Polymer Reflow Research Articles

Autonomous self‐healing 3D micro‐suction adhesives for multi‐layered amphibious soft skin electronics

AbstractAutonomously self‐healing, reversible, and soft adhesive microarchitectures and structured electric elements could be important features in stable and versatile bioelectronic devices adhere to complex surfaces of the human body (rough, dry, wet, and vulnerable). In this study, we propose an autonomous self‐healing multi‐layered adhesive patch inspired by the octopus, which possess self‐healing and robust adhesion properties in dry/underwater conditions. To implement autonomously self‐healing octopus‐inspired architectures, a dynamic polymer reflow model based on structural and material design suggests criteria for three‐dimensional patterning self‐healing elastomers. In addition, self‐healing multi‐layered microstructures with different moduli endows efficient self‐healing ability, human‐friendly reversible bio‐adhesion, and stable mechanical deformability. Through programmed molecular behavior of microlevel hybrid multiscale architectures, the bioinspired adhesive patch exhibited robust adhesion against rough skin surface under both dry and underwater conditions while enabling autonomous adhesion restoring performance after damaged (over 95% healing efficiency under both conditions for 24 h at 30°C). Finally, we developed a self‐healing skin‐mountable adhesive electronics with repeated attachment and minimal skin irritation by laminating thin gold electrodes on octopus‐like structures. Based on the robust adhesion and intimate contact with skin, we successfully obtained reliable measurements during dynamic motion under dry, wet, and damaged conditions.image

Azimuthal rotation-controlled nanoinscribing for continuous patterning of period- and shape-tunable asymmetric nanogratings

We present an azimuthal-rotation-controlled dynamic nanoinscribing (ARC-DNI) process for continuous and scalable fabrication of asymmetric nanograting structures with tunable periods and shape profiles. A sliced edge of a nanograting mold, which typically has a rectangular grating profile, slides over a polymeric substrate to induce its burr-free plastic deformation into a linear nanopattern. During this continuous nanoinscribing process, the “azimuthal angle,” that is, the angle between the moving direction of the polymeric substrate and the mold’s grating line orientation, can be controlled to tailor the period, geometrical shape, and profile of the inscribed nanopatterns. By modulating the azimuthal angle, along with other important ARC-DNI parameters such as temperature, force, and inscribing speed, we demonstrate that the mold-opening profile and temperature- and time-dependent viscoelastic polymer reflow can be controlled to fabricate asymmetric, blazed, and slanted nanogratings that have diverse geometrical profiles such as trapezoidal, triangular, and parallelogrammatic. Finally, period- and profile-tunable ARC-DNI can be utilized for the practical fabrication of diverse optical devices, as is exemplified by asymmetric diffractive optical elements in this study.

Designer patterned functional fibers via direct imprinting in thermal drawing

Creating micro/nanostructures on fibers is beneficial for extending the application range of fiber-based devices. To achieve this using thermal fiber drawing is particularly important for the mass production of longitudinally uniform fibers up to tens of kilometers. However, the current thermal fiber drawing technique can only fabricate one-directional micro/nano-grooves longitudinally due to structure elongation and polymer reflow. Here, we develop a direct imprinting thermal drawing (DITD) technique to achieve arbitrarily designed surface patterns on entire fiber surfaces with high resolution in all directions. Such a thermal imprinting process is simulated and confirmed experimentally. Key process parameters are further examined, showing a process feature size as small as tens of nanometers. Furthermore, nanopatterns are fabricated on fibers as plasmonic metasurfaces, and double-sided patterned fibers are produced to construct self-powered wearable touch sensing fabric, revealing the bright future of the DITD technology in multifunctional fiber-based devices, wearable electronics, and smart textiles.

Scalable fabrication of sub-10\u2009nm polymer nanopores for DNA analysis

We present the first fabrication of sub-10 nm nanopores in freestanding polymer membranes via a simple, cost-effective, high-throughput but deterministic fabrication method. Nanopores in the range of 10 nm were initially produced via a single-step nanoimprinting process, which was further reduced to sub-10 nm pores via a post-NIL polymer reflow process. The low shrinkage rate of 2.7 nm/min obtained under the conditions used for the reflow process was the key to achieving sub-10 nm pores with a controllable pore size. The fabricated SU-8 nanopore membranes were successfully employed for transient current measurements during the translocation of DNA molecules through the nanopores.

