Nanosphere Lithography Process Research Articles

High aspect ratio silicon and polyimide nanopillars by combination of nanosphere lithography and intermediate mask pattern transfer

Graphical abstractDisplay Omitted Highlights? Novel multilayer mask stack for high aspect ratio etching using nanosphere lithography. ? Silicon nanopillars with 75nm diameter and an aspect ratio of 17. ? High aspect ratio polyimide nanopillars with sub-100nm diameter. ? Silicon pillar covered surface showed superhydrophilic properties under repeated wetting conditions. Silicon and polymer nanopillar structures with sub-100nm diameter and high aspect ratios were fabricated using a modified nanosphere lithography process. A thin silicon film was sputtered onto spin-coated polyimide films. A self-assembled layer of nanospheres was then formed on the silicon. Reactive ion etching with SF6/C4F8 was used to transfer the nanosphere pattern to the silicon. Oxygen plasma etching then transferred the silicon pattern into the polymer to create nanopillars. The nanopillar diameter could be finely tuned by oxygen plasma thinning of the nanospheres and the conditions for silicon mask etching. In the last step the polyimide pillars served as etch masks to transfer the structures back into the silicon substrate to give well-ordered pillars of 1.3µm height and 75nm diameter.

Self-Organized Nanostructure Formation for Anti-Reflection Glass Surfaces

ABSTRACTTwo methods to create biomimetic anti-reflection nanostructures in ordinary glass microscope object slides are presented. One technique is based on a nanosphere lithography process combined with physical vapour deposition of nickel and reactive ion etching (RIE). The other uses plasma induced dewetting of a smooth nickel surface. The amount of reflected light was measured and a method to simulate the reflectivity from an atomic force microscopy (AFM) topography scan of the glass surface is presented. The reflectivity for visible light at normal incidence was reduced to 20-50 % of the original value with both methods and the simulation gives results in good agreement to the measurement.

Realization of multi-paired photonic crystals by the multiple-exposure nanosphere lithography process

Arrays of two-dimensional multi-paired photonic crystals (PCs) have been fabricated by a multiple-exposure nanosphere lithography (MENSL) method utilizing a self-assembled nanosphere as a lens mask and an expanded He–Cd laser. The nanospheres were self-assembled on a photoresist. The masked PR was then multi-exposed with changing rotation angle (θ) and tilt angle (γ). Scanning electron microscopy reveals that MENSL is a useful tool for fabricating multi-paired PCs with various lattice structures.

High-Q whispering-gallery mode lasing from nanosphere-patterned GaN nanoring arrays

A hexagonal-close-packed ordered array of nanorings was fabricated on GaN with a modified nanosphere lithography process. The spheres initially served as etch masks for the formation of closed-packed nanopillars. The spheres were then shrunk and, with a layer of oxide deposited, the roles of the spheres became masks for liftoff. The final etch produced nanorings with wall widths of 140 nm. Photopumped lasing with splitting modes was observed at room temperature, with a low lase threshold of ∼10 mJ/cm2 and high quality factor of ∼5000, via whispering-gallery modes. The resonant frequencies were verified through finite-difference time-domain simulations.

Aqueous phase Ag nanoparticles with controlled shapes fabricated by a modified nanosphere lithography and their optical properties

We have developed a modified nanosphere lithography (NSL) process to fabricate surface-confined Ag nanoparticles (NPs) with controlled shapes. NPs with different shapes, such as triangular, quadrilateral, pentagon or trapezoidal with rounded tips or edges, can be fabricated by this process. These Ag NPs can be dislodged into water forming NPs in an aqueous environment. The developed process results in better NP shape retaining than those obtained using the routine NSL process. The UV–vis absorption of the surface-confined Ag NPs show distinct blue shift and reduced intensity after surface modification. The NPs produced by the modified NSL and dislodged in water have significantly less density of debris as observed by transmission electron microscopy and UV–vis absorption spectrum.

