Thermal Nanoimprint Lithography Research Articles

Analysis of polymer flow in embossing stage during thermal nanoimprint lithography

AbstractViscous polymer flow in embossing stage during thermal nanoimprint lithography was investigated. Flow behavior of the thermoplastic polymer was analyzed numerically and experimentally. Fixed grid scheme along with the finite element method was used for numerical analysis. Effects of process parameters such as the temperature, the stamp speed, and the stamp geometry on the filling pattern for single cavity were studied. Also the patterns with several cavities were analyzed to understand the flow behavior between adjacent cavities. Experiments were performed using PMMA sheets to confirm the effects of parameters obtained from numerical analyses. Two different types of geometries were considered and the filled patterns were observed using a scanning electron microscopy. Numerical results exhibited a similar tendency as the experimental observation. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers

Low loss Chalcogenide glass waveguides by thermal nano-imprint lithography

We report the fabrication of low loss rib waveguides from chalcogenide glass films by thermal nano-imprint using a soft stamp. Waveguides 2-4 µm wide and 1 µm high were fabricated with extremely smooth sidewalls and optical losses limited by Rayleigh scattering to values of 0.26 dB/cm for the TM and 0.27 dB/cm for TE polarizations at 1550 nm.

Measurement of pull-off force on imprinted nanopatterns in an inert liquid

We report on the measurement of the pull-off force on nanoscale patterns that are formedby thermal nanoimprint lithography (t-NIL). Various patterns with feature sizesin the range of 50–900 nm were fabricated on silicon substrates using a rigiflexpolymeric mold of ultraviolet curable polyurethane acrylate (PUA, Young’s modulus ∼ 1 GPa) or perfluoropolyether (PFPE, Young’s modulus ∼ 10.5 MPa) and a resist layer of polystyrene (PS) of three different molecular weights (Mw = 18 100, 211 600 and 2043 000). The pull-off force was measured in non-polar, non-reactiveperfluorodecalin (PFD) solvent between a sharp atomic force microscopy (AFM) tip and animprinted pattern. Our experimental data demonstrated that the measured pull-off forceswere in good agreement with a simple adhesion model based on Lifshitz theory. Also, theforce on the pressed region (valley) is higher than that on the cavity region (hill), with theratio (hill/valley) decreasing with the decrease of pattern size and the increase of molecularweight. The confinement effects were more pronounced for smaller patterns (<300 nm) and highermolecular weights (Mw = 211 600 and 2043 000) presumably due to sluggish movement of polymer chains into nano-cavities.Finally, the experimental observations were compared with molecular dynamic simulationsbased on a simplified amorphous polyethylene model.

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.

Resist Properties of Thin Poly(methyl methacrylate) and Polystyrene Films Patterned by Thermal Nanoimprint Lithography for Au Electrodeposition

Gold (Au) line patterns of 0.5 and 1 µm widths were fabricated on a silicon substrate covered with a Au thin film by thermal nanoimprint lithography with poly(methyl methacrylate) (PMMA) and polystyrene (PS), followed by Au electrodeposition with their thin films as resist masks. We described the differences in the fidelity of the deposited Au patterns for linewidth and linewidth roughness and differences in undesired Au deposition in the masked regions between thin PMMA and PS films. The linewidths of the convex deposited Au pattern were larger than those of the concave nanoimprinted polymer pattern, and the deviation from the polymer concave pattern in the case of PS was significantly smaller than that in the case of PMMA. The linewidth roughness of the Au lines deposited with a PMMA mask was markedly high owing to particle-like Au deposition in comparison with that deposited with a PS mask. Undesired Au deposition occurred even on electrode surfaces masked with the thin PMMA and PS films. It was considered from these results that the difference in the size accuracy of Au electrodeposition between PMMA and PS was responsible for the resistance in the oxygen dry-etching step of a residual layer during thermal nanoimprint lithography in addition to polymer water absorbability in Au electrodeposition.

Enhanced Biomolecular Detection Based on Localized Surface Plasmon Resonance (LSPR) Using Enzyme-Precipitation Reaction

An enzyme-catalyzed precipitation reaction was employed as a means to increase the change in the LSPR signal after intermolecular bindings between antigens and antibodies occurred on gold nanodot surfaces. The gold nanodot array with an diameter of 175 nm and a thickness of 20 nm was fabricated on a glass wafer using thermal nanoimprint lithography. The human interleukin (hIL) 5 antibody was immobilized on the gold nanodot, followed by binding of hIL 5 to the anti-hIL 5. Subsequently, a biotinylated anti-hIL 5 and a alkaline phosphatase conjugated with streptavidin were simultaneously introduced. A mixture of 5-bromo-4-chloro-3-indolyl phosphate p-toluidine (BCIP) and nitro blue tetrazolium (NBT) was then used for precipitation, which resulted from the biocatalytic reaction of the alkaline phosphatase on gold nanodot. The LSPR spectra were obtained after each binding process. Using this analysis, the enzyme-catalyzed precipitation reaction on gold nanodots was found to be effective in amplifying the change in the peak wavelength of LSPR after molecular bindings.

