Submicron Features Research Articles

Focus on lasers, imaging, and microscopy

Focus on lasers, imaging, and microscopy

Understanding resolution limit of displacement Talbot lithography

Displacement Talbot lithography (DTL) is a new technique for patterning large areas with sub-micron periodic features with low cost. It has applications in fields that cannot justify the cost of deep-UV photolithography, such as plasmonics, photonic crystals, and metamaterials and competes with techniques, such as nanoimprint and laser interference lithography. It is based on the interference of coherent light through a periodically patterned photomask. However, the factors affecting the technique's resolution limit are unknown. Through computer simulations, we show the mask parameter's impact on the features' size that can be achieved and describe the separate figures of merit that should be optimized for successful patterning. Both amplitude and phase masks are considered for hexagonal and square arrays of mask openings. For large pitches, amplitude masks are shown to give the best resolution; whereas, for small pitches, phase masks are superior because the required exposure time is shorter. We also show how small changes in the mask pitch can dramatically affect the resolution achievable. As a result, this study provides important information for choosing new masks for DTL for targeted applications.

Scalably manufactured textured surfaces for controlling wettability in oil-water systems

Competitive wettability in oil-water systems influences applications such as oil-water separation and enhanced oil recovery. Here, we study the wettability of water (in oil) and oil (in water) on sub-millimeter/micro/nano textured surfaces fabricated on a variety of substrates (metals, polymers, elastomers). Importantly, all the fabrication processes employed involved non-cleanroom-based scalable techniques. Metal surfaces were fabricated via wet etching processes and polymer/elastomer surfaces were fabricated via laser etching. These fabrication techniques can enable texturing with sub-millimeter, micron and sub-micron feature sizes. Wettability was characterized by measuring static contact angle and dynamic contact angle (roll-off angle). Several insights into wettability are obtained from this work. Firstly, textured metal surfaces with low energy surface chemistry showed the lowest adhesion to water and oil droplets. Textured metal surfaces coated with Teflon AF were superhydrophobic (in oil) with very low roll-off angles (4°–7°). Uncoated textured metal surfaces were superoleophobic (in water) with roll-off angles of 3°–9°. Secondly, textured polymer and elastomer surfaces exhibited utrahydrophobicity (in oil); however not all textured elastomers exhibited superoleophobicity (in water). Thirdly, no droplet roll-off was observed on any textured elastomer and polymer surface, despite very favourable contact angles. This indicates that high contact angles do not always translate to superhydrophobicity/oleophobicity. Fourthly, it is seen that competitive wettability of a surface can be understood by analysing the corresponding water and oil wettability of that surface in an air environment. Additionally, the initial state of the surface can be important, as the first fluid in contact with the surface fills the porous textures, and dictates subsequent wettability. All these findings and insights position this work as the foundation for more detailed studies on the development of surfaces for specific applications.

Sub-micron features from polymer-derived SiOC via imprint lithography

Silicon oxycarbide (SiOC) is a unique class of materials with great potential for facile manufacturing of complex shaped high temperature parts and devices. In this study, we examine the characteristics of micron-sized ridge and rod patterns of SiOC created by imprint lithography. Feature fidelity, shape change, and shrinkage are studied as a function of pyrolysis condition and feature size. All the features have acceptable surface fidelity under the pyrolysis conditions studied. However, pronounced rounding and flattening of patterned features are observed as the pyrolysis temperature increases or the feature size decreases. Based on the Kelvin and Gibbs-Thomson equations, we can predict the feature evolution and show that the feature rounding and flattening are due to surface diffusion and evaporation-condensation. As a result, the features also have more linear shrinkage than the bulk.

A compact method for efficient evaluation of acoustic absorbers with submicron materials.

