Microhole Array Research Articles

Performance Optimization of Microvalves Based on a Microhole Array for Microfluidic Chips

A microfluidic chip with a microvalve based on a microhole array is proposed in this paper for the POCT of tumor marker proteins. In order to control the biochemical reaction time accurately and obtain a higher testing sensitivity, the parameters of the microhole array are optimized basing on the investigation of the effects of the variation of those parameters on the fluid rate and the residual liquid value in the microvalve region. By conducting liquid flow experiments using microvalves based on microhole arrays with varying microstructural parameters, the residual rate of reaction products is demonstrated to be proportional to the depth and diameter of the microholes and inversely proportional to the distance between the microhole centers. A comprehensive analysis indicates that a microhole depth of 95 μm, a microhole diameter of 230 μm, and a distance between microhole centers of 250 μm not only ensure a sufficiently long delay time, but also reduce the residual rate of reaction products, thereby providing an optimum microvalve performance that maximizes the detection efficiency and accuracy of microfluidic chips.

Controllable preparation of micro-textures on WC/Co substrate surface by an integrated laser-dry etching process for improving PVD coatings adhesion

The micro-texture fabricated on the substrate surface of coatings is an established method for enhancing the Physical Vapor Deposition (PVD) coatings adhesion. Currently, laser micromachining is widely used to prepare the micro/nano-texture. Nevertheless, it has a problem of low processing accuracy, such as irregular texture edges and irregular protrusions inside the textures. Here, we propose an ion beam etching (IBE)-assisted laser machining approach to fabricate micro-texture, and the effect of IBE/laser textures on the PVD coatings adhesion is revealed for the first time. The profile evolution mechanism of the micro-textures on the cemented carbide during etching process was investigated. By IBE-assisted laser machining, a ring-shaped microholes array with high precision and controllability was fabricated on the cemented carbide substrate surface. Then, the textured substrates were coated with TiAlN materials using cathode arc-evaporation technique. The scratch tests and dry wear tests on the coatings with the textured substrates were carried out, demonstrating that the IBE/laser textured coatings exhibited higher adhesive strength compared to that of the untextured and laser textured ones. Thus, the IBE-assisted laser machining shows a great potential in the preparation of high precision micro-textures for enhancing PVD coatings adhesion.

Bistable Terahertz Switch Designed by Integration of a Graphene Plasmonic Crystal into Fabry-Perot Resonator

An appropriately designed periodic lattice in graphene can cause plasmonic modes, absorbing a specific THz frequency band, minimizing transmission. A suitable Fabry-Perot cavity, on the other hand, can resonate at another desired frequency in the nearby THz band, enhancing the transmittance. Integrating, these two types of structures into a THz switch, in which the peaks of the plasmonic absorption and the Fabry-Perot resonance band play the role of the OFF and ON states, yielding a bistable switch with a very high output due to the resonance and a high extinction (ON/OFF) ratio. By placing a patterned with an array of circular micro-holes with a plasmonic absorption band centered about 4.1 THz in the middle of a Fabry-Perot vertical cavity of resonance band centered about 3.9 THz, we have designed a bistable THz switch with 87% transmission in the ON state and the extinction ratio of 23 dB. The optical bistability, in this switch, is the consequence of graphene nonlinear property invoked by the enhanced optical intensity due to the cavity resonance, adding an intensity-dependent term to the graphene surface conductivity.

Terahertz Time-Domain Spectroscopy for Ultrafast and Quasi-Static Characterizations of Germanium

In this work, we carry out a comprehensive study of charge carrier dynamics in germanium spanning its ultrafast to quasi-static timescales. The study makes use of a continuous-wave (CW) near-infrared (NIR) pump and terahertz (THz) probe beams to realize a CW NIR pumped THz-time-domain spectroscopy (THz-TDS) system with frequency-selective homodyne detection. This enables characterizations of ultrafast charge carrier scattering, while the detection is locked to the THz probe beam, and quasi-static charge carrier recombination, while the detection is locked to the CW NIR pump beam. The ultrafast THz-TDS characterization reveals scatter times of 239 fs for electrons and 204 fs for holes at low pump intensities, which reduce to scatter times of 186 fs for electrons and 159 fs for holes at higher pump intensities. These scatter times give good agreement to the literature mobility of germanium at low intensities and indicate carrier–carrier scattering at higher intensities. The quasi-static THz-TDS characterization suggests that the charge carrier lifetime is dominated by surface states. The manifestation of surface states is gleaned by characterizing germanium samples with varying forms of microhole arrays. This reveals lifetimes ranging from 1.5 to 8.6 μ s, in good agreement with a linearized model with a surface recombination velocity of 14 200 cm/s. Ultimately, the experimental results from the CW NIR pump THz-TDS system and the applied theoretical models give an accurate depiction of the charge carrier dynamics that evolve in germanium over its ultrafast to quasi-static timescales.