Fabrication and application of a microfluidic‐embedded silicon nanowire biosensor chip

AbstractWe report the fabrication of a silicon nanowire (SiNW) transistor array realized in a top‐down process on 4′′ silicon‐on‐insulator (SOI) wafers. This process is a second, optimized version combining thermal nanoimprint lithography and wet chemical etching to define planar nanowires and contact lines out of the top silicon layer in one etching step. We used ion implantation to reduce series resistance of source and drain contact lines. In total 56 individually addressable SiNWs were aligned in two lines. Wires dimensions were down to 60 nm in width and 40 nm in height having lengths up to 40 µm. Due to an improved processing we obtained almost identical wire structures and electronic properties throughout the 4′′ wafer enabling direct comparison of signals from one chip. The wires were capped by a microfuidic structure, which was used to administer different analyte solutions. First pH characterization measurements and molecular detection experiments using charged polyelectrolytes (PEs) and poly(D)lysine were successful. In future these devices could be used to observe diffusion of analytes in a time‐dependent, spatially‐ and quantitatively‐resolved readout.

Step-and-repeat process for thermal nanoimprint lithography

We present a step-and-repeat process for thermal nanoimprint lithography. For the selective heatingand imprinting, a spin-coated polystyrene layer is exposed to infra-red rays from a halogen lamp (intensity ∼ 500 W) with a metal-covered glass while pressed with a transparent polymer mold (Young’smodulus ∼ 300 MPa) under apressure of ∼ 4 bar for 60–120 s. During imprinting, the non-irradiated region is protected by ametal screen and a heat sink consisting of a copper block at the bottom whichprevents the pattern collapse by lateral heat conduction from the irradiated region.

Reactive-monolayer-assisted Thermal Nanoimprint Lithography with a Benzophenone-containing Trimethoxysilane Derivative for patterning Thin Chromium and Copper Films

Reactive-monolayer-assisted thermal nanoimprint lithography was applied for patterning thin films of chromium (Cr) and copper (Cu). The metal surfaces with the oxide layer were modified with a benzophenone-containing trimethoxysilane derivative to induce a surface graft reaction of the benzophenone moiety with poly(styrene) (PS) used for a resist polymer. A thin PS film was successfully prepared as a resist layer for wet etching on the modified metal surfaces by spin-coating, followed by exposure to ultraviolet light and annealing without dewetting the resist layer. The thin PS film could be patterned by thermal nanoimprinting involving a removal of the residual layer by exposure to UV/ozone. The patterned PS film worked as a resist mask for acidic aqueous wet etchants for Cr and Cu. We demonstrated that thin patterned films of Cr and Cu on micrometer scales could be fabricated by simple wet etching.

Synthesizing a kind of controllable nanopillar arrays

On the basis of the first oxide prepared anodized aluminum oxide template (AAO), the second, third and fourth oxidation preparation are implemented in turn. Then, the surface morphology and current curve changes of the AAO template with multiple oxidation preparation were compared and analyzed systemically. The apertures of nanopore made by the second, third and forth oxidation increase in order, the intervals between neighboring apertures reduce. However, the scale of ordered distribution of nanopores on the whole template surface does not change significantly. Then, carefully controlling the working conditions for the membrane-removed of the one-step oxidation AAO, the nanoporous with regular apertures and highly ordered was obtained after getting rid of the top oxidation layer about 10 minutes. Finally, based on those prepared AAO templates, integrated with the thermal nanoimprint lithography, we achieved the PMMA nano-pillar arrays with controllable lengths and diameters.

Patterning of Polyimide/Single-Walled Carbon Nanotube Composite and Its Electrical Properties

We have investigated the electrical properties of solution-processed polyimide/carboxylic acid functionalized single-walled carbon nanotube (SWCNT-COOH) composites. The electrical conductivity of the polyimide/SWCNT-COOH composite markedly increased with SWCNT-COOH loading significantly lower than 1 wt %. The conductivity of the polyimide/SWCNT-COOH composite further increased with electric-field treatment of half-dried polyimide precursor films. The polyimide/SWCNT-COOH composite was also patterned by low-temperature thermal nanoimprint lithography of uncured polyamic acid/SWCNT-COOH films by a silicone elastomer. The current was observed only in the direction parallel to the line patterns and could not flow across the lines, thereby indicating the presence of negligibly thin residual layers. The field-emission (FE) properties of the polyimide/SWCNT-COOH composite deposited by ac-coupled electrophoresis were also investigated and excellent FE properties were obtained, especially for deposition on an indium tin oxide (ITO) electrode that is selectively coated by micropatterned polymer banks.

