Junseo Choi

Simulation study on stress and deformation of polymeric patterns during the demolding process in thermal imprint lithography

Thermal imprint lithography or hot embossing is a processing technique using molding to produce surface patterns in polymer resist at micro- and nanoscales. While fast molding is important to improve the yield of the process, the process step that determines the success of imprinting high aspect ratio structures is demolding, a process to separate the mold insert from the patterned resist after conformal molding. In this paper the authors studied the stress and deformation behavior in polymer resist during the cooling and demolding process of thermal imprint lithography via finite element method. A simple model structure of the Si stamp/poly(methyl methacrylate) (PMMA) resist/Si substrate was used for the simulation, assuming that PMMA is viscoelastic. As demolding proceeds, Von Mises stress in the PMMA layer is highly localized in two locations, one at the transition corner zone between the residual layer and the replicated PMMA pattern and the other close to the contact region with the moving stamp edge...

A microfluidic platform with a free-standing perforated polymer membrane

A membrane architecture that facilitates access from both sides in microfluidic environments provides a flexible platform for the study of biosystems. Here, we report for the first time on a simple and low cost fabrication process via nanoimprint lithography (NIL) for a thin, fully released SU-8 membrane with perforated micro- and sub-micron pores and a modular microfluidic system integrated with the membrane. A modified NIL process which combines thermal and UV NIL was employed to define the pore structures in an SU-8 layer coated on a sacrificial layer. We have demonstrated the production of large area SU-8 membranes of as large as 4 inch diameter that are fully covered with perforated micropores. The released SU-8 membrane was easily integrated as a modular component into a microfluidic system by sandwiching the membrane between two microfluidic chips. Important aspects to reliably produce the membrane architecture such as materials selection and process conditions for fabrication are discussed. After demonstrating selective adsorption of lipid vesicles at the micropore sites of the SU-8 membrane, we have reconstituted lipid bilayers at the micropores within the microfluidic system following the method developed by Suzuki et al (2004 Lab Chip 4 502–5). This implies that the membrane architecture can potentially be used as a microfluidic platform with lipid bilayers that can sustain external mechanical stress for biophysical studies of membrane proteins.

Reduction of Nanowire Agglomeration via an Intermediate Membrane in Nanowires Preparation for Nanosensors Application

Nanowires are widely used as sensing components for lab-on-a-chip devices. One major problem in utilizing pre-grown nanowires in lab-on-a-chip applications is the agglomeration of nanowires during their preparation process. The common methods to reduce the agglomeration of nanowires include stirring, sonication and using of surfactants. However, these methods break the long nanowires and are not efficient to produce enough single nanowires. This paper shows a new method to improve the deposition process of individual nanowires. An intermediate membrane was used for the deposition of the nanowires after their preparation process. The membrane helps to filter the nanowire agglomerates and to deposit separated individual nanowires over a silicon surface underneath. The study also shows that the number of single nanowires is increased by increasing the tilt angle of the membrane. The method also helps achieving single long nanowires.Copyright

Fabrication of polymeric dual-scale nanoimprint molds using a polymer stencil membrane

We report on a simple and effective process that allows fabricating polymeric dual-scale nanoimprinting molds. The key for the process is the use of a thin flexible SU-8 stencil membrane, which was fabricated by either photolithography or thermal nanoimprint lithography (NIL). The stencil membrane with microscale pores was assembled into a nanopatterned substrate, producing a dual-scale structure. The assembled structure was used as a template to produce polymeric imprinting molds via UV-NIL. With this method, we demonstrated dual-scale nanoimprint molds having nano-pillars of 251 nm diameter and 146 nm high on top of microscale square protrusions of 5 μm wide and 3.6 μm high. The resin mold with the dual-scale structure was successfully used to produce a freestanding membrane with dual-scale perforated pores via UV-NIL. After metal coating and integrated into microfluidic devices, this freestanding membrane can potentially be used as a substrate for surface plasmon resonance sensors.

Patterned electromagnetic alignment of magnetic nanowires

A combination of electromagnetic alignment and topological pattern assisted alignment to position magnetic nanowires, which is referred to as the Patterned Electromagnetic Alignment (PEA), is developed and examined. Electrodeposited, FeNiCo nanowires with different lengths were used as the test nanomaterial, and the microscale grooved surface was formed by UV nanoimprint lithography. The accuracy of the PEA with FeNiCo nanowires was evaluated by measuring the deviation angle from the direction of the magnetic field line for different magnetic field strengths and nanowire lengths, and a statistical alignment distribution was reported for different nanowire length groups. The results were compared with those of the electromagnetic alignment on flat surfaces and in grooved-patterned substrates without electromagnetic alignment. Overall, the deviation angle for the PEA was lower than that for the electromagnetic alignment when all other experimental conditions were identical, indicating that the alignment accuracy along the direction of the magnetic field lines was enhanced in the presence of surface micro grooves. This can be attributed to the fact that, upon attachment of nanowires to the substrate surface, the surface micro grooves in the PEA add additional deterministic characteristics to the otherwise stochastic nature of the nanowire deposition and solvent evaporation processes compared to the sole electromagnetic alignment.