Flexible fabrication of optical glass micro-lens array by using contactless hot embossing process

Abstract This paper reported the fabrication of optical glass micro-lens array (MLA) by using contactless micro-embossing technology. Firstly, the micro-hole array in the tungsten carbide (WC) substrate was made by the micro-electrical discharge machining (micro-EDM). Then, to improve the oxidation resistance, anti-sticking properties and chemical inertness of mold surface, Cr-W-N coating was deposited on the mold insert by plasma-enhanced sputtering technique. Finally, controllable MLA were fabricated with the coated EDM-mold by using the precision glass molding machine from Shenzhen University (SZU’s PGMM30). The surface roughness, three-dimensional morphology and optical performance of glass MLA were measured by ContourGT-X 3D optical profiler, ultraviolet spectrophotometer, and optical imaging system, respectively. The experimental results indicated that the contactless hot embossing process has the capability of fabricating high-quality and multi-scale glass MLA. This work provides an effective method for the fabrication of optical glass MLA.

Silicon-based microhole arrays for spectral tuning of white light emitted from InGaN nanocolumns grown by molecular-beam epitaxy

In this paper, the molecular beam epitaxy of InGaN nanocolumns on silicon-based microhole arrays has been investigated experimentally. A low-cost method has been designed, which contains the processes of photolithography and wet etching. Tuning uniform microhole arrays show the ability in modulating the luminescence spectra of InGaN nanocolumns, which could lead to the modulation of tristimulus coordinates and correlated color temperatures. Due to the shadow effect, it is found that the size of microholes and the area of flat Si surface have more obvious effects, while the configuration of microhole arrays has limited effect on the modulation of peak wavelength.

Engineering Micropatterned Surfaces for Controlling the Evaporation Process of Sessile Droplets

Controlling the evaporation process of a droplet is of the utmost importance for a number of technologies. Also, along with the advances of microfabrication, micropatterned surfaces have emerged as an important technology platform to tune the wettability and other surface properties of various fundamental and applied applications. Among the geometrical parameters of these micropatterns, it is of great interest to investigate whether the arrangement of the patterns would affect the evaporation process of a sessile liquid droplet. To address this question, we fabricated four microhole arrays with different arrangements, quantified by the parameter of “eccentricity”. The results suggested that, compared to smooth substrates, the evaporation mode was not only affected by engineering the microhole arrays, but also by the eccentricity of these micropatterns. The values of contact angle hysteresis (CAH) were used to quantify and test this hypothesis. The CAH could partially explain the different evaporation modes observed on the microhole arrays with zero and non-zero values of eccentricity. That is, on microhole arrays with zero eccentricity, CAH of water droplets was comparatively low (less than 20 ° ). Consistently, during the evaporation, around 60% of the life span of the droplet was in the mixed evaporation mode. Increasing the eccentricity of the microhole arrays increases the values of CAH to above 20 ° . Unlike the increasing trend of CAH, the evaporation modes of sessile droplets on the microhole array with non-zero values of eccentricity were almost similar. Over 75% of the life span of droplets on these surfaces was in constant contact line (CCL) mode. Our findings play a significant role in any technology platform containing micropatterned surfaces, where controlling the evaporation mode is desirable.

Fabrication of Cu/graphite film/Cu sandwich composites with ultrahigh thermal conductivity for thermal management applications

Effective thermal management of electronic integrated devices with high powder density has become a serious issue, which requires materials with high thermal conductivity (TC). In order to solve the problem of weak bonding between graphite and Cu, a novel Cu/graphite film/Cu sandwich composite (Cu/GF/Cu composite) with ultrahigh TC was fabricated by electro-deposition. The micro-riveting structure was introduced to enhance the bonding strength between graphite film and deposited Cu layers by preparing a rectangular array of micro-holes on the graphite film before electro-deposition. TC and mechanical properties of the composites with different graphite volume fractions and current densities were investigated. The results showed that the TC enhancement generated by the micro-riveting structure for Cu/GF/Cu composites at low graphite content was more effective than that at high graphite content, and the strong texture orientation of deposited Cu resulted in high TC. Under the optimizing preparing condition, the highest in-plane TC reached 824.3 W·m−1K−1, while the ultimate tensile strength of this composite was about four times higher than that of the graphite film.

