Free-standing Microstructures Research Articles

Liquid Crystalline Network Microstructures for Stimuli Responsive Labels with Multi-Level Encryption.

Two-photon direct laser writing enables the fabrication of shape-changing microstructures that can be exploited in stimuli responsive micro-robotics and photonics. The use of Liquid Crystalline Networks (LCN) allows to realize 3D micrometric objects that can contract along a specific direction in response to stimuli, such as temperature or light. In this paper, the fabrication of free-standing LCN microstructures is demonstrated as graphical units of a smart tag for simple physical and optical encryption. Using an array of identical pixels, information can be hidden to the observer and revealed only upon application of a specific stimulus. The reading mechanism is based on the shape-change of each pixel under stimuli and their color that combinetogether in a two-level encryption label. Once the stimulus is removed, the pixels recover their original shape and the message remains completely hidden. Therefore, an opto-mechanical equivalent of an "invisible ink" is realized. This new concept paves the way for introducing enhanced functionalities in smart micro-systems within a single lithography step, spanning from storage devices with physical encryption to complex motion actuators.

Laser micromachined 3D glass photonics platform demonstrated by temperature compensated strain sensor

A platform for integrated glass solutions with full 3D capabilities based on femtosecond laser micromachining is presented. The design and fabrication of different building blocks, such as optical waveguides, bragg gratings, V-grooves and (freestanding) microstructures are described. The capabilities of the platform are exemplified by designing a temperature compensated strain sensor composed of two Bragg grating sensors, where one Bragg grating sensor is mechanically isolated by means of a cantilever, while the other is embedded in the bulk of the glass. Both Bragg gratings exhibit a similar temperature sensitivity of about 10.5pm/°C. It is proven that the isolated Bragg grating does not experience strain, while the other one does with a sensitivity of 1.1 pm/μϵ; thus allowing temperature compensation. Integrated V-grooves allow for a fully fiber-packaged strain sensing solution, directly applicable in many structural health monitoring applications. The versatility of the platform is further illustrated by the widely varying building blocks.

Fabrication of silver helix microstructures in a large area by a two-photon absorption DLW method

Large-area helix microstructures intended for metamaterials were fabricated using a negative photoresist, SU-8 using a two photon absorption direct laser writing (TPA-DLW). Two types of helix structures were fabricated. One type is those with no neighboring distance. In this case, compact helix structures with radius of 2.5 and 1.0 μm were fabricated. Another type is those with enough neighboring distance. The helix structures with shorter neighboring distance below 6.0 μm were collapsed, whereas those with longer neighboring distance more than 6.5 μm, free-standing helix structures could successfully be built. To stabilize the fabricated free-standing helix microstructures with a 1 μm radius, circular foundations with a radius of 1.3 μm and elevation angle of 10, 12, or 14° were built in advance. The foundation is useful to avoid collapsing the helix microstructures. Due to the useful foundation, over 18,000 helical structures were fabricated in a large area. The fabricated helical structures were coated with silver using an electroless plating method to produce 3D metallic helix structures. Silver coating was measured using a EDX measurement. The obtained helical structures have the potential for metamaterials to control the handedness of a circularly polarized infrared beam.

The role of etching anisotropy in the fabrication of freestanding oxide microstructures on SrTiO3(100), SrTiO3(110), and SrTiO3(111) substrates

The release process for the fabrication of freestanding oxide microstructures relies on appropriate, controllable, and repeatable wet etching procedures. SrTiO3 (STO) is among the most employed substrates for oxide thin films growth and can be decomposed in HF:water solution. Such a process is strongly anisotropic and is affected by local defects and substrate cut-planes. We analyze the etching behavior of SrTiO3 substrates having (100), (110), and (111) cut-planes during immersion in a 5% HF:water solution. The etching process over the three substrates is compared in terms of pitting, anisotropy, macroscopic etch rate, and underetching effects around HF-resistant (La,Sr)MnO3 thin film micropatterns. The release of targeted structures, such as the reported (La,Sr)MnO3 freestanding microbridges, depends on the substrate crystallographic symmetry and on the in-plane orientation of the structures themselves along the planar directions. By comparing the etching evolution at two different length scales, we distinguish two regimes for the propagation of the etching front: an intrinsic one, owing to a specific lattice direction, and a macroscopic one, resulting from the mixing of different etching fronts. We report the morphologies of the etched SrTiO3 surfaces and the geometries of the underetched regions as well as of the microbridge clamping zones. The reported analysis will enable the design of complex MEMS devices by allowing to model the evolution of the etching process required for the release of arbitrary structures made of oxide thin films deposited on top of STO.