Fabrication of astronomical x-ray reflection gratings using thermally activated selective topography equilibration

Thermally activated selective topography equilibration (TASTE) enables the creation of 3D structures in resist using grayscale electron-beam lithography followed by a thermal treatment to induce a selective polymer reflow. A blazed grating topography can be created by reflowing repeating staircase patterns in resist into wedgelike structures. Motivated by astronomical applications, such patterns with periodicities 840 and 400 nm have been fabricated in 130 nm-thick poly(methyl methacrylate) using TASTE to provide a base for x-ray reflection gratings. A path forward to integrate this alternative blazing technique into grating fabrication recipes is discussed.

Ion-Induced Localized Nanoscale Polymer Reflow for Three-Dimensional Self-Assembly.

Thermal reflow of polymers is a well-established phenomenon that has been used in various microfabrication processes. However, present techniques have critical limitations in controlling the various attributes of polymer reflow, such as the position and extent of reflow, especially at the nanoscale. These challenges primarily result from the reflow heat source supplying heat energy to the entire substrate rather than a specific area. In this work, a focused ion beam (FIB) microscope is used to achieve controllable localized heat generation, leading to precise control over the nanoscale polymer reflow. Through the use of the patterning capability of FIB microscopy, dramatically different reflow performances within nanoscale distances of each other are demonstrated in both discrete periodic and continuous polymer structures. Further, we utilize a self-assembly process induced by nanoscale polymer reflow to realize 3D optical devices, specifically, vertically aligned nanoresonators and graphene-based nanocubes. HFSS and Comsol simulations have been carried out to analyze the advantages of the polymer-based 3D metamaterials as opposed to those fabricated with a metallic hinge. The simulation results clearly demonstrate that the polymer hinges have a dual advantage; first, the removal of any interference from the transmission spectrum leading to strong and distinct resonance peaks and, second, the elimination of parasitic leeching of the enhanced field by the metallic hinge resulting in stronger volumetric enhancement. Thus, the 2-fold advantages existing in 3D polymer-hinge optical metamaterials can open pathways for applications in 3D optoelectronic devices and sensors, vibrational molecular spectroscopy, and other nanoscale 3D plasmonic devices.

Controlled Sub‐Micrometer Hierarchical Textures Engineered in Polymeric Fibers and Microchannels via Thermal Drawing

The controlled texturing of surfaces at the micro‐ and nanoscales is a powerful method for tailoring how materials interact with liquids, electromagnetic waves, or biological tissues. The increasing scientific and technological interest in advanced fibers and fabrics has triggered a strong motivation for leveraging the use of textures on fiber surfaces. Thus far however, fiber‐processing techniques have exhibited an inherent limitation due to the smoothing out of surface textures by polymer reflow, restricting achievable feature sizes. In this article, a theoretical framework is established from which a strategy is developed to reduce the surface tension of the textured polymer, thus drastically slowing down thermal reflow. With this approach the fabrication of potentially kilometers‐long polymer fibers with controlled hierarchical surface textures of unprecedented complexity and with feature sizes down to a few hundreds of nanometers is demonstrated, two orders of magnitude below current configurations. Using such fibers as molds, 3D microchannels are also fabricated with textured inner surfaces within soft polymers such as poly(dimethylsiloxane), at dimensions and a degree of simplicity impossible to reach with current techniques. This strategy for the texturing of high curvature surfaces opens novel opportunities in bioengineering, regenerative scaffolds, microfluidics, and smart textiles.

Fabrication of microfluidic devices: improvement of surface quality of CO2 laser machined poly(methylmethacrylate) polymer

Laser engraving has considerable potential for the rapid and cost effective manufacturing of polymeric microfluidic devices. However, fabricated devices are hindered by relatively large surface roughness in the engraved areas, which can perturb smooth fluidic flow and can damage sensitive biological components. This effect is exacerbated when engraving at depths beyond the laser focal range, limiting the production of large aspect ratio devices such as microbioreactors. This work aims to overcome such manufacturing limitations and to realise more reproducible and defect free microfluidic channels and structures. We present a strategy of multiple engraving passes alongside solvent polymer reflow for shallow depth (<500 µm) and a layer cutting with laminate bonding for larger depth (>500 µm) features. To examine the proposed methodologies, capillary action and bioreactor microfluidic devices were fabricated and evaluated. Results indicate that the multiple engraving technique could reproduce engraved microfluidic channels to depths between 50–470 µm, both rapidly (6–8 min) and with low average surface roughness (1.5–2.5 µm). The layer cutting approach was effective at manufacturing microfluidic devices with depths <500 µm, rapidly (<1 min) and with low surface roughness. Ultimately, the proposed methodology is highly beneficial for the rapid development of polymer-based microfluidic devices.