Manipulations of light by ordered micro-holes in silicon substrates.

Ordered micro-holes with controllable period, diameter and depth are fabricated in Si (001) substrates via a feasible approach based on nanosphere lithography. They dramatically reduce the reflectance in a broad wavelength range of 400-1000 nm, which can be deliberately modulated by tailoring their geometrical parameters. The simulated reflectance via finite-difference time-domain (FDTD) method agrees well with the experimental data. The FDTD simulations also demonstrate substantially enhanced light absorption of a Si thin film with ordered micro-holes. Particularly, the light-filled distributions around micro-holes disclose fundamental features of two types of modes, channel modes and guided modes, involving the wavelength-dependence, the origin, the dominant location region and the interference pattern of the light field around micro-holes. Our results not only provide insights into the antireflection and the substantially enhanced absorption of light by ordered micro-holes, but also open a door to optimizing micro-hole arrays with desired light field distributions for innovative device applications.

Stress Release Effect of Micro-hole Arrays for Flexible Electrodes and Thin Film Transistors.

The effects of micro-hole arrays in the thin metal films were studied as a method to release bending stress in flexible electrodes and flexible thin film transistors (TFTs). Interest in flexible electronics is increasing, and many approaches have been suggested to solve the issue of the electrical failure of electrodes or electrical components such as TFTs after repeated bending. Here, we demonstrate a micro-hole array structure as a common solution to release bending stress. Although micro-size cracks were generated and propagated from the hole edges, the cracks stopped within a certain range when enough stress was released. Moreover, since the crack sites were predictable and controllable, a fatal electrical breakdown in a conductive layer such as a metal electrode or the semiconducting junction of a TFT can be prevented by specifically arranging the hole arrays. Thin film layers fabricated without holes suffered an electrical breakdown due to random crack propagation during bending tests. Aluminum thin film electrodes prepared with arrays of 3 μm diameter holes and 25% hole area showed excellent durability after 300,000 bending cycles. The change in resistance was below 3%. The electrical characteristics of an a-IGZO TFT with the micro-hole structure were almost equivalent to a standard a-IGZO TFT. After 10,000 bending cycles, ION and the ratio of ION/IOFF remained >107 A and ∼107, respectively. Since the effective hole diameter is micrometer in size, fabrication does not require additional process steps or expensive process equipment. Therefore, the approach can be an important way to enhance the reliability of various electrical devices in flexible and wearable applications.

A novel micro-EDM method to improve microhole machining performances using ultrasonic circular vibration (UCV) electrode

The difficulties in circulating working fluid and expelling discharge debris are major challenges for microhole machining by micro electrical discharge machining (micro-EDM). To solve this issue, a novel micro-EDM method that adopts ultrasonic circular vibration (UCV) electrode was proposed in this study, aiming at effectively improving the microhole machining performances. A piezoelectric device for UCV electrode with 32.85 kHz frequency and 3 μm trajectory radius was developed and its basic principle was analyzed. To better understand the effect of UCV electrode on microhole machining process, the solid-liquid two-phase flow simulation was conducted, and the flow field distribution and discharge debris movement in machining gap were studied. Machining experiments of deep microhole (aspect ratio = 9.4) showed that the UCV electrode can effectively lower the electrode retreat frequency and prevent the debris adhesion at the inner wall. 10 × 10 microhole array machining experiments were also carried out in this study. Results showed that the inlet and outlet of microhole array have better morphologies when using UCV electrode, and the diameter consistency of inlet and outlet were improved by 22% and 2.8% compared with rotating electrode. The advantages of UCV electrode in improve microhle machining performances can be mainly attributed to the significantly enlarged flow velocity of working fluid and enhanced capacity of working fluid to expel debris particles in terms of number and efficiency. Meanwhile, the increased relative velocity between electrode and workpiece is also effective to discontinue the abnormal discharges and achieve stable machining process.

Proposal of a new diffractive corneal inlay to improve near vision in a presbyopic eye.

