Micropyramid Arrays Research Articles

Monolithic Integrations of Slanted Silicon Nanostructures on 3D Microstructures and Their Application to Surface-Enhanced Raman Spectroscopy

We demonstrated fabrication of black silicon with slanted nanocone array on both planar and 3D micro and meso scale structures produced by a high-throughput lithography-free oblique-angle plasma etching process. Nanocones with gradual change in height were created on the same piece of silicon. The relation between the slanted angle of nanocones and incident angle of directional plasma is experimentally investigated. In order to demonstrate the monolithic integration of nanostructures on micro and meso scale non-planar surfaces, nanocone forest is fabricated on non-planar silicon surfaces in various morphologies such as silicon atomic force microscopy (AFM) tips and pyramidal pits. By integrating nanocones on inverse silicon micro-pyramid array devices, we further improved the surface enhanced Raman scattering (SERS) enhancement property of this optimized commercial SERS substrate by several folds even when using 66% less noble metal coating. We investigated the length gradient dependence and asymmetric properties of SERS effects for slanted nanocone with polarized excitation. This versatile and angle-controllable nanocone fabrication and monolithic 3D nano-micro-meso integration method provides new dimensions for production and optimization of SERS and other nanophotonic sensors.

Synthesis of CNT film on the surface of micro-pyramid array and its intense pulsed emission characteristics

We developed a new scheme to suppress the electric-field-screening effect in high growth density of a carbon nanotube (CNT) film during its intense pulsed emission. We synthesize the CNT film on a tridimensional surface (t-CNT film). The tridimensional surface includes wet etched silicon pyramids, and the Ni layer is electroless plated thereon. The intense pulsed emission characteristics of the t-CNT and planar-grown CNT (p-CNT) films were measured using a diode structure in single-pulse mode. The even turn-on field decreased from 5.5 V/μm for p-CNTs to 2.8 V/μm for t-CNTs, and the peak emission current increased from 232 A for p-CNTs to 324 A for t-CNTs at a peak field intensity ∼12.2 V/μm. The peak current of the t-CNT film increased by ∼39.7% over the p-CNT film. It is clear that the micro-pyramid array can effectively suppress the field screening effect to improve the electron-emission of CNT films.

Fabrication of Polymer Micropyramid/Nanolens Combined Structures Using Microstructured Nanodimpled Aluminum Stamp

In this study, we present an efficient and low-cost fabrication technique of polymer micropyramid/nanolens combined structures (MPNL-CS). The present fabrication technique is based on hot embossing of polymer substrates using a microstructured nanodimpled aluminum (MNA) stamp. The MNA stamp for the replication of polymer MPNL-CS was realized by combining three fabrication steps. (1) A nickel stamp with micropyramid array was manufactured by electroforming onto an anisotropically wet-etched silicon substrate. (2) A reverse micropyramid cavity array was transcribed on an electropolished aluminum substrate by microindentation with the nickel stamp. (3) The MNA stamp was finally realized by forming nanodimple arrays onto the reverse micropyramid array on the aluminum substrate through an electrochemical oxidation (anodization) process. In the final step, we have optimized the electrochemical oxidation process to realize nanodimple arrays at the sharp edge reverse micropyramid without defects. The MPNL-CS were successfully replicated on polymer substrates via a hot embossing process with the fabricated MNA stamp through investigating the effect of embossing pressure on the transcription quality of MPNL-CS. The present fabrication technique can be widely applied to the realization of various types of three-dimensional micro/nano combined structures.

Highly Integrated InGaN/GaN Semipolar Micro-Pyramid Light-Emitting Diode Arrays by Confined Selective Area Growth

We report on the fabrication of highly integrated semipolar {101} GaN micro-pyramid light-emitting diode (LED) arrays on a c-plane sapphire substrate obtained via confined selective area growth (SAG). The cathodoluminescence (CL) revealed that the micro-pyramid arrays were under a severe local strain and potential fluctuations were observed depending on the location of the pyramid facets. The emissive region in the micro-pyramid facets was also found to be different in the monochromatically captured CL images. Furthermore, the electroluminescence (EL) of the micro-pyramid LED arrays had a palpable emissive spectra for high indium composition compared to c-plane LEDs, and optical polarization switching was also observed.