Functional Microarray Platform with Self-Assembled Monolayers on 3C-Silicon Carbide.

Currently available bioplatforms such as microarrays and surface plasmon resonators are unable to combine high-throughput multiplexing with label-free detection. As such, emerging microelectromechanical systems (MEMS) and microplasmonics platforms offer the potential for high-resolution, high-throughput label-free sensing of biological and chemical analytes. Therefore, the search for materials capable of combining multiplexing and label-free quantitation is of great significance. Recently, interest in silicon carbide (SiC) as a suitable material in numerous biomedical applications has increased due to its well-explored chemical inertness, mechanical strength, bio- and hemocompatibility, and the presence of carbon that enables the transfer-free growth of graphene. SiC is also multifunctional as both a wide-band-gap semiconductor and an efficient low-loss plasmonics material and thus is ideal for augmenting current biotransducers in biosensors. Additionally, the cubic variant, 3C-SiC, is an extremely promising material for MEMS, being a suitable platform for the easy micromachining of microcantilevers, and as such capable of realizing the potential of real time miniaturized multiplexed assays. The generation of an appropriately functionalized and versatile organic monolayer suitable for the immobilization of biomolecules is therefore critical to explore label-free, multiplexed quantitation of biological interactions on SiC. Herein, we address the use of various silane self-assembled monolayers (SAMs) for the covalent functionalization of monocrystalline 3C-SiC films as a novel platform for the generation of functionalized microarray surfaces using high-throughput glycan arrays as the model system. We also demonstrate the ability to robotically print high throughput arrays on free-standing SiC microstructures. The implementation of a SiC-based label-free glycan array will provide a proof of principle that could be extended to the immobilization of other biomolecules in a similar SiC-based array format, thus making potentially significant advances to the way biological interactions are studied.

Systematic study of the etching characteristics of Si{111} in modified TMAH

Wet bulk micromachining on Si{111} is done to fabricate simple to complex microstructures for applications in sensors and actuators. In this work, it has been performed a systematic study of Si{111} in modified 5 wt% tetramethyl-ammonium hydroxide (TMAH) with varying concentration of NH 2 OH for achieving improved etching characteristics especially high lateral undercutting at mask edges aligned along non-〈110〉 directions. The concentration of NH 2 OH is varied from 5 to 20% in step of 5%. The lateral undercutting, which is highly desirable for the quick release of freestanding microstructures from the substrate, is increased considerably with the addition of NH 2 OH in TMAH solution. In addition, the incorporation of NH 2 OH improves etched surface morphology. Moreover, the effect of etchant ageing on the etching characteristics is investigated. Suspended microstructures are fabricated to demonstrate the application of modified TMAH solution for silicon wet bulk micromachining. The results presented in this work are highly useful where Si{111} wafer is used for the fabrication of microstructures.

Plasma-based processes for planar and 3D surface patterning of functional nanoparticles

We present a gas-phase process for surface patterning and 3D self-assembly of nanoparticles (NPs) of functional materials such as metals, oxides, and nitrides. The method relies on electrostatic assembly of free-flying NPs with unipolar charge produced in plasma sources. We demonstrate the capability of the process in self-assembly of NPs, with the size in the range 10–60 nm, into arrays of free-standing 3D microstructures with complex morphologies. Considering that the plasma nanoparticle sources are compatible with synthesis of a large library of material NPs, the process introduces a novel approach for 3D printing of various functional NPs, high-precision device integration of NPs on sub-micrometer scales, and large-area parallel surface patterning of NPs.

Highly Efficient Active All-Dielectric Metasurfaces Based on Hybrid Structures Integrated with Phase-Change Materials: From Terahertz to Optical Ranges.