Materials with submicron or nano-scale features possess unique physical properties and are well-suited for application in noise reduction. To date, experimental characterization of the sound absorbing ability of the submicron/nano materials is still a challenging task, because the measuring of sound absorptivity usually requires bulky samples, while the preparation of large quantities of submicron/nano materials or structures is generally costly and laborious. In this work, an acoustic testing method is proposed to evaluate the acoustic absorptivity of submicron/nano materials using small samples. Based on the transfer-matrix algorithm, the method establishes correlations among acoustic-related parameters of a large sensor fixture and a small sample holder. A proof-of-principle experimental setup was developed to test absorbers with well-known acoustic behavior to verify accuracy of the method. Finally, the sound absorption properties of two submicron materials are characterized, with one comprising dispersed silver submicron fibers and the other comprising electrospinning submicron fibers. The results indicate that acoustic absorption coefficients can be effectively retrieved using only 1/200 of the amount of materials that are typically required in the standard test.

Bio-Inspired Fluorine-Free Self-Cleaning Polymer Coatings

Bio-inspired fluorine-free and self-cleaning polymer coatings were developed using a combination of self-assembly and UV-printing processes. Nasturtium and lotus leaves were selected as natural template surfaces. A UV-curable acrylate oligomer and three acrylated siloxane comonomers with different molecular weights were used. The spontaneous migration of the comonomers towards the polymer–air interface was found to be faster for comonomers with higher molecular weight, and enabled to create hydrophobic surfaces with a water contact angle (WCA) of 105°. The replication fidelity was limited for the nasturtium surface, due to a lack of replication of the sub-micron features. It was accurate for the lotus leaf surface whose hierarchical texture, comprising micropapillae and sub-micron crystalloids, was well reproduced in the acrylate/comonomer material. The WCA of synthetic replica of lotus increased from 144° to 152° with increasing creep time under pressure to 5 min prior to polymerization. In spite of a water sliding angle above 10°, the synthetic lotus surface was self-cleaning with water droplets when contaminated with hydrophobic pepper particles, provided that the droplets had some kinetic energy.

Evolution of Electrochemical Cell Designs for In-Situ and Operando 3D Characterization.

Lithium-based rechargeable batteries such as lithium-ion (Li-ion), lithium-sulfur (Li-S), and lithium-air (Li-air) cells typically consist of heterogenous porous electrodes. In recent years, there has been growing interest in the use of in-situ and operando micro-CT to capture their physical and chemical states in 3D. The development of in-situ electrochemical cells along with recent improvements in radiation sources have expanded the capabilities of micro-CT as a technique for longitudinal studies on operating mechanisms and degradation. In this paper, we present an overview of the capabilities of the current state of technology and demonstrate novel tomography cell designs we have developed to push the envelope of spatial and temporal resolution while maintaining good electrochemical performance. A bespoke PEEK in-situ cell was developed, which enabled imaging at a voxel resolution of ca. 230 nm and permitted the identification of sub-micron features within battery electrodes. To further improve the temporal resolution, future work will explore the use of iterative reconstruction algorithms, which require fewer angular projections for a comparable reconstruction.

Identifying the structures retained when transforming wood into biocarbon

Biocarbon is a carbon-rich, porous material derived from biomass, and a promising electrode material for supercapacitors. Supercapacitors are fast-charging, long-lasting energy storage devices that have high power density and low energy density, relative to batteries. Many of the most recent efforts to raise the energy density of supercapacitors focus on increasing the gravimetric capacitance (F/g) of the electrode material. However, the electrodes in commercial supercapacitors are thin films (<50 μm), limiting the volume fraction of the electrode material within a device and, ultimately, its energy density. The use of thick monolithic electrodes (>1 mm thick) may alleviate this limitation, but only if monoliths can be made electrically conductive and facilitate ion migration in electrolyte. Whilst electrode thickness inevitably raises the overall resistance, the extent to which resistance is raised depends on the structure of the pores. This is also the case with electrolyte transport, making the pore structure of a monolith a key factor in improving performance. We use electron deceleration with a scanning electron microscope, physisorption, and x-ray diffraction to better understand what wood structures may be retained in biocarbon, and what new structures may be created when transforming wood to biocarbon. The features in four biocarbon samples are studied: one commercial hard-wood biocarbon, and three in-house biocarbon made from poplar, black locust, and pine via a slow pyrolysis procedure. The macro features of wood (those >1 μm, such as vessels, rays tracheids and pits) are retained in all four biocarbon samples. However, the retention of submicron wood features (e.g. layered structures within cell wall) is determined by pyrolysis conditions. Fast heating retained some submicron morphological structures in the commercial biocarbon, while the slow pyrolysis biocarbons did not have any distinguishable submicron features. The physisorption data suggested that the distributions of mesopores (2–50 nm) varied between biocarbons. X-ray diffraction showed the graphite nano-crystallites in all four biocarbon samples, but with significant size variations. Further study is necessary to learn how to tune the distribution of pore sizes, in order to create biocarbons with desirable pore structures.

