reduction rate. The final surface roughness after ion beam planarization is 30%~40% of the original roughness, irrespective of spatial wavelength and depth of turning marks. Extending planarization time does not alter surface quality after photoresist is etched away. These results show that the IBP is applicable to roughness minimization of diamond turned surfaces.

Silicide induced ion beam patterning of Si(001).

Low energy ion beam pattern formation on Si with simultaneous co-deposition of Ag, Pd, Pb, Ir, Fe or C impurities was investigated by in situ scanning tunneling microscopy as well as ex situ atomic force microscopy, scanning electron microscopy, transmission electron microscopy and Rutherford backscattering spectrometry. The impurities were supplied by sputter deposition. Additional insight into the mechanism of pattern formation was obtained by more controlled supply through e-beam evaporation. For the situations investigated, the ability of the impurity to react with Si, i.e. to form a silicide, appears to be a necessary, but not a sufficient condition for pattern formation. Comparing the effects of impurities with similar mass and nuclear charge, the collision kinetics is shown to be not of primary importance for pattern formation. To understand the observed phenomena, it is necessary to assume a bi-directional coupling of composition and height fluctuations. This coupling gives rise to a sensitive dependence of the final morphology on the conditions of impurity supply. Because of this history dependence, the final morphology cannot be uniquely characterized by a steady state impurity concentration.

Is keV ion-induced pattern formation on Si(001) caused by metal impurities?

We present ion beam erosion experiments performed in ultrahigh vacuum using a differentially pumped ion source and taking care that the ion beam hits the Si(001) sample only. Under these conditions no ion beam patterns form on Si for angles theta < or = 45 degrees with respect to the global surface normal using 2 keV Kr+ and fluences of approximately 2 x 10(22) ions m(-2). In fact, the ion beam induces a smoothening of preformed patterns. Simultaneous sputter deposition of stainless steel in this angular range creates a variety of patterns, similar to those previously ascribed to clean ion-beam-induced destabilization of the surface profile. Only for grazing incidence with 60 degrees < or = theta < or = 83 degrees do pronounced ion beam patterns form. It appears that the angular-dependent stability of Si(001) against pattern formation under clean ion beam erosion conditions is related to the angular dependence of the sputtering yield, and not primarily to a curvature-dependent yield as invoked frequently in continuum theory models.

Solvent influence on the surface morphology of P3HT thin films revealed by photoemission electron microscopy.

Only rigorous understanding of the relationship between the nanoscale morphology of organic thin films and the performance of the devices built from them will ultimately lead to design rules that can guide a structured development on the field of organic electronics. Despite great effort, unraveling the nanoscale structure of the films is still a challenge in itself. Here we demonstrate that photoemission electron microscopy can provide valuable insights into the chain orientation, domains size and grain boundary characteristics of P3HT films spun cast from different solvents at room as well as at elevated temperatures.

Ripple coarsening on ion beam-eroded surfaces.

The temporal evolution of ripple pattern on Ge, Si, Al 2 O 3, and SiO 2 by low-energy ion beam erosion with Xe (+) ions is studied. The experiments focus on the ripple dynamics in a fluence range from 1.1 × 10(17) cm(-2) to 1.3 × 10(19) cm(-2) at ion incidence angles of 65° and 75° and ion energies of 600 and 1,200 eV. At low fluences a short-wavelength ripple structure emerges on the surface that is superimposed and later on dominated by long wavelength structures for increasing fluences. The coarsening of short wavelength ripples depends on the material system and angle of incidence. These observations are associated with the influence of reflected primary ions and gradient-dependent sputtering. The investigations reveal that coarsening of the pattern is a universal behavior for all investigated materials, just at the earliest accessible stage of surface evolution.