Application of Nanosphere Lithography to LED Surface Texturing and to the Fabrication of Nanorod LED Arrays

In this study, the process of nanosphere lithography was developed and applied to LED surface texturing and nanorod device fabrication. We observed a texture-size-dependent improvement of total light output. While the increase of output optical power from the textured LEDs can be attributed to surface roughening in the GaN-air surface and to the increase of internal quantum efficiency as the strain is relaxed with the surface texturing, the size-dependent device performance is related to the interaction of generated photons with the textured surface. We further etched through the p-GaN and quantum well region to form p-i-n nanorods on the sample. By inserting a spacer to prevent p-type contact from shorting the n-GaN, we successfully demonstrated nanorod LED arrays. For such a device, a narrower radiation profile was demonstrated from the nanorod LED array as compared with that from the planar LED. The result is associated with the vertical guiding effect along the nanorod cylinder and the Bragg scattering of photons extracted from the sidewall by the rest of the rods. Furthermore, the electroluminescence spectra showed a nearly constant peak wavelength of the nanorod LED arrays, which is due to the suppression of the effect of quantum confined Stark effect.

FABRICATION OF TWO-DIMENSIONAL PHOTONIC CRYSTALS WITH GE2SB2TE5 NANOHOLE ARRAYS BY NANOSPHERE LITHOGRAPHY

Two-dimensional photonic crystals (2D-PCs) with Ge 2 Sb 2 Te 5 ( GST ) nanohole arrays were prepared by the nanosphere lithography (NSL) process. A primary factor of PCs is that the refractive index (n) and the n-modulation can be realized by using the GST films, which exhibit a reversible phase transformation between amorphous and crystalline states by laser illumination. The polystyrene (PS) spheres with a diameter of 500 nm were spin-coated on Si substrate and subsequently reduced by O 2-plasma treatment. The reduced spheres were utilized as a lift-off mask of the NSL process and their size and separation could be precisely controlled. Amorphous GST films were thermally evaporated and then the reduced PS spheres were removed. The fabricated GST nanohole arrays were observed by SEM and AFM. The nanohole diameters are nearly linearly reduced with increasing plasma-treatment time (t). The reduction rate (δ) for the conditions of this work was evaluated to be ~ 0.92 nm/s. The period (Λ) and filling factor (η) of PCs are structure parameters that determine their photonic bandgaps (PBGs). η-modulation can be easily achieved via a control of t and the Λ can be also modulated by the use of PS spheres with specific diameter. In addition, the PBGs for the fabricated GST 2 D PC were calculated by considering the amorphous and crystalline states of GST .

Wafer Surface Charge Reversal as a Method of Simplifying Nanosphere Lithography for Reactive Ion Etch Texturing of Solar Cells

A simplified nanosphere lithography process has been developed which allows fast and low-waste maskings of Si surfaces for subsequent reactive ion etching (RIE) texturing. Initially, a positive surface charge is applied to a wafer surface by dipping in a solution of aluminum nitrate. Dipping the positive-coated wafer into a solution of negatively charged silica beads (nanospheres) results in the spheres becoming electrostatically attracted to the wafer surface. These nanospheres form an etch mask for RIE. After RIE texturing, the reflection of the surface is reduced as effectively as any other nanosphere lithography method, while this batch process used for masking is much faster, making it more industrially relevant.

Advances in nanolithography using molecular rulers

The combination of conventional lithographic techniques with chemical self-assembly allows for the creation of nanostructures whose spacing and edge resolution reach nanometer-scale precision. The controlled placement and thickness of self-assembled multilayers composed of alternating layers of α,ω-mercaptoalkanoic acids and coordinated metal ions form precise “molecular ruler” resists that enable the production of tailored and lithographically defined metal patterns. Initial structures created by conventional techniques are referred to as parents and subsequent structures generated by the molecular ruler process are identified as daughters. We report the further creation of subsequent generation structures (granddaughters) that have sub-100 nm dimensions. The granddaughter structures are created by forming molecular rulers on parent and daughter structures, and can be isolated by removing sacrificial parent and/or daughter structures. This process has also been utilized in combination with parent structures created by the process of nanosphere lithography to produce arrays of metal features with 10 nm spacings. Since our original report, we have improved the throughput and reproducibility of the molecular ruler process by automating its iterative nature and by utilizing appropriate chemical lift-off solutions.