Fabrication Of Three-dimensional Microstructures Research Articles

Fabrication of three-dimensional microstructures on GaN surfaces through the integration of femtosecond laser ablation and ICP etching

Gallium nitride (GaN) surface microstructures are crucial for enhancing the performance and reliability of high-power devices. However, the direct fabrication of patterned three-dimensional (3D) microstructures on GaN surfaces remains a significant challenge. This paper introduces an innovative approach, employing a maskless femtosecond laser combined with inductively coupled plasma (ICP) etching, to efficiently fabricate petal-shaped 3D microstructures on GaN surfaces. By fine-tuning the ICP etching and femtosecond laser parameters, we have obtained the regularities of the dimensional variations of these 3D microstructures. Additionally, our investigation reveals that the design of microstructural concavity and convexity characteristics can be flexibly manipulated by controlling the ICP etching rate, which is influenced by the oxide content in the laser-modified zone. This study presents a novel method for straightforward fabrication of microstructures on gallium nitride surfaces.

U-Net convolutional neural network-based modification method for precise fabrication of three-dimensional microstructures using laser direct writing lithography.

In this paper, a modification method based on a U-Net convolutional neural network is proposed for the precise fabrication of three-dimensional microstructures using laser direct writing lithography (LDWL). In order to build the correspondence between the exposure intensity distribution data imported to the laser direct writing system and the surface profile data of the actual fabricated microstructure, these two kinds of data are used as training tensors of the U-Net convolutional neural network, which is proved to be capable of generating their accurate mapping relations. By employing such mapping relations to modify the initial designed exposure intensity data of the parabolic and saddle concave micro-lens with an aperture of 24µm×24µm, it is demonstrated that their fabrication precision, characterized by the mean squared error (MSE) and the peak signal-to-noise ratio (PSNR) between the fabricated and the designed microstructure, can be improved significantly. Specifically, the MSE of the parabolic and saddle concave micro-lens decreased from 100 to 17 and 151 to 50, respectively, and the PSNR increased from 22dB to 29dB and 20dB to 25dB, respectively. Furthermore, the effect of laser beam shaping using these two kinds of micro-lens has also been improved considerably. This study provides a new solution for the fabrication of high-precision three-dimensional microstructures by LDWL.

Three-dimensional structures fabricated after laser-induced free radical generation in azoaromatic compounds

Three-dimensional microstructures with selective properties have often been developed for optical and photonic applications. The simplest way to achieve optically active structures is by the functionalization of host matrices using, for instance, organic dyes. Laser fabrication techniques, such as two-photon polymerization, allow manufacturing devices quickly, with high resolution, without shape limitation, among other many advantages. In this work, we demonstrate the fabrication of three-dimensional microstructures via two-photon polymerization, using azobenzene-based dyes as photoinitiator, the compound responsible for absorbing the light and start the polymerization. When a high photoinitiator concentration is used (for instance 1.00 wt%), a significant amount of dye remains unchanged into the final structures, and its optically induced birefringence could be investigated. Therefore, the azobenzene-based dye acts, at the same time, as the photoinitiator and functionalizer.

Rapid Fabrication of Continuous Surface Fresnel Microlens Array by Femtosecond Laser Focal Field Engineering.

Femtosecond laser direct writing through two-photon polymerization has been widely used in precision fabrication of three-dimensional microstructures but is usually time consuming. In this article, we report the rapid fabrication of continuous surface Fresnel lens array through femtosecond laser three-dimensional focal field engineering. Each Fresnel lens is formed by continuous two-photon polymerization of the two-dimensional slices of the whole structure with one-dimensional scan of the corresponding two-dimensional engineered intensity distribution. Moreover, we anneal the lens array to improve its focusing and imaging performance.

Approximating gecko setae via direct laser lithography

The biomimetic replication of dry adhesion present in the gecko’s foot has attracted great interest in recent years. All the microfabrication techniques used so far were not able to faithfully reproduce the hierarchical and complex three-dimensional geometry of the gecko’s setae, with features at the micro- and nano-scale, thus reducing the effectiveness that such conformal morphology could provide. By means of direct laser lithography we fabricated artificial hairs that faithfully reproduce the natural model. This technique allows the fabrication of three-dimensional microstructures with outstanding results in terms of reproducibility and resolution at the micro- and nano-scale. It was possible to get very close to the morphology of the natural gecko setae, especially concerning the hierarchical shape. We designed several morphologies for the setae and studied the effects in terms of adhesion and friction performances compared to the natural counterpart, showing the interplay between morphology, dimensional scaling and materials. Direct laser lithography promises great applications in the biomimetics field, paving the way to the implementation of the concept of hierarchical bioinspired dry adhesives.