Sub-micron and nanosized features in laser-induced periodic surface structures

The present report concentrates on little known peculiarities of laser-induced periodic surface structures (LIPSS) on metals and alloys formed upon irradiation by linearly polarized Ti/sapphire femtosecond laser pulses with the energy density of 0.17–1.0 J/cm2 in air environment. The peculiarities discussed are spontaneous twofold reduction in period and appearance of dislocations in the quasi-grating LIPSS. The twofold reduction in period is interpreted as a result of the second harmonic generation (SHG) of the laser light on the surface, stimulated and enhanced by surface roughness. LIPSS with two times shorter period stimulate SHG, and in this way a positive feedback mechanism works. The dislocations in the LIPSS are explained as the interference of scattered wave, which may contain optical vortices, with the incident plane wave. The quasi-grating with the dislocations works as a spatial phase modulator for the scattered wave and provides a positive feedback for enhancement of the dislocations. A successful example of application of LIPSS on noble metals as a SERS substrate and a discussion of their features important for such application is presented.

Superhydrophobicity on hierarchical periodic surface structures fabricated via direct laser writing and direct laser interference patterning on an aluminium alloy

Multi-scaled hierarchical surface microstructures are fabricated on Al2024 alloy using Direct Laser Writing and Direct Laser Interference Patterning. During the manufacturing procedure two distinct laser sources are used, including an ultra-violet nanosecond laser system that produces microcell structures with different dimensions through Direct Laser Writing. Posteriorly, an infrared picosecond laser source is used to fabricate sub-micron features on the previously patterned microcells. Thus, the fabricated multi-scaled hierarchical structures are a closer representation of what is observed in nature. Water contact angle measurements are carried out in order to assess the wettability response, which revealed superhydrophobicity one week posterior to the fabrication, with measured contact angles up to 161.5 ± 3°.

Printing sub-micron structures using Talbot mask-aligner lithography with a 193 nm CW laser light source.

A continuous improvement of resolution in mask-aligner lithography is sought after to meet the requirements of an ever decreasing minimum feature size in back-end processes. For periodic structures, utilizing the Talbot effect for lithography has emerged as a viable path. Here, by combining the Talbot effect with a continuous wave laser source emitting at 193 nm, we demonstrate successfully the fabrication of periodic arrays in silicon substrates with sub-micron feature sizes. The excellent coherence and the superior brilliance of this light source, compared to more traditional mercury lamps and excimer lasers as light source, enables the efficient beam shaping and a reduced minimum feature size at a fixed gap of 20 µm. We present a comprehensive study of proximity printing with this system, including simulations and selected experimental results of prints in up to the fourth Talbot plane. This printing technology can be used to manufacture optical metasurfaces, bio-sensor arrays, membranes, or microchannel plates.

Laser surface texturing of SS316L for enhanced adhesion of HUVECs

ABSTRACT This study investigates the effects of the structured surface on the adhesion, proliferation and alignment of endothelial cells (HUVECs). The chemical state of laser-structured surface was also investigated in comparison with the non-treated surface. A novel design for biomedical applications consisting of parallel chain-like structures was realised on stainless steel surface by laser micromachining. The structures were designed to employ surface topography in the presence of micron and sub-micron features and to avoid intensive surface modification that could compromise the mechanical properties of thin devices like cardiovascular stents. The results showed that the structured surfaces favored the adhesion, proliferation and alignment of HUVECs. The proliferation and the alignment of HUVECs were pronounced when the periodic distance between two consecutive chain-like structures was 25 µm. Moreover, there was no significant difference in chemical composition on the structured surface suggesting that the cell proliferation and alignment were mainly influenced by the surface topography.

