Microimprint Lithography Research Articles

Surface Microfabrication of Lactic Acid–Glycolic Acid Copolymers Using a Gas-Permeable Porous Mold

We attempted to perform surface microfabrication of the bioabsorbable material lactic acid–glycolic acid copolymer (LG-80) using a micro-imprint lithography technique with a gas-permeable porous mold at less than 5 °C. As a result, high-resolution surface micromachining with a height of 1.26 μm and a pitch of 2.97 μm was achieved using a convex sapphire mold with a height of 1.3 μm and a pitch of 3 μm. After processing, the LG-80 exhibited high water repellency, and FT-IR analysis of the surface showed no significant change in its chemical structure, confirming that the surface microfabrication was successful, while retaining the properties of the material. This demonstrated new possibilities for surface microfabrication technology for bioabsorbable materials, which are expected to be applied in the medical and life science fields in products such as surgical implants, tissue regeneration materials, and cell culture scaffold materials. In particular, the use of micro-imprint lithography enables low-cost and high-precision processing, which will be a major step toward the practical application of bioabsorbable materials.

Attenuated Glial Reactivity on Topographically Functionalized Poly(3,4-Ethylenedioxythiophene):P-Toluene Sulfonate (PEDOT:PTS) Neuroelectrodes Fabricated by Microimprint Lithography.

Following implantation, neuroelectrode functionality is susceptible to deterioration via reactive host cell response and glial scar-induced encapsulation. Within the neuroengineering community, there is a consensus that the induction of selective adhesion and regulated cellular interaction at the tissue-electrode interface can significantly enhance device interfacing and functionality in vivo. In particular, topographical modification holds promise for the development of functionalized neural interfaces to mediate initial cell adhesion and the subsequent evolution of gliosis, minimizing the onset of a proinflammatory glial phenotype, to provide long-term stability. Herein, a low-temperature microimprint-lithography technique for the development of micro-topographically functionalized neuroelectrode interfaces in electrodeposited poly(3,4-ethylenedioxythiophene):p-toluene sulfonate (PEDOT:PTS) is described and assessed in vitro. Platinum (Pt) microelectrodes are subjected to electrodeposition of a PEDOT:PTS microcoating, which is subsequently topographically functionalized with an ordered array of micropits, inducing a significant reduction in electrode electrical impedance and an increase in charge storage capacity. Furthermore, topographically functionalized electrodes reduce the adhesion of reactive astrocytes in vitro, evident from morphological changes in cell area, focal adhesion formation, and the synthesis of proinflammatory cytokines and chemokine factors. This study contributes to the understanding of gliosis in complex primary mixed cell cultures, and describes the role of micro-topographically modified neural interfaces in the development of stable microelectrode interfaces.

Gas‐Permeable Cellulose Template for Reduction of Template Damage and Gas Trapping in Microimprint Lithography of High Volume Manufacturing

A gas‐permeable cellulose template for microimprint lithography has been synthesized and characterized for the reduction of template damage and gas trapping caused by solvents and oxygen generated from cross‐linked materials. The 5 μm line‐pattern failure of the microimprinted UV cross‐linked liquid materials with 4.7 wt% acetone as a volatile solvent is solved by using the gas‐permeable cellulose template because of its increased oxygen permeability. The gas‐permeable cellulose template also allows the use of volatile solvents with high coating property and solubility into the microimprinted materials instead of the compounds and plastic resins conventionally used in mold injection. image

UV micro-imprint patterning for tunable light trapping in p-i-n thin-film silicon solar cells

In this paper, we used UV micro-imprint lithography periodic patterning combined with self-textured BZO films with a wide range of texture distributions for light trapping in thin-film silicon solar cells. It was found that the feature size of the periodic textures has a significant influence on the light trapping capacity of the glass substrate and the external quantum efficiency (EQE) of microcrystalline silicon (μc-Si:H) solar cells. Microcrystalline silicon solar cells, deposited on periodic textures of 5μm, showed an improved photocurrent density without any loss in the open-circuit voltage and fill factor; hence, resulting in an overall efficiency increase of 6.28%.

Ultrananocrystalline diamond-CMOS device integration route for high acuity retinal prostheses.