A new class of diffraction-based corneal inlays for treatment of presbyopia is described. The inlay is intended to achieve an improvement of the near focus quality over previous designs. Our proposal is a two-zone hybrid device with separated amplitude and phase areas having a central aperture and no refractive power. An array of micro-holes is distributed on the surface of the inlay conforming a binary photon sieve. In this way, the central hole of the disk contributes to the zero order of diffraction, and the light diffracted by the micro-holes in the peripheral photon sieve produces a real focus for near vision. We employed ray-tracing software to study the performance of the new inlay in the Liou-Brennan model eye. The modulation transfer functions (MTFs) at the distance and near foci, and the area under the MTFs for different object vergences, were the merit functions used in the evaluation. The results were compared with those obtained with previous pure amplitude designs. Additionally, image simulations were performed with the inlays in the model eye to show the good performance of our proposal in improving the quality of the near vision.

An AC-driven desalination/salination system based on a Nafion cationic rectifier

Cationic diodes based on Nafion ionomer coated microhole array substrates are combined with an anion conducting membrane to give an AC-electricity driven desalination or salination system. The combination of cationic diode with anionic resistor allows a 3D-printed four-chamber system to be configured with two internal chambers to extract and/or accumulate salt. Three configurations are demonstrated combining desalination and salination for an aqueous 250 mM NaCl solution. For a proof-of-concept device, 50% desalination/salination is demonstrated. The charge efficiency is estimated to be only 12%, limited mainly due to (i) resistivity lowering rectification, (ii) rectification ratio limitations for the array of microholes, (iii) insufficient stability of ionomer membranes under AC-pulse-driven operation. Perspectives for improvements and for future desalination/salination applications are given.

Self-Assembled Microlens Array with Controllable Focal Length Formed on a Selective Wetting Surface.

We report a microlens array with a controllable focal length that is based on a selective wetting surface. A substrate modified with a hydrophobic layer that has an array of microholes exhibits different wettability for inside holes (hydrophilic) versus outside holes (hydrophobic). When liquid flows over the surface, a small amount of liquid is adhered to hydrophilic holes and forms a lens-shaped droplet array that self-assembles because of surface tension. A large-sized plano-convex lens array that has good uniformity is obtained via the blade coating method, and controlling the amount of liquid enables control of the focal length. Our self-assembled microlens array has the merits of high optical performance, a simple fabrication procedure, and good mechanical stability, and thus, it has potential applications in imaging processing, light extraction, protein detection, light-emitting diodes, sensors, and displays.

The Tool Electrode Wear and Gap Fluid Field Simulation Analysis in Micro-EDM Drilling of Micro-hole Array

With the micro-hole array structure has been widely used in industrial areas, such as inkjet printer nozzle, oil nozzle, spinneret and so on, its processing accuracy significantly affects the performance of the product. In the machining of micro-hole array by micro-EDM drilling, unavoidable tool electrode wear and frequent occurrence of abnormal discharges such as short circuit and secondary discharge caused by the difficulty in removing discharge debris are the main factors affecting machining geometric and dimensional accuracy. In this study, the flow velocity simulation analysis of working fluid playing a crucial role in the flushing of discharge debris and the experimental study on the axial wear of tool electrode are explored in order to ensure the machining accuracy of micro-hole array. By changing the parameters of tool electrode diameter and machining depth in the fluid field simulation analysis, the gap velocity distribution under different depth-diameter ratios is obtained. Simulation results show that the gap flow velocity decreases with depth-diameter ratios increasing. For different depth-diameter ratios in the micro-hole array processing, the tip morphology evolution and the wear of tool electrode are considered, which show that the electrode tip morphology tends to be stable and the axial wear of the tool electrode could be compensated evenly. In addition, the characteristics of tool electrode axial wear are explained based on the simulation results of gap flow field, which verifies the effectiveness of the flow field simulation and provides a reference for formulating electrode wear compensation strategy and improving machining accuracy to some extent.

INVESTIGATION OF DEBRIS PARTICLES DISTRIBUTION IN ELECTRICAL DISCHARGE MACHINING OF MICRO-HOLES ARRAY

Micro-holes array and micro-electrodes array are widely used in biomedical industry, aerospace industry and micro-electromechanical systems. The machining of micro-holes array is a difficult problem for traditional machining technology. Currently electrical discharge machining technology (EDM) is an alternative machining technology for the fabricating of micro-holes array in high strength and hardness of metal material because of it is free from mechanical force. However, distribution and concentration of debris particles in the discharge gap easily leads to the intensive distribution of discharge positions, which significantly influences machining performance of EDM of micro-holes array. In this study, a three-dimensional flow field simulation model of ultrasonic assisted EDM of micro-holes array is proposed by FLUENT software. The effect of ultrasonic vibration and the depth-to-diameter ratio on debris particles distribution in ultrasonic assisted EDM of micro-holes array will be investigated by numerical simulation. It is shown that reducing the depth-diameter ratio and increasing the ultrasonic amplitude and frequency are beneficial to enhance the exhaustion of debris particles out of the discharge gap and decrease the concentration of debris particles in the discharge gap. In addition, machining experiments are carried out.