Recently, all-dielectric metasurfaces (AMs) have emerged as a promising platform for high-efficiency devices ranging from the terahertz to optical ranges. However, active and fast tuning of their properties, such as amplitude, phase, and operating frequency, remains challenging. Here, a generic method is proposed for obtaining high-efficiency active AMs from the terahertz to optical ranges by using "hybrid structures" integrated with phase-change materials. Various phase-change mechanisms including metal-insulator phase change, nonvolatile phase change, and ferroelectric phase change are investigated. We first experimentally demonstrate several high-efficiency active AMs operating in the terahertz range based on hybrid structures composed of free-standing silicon microstructures covered with ultrathin phase-change nanofilms (thickness d ≪ λ). We show that both the frequencies and the strength of the Mie resonances can be efficiently tuned, resulting in unprecedented modulation depth. Furthermore, detailed analyses of available phase-change materials and their properties are provided to offer more options for active AMs. Finally, several feasible hybrid structures for active AMs in the optical range are proposed and confirmed numerically. The broad platform built in this work for active manipulation of waves from the terahertz to optical ranges may have numerous potential applications in optical devices including switches, modulators, and sensors.

Alleviation of the adhesive protrusion problem at the bonding interface of free-standing microstructures

The vertically allocated free-standing SU-8 microstructures are typically bonded to a glass cover by the usage of uncrosslinked SU-8 adhesives. Such a phenomenon can easily cause SU-8 protrusion and eventually result in the SU-8 cantilever to be immovable. Traditional methods are sensitive to the bonding conditions and have a short bondable thickness of SU-8 adhesives. In this study, we propose an approach, that is, improved structural features, to alleviate the protrusion problem while extending the bondable thickness for the freestanding SU-8 microstructures in an enclosed channel. We used concave and moat microstructures as solutions of the improved structural features. We investigated the influence of both microstructures on the bonding quality and compared the bondable thickness with the previous one. THB-151N was used in another example to demonstrate the availability of our method. The bonding quality at the interfaces was evaluated by SEM images and direct inspection through a transparent glass cover. The bonding method is advantageous to other microfluidic systems, particularly those with long narrow channels.

Glass‐Channel Molding Assisted 3D Printing of Metallic Microstructures Enabled by Femtosecond Laser Internal Processing and Microfluidic Electroless Plating

Abstract3D printing of metallic microstructures is highly desirable for many practical applications such as microelectronics, terahertz photonics, microelectromechanical systems, and electrochemisty. Unfortunately, high resolution microprinting of 3D metal structures with widely tunable feature sizes, high conductivities, high melting points, and highly smooth surfaces remains challenging for current microfabrication technologies. Herein, 3D printing of metallic microstructures of feature sizes ranging from ≈10 to ≈200 μm by combining femtosecond laser micromachining of 3D glass microchannels and microfluidic electroless plating is demonstrated. The proposed technique allows for producing 3D metallic structures of almost arbitrary geometries embedded in glass since the continuous flow of the plating solution enables controllable deposition of metal films inside the through microchannels. Moreover, freestanding metallic 3D microstructures are fabricated by removing the glass matrix with a wet chemical etching. As a nonthermal processing, the surface roughness of the fabricated metallic structures is as low as ≈20 nm. The 3D microstructures can be made of either silver or copper covered with a thin layer of silver, which are shown to have a high conductivity. As a proof‐of‐concept demonstration, a 3D metal scaffold structure with a size of ≈5 × 5 × 2 mm3 is printed.

Printing of complex free-standing microstructures via laser-induced forward transfer (LIFT) of pure metal thin films

A combined approach of laser-induced forward transfer (LIFT) and chemical etching of pure metal films is studied to fabricate complex, free-standing, 3-dimensional gold structures on the few micron scale. A picosecond pulsed laser source with 515 nm central wavelength is used to deposit metal droplets of copper and gold in a sequential fashion. After transfer, chemical etching in ferric chloride completely removes the mechanical Cu support leaving a final free-standing gold structure. Unprecedented feature sizes of smaller than 10 μm are achieved with surface roughness of 0.3 to 0.7 μm. Formation of interfacial mixing volumes between the two metals is not found confirming the viability of the approach.