High density electrodes are a new frontier for biomedical implants. Increasing the density and the number of electrodes used for the stimulation of retinal ganglion cells is one possible strategy for enhancing the quality of vision experienced by patients using retinal prostheses. The present work presents an integration strategy for a diamond based, high density, stimulating electrode array with a purpose built application specific integrated circuit (ASIC). The strategy is centered on flip-chip bonding of indium bumps to create high count and density vertical interconnects between the stimulator ASIC and an array of diamond neural stimulating electrodes. The use of polydimethylsiloxane (PDMS) housing prevents cross-contamination of the biocompatible diamond electrode with non-biocompatible materials, such as indium, used in the microfabrication process. Micro-imprint lithography allowed edge-to-edge micro-scale pattering of the indium bumps on non-coplanar substrates that have a form factor that can conform to body organs and thus are ideally suited for biomedical applications. Furthermore, micro-imprint lithography ensures the compatibility of lithography with the silicon ASIC and aluminum contact pads. Although this work focuses on 256 stimulating diamond electrode arrays with a pitch of 150μm, the use of indium bump bonding technology and vertical interconnects facilitates implants with tens of thousands electrodes with a pitch as low as 10μm, thus ensuring validity of the strategy for future high acuity retinal prostheses, and bionic implants in general.

Ordered silicon microwire arrays grown from substrates patterned using imprint lithography and electrodeposition.

Silicon microwires grown by the vapor-liquid-solid process have attracted a great deal of interest as potential light absorbers for solar energy conversion. However, the research-scale techniques that have been demonstrated to produce ordered arrays of micro and nanowires may not be optimal for use as high-throughput processes needed for large-scale manufacturing. Herein we demonstrate the use of microimprint lithography to fabricate patterned templates for the confinement of an electrodeposited Cu catalyst for the vapor-liquid-solid (VLS) growth of Si microwires. A reusable polydimethylsiloxane stamp was used to pattern holes in silica sol-gels on silicon substrates, and the Cu catalyst was electrodeposited into the holes. Ordered arrays of crystalline p-type Si microwires were grown across the sol-gel-patterned substrates with materials quality and performance comparable to microwires fabricated with high-purity metal catalysts and cleanroom processing.

Investigation of resist filling profile evolution in microimprint lithography

In micro-/nanoimprint lithography, the quality of the imprinted patterns can be reflected by their final profile shapes. The evolution of resist filling profiles was investigated through numerical simulations and a visualization experiment. A numerical model based on computational fluid dynamics was built to predict the resist filling process. Meanwhile, a 3D defocusing digital particle image velocimetry system was developed to capture the spatial coordinates of the fluorescent tracer particles at different filling times and reconstruct the filling profiles according to the particles' coordinates. The three-dimensional filling profiles of the resist can help understand this microflow phenomenon and describe the resist filling modes. A comparison of filling profiles for a single mold geometry and a single initial thickness revealed good consistency between the model prediction and experiment. The critical range of conversion from single-peak filling mode to double-peak filling mode was determined, which will provide qualitative information for optimizing mold geometries and process parameters.

Selectively Patterning Polymer Opal Films via Microimprint Lithography

Large-scale structural color flexible coatings have been hard to create, and patterning color on them is key to many applications, including large-area strain sensors, wall-size displays, security devices, and smart fabrics. To achieve controlled tuning, a micro-imprinting technique is applied here to pattern both the surface morphology and the structural color of the polymer opal films (POFs). These POFs are made of 3D ordered arrays of hard spherical particles embedded inside soft shells. The soft outer shells cause the POFs to deform upon imprinting with a pre-patterned stamp, driving a flow of the soft polymer and a rearrangement of the hard spheres within the films. As a result, a patterned surface morphology is generated within the POFs and the structural colors are selectively modified within different regions. These changes are dependent on the pressure, temperature, and duration of imprinting, as well as the feature sizes in the stamps. Moreover, the pattern geometry and structural colors can then be further tuned by stretching. Micropattern color generation upon imprinting depends on control of colloidal transport in a polymer matrix under shear flow and brings many potential properties including stretchability and tunability, as well as being of fundamental interest.