Experimental study on fabrication and evaluation of micro pyramid-structured silicon surface using a V-tip of diamond grinding wheel

A mechanical fabrication of micro pyramid-structured silicon surface is proposed using crossed grooving with a 60° V-tip of diamond grinding wheel. It can obtain high form-accuracy, good surface quality and efficient productivity in contrast to laser machining and etching, and also assure a high aspect ratio in contrast to other mechanical processes. In order to describe its micro-structured topography, a white-light interferometer was employed, and its measured point cloud was matched using an Iterative Closest Point (ICP) algorithm. In micro grinding, a novel CNC mutual-wear truing was first developed to sharpen the wheel V-tip; then, the effects of microscopic wheel topography, silicon crystal-orientation and grinding parameter were investigated on ground micro-topography, truing ratio and material removal ratio; finally, its form-accuracy, pyramid top radius, groove tip radius, surface roughness and aspect ratio were evaluated. It is shown that better microscopic grain protrusion topography on wheel V-tip produces much larger material removal ratio and much better micro-structured topography in micro grinding, but it leads to much less truing ratio in finer GC truing. In micro grinding, silicon crystal-orientation has little effect on micro-structured topography due to diamond crystal-orientations that are randomly distributed on wheel V-tip. Although the micro pyramid-structured form error is only about 3.4 μm, its V-groove bottom and pyramidal top have very large form errors (23.1–47.9 μm) due to the sharpness of wheel V-tip and the frangibility of micro pyramid top. On increasing feed speed, its pyramid top radius decreases and its groove tip radius slightly increases, ultimately leading to an increase in aspect ratio, whereas its surface quality descends. It is concluded that the micro-pyramid arrays may be precisely patterned on silicon surface using a SD600 wheel with crossed tool paths, on-machine V-tip truing and the depth of cut in 1 μm.

Comparing Microgroove Array Forming with Micropyramid Array Forming in the Glass Molding Press

This paper presents a glass molding press (GMP) method to fabricate microgroove array and micropyramid array on the glass plate by replicating the shape of the mold to the glass surface. The differences between microgroove forming and micropyramid forming were investigated by experiments and finite element method (FEM) simulations. Microgroove arrays and micropyramid arrays were generated on the flat glass plate in the GMP process by using an electroless-plated Nickel Phosphorus (Ni-P) mold, on which the microstructures are fabricated by micro cutting. Furthermore, FEM simulations were used to trace the stress distribution and the strain distribution during the glass deformation, which illustrates the glass material flow in the microgrooves and the micropyramids on the mold during pressing. By comparing the processes between microgroove forming and micropyramid forming, the differences between them observed in the experiments were explained by the simulation results. Finally, some techniques to improve the forming accuracy were proposed.

Fabricating micro-structured surface by using single-crystalline diamond endmill

A ball endmill made of single-crystalline diamond was used for cutting micro-structures on two kinds of mold materials, oxygen-free copper, and reaction-bonded silicon carbide (RB-SiC). The cutting performance of the ball endmill was investigated by examining surface roughness and form accuracy of the machined workpiece as well as tool wear characteristics. Micro-dimple arrays, micro-grooves, and micro-pyramid arrays with extremely smooth surface and high-accuracy profile could be obtained on oxygen-free copper without remarkable tool wear. When machining RB-SiC, however, tool flank wear takes place, leading to a rough surface finish. After the tool has worn off, the cutting performance of the endmill significantly depended on the tool feed direction. The optimum tool feed direction for micro-grooving was experimentally investigated.

Interferometric technique for faceted microstructure metrology using an index matching liquid

Microstructured optical products are becoming more widespread due to advances in manufacturing. Many of these structures contain faceted surfaces with steep slopes. Adequate metrology for such surfaces is lacking. We describe an interferometric technique that combines plane wave illumination with an index matching liquid to achieve high quality, high speed measurements of such faceted microstructures. We account for refraction at the interfaces, rather than consider only optical path length changes due to the index liquid, and this significantly improves the facet angle measurement. We demonstrate the technique with the measurement of an array of micropyramids and show that our results are in good agreement with measurements taken on a contact profilometer. We also extend the technique to measure opaque microcorner cubes by implementing an intermediate replication step.