Creation of film-free freestanding 3D microstructures by aligned mask micromolding

There are established macroscale fabrication methods to create parts of virtually any shape or size; but for components in the microscale range (thousands to tens of microns) there is a dearth of options for making 3D parts at high enough throughput to make them practical. Here we use traditional photolithography with an aligned mask micromolding method to fabricate freestanding 3D microstructures that can be used as components of microrobotics and microfluidics devices for biomedical applications. The overall goal of this research is to create novel micromolding processes that are capable of creating defect-free microscale structures with controllable 3D geometry, a variety of material properties, and high fabrication throughput. While some methods are available that are capable of meeting a few of these goals, nothing exists in the current microfabrication repertoire that can accomplish all simultaneously.

Supercritical CO2 drying of poly(methyl methacrylate) photoresist for deep x-ray lithography: a brief note

Poly(methyl methacrylate) (PMMA) is an extensively used positive photoresist for deep x-ray lithography. The post-development release of the microstructures of PMMA becomes very critical for high aspect ratio fragile and freestanding microstructures. Release of high aspect ratio comb-drive microstructure of PMMA made by one-step x-ray lithography (OXL) is studied. The effect of low-surface tension Isopropyl alcohol (IPA) over water is investigated for release of the high aspect ratio microstructures using conventional and supercritical (SC) CO2 drying. The results of conventional drying are also compared for the samples released or dried in both in-house developed and commercial SC CO2 dryer. It is found that in all cases the microstructures of PMMA are permanently deformed and damaged while using SC CO2 for drying. For free-standing high aspect ratio microstructures of PMMA made by OXL, it is advised to use low-surface tension IPA over DI water. However, this brings a limitation on the design of the microstructure.

Polymer Microstructures through Two-Photon Crosslinking.

Two-photon crosslinking of polymers (2PC) is proposed as a novel method for the fabrication of freestanding microstructures via two-photon lithography. During this process in the confocal volume, two-photon absorption leads to (formal) C,H-insertion reactions, and consequently to a strictly localized crosslinking of the polymer. To achieve this, the polymer is coated as a solvent-free (glassy) film onto an appropriate substrate, and the desired microstructure is written by 2PC into this glass. In all regions outside of the focal volume where no two-photon process occurs, the polymer remains uncrosslinked and can be washed away during a developing process. Using a self-assembled monolayer containing the same photoreactive group allows covalent attachment of the forming freestanding structures to the substrate, and thus guarantees an improved stability of these structures against shear-induced detachment. As the two photon process is carried out in the glassy state, in a simple way, multilayer structures can be used to write structures having a varying chemical composition perpendicular to the surface. As an example, the 2PC process is used to build a structure from both protein-repellent and protein-adsorbing polymers so that the resulting 3D structure exhibits spatially controlled protein adsorption.

Effect of Surfactant and Alcohol Additives on Etching Characteristics in Aqueous Potassium Hydroxide Solutions

Anisotropic wet etching is a low cost technique for silicon bulk micromachining to fabricate MEMS components such as cantilevers, diaphragms, etc. It is inevitable for the fabrication of microstructures with slanted sidewalls and to remove underneath material for the realization of freestanding microstructures. Potassium hydroxide (KOH) is a popular etchant as it provides high anisotropy between {100}/{110} and {111} planes. In order to alter the etching characteristics, the effect of various kinds of additives such as isopropyl alcohol (IPA), surfactants (Triton X-100) in pure KOH is studied [1-3]. In this work, we studied the etching characteristics of ternary solutions composed of KOH + Triton X-100 + IPA. The concentration of KOH is varied from 5-40 wt% in step of 5 wt%, while the concentrations of Triton and IPA are kept fixed at 0.1 vol% and 15 vol%, respectively. In the different compositions of ternary solutions, the etch rate and etched surface morphologies of Si{100}, Si{110} and Si{111} are investigated. In addition to that the undercutting at convex corners, which is very important parameter for designing compensation structure to protect convex corner and to estimate etching to release the structure from the substrate, is measured. In order to explore the thermal oxide as mask and structural material, the etch rate of silicon dioxide is also measured. In the ternary solution comprising low concentration KOH, the etching characteristics are more significantly affected by Triton than IPA. However, in the case of ternary solution containing high concentration KOH, IPA influences etching characteristics more prominently than Triton. The systematic study of etch rate, etched surface roughness, and undercutting at convex corners in ternary solution presented in this work is very useful to select the composition of ternary solution for MEMS applications as per the requirement.