The Design and Fabrication of Dimples Pattern on the Surface of UHMWPE

Tribological problems are the major obstacle that limits the long-term using of ultra high molecular weight polyethylene (UHMWPE) in artificial joints. Many efforts have been made to improve the tribological properties of UHMWPE mainly by promoting the structure, morphology and mechanical properties of UHMWPE. In this paper, inspired from the natural joint, the micro-scale texture has been introduced to improve the tribological properties of UHMWPE. The micro-imprint lithography technology has been adopted to fabricate the textured UHMWPE. The profile of textured UHMWPE shows that the quality of textured UHMWPE is good. The result of tribological tests shows that the micro-scaled surface texture can remarkably reduce friction on UHMWPE.

Resist deformation and flow fields in microimprint lithography

In microimprint lithography, the resist deformation directly influences the quality of the final imprinted patterns. The resist velocity field was investigated through numerical simulations and visualization experiments. A numerical model based on the computational fluid dynamics was built to predict the resist filling behavior. Meanwhile, a 3D defocusing digital particle image velocimetry (DDPIV) system was developed to achieve the microscale velocity field of resist. The spatial coordinates of the fluorescent tracer particles were derived from their defocused images, and then the three-dimensional particle field and velocity field inside the resist were obtained according to the particles' spatial coordinates and time interval recording the particle images. The investigation of the velocity field, including the horizontal and vertical velocity history, was performed to help describe the filling mode and flow behavior of the resist. The experimental results agreed well with the simulation prediction, which justified the use of the micro DDPIV system to investigate the resist filling behavior and verified the numerical model. The combined effect of the mold's local asymmetric geometries and resist's initial thickness on its own deformation was further analyzed by numerical simulation.

Investigation of resist filling behavior in microimprint lithography by computational fluid dynamics simulation and defocusing digital particle image velocimetry

Resist filling behavior is crucial to the quality of final imprinted patterns in microimprint lithography (MIL). This article investigates the velocity field of the resist in microimprint lithography through numerical simulations and visualization experiments. To achieve the microscale velocity field of resist, a numerical model based on the computational fluid dynamics was built to predict the resist filling behavior, and the surface tension and contact angle were considered in this model. Meanwhile, a 3-D defocusing digital particle image velocimetry (DDPIV) was established. The spatial coordinates of the fluorescent tracer particles were derived from their DDPIV images. Time-resolved 3-D particle field inside the resist was obtained with the spatial coordinates. Particle tracking velocimetry was utilized to derive the velocity field from the particles' spatial position in the imprinting process. The investigation of the velocity field, including the horizontal and vertical velocity history, was carried out to directly describe the filling mode of the resist and hence determining the resist filling mechanism. The results of the visualization experiments and the numerical simulations were compared to obtain an in-depth understanding of the resist flow in MIL.

Experimental research on resist filling behavior in microimprint lithography by using defocusing digital particle image velocimetry

The resist filling behavior is crucial to the quality of the final imprinted patterns in microimprint lithography. To achieve the microscale velocity field of resist, a 3-D defocusing digital particle image velocimetry (DDPIV) system was established. The spatial coordinates of 500 nm fluorescent tracer particles were inferred from their DDPIV images generated by a mask with three apertures forming an equilateral triangle. Time-resolved 3-D particle field inside the resist was obtained with the spatial coordinates of particles. Particle tracking velocimetry was utilized to derive the velocity field from the particle spatial position in the imprinting sequence. Velocity history of particles along the extracted track was described. The results showed that the maximum velocity of the resist was always located in the region between the mold corner and the supporting substrate; meanwhile, the velocity component of the resist in the vertical direction is approximate to zero. The volume transfer characteristics of resist was illustrated from the view of particle field and velocity field, which will help reveal the resist filling mechanism and provide useful guidance for the mold structure optimization.

Tuning surface micropattern features using a shape memory functional polymer

Living cells can alter their shapes and functions in response to physical cues at the cell–substrate interface, and consequently the temporal variations of the substrate surface are thought to be very important in governing cell behaviors. In this study, a thermally-activated shape memory function was employed to tune the dynamic change of the surface topography features on a polymer nanocomposite substrate. The polymer nanocomposite is composed of chemically cross-linked poly(ε-caprolactone) with allyl alcohol and Fe3O4 nanoparticles. The regular arrays of surface micropillar patterns were fabricated through thermal embossing micro-imprint lithography. They exhibited excellent shape memory performance with a recovery from a temporary shape at 34 °C to a permanent shape at 41 °C. The static and the dynamic surface micropillars as well as a smooth flat surface were used to study the behaviors of rat bone marrow mesenchymal stem cells. During this process, the surface micropillars switched from a temporal zero degree overwhelmed shape at 32 °C gradually to a permanent 90 degree standing shape at 41 °C due to the shape memory recovery. The results showed that the dynamic changes of the surface topography transition and the resultant force could effectively regulate cell adhesion, spread and alignment compared to the static surface micropillars and the smooth flat surface.