Modeling and measurement of curing properties of photocurable polymer containing magnetic particles and microcapsules

In microstereolithography, three-dimensional microstructures are created by scanning an ultraviolet laser on a photocurable resin and stacking several such layers to form the desired structure. By mixing different types of particles in the resin, the formed microstructures exhibit various physical properties. For example, the magnetism and density of the microstructure can be controlled by adding magnetic particles and microcapsules to the resin. This method has been used to fabricate magnetic micromachines. Although such functional resins are useful, the incorporated magnetic particles and microcapsules can affect the fabrication resolution, making it difficult to fabricate microstructures with high precision. Thus, it is necessary to understand the effects of such microparticles and microcapsules on the fabrication process. In this study, we propose a simple model of curing resins containing magnetic particles and microcapsules to explain the effects of the magnetic particles and microcapsules. The proposed model can explain the observed curing characteristics of a resin that contains particles for all concentrations as well as for different types of magnetic particles and microcapsules. Finally, using the proposed model, we discuss how to improve the characteristics of resins containing microparticles to realize the high-resolution fabrication of three-dimensional microstructures with desirable material properties.

Fabrication of three-dimensional microstructures in positive photoresist through two-photon direct laser writing

Three-dimensional (3D) microstructures with micron scale are fabricated in photoresist using two-photon direct laser writing with an infrared femtosecond laser at 800 nm. The positive photoresist of Novolak/diazonaphthoquinone (DNQ) is used for the fabrication of line structures and 3D microstructures. Linewidths of line structures are fabricated with laser power ranging from 1 to 15 mW and scanning speeds ranging from 5 to 50 μm s−1. The obtained linewidth is analyzed using an exposure kinetics model of DNQ for two-photon absorption. Both 3D inversed woodpile structures and helical structures are fabricated. Two-photon direct laser writing is used to fabricate microstructures in positive photoresist. Microstructures with helical and inversed woodpile shapes were fabricated with positive photoresist Novolak/DNQ using two-photon direct laser writing.

Two-photon polymerization of microstructures by a non-diffraction multifoci pattern generated from a superposed Bessel beam.

In this Letter, superposed Bessel beams (SBBs) are realized by alternatively imprinting holograms of opposite-order Bessel beams along the radial direction on a spatial light modulator. The propagation invariance and non-rotation properties of SBBs are theoretically predicted and experimentally demonstrated. The focusing property of SBBs with a high numerical aperture (NA) objective is investigated with the Debye vectorial diffraction theory. Near the focal plane, a circularly distributed multiple foci pattern is achieved. The multiple foci generated from SBBs are adopted in a two-photon fabrication system, and micropattern fabrication by a single exposure is demonstrated. Facile fabrication of three-dimensional microstructures with SBBs is realized by dynamically controlling the number of focal spots, and the diameter and rotation of the focal pattern.

Photolysis of a peptide with N-peptidyl-7-nitroindoline units using two-photon absorption.

N-acyl-7-nitroindolines have been used as caged compounds to photorelease active molecules by a one- or two-photon excitation mechanism in biological systems. Here, we report the photolysis of a polypeptide that contains 7-nitroindoline units as linker moieties in its peptide backbone for potential materials engineering applications. Upon two-photon excitation with femtosecond laser light at 710 nm the photoreactive amide bond in N-peptidyl-7-nitroindolines is cleaved rendering short peptide fragments. Thus, this photochemical process changes the molecular composition at the laser focal volume. Gel modifications of this peptide can potentially be used for three-dimensional microstructure fabrication.

Fabrication of microstructured materials based on chitosan and D,L-lactide copolymers using laser-induced microstereolithography

The graft copolymers of chitosan and oligo(D,L-lactide) obtained by solid-phase synthesis have been used as the basis of photosensitive compositions for the fabrication of three-dimensional microstructures by laser-induced stereolithography. The electronic absorption spectra of the copolymers are close to the sum of the spectra of native chitosan and polylactide, which has been chosen as a model of grafted oligolactide chains. The fundamental absorption bands of the copolymers lie in a range to 500 nm, and their contribution to the absorption intensity of a photosensitive composition based on the copolymers at second harmonic laser frequency is insignificant. Depending on the macromolecular characteristics of the copolymers, the three-dimensional crosslinking of photosensitive compositions on their basis in the course of microstructuring occurs with different efficiency.