The role of hydrophobic silane coating on Si stamps in nanoimprint lithography

Hydrophobic silane coatings have been successfully applied to the surface of Si stamps to improve demolding in nanoimprint lithography (NIL). However, the role of the silane coating has only been studied either indirectly, by measuring adhesion or friction coefficients for Si and substrate surfaces without patterns, or collectively, by measuring the overall demolding force that does not differentiate contributions of friction dissipation, stored elastic energy, and adhesion. Here, for the first time, we present experimental evidence on the role of the silane coating in improving demolding in UV-NIL by using different silane coatings. The silane coatings were characterized by x-ray photoelectron spectroscopy, water contact angle, and friction force measurements. Then, the work of demolding was systematically measured for different silane coatings using stamps with the same micropattern but different pattern depths. Comparison of the results to the theoretical model developed for fiber-matrix debonding energy by Sutcu and Hillig [Acta Metall. Mater. 38(12), 2653-2662] indicated that with a hydrophobic silane coating, the main parameter contributing to overall demolding work shifts from adhesion to stored elastic energy and frictional dissipation as surface adhesion keeps decreasing. The results confirm that the main role of the silane coating in reducing the demolding is to reduce surface adhesion rather than friction at the stamp/substrate interface.

Electrodeposition of Long Silver Nanowires in Highly Ordered Polymer-Based Template

Template synthesis of silver nanowires is studied in this paper. Unlike previous studies, highly ordered polymer-based template fabricated by nanoimprinting lithography was used instead of porous polycarbonate (PC) and anodic aluminum oxide (AAO) templates. This new template combines the advantages of the traditional templates and overcomes their drawbacks at the same time. Electrodeposition was carried out under different overpotential and time periods. Long single silver nanowires were found in the templates. The average length of the nanowires was much shorter than the length of the template, which can be explained by difficulty of plating solution diffusion. Wettability of the template and how to increase channel filling rate were also discussed in this paper.Copyright

Fabrication of Perforated Conical Nanopores in Freestanding Polymer Membranes Using Nanoimprint Lithography and Pressed Self-Perfection Method

Nanopores have proven to be an important sensing element in biosensors to detect and analyze single biomolecules such as DNAs, RNAs, or proteins. The charged biomolecules are driven by an electric field and detected as transient current blocks associated with their translocation through the pores. While protein nanopores, such as alpha-hemolysin and MspA protein nanopores embedded within a lipid bilayer membrane [1], promise to be a rapid, sensitive and label-free sensing paradigm, their duration of usage is too short to perform repetitive experiments due to the mechanical instability of the lipid bilayer. A variety of methods have been developed to prepare synthetic nanopores, which can substrate for protein nanopores, including a direct milling with a focused high-energy electron or ion beam in insulating substrates, an ion track etching in polymer substrates, and an anodizing in aluminum substrates. However, those methods do not allow for control over both the size and location of pores and the high yield of production.Copyright

Perforated Micro- and Nanopores in Free-Standing Polymer Membranes Fabricated by Nanoimprint Lithography and Pressed Self-Perfection Method

This study presents a stable and flexible method for fabricating a free-standing polymer membrane with perforated micro- and nanopores using an imprint lithography combined with a pressed self-perfection method and a sacrificial layer technique. For the fabrication, micropores were initially patterned on a double resist layer: the upper SU-8 resist layer as an active membrane layer and the lower life-off resist used as a sacrificial layer. The membrane with micropores was then pressed with a flat quartz wafer to reduce pore size down to sub-micrometer. Finally, a free-standing SU-8 membrane with perforated micro- and nanopores was successfully lifted-off from the substrate by dissolving the sacrificial layer.Copyright

Fabrication of Free-Standing Polymer Membranes via Imprint Lithography and Selective Immobilization of Lipid Vesicles at Micropores in the Membrane

This study presents fabrication of free-standing, perforated membranes in polymer with the pore diameter down to 500 nm via novel imprint lithography and use of the membrane to selectively immobilize lipid vesicles at the micropores in the membrane. For the fabrication, a combination of imprint lithography and a sacrificial layer technique was employed in order to get a clean, fully released, and mechanically stable membrane with perforated pores. Si molds with microscale pillar structures fabricated via photolithography were used to define patterns in SU-8 resist layer which was spin-coated on lift-off resist (LOR) used as a sacrificial layer. This was followed by a UV curing process to achieve enough mechanical strength in the SU-8 layer to be fee-standing. Release of the SU-8 layer from the LOR sacrificial layer by lift-off results in the free-standing, perforated membranes with pore diameter down to sub-micrometer range. Prior to the application of lipid vesicles, the SU-8 membrane surface was treated with poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) which prevents non-specific adsorption of the lipid vesicles on the membrane surface. As a result, lipid vesicles were found to selectively adsorb at the pore sites in the membrane, as observed with an evanescence fluorescence microscope. This result indicates that the perforated polymer membranes with micro- and nanoscale pores have potential as a platform for fundamental study of biological systems.Copyright