Bio-inspired 3D funnel structures made by grayscale electron-beam patterning and selective topography equilibration

Electron beam grayscale lithography and selective thermal polymer reflow were combined to realize replication master having a bio-inspired surface topography. It consists of an array of asymmetric denticles with two-directional gradients on different length-scales. Each denticle has a sharp pin of 2μm height on one and a smooth taper on the other side. In contrast to funnel-like structures having only vertical sidewalls, master with a sidewall angle smoothly changing between 15° and 70° were prepared. Discrete patterns, predefined by electron beam lithography, were transformed by reflow into well-defined, continuous, final device structures. Starting from stepped structures with only a few levels, this new approach of tapering allowed a very smooth transition from steep, sub-500nm regions with aspect ratios up to four towards very broad and shallow regions within the same structure. This enabled the origination of master structures to be used for replication as low-frictional, durable, bio-inspired surfaces by ceramic powder injection molding.

Mobility based 3D simulation of selective, viscoelastic polymer reflow using surface evolver

This work demonstrates implementation of the main effects of viscoelastic thermal polymer reflow in an efficient energy and mobility based simulation. The concept is based on a finite-element, soap-film method using the free software surface evolver. Properties of a homogeneous 3D volume are thereby represented by a corresponding 2D surface. The simulation only requires the contact angle between polymer and substrate for infinite long reflow times, obtained from fingerprint experiments, and a mobility value as input. The mobility value is a measure for the polymer-chain mobility and is directly linked to the polymer viscosity. This concept allows for an accurate and fast treatment of the thermomechanically complex polymer behavior close to the glass transition. The simulation time scale is linearly related to the experimental time scale allowing for accelerated-time simulations. Simulation and experiment showed a very good agreement. As a generalized concept, the approach presented here can be used for fast and full 3D shape computation during any complex, energy driven geometry optimization process like polymer reflow, viscoelastic wetting or dewetting and droplet coagulation. This simulation may facilitate a faster uptake of grayscale reflow technologies for industrial processes. Supplementary material supports a quick grasp of the simulation approach.

A novel Surface Tension Assisted Lithography (STAL) technique for microfabrication of 3D structures

A novel Surface Tension Assisted Lithography (STAL) technique is proposed to fabricate 3D structures in the photo-patternable polymer, SU-8, using a sequence of soft-imprint lithography, photo-lithography, and polymer reflow. The dimensions of the reflowed 3D structures are controlled by the volume of material initially displaced by soft-imprinting, and the lateral dimensions of the reflow container, defined by photo-lithography.

Scatterometry analysis of sequentially imprinted patterns: Influence of thermal parameters

Graphical abstractDisplay Omitted Highlights? Thermal Step and Repeat NIL processes characterized by scatterometry. ? Scatterometry demonstrates high accuracy to quantify imprinted lines on silicon. ? Polymer reflow due to heat diffusion has been studied as a function of printing parameters. Scatterometry technique has been used to characterize Thermal Step and Repeat NIL processes in the framework of NAPANIL project. Two hundred and fifty nanometers dense lines were imprinted in mr-7030 polymer and their profiles have been analyzed and compared to the mold one. It has been demonstrated that scatterometry on silicon exhibits a very high accuracy and that a change of the sidewall verticality of only few degrees can been measured. The results show that some reflow occurs in lines during the imprint of neighboring dies due to heat diffusion. This phenomenon has been studied as a function of printing temperature, demolding temperature, and space between adjacent dies. The conclusion is that a trade off has to be made between imprint temperature and chuck temperature to be able to fill mold cavities with a limitation of the reflow.

Domed and Released Thin-Film Construct—An Approach for Material Characterization and Compliant Interconnects

An approach for microfabricating precisely defined spherical 3-D domes through a simple low-temperature polymer reflow process was developed. Release of a thin metal film patterned over the domed structure was accomplished by the removal of the underlying polymer using two different methods: dry thermal decomposition and wet supercritical release. The domed shape impacted the effect of stiction during the release step and assisted in the release of large millimeter-size film geometries. The dome and release procedures were used to fabricate an experimental test specimen that functions as either a tensile or interfacial fracture test for thin films on rigid substrates. Other potential applications of the domed metal structures such as compliant electrical interconnects are discussed.

Gradual pressure release for reliable nanoimprint lithography

Reliable generation of nanometer-scale features is one of the most critical subjects for the industrial use of nanoimprint lithography. In this brief report, the authors investigate the influence of imprinting pressure history on the fidelity of a molded pattern in thermal nanoimprint lithography. The authors demonstrate that the morphology of molded patterns can be drastically improved by altering the pressure applied during the cooling step. A gradual release of imprinting pressure is found to augment reliable pattern transfer of high-density nanodot arrays without any noticeable polymer reflow or delamination of the patterned layer.

Nanofluidic channels fabricated by e-beam lithography and polymer reflow sealing

The authors developed a facile approach for creating nanofluidic channels by electron beam lithography that used a bilayer e-beam resist consisting of poly(methyl methacrylate) (PMMA) on top of poly(dimethyl glutarimide) (PMGI). In the process, the more sensitive PMGI was fully exposed with channel patterns, and the less sensitive PMMA was only fully exposed with a chain of dot patterns right above the channel patterns. PMMA was then developed to form a chain of holes through which PMGI channels were developed. After closing the holes by thermal reflowing PMMA, channels in PMGI were sealed with PMMA. The current method is capable of fabricating simultaneously channels with different channel widths.

Shape control of polymer reflow structures fabricated by nanoimprint lithography

Nanoimprinted resist pre-forms were modified using thermal reflow. This post-processing of binary structures enabled us to generate lens-like 3-D structures with different shapes by time- and surface chemistry controlled spreading. The method was extended to feature dimensions down to 100 nm. Surfactant coated line nanostructures were found to be limited by a maximum aspect ratio for imprinted pre-forms. This powerful post-processing method can be used either directly as post-processing step in production or for the fabrication of 3-D stamp copies with the desired shapes.

Reduction of surface roughness for optical quality microfluidic devices in PMMA and COC

A rapid and low-cost technique is presented for the fabrication of optical quality microfluidic devices in poly(methyl methacrylate) (PMMA) or cyclic olefin copolymer (COC). When polymer microfluidic devices are manufactured by rapid prototyping techniques, such as micromilling, the surface roughness is typically in the region of hundreds of nanometres reducing the overall optical efficiency of many microfluidic-based systems. Here we demonstrate a novel solvent vapour treatment that is used to irreversibly bond microfluidic chips while simultaneously reducing the channel surface roughness, yielding optical grade (less than 15 nm surface roughness) channel walls. We characterize this vapour bonding method and optimize the process parameters to avoid channel collapse, while achieving reflow of polymer and uniformity of bonding. The reflow of polymer is the key to enabling a fabrication process that takes less than a day and produces optical quality surfaces with low-cost rapid prototyping tools.

Fabrication of polymer-based reflowed microlenses on optical fibre with control of focal length using differential coating technique

Thermal reflow of polymer to generate spherical profile has been used to fabricate microlenses in this paper. A simple model based on the volume conservation (before and after reflow) and geometrical consideration of lens profile, shows that the focal length of the microlens produced by reflow technique is a function of the initial geometry of microcylinders, i.e. diameter and thickness. This relationship of focal length with diameter and thickness is used as a basis to control focal length. A simple spin coating technique on dual surface is used to achieve differential thickness, to control the focal length of microlenses produced on the same substrate. A biomedical application of such combination of microlenses is endoscopy where the lenses of varying diameter and equal focal length are needed on top of optical fibre bundles to provide independent function of illumination and imaging. This paper incorporates the differential thickness technique to show a micro fabrication process to produce the polymer reflowed microlenses, with a control of focal length based on thickness. The design also helps to integrate these microlenses on top an optical fibre with accurate alignment.