Study of the behaviour of monomers in thermal nanoimprint lithography

Thermoplastics are commonly used in thermal nanoimprint lithography (NIL) but their high viscosity leads to inhomogeneities of residual thickness in patterns with various densities. Monomers exhibit low viscosity and are imprinted easily and polymerized with UV–NIL processes. These monomers can be also used for thermal NIL. We have imprinted A-POSS material which is spontaneously polymerized at 170 °C. The inorganic part of this monomer is interesting for pattern transfer and for permanent applications. Thermal properties of this molecule are presented in this paper. It is shown that polymerization occurs at 170 °C, and that the viscosity is 1330 mPa s at ambient temperature. Imprint experiments have demonstrated that A-POSS flows over larger surfaces during imprint step, compared to thermoplastics. Patterns with different densities have been studied and different filling regimes have been observed depending on material viscosity. They are induced by a competition between material flow and mold deformation. Finally, we imprinted some nanoelectrodes simultaneously with millimetric large connection pads, and it was demonstrated that complete filling was obtained with monomers whereas this was not possible with thermoplastics.

A versatile pattern inversion process based on thermal and soft UV nanoimprint lithography techniques

Si master molds are generally patterned by electron-beam lithography (EBL) that is known to be a time-consuming nanopatterning technique. Thus, developing mold duplication process based on high throughput technique such as nanoimprint lithography can be helpful in reducing its fabrication time and cost. Moreover, it could be of interest to get inverted patterns (holes instead of pillars) without changing the master EBL process. In this paper, we propose a two step process based on thermal nanoimprint lithography (T-NIL) (step 1) and soft UV assisted nanoimprint lithography (UV-NIL) (step 2) to invert a master EBL mold. After the two inversion steps, the grand-daughter Si mold exhibits the same pattern polarity as the EBL mold. For step 1, pattern transfer using ion beam etching (IBE) of a thin metallic underlayer is the critical step for dimension control due to the low NXR1020 resistance. For step 2, the optimized reactive ion etching (RIE) step allows transfer with good anisotropy even for nanostructures at the 50 nm-scale. For structures larger than 100 nm, this inversion process has been successfully applied to large field replication (up to 1.5 cm 2) on whole wafer.

Highly photostable solid-state organic distributed feedback laser fabricated via thermal nanoimprint lithography

A highly photostable distributed feedback (DFB) laser has been fabricated by spin-coating a polystyrene film doped with a perylenediimide derivative on a SiO 2 grating. The second-order grating, with a period of 368 nm, was fabricated via thermal nanoimprint lithography and pattern transfer. When the DFB laser is photopumped at 533 nm, the laser emission spectrum shows two peaks at 573 and 581 nm. The laser threshold ( I th) is 30 times lower than the amplified spontaneous emission threshold for a sample without grating. These devices are very photostable: the halflife is longer than 100,000 pump pulses at a pump intensity of 6 μJ/pulse.

Recovery prevention via pressure control in thermal nanoimprint lithography

In order to investigate the nonuniformity occurring below wider patterns during thermal nanoimprint lithography, the pressure situation is analyzed in detail. A balance of vertical forces shows that the gas pressure within the cavities is negligible, whereas the effective pressure acting on the stamp structures in contact to the polymer is increased compared to the externally applied pressure. This effective pressure has a high hydrostatic component, which may result in local stamp deformation. Imprint with a low molecular weight polymer provides evidence that the stamp structures become compressed under high pressure, resulting in a nonuniform residual layer beneath the imprinted patterns. In contrast, bending deformation of single stamp structures is found to be negligible. Pressure reduction is effective to reduce stamp compression, improving the uniformity of the residual layer. With typical imprint polymers of medium molecular weight, however, pressure reduction reduces the overall imprint depth.

Direct indium tin oxide patterning using thermal nanoimprint lithography for highly efficient optoelectronic devices

Indium tin oxide (ITO) is the most commonly used transparent conducting oxide (TCO) material on account of its high conductivity and transmittance in a visible range. In order to improve the efficiency of optoelectronic devices using TCO materials, the transmittance and conductivity of the TCO layers need to be improved. Recently, various techniques, which use nanostructures on the surface to improve the transmittance, have attracted considerable attention. In this study, a direct ITO nanopatterning technique using thermal nanoimprint lithography was presented. An ITO nanoparticle solution is used as an imprint resin and postimprint annealing process was followed. The optical transmittance of the ITO glass was increased by forming periodic ITO dot patterns on the ITO glass. UV-visible near infrared spectra showed that the transmittance at 485nm of the ITO glass with the directly patterned ITO layer was 