Fabrication and Wettability Analysis of Hydrophobic Stainless Steel Surfaces With Microscale Structures From Nanosecond Laser Machining

In this work, nanosecond laser machining is used to fabricate hydrophobic 17-4 PH stainless steel surfaces with microscale and submicron structures. Four surface structures were designed, with microscale channels and pillars (100 μm pitch size) of uniform heights (100 μm) or alternating heights (between 100 μm and 50 μm). During fabrication, the high-power laser beams also created submicron features on top of the microscale ones, leading to hierarchical, multiscale surface structures. Detailed wettability analysis was conducted on the fabricated samples. Measured static contact angles of water on these surfaces are over 130 deg without any coating, compared to ∼70 deg on the original steel surface before laser machining. Slightly lower contact angle hysteresis was also observed on the laser machined surfaces. Overall, these results agree with a simple Cassie–Baxter model for wetting that assumes only fractional surface area contact between the droplet and the surface. This work demonstrates that steel surfaces machined with relatively inexpensive nanosecond laser can achieve excellent hydrophobicity even with simple microstructural designs.

Kinematic fixtures to enable multi-material printing and rapid non-destructive inspection during two-photon lithography

Two-photon lithography (TPL) is a polymerization based technique that enables additive manufacturing of millimeter scale parts with submicron features. TPL equipment is often based on retrofitted optical microscopes that lack precise registration capabilities. Consequently, slow and error-prone visual alignment to fiducials is necessary if registration to pre-existing features is required. Herein, we have designed, built, and tested precise kinematic fixtures that are repeatable to within ±315 nm (3σ value) and passively register the build surface to TPL equipment with an accuracy of ±1.7 μm. This enables one to sequentially print with multiple materials by building the structures directly on top of the kinematic fixtures. In addition, the same fixtures passively register to an X-ray computed tomography (CT) system to enable non-destructive 3D inspection that is integrated with the fabrication process. These fixtures (i) provide a practical means to handle micro-scale parts during non-destructive imaging, (ii) reduce the set-up time for X-ray CT from more than an hour to less than a few minutes, and (iii) eliminate operator uncertainty from the multi-material printing and imaging process. As such, these fixtures enable new printing and imaging functionalities that are critical for high-quality additive manufacturing of multi-material polymer parts with microscale and submicron features.

Adaptive stitching for meso-scale printing with two-photon lithography

An inevitable trade-off between resolution and total size exists when 3D printing objects. This is also the case for two-photon or multi-photon lithography. While it is capable of reaching a sub-micron feature size, it needs to combine a high precision movement mechanism with a lower precision one when writing centimetric size objects. As a result, the final part consists of a combination of many individual blocks, where the overlapping areas often create bumps and imperfections in the final object. In the current contribution, we demonstrate how an adaptive stitching algorithm can lead to a reduction in the total amount of stitching blocks of up to 40% for slender objects (i.e. with features smaller than the individual block size). As is demonstrated on a winding microfluidic channel, this can lead to substantial manufacturing time gains of up to 25%. As our tiling is irregular, we also explain how to create the writing masks for the blocks and decide on their order. In the special case in which one does not need to print one single large continuous part, but rather many individual ones (such as a microlens array), we demonstrate how zero-overlap stitching can be achieved with a modified version of our adaptive algorithm.

Informatics-Aided Raman Microscopy for Nanometric 3D Stress Characterization

The advanced confocal Raman microscopy provides a unique nondestructive, submicron feature resolving, and utility-convenient methodology for material stress analysis. Conventionally, only materials transparent to the excitation laser are allowed for depth-resolved 3D characterization. For opaque materials, the collected Raman scattering signal is inevitably an ensemble of contributions from multiple specimen layers. In this work, an informatics approach was adopted while considering both laser attenuation and confocal exclusivity to decompose true local stress components in three dimensions with submicron resolution. Such analytical power was demonstrated on a laser-opaque silicon wafer, where a depth-dependent study of stress dynamics was made possible. The technique extends to narrow band semiconductor-based microelectronic, solar cell, and battery materials, where stress plays a key role in the device performance and durability.