Multi-Component Microscaffold With 3D Spatially Defined Proteinaceous Environment.

In this paper, we present a multicomponent microenvironment consisting of proteinaceous networks with submicron-sized features optionally embedded into a photoresist microscaffold. By two-photon direct laser writing, free-standing 3D proteinaceous microstructures were fabricated for cell culture application, demonstrated with NIH/3T3 fibroblast cells. A Young's modulus of megapascal-order contributes to the challenge of structural sustainability of the proteinaceous microstructures for experiments as well as sequential fabrication steps. We propose to embed proteinaceous networks into a mechanically robust photoresist microscaffold. We investigate the limits of this 3D microfabrication of embedded proteinaceous networks and demonstrate the embedment of two different proteinaceous networks within one microscaffold. Performing cell culture of PC12 cells, we observe cell adhesion and cell motility on embedded proteinaceous networks of collagen type-IV mixed with bovine serum albumin into a photoresist microscaffold. The ability to structure proteinaceous elements for 3D spatial control of microenvironment might be a key feature in cell culture to decouple environmental cues to control cellular behavior.

Fabrication and Bonding of SU-8 Free-standing Microstructures

Fabrication and Bonding of SU-8 Free-standing Microstructures

Microfabrication techniques for producing freestanding multi-dimensional microstructures

Freestanding microstructures are widely used as components in micro-electro-mechanical systems (MEMS) applications or its functions as a microdevice itself. Microfabrication techniques and materials were originally borrowed from the integrated circuit industry; but nowadays, various microfabrication techniques for producing multi-dimensional microstructures are developed with lots of available alternative materials. To obtain freestanding microstructures for assembling MEMS devices, sacrificial layer and release techniques are developed to release the fabricated microcomponents from substrate. This review presents a detailed description of current existing microfabrication and release techniques, fabrication structural and sacrificial material, and achieved multi-dimensional microstructures.

A simple method for producing freestanding 3D microstructures by integrated photomask micromolding

Freestanding three-dimensional (3D) microstructures are widely used in micro-electro-mechanical system (MEMS) applications or can function as microdevices themselves. However, microfabrication methods for freestanding 3D microstructures have limitations in shape, size, cost, and mass production, etc. In this work, integrated photomask micromolding is demonstrated, which uses a portable UV light source and chrome glass micromolding to fabricate 3D microstructures without alignment. Specifically, a chrome layer on one side of the glass micromold shields the excess filling SU-8 photoresist from UV exposure and only the SU-8 photoresist in mold cavities is crosslinked. The 3D microstructures produced using this method have very high dimensional accuracy and the profile error is approximately 1.5%. This method can be used with features of virtually any size and shape and can be integrated into highly-parallel micromolding processes and has potential for MEMS applications.

Soft lithographic printing and transfer of photosensitive polymers: facile fabrication of free-standing structures and patterning fragile and unconventional substrates

Dry film photoresists (DF PRs) are widely used to perform photolithography on non-traditional substrates such as printing circuit boards, plastic sheets, or non-planar surfaces. Commercially available DF PRs are usually in a negative tone and rather thick, limiting lithographic resolution and versatility. The relatively large pressure required for lamination also prevents the technology from being used for delicate substrates. Here we present a modified soft-lithographic process, namely photoresist blanket transfer (PR BT), transferring a spin-coated PR film from a flat elastomeric stamp to a substrate. The elastomeric stamp is highly compliant, bringing the PR film into intimate contact with the substrate and eliminating the need for a large lamination pressure. Photolithography on unconventional substrates such as etched, fragile, and porous ones is demonstrated. Single or multiple transfers of PRs by BT are utilized to fabricate multilayer, free-standing, and re-entrant polymeric microstructures. A fragile and porous substrate such as an anodized aluminum oxide membrane can also be patterned using PR BT. Moreover, a reliable method to create metal electrodes and high surface area catalysts inside microchannels is discussed for novel microfluidic applications.