On-chip enucleation of an oocyte by untethered microrobots

We propose a novel on-chip enucleation of an oocyte with zona pellucida by using a combination of untethered microrobots. To achieve enucleation within the closed space of a microfluidic chip, two microrobots, a microknife and a microgripper were integrated into the microfluidic chip. These microrobots were actuated by an external magnetic force produced by permanent magnets placed on the robotic stage. The tip of the microknife was designed by considering the biological geometric feature of an oocyte, i.e. the oocyte has a polar body in maturation stage II. Moreover, the microknife was fabricated by using grayscale lithography, which allows fabrication of three-dimensional microstructures. The microgripper has a gripping function that is independent of the driving mechanism. On-chip enucleation was demonstrated, and the enucleated oocytes are spherical, indicating that the cell membrane of the oocytes remained intact. To confirm successful enucleation using this method, we investigated the viability of oocytes after enucleation. The results show that the production rate, i.e. the ratio between the number of oocytes that reach the blastocyst stage and the number of bovine oocytes after nucleus transfer, is 100%. The technique will contribute to complex cell manipulation such as cell surgery in lab-on-a-chip devices.

Direct laser writing of synthetic poly(amino acid) hydrogels and poly(ethylene glycol) diacrylates by two-photon polymerization

The additive manufacturing technique of direct laser writing by two-photon polymerization (2PP-DLW) enables the fabrication of three-dimensional microstructures with superior accuracy and flexibility. When combined with biomimetic hydrogel materials, 2PP-DLW can be used to recreate the microarchitectures of the extracellular matrix. However, there are currently only a limited number of hydrogels applicable for 2PP-DLW. In order to widen the selection of synthetic biodegradable hydrogels, in this work we studied the 2PP-DLW of methacryloylated and acryloylated poly(α-amino acid)s (poly(AA)s). The performance of these materials was compared to widely used poly(ethylene glycol) diacrylates (PEGdas) in terms of polymerization and damage thresholds, voxel size, line width, post-polymerization swelling and deformation. We found that both methacryloylated and acryloylated poly(AA) hydrogels are suitable to 2PP-DLW with a wider processing window than PEGdas. The poly(AA) with the highest degree of acryloylation showed the greatest potential for 3D microfabrication.

Characterization of “Bulk Lithography” Process for Fabrication of Three-Dimensional Microstructures

Various ways of fabricating a three-dimensional (3D) component in a single-layer exposure using spatial variation of exposure dose have been presented in the literature. While some of them are based on dynamic mask process, more recently, a process based on varying intensity of a scanning Gaussian laser beam termed as “bulk lithography” has been proposed. In bulk lithography, the entire varying depth 3D microstructure gets fabricated because of spatial variation of intensity of laser imposed at every point in single layer scan. For the bulk lithography process, this paper first presents experimental characterization of unconstrained depth photopolymerization of resin upon exposure to Gaussian laser beam. Experimental characterization carried out for two resins systems: namely 1,6 hexane diol-diacrylate (HDDA) and trimethylolpropane triacrylate (TMPTA), over relatively wider range of Ar+ laser exposure dose and time, show behavior well beyond Beer–Lambert law. A unified empirical model is proposed to represent the nondimensional depth variation with respect to the time and energy of exposure for both resins. Finally, using these models, successful fabrication of several microstructures including micro-Fresnel lens, textured curved surface, otherwise difficult or impossible to fabricate, is demonstrated. Several advantages of the bulk lithography as compared to other similar processes in the literature are highlighted.

Motion-path-based three-dimensional x-ray free-form micromachining system

This paper presents an x-ray machining system for the fabrication of three-dimensional microstructures with free-form surfaces. The structures with different sizes and various shapes can be generated by a single mask combining with precise moving paths. By adjusting the motions such as path types, overlaps and velocities, various 3D microstructures with complex features can be fabricated. The idea and the construction of this system, fabrication details and successful preliminary experimental results are introduced in this paper.

Materials and technologies for fabrication of three-dimensional microstructures with sub-100 nm feature sizes by two-photon polymerization

The fabrication of sub-100 nm feature sizes in large-scale three-dimensional (3D) geometries by two-photon polymerization requires a precise control of the polymeric reactions as well as of the intensity distribution of the ultrashort laser pulses. The authors, therefore, investigate the complex interplay of photoresist, processing parameters, and focusing optics. New types of inorganic– organic hybrid polymers are synthesized and characterized with respect to achievable structure sizes and their degree of crosslinking. For maintaining diffraction-limited focal conditions within the 3D processing region, a special hybrid optics is developed, where spatial and chromatic aberrations are compensated by a diffractive optical element. Feature sizes below 100 nm are demonstrated.