Effects of PDMS curing ratio and 3D micro-pyramid structure on the formation of an in vitro neural network

An in vitro neural network can provide a model to investigate the signaling processes that regulate the body functions. Using polydimethylsiloxane (PDMS) multilayer micro-structures to construct a neural network, the bonding between the layers can be improved by modulation of the curing ratio. In this study, we found that as the curing ratio increased from 10:1 to 10:4, the contact angle decreased from 111.69° to 102.08° and the surface energy and roughness increased. For adhesion and proliferation, the hippocampal primary neural cells preferred a PDMS surface with a 10:1 curing ratio to other surfaces. In addition, 3D PDMS micro-pyramid array allows the primary hippocampal neuron to pattern a network without any surface treatment and the network was verified by immunocytochemistry. These results may suggest the optimum PDMS curing ratio to apply in constructing a cell device for in vitro neural network formation as well as the potential of 3D structure fabrication which allows us to construct neural network without any surface treatment.

Precision machining of microstructures on electroless-plated NiP surface for molding glass components

Micro-cutting experiments were conducted on electroless-plated nickel phosphorous (NiP) surfaces to fabricate microstructures such as microgrooves and micropyramid arrays. Burr formation behavior and cutting force characteristics were investigated experimentally and simulated by the finite element method (FEM) under various conditions. A simple two-step cutting process was proposed to improve the surface quality. The machined microstructure arrays were used as molds for hot-press glass molding experiments and good geometrical transferability was confirmed. The results from this study verify that diamond-machined NiP microstructure molds are applicable to glass molding processes for mass production of precision micro-mechanical and optical components.

Studies of the photonic and optical-frequency phonon properties of arrays of selectively grown GaN micropyramids

An array of GaN micropyramids containing a near-surface InxGa1−xN∕GaN single quantum well has been fabricated using selective area epitaxial overgrowth above a patterned silica mask. The pyramid array has been studied by means of angle-resolved reflection measurements using s- and p-polarized incident light in the near- and mid-infrared optical ranges. We have found that the periodic array of flat-topped pyramids shows marked resonances in the near-infrared optical range due to resonant Bloch modes within the extraction cone and that the angular dispersion of these modes exhibits strong photonic crystal characteristics. The experimental results are in good agreement with the photonic band structure calculated using a scattering matrix formalism. The mid-infrared optical anisotropy properties of the micropyramids were investigated to probe the infrared active phonons of the pyramid array. The A1(LO) phonon of the InxGa1−xN∕GaN single quantum well was identified and the InN mole fraction was estimated from ...

Photonic crystals comprising selectively grown flat‐topped and sharp‐tipped GaN pyramids

Selective area growth offers the promise of producing III-nitride photonic crystals (PhCs) without dry etching and concomitant surface damage. Two PhC structures were studied, one comprising an array of flat-topped GaN micropyramids with micrometre-scale periodicity, and the other made up of sharp-tipped pyramids with sub-micrometre periodicity. Angularly resolved reflection and transmission measurements revealed the presence of sharp resonances associated with resonant Bloch modes. As a result, the photonic band structure was determined along symmetry directions of the reciprocal lattice for the two PhC structures.

Quantum dot emission from site-controlled InGaN∕GaN micropyramid arrays

In x Ga 1 − x N quantum dots have been fabricated by the selective growth of GaN micropyramid arrays topped with InGaN∕GaN quantum wells. The spatially, spectrally, and time-resolved emission properties of these structures were measured using cathodoluminescence hyperspectral imaging and low-temperature microphotoluminescence spectroscopy. The presence of InGaN quantum dots was confirmed directly by the observation of sharp peaks in the emission spectrum at the pyramid apices. These luminescence peaks exhibit decay lifetimes of approximately 0.5ns, with linewidths down to 650μeV (limited by the spectrometer resolution).