Influences of Microhole Depth and SiO2 Nanoparticle/Microsphere Passivation Layer on the Performance of GaN-Based Light-Emitting Diodes

A hybrid structure, including 45° sidewalls, a microhole array, and a thin SiO2 nanoparticle (NP)/microsphere (MS) passivation layer, is used to produce GaN-based light-emitting diodes (LEDs). The influences of the microhole depth and SiO2 NP/MS passivation layer on the LED performance are studied. A shallower depth of a microhole array shows better optical properties due to the complete preservation of the GaN/InGaN multiple quantum well (MQW) region. In addition, the use of a thin SiO2 NP/MS passivation layer gives a remarkably reduced reverse-biased leakage current and improved optical performance. Experimentally, under an injection current density of 110 A/cm2, the studied device, with a proper hybrid structure, shows enhancements of 25.9%, 29.2%, and 29.4% in light output power (LOP), external quantum efficiency (EQE), and wall-plug efficiency, respectively, as compared to a LED device with a deeper depth of microhole array. These improvements are mainly caused by the reduced total internal reflection (TIR) and the Fresnel reflection, which increase scattering probability and the opportunity to find photon escape cones. Hence, the studied hybrid structure in this work is a promising route to fabricate high-performance GaN-based LEDs.

High Efficiency Flip Chip-Based UV Emitter with Indium Tin Oxide Nano Grains/Al Reflector and Periodic Microhole Arrays.

We propose a high efficiency flip chip-based ultraviolet (UV) emitter with aluminum (Al) reflector that includes indium tin oxide (ITO) nano grains for current injection between the Al and p-AlGaN layer. Al has attracted attention as a reflector for high efficiency UV emitters because of its high reflectance in the UV region. To improve the efficiency of UV emitter, we generated periodic microhole arrays on the p-AlGaN layer, which serve as a scattering center in the flip chip structure and enhance the light extraction efficiency. The light output power of the fabricated flip chip-based UV emitter with ITO nano grains/Al reflector and microhole arrays on the p-AlGaN layer is significantly improved by 72% and 45% at an injection current of 20 mA, compared to that of UV emitter with only Al reflector and ITO nano grains/Al reflector.

Plasmonic micropipe spectral filters in mid-infrared.

Multispectral analyzers based on nanostructured plasmonic spectral filters can potentially find a wide range of applications. However, spectral filters based on the widely reported microhole or ring arrays suffer from relatively wide filtering bands, resulting in a relatively low spectral resolution. In this work, we fabricate high-performance spectral filters based on vertically standing micropipes on a silver film. An infrared spectral microscope is used to investigate the properties of these micropipe spectral filters. The results indicate that the micropipe spectral filters have a full width at half-maximum ∼5 times smaller than the microhole filters at the same wavelength. Micropipe spectral filters are expected to significantly improve the spectral resolution of multispectral analyzers.

Self‐Organized Periodic Microholes Array Formation on Aluminum Surface via Femtosecond Laser Ablation Induced Incubation Effect

AbstractLaser surface structuring has been demonstrated to be a versatile technology to create various functional materials by modifying solid surface properties. An interesting experimental phenomenon of self‐organized periodic microholes array formation is demonstrated by exposing an aluminum surface to femtosecond laser irradiation. The microholes with a diameter much smaller than focal laser spot size are produced along laser scan paths due to the incubation effect of multiple laser scans, and they spontaneously form a highly ordered microholes array after 80 scans. It is found that the microhole period and diameter are highly dependent on laser fluence, and controllable microholes arrays with different periods and diameters are achieved by adjusting laser fluence. The physical mechanism behind the formation of the microholes array is attributed to femtosecond laser‐induced melting and Marangoni effect. The research provides a novel processing approach to achieve controllable microholes surface materials fabrication at a high speed on metal substrates, which are of great interests for various technological applications.