A Study on the Cross-Sectional Shape of Kerf Cut with Micro Abrasive Air Jet

Micro abrasive air jet (MAAJ) cutting is a promising technology for the fabrication of three-dimensional microstructures in hard and brittle materials. In this paper, a study on the cross-sectional shape of the kerf cut with MAAJ is presented. It shows that the machining depth and slope of the sidewall increase with an increase in air pressure, abrasive flow rate and jet incidence angle, while decrease with an increase in nozzle traverse speed. Using a dimensional analysis technique, predictive model for cross-sectional profile is developed. The research results may be meaningful to the highly precision three-dimensional micro-structural cutting.

フォトリソグラフィ−陽極酸化−化学エッチングによるアルミニウム圧延板の装飾的表面加工

Surface patterning is an important technology for improving product design. This paper describes the fabrication of three-dimensional microstructures on aluminum plates using photolithography, anodization, and chemical etching. Photoresist patterns were prepared on an aluminum plate using a standard photolithography technique. The aluminum plate was anodized in sulfuric acid at a controlled concentration and bath voltage. During anodization, an anodic oxide film was formed on the photoresist/aluminum plate interface and in an open region. The oxide film formation was sensitive to both sulfuric acid concentration and bath voltage. The aluminum plate was expanded by anodization. The volume expansion factor (calculated as the ratio of the oxide film thickness to the thickness of the consumed aluminum) was 1.44-1.69.The oxide film was removed by chemical etching in phosphoric acid. Thereby, the aluminum surface was exposed, and convex shapes were formed. The surface roughness of the plate after chemical etching was comparable to its value before fabrication. The mean roughness Ra was <20 nm. Quadrangular pyramids, cones, and hexagonal pyramids were fabricated successfully on the aluminum plate by changing the photoresist mask pattern. Anodization and chemical etching conditions were controlled for accurate shape control of the three-dimensional metallic microstructures.

A predictive divergence compensation approach for the fabrication of three-dimensional microstructures using focused ion beam machining

In this paper, the major factors which cause fabrication divergence in the focused ion beam (FIB) milling process are discussed. A divergence compensation approach is outlined which calculates the corrected dwell time in order to allow for the divergence caused by the FIB milling process; this is due to overlap effects and the angular-dependent sputter yield. A multi-pass scanning method is used to reduce the fabrication divergence precipitated by atom redeposition. Microstructures, such as parabolic, hemispherical and sinusoidal shapes and a nano hemispherical structure have been produced during the FIB milling experiments using conventional bitmap milling and proposed divergence compensation approaches. A flat top/bottom surface is obtained in convex/concave structures when using the conventional bitmap FIB milling approach. Further research shows that the reasons for this phenomenon are mainly related to both the aspect ratio of the structures and the existence of an oxide layer on the substrate surface. The flat top/bottom phenomenon can be removed by using the combination of a novel divergence compensation approach and the removal of the oxide layer from the substrate surface prior to FIB machining. The experimental results show that the surface form accuracy has been greatly improved by using this method and the overlap effect can be suppressed by carefully choosing the normalized pixel spacing.

Enhancement of the Two-Photon Initiating Efficiency of a Thioxanthone Derivative through a Chevron-Shaped Architecture

We investigated the one- and two-photon excitation properties of a new hybrid anthracene–thioxanthone (ANTX) system, which was assembled into a chevron-shaped molecular architecture. Such a geometrical association first preserves the high efficiency of the free radical photoinitiator and leads, in the same time, to a strong increase in the two-photon absorption (2PA) cross section by more than a factor of 30 as compared to thioxanthone (TX). A four-state model was required to fit the 2PA spectrum of ANTX, which clearly undergoes a strong enhancement in the low-frequency region. As a consequence, ANTX constitutes a suitable two-photon initiating chromophore with a much higher efficiency as TX used as reference. At λexc = 710 nm for instance, we showed that the two-photon polymerization threshold of ANTX is five times lower with respect to that of TX. Additionally, we demonstrated by the fabrication of three-dimensional microstructures that this new hybrid system remains two-photon activable when excited at...

Maskless fabrication of three-dimensional microstructures with high isotropic resolution: practical and theoretical considerations

A maskless three-dimensional (3D) microfabrication method based on a digital micromirror device (DMD) is proposed for high lateral and vertical resolution. A substrate is scanned laterally under virtual masks of the DMD. The masks are allocated to a large number of virtual slices, all of which are projected in a single scan of the stage. A theoretical model for the cumulative dose distribution in a photoresist is derived and used to predict the resulting 3D profile. Experiments showed that the proposed method is promising for avoiding the stair-step problem and preventing misalignment errors.