Energy‐efficient fault tolerant technique for deflection routers in two‐dimensional mesh Network‐on‐Chips

New generation multi-processor system-on-chips integrate hundreds of processing elements in a single chip which communicate with each other through on-chip communication networks, commonly known as network-on-chip (NoC). Routers are the most critical NoC components and deflection routing is a technique used in buffer-less routers for better energy efficiency. Massive integration of devices along with fabrication at deep sub-micron level feature sizes increases the possibility of wear out and damage to various components resulting in unreliable operation of the chip. Hence NoC fabric in general and routers, in particular, should be equipped with built-in fault tolerance mechanisms to ensure the reliability of the chip in the presence of faults. The authors propose an energy-efficient routing technique that can tolerate permanent faults in NoC links by introducing a simple logic unit placed next to the output port allocation stage of the deflection router pipeline. This technique incurs minimum wiring overheads and promises a stable network throughput for high fault rates. Evaluation of the proposed method on 8 × 8 mesh NoC for various fault rates reports reduced flit deflection rate and hop power which brings about a significant reduction in dynamic power consumption at the inter-router links compared to state-of-the-art fault tolerance techniques.

Characterization and evaluation of femtosecond laser-induced sub-micron periodic structures generated on titanium to improve osseointegration of implants

Reproducible and controllable methods of modifying titanium surfaces for dental and orthopaedic applications are of interest to prevent poor implant outcomes by improving osseointegration. This study made use of a femtosecond laser to generate laser-induced periodic surface structures with periodicities of 300, 620 and 760 nm on titanium substrates. The reproducible rippled patterns showed consistent submicron scale roughness and relatively hydrophobic surfaces as measured by atomic force microscopy and contact angle, respectively. Transmission electron microscopy and Auger electron spectroscopy identified a thicker oxide layer on ablated surfaces compared to controls. In vitro testing was conducted using osteosarcoma Saos-2 cells. Cell metabolism on the laser-ablated surfaces was comparable to controls and alkaline phosphatase activity was notably increased at late time points for the 620 and 760 nm surfaces compared to controls. Cells showed a more elongated shape on laser-ablated surfaces compared to controls and showed perpendicular alignment to the periodic structures. This work has demonstrated the feasibility of generating submicron features on an implant material with the ability to influence cell response and improve implant outcomes.

Clinical relevance of sub-micron textured calcium phosphates for bone tissue regeneration

Osteoinductive biomaterials are synthetic ceramics of non-biological origins able to stimulate bone formation in non-bony sites (e.g. in muscle or under skin). Although the mechanism of osteoinduction by synthetic biomaterials is not yet completely understood, the pivotal role of several physico chemical parameters have been identified, i.e. surface structure at the nano- and micro- scale. Osteoinductive resorbable Calcium Phosphate (CaP) ceramics are particularly relevant as bone graft as they have a composition comparable to the inorganic component of bone, and as they can resorb overtime to be replace by bone. The superiority of osteoinductive CaP with sub-micron features has been proven in several challenging preclinical models. The preliminary clinical results show clear benefits for patients in need for bone grafting.

A Thermodynamic Study of Adsorption of Benzyl Viologen and Polyethylene Glycol and Their Displacement by 3-Mercapto-1-Propanesulfonate during Copper Electrodeposition

In copper-plating baths for filling of submicron features, a combination of additives regulates the distribution of deposition rate, and in the proper concentrations produces superconformal filling. The present study is built on the competitive adsorption model of superconformal filling in which an adsorbed suppressor, in this instance benzyl viologen or polyethylene glycol, is displaced from the surface by the accelerant 3-mercapto-1-propane sulfonic acid (MPS) during copper electrodeposition. The change in deposition current after progressive additions of suppressor or accelerant was used to determine the surface coverage of each additive as a function of its concentration in solution and of temperature. The data were fitted to the Langmuir isotherm, and the free energy and enthalpy of adsorption or displacement were determined. It was shown that adsorption of PEG or BV is a spontaneous exothermic process, and the displacement of PEG or BV by MPS is a spontaneous endothermic process. Although the suppressors form stronger bonds with the surface, the accelerant displaces them due to the resulting increase in surface-excess entropy.