Biomimetic surface design for ultrahigh molecular weight polyethylene to improve the tribological properties

Tribological problem is a major obstacle that limits the using of ultrahigh molecular weight polyethylene (UHMWPE) in industrial applications and orthopedic surgeries. Many efforts have been made to improve the tribological properties of UHMWPE, such as promoting the structure, morphology, and mechanical properties of the polymer. Inspired by the features of articular surface, micro-scaled texture is introduced to improve the tribological properties of UHMWPE using micro-imprint lithography. Friction and wear experiments are conducted on textured and untextured specimens using ring-on-disc test apparatus under water lubrication. The experimental results demonstrate that the micro-scaled surface texture can remarkably improve the tribological properties of UHMWPE. Friction force can be effectively reduced by selecting suitable dimple parameters. Compared with an untextured UHMWPE, the textured one with optimum parameters shows a reduction in the friction coefficient as much as 66.7–85.7% on different load–speed conditions. The optimized area density of surface textured UHMWPE ranges from 22.9% to 29.9%, which is obviously higher than that of stiff materials such as metals and ceramics. The textured UHMWPE with area density 29.9%, diameter 50 µm, and depth 15 µm presents a significant effect of wear resistance. The average wear depth of textured UHMWPE is 35.5% of that of untextured one.

Technological Development of Advanced Asymmetric Magnetic Soft Mold Micro-Imprint Lithography

This study proposed an advanced micro-imprint lithography (MIL), which integrates electromagnetic field-aided hot embossing and PDMS asymmetric magnetic flexible soft mold imprinting techniques, to imprint and replicate the microlens array structure. It is similar to the continuous grayscale technique; however, it is smoother than the structural form defined by the semi-conductor grayscale mask technique, and the process is simpler with lower costs, making it a good alternative for imprinting techniques and applications. This study also employed prescale films to measure and discuss the distribution of imprinting force on asymmetric magnetic soft molds. The results indicated that the magnetic soft mold and the substrate surface can be fully contacted. Since the magnetic powder reveals a skewed distribution, the test results of prescale film indicated that color depth is related to the concavo-convex of magnetic powder. Thus, the depth and accuracy of structural molding can be controlled in advance by casting the inclining platform. Finally, the SEM observation showed that if an inclining platform is used for composite magnetic PDMS casting, the microlens array asymmetric magnetic soft mold structure, which is complementary to it, could be obtained. The asymmetric magnetic soft mold was applied together with electromagnetic field-aided hot embossing equipment to imprint and replicate different continuous and smooth microlens array structures with foreseeable depth.

A Zero Residual Layer Process for Microimprint Lithography Using a Soft Mold

We studied the formation of a resist pattern by a zero residual layer process with microimprint lithography (MIL) using a soft mold. Polyethylene glycol-dimethacrylate as an UV hardening-type resist and polydimethylsiloxane as a soft mold were used for the MIL process. The stripe and square patterns with a zero residual layer process were achieved on Cr/glass. UV ozone was treated on the Cr to achieve a hydrophilic surface for a zero residual layer process, which was confirmed by the measurements of cross-sectional scanning electron microscopy and energy-dispersive X-ray microanalysis.

Development of a step micro-imprint lithography tool

This paper proposes a UV imprint lithography process which uses a soft PDMS mold to transfer high resolution patterns at room temperature and low pressures conditions, and emphasizes reporting the development of the step micro-imprint lithography (SIL) tool for sub-micrometer patterning. The overall design of the machine and implementation of key units are described in detail. To eliminate the distortion of the soft mold in the loading process, distortion reduction by a load release process was proposed and applied in the SIL machine. By optimizing the loading process curve, an imprinting process curve was established. Furthermore, an overlay process using the load release and alignment error pre-compensation method was proposed to achieve high precision overlay for the SIL tool. Finally, various experiments were conducted to test the performance of the SIL apparatus. The experimental results show that the step micro-imprint lithography tool together with the proposed processes can replicate the sub-micrometer patterns with different feature sizes, different structures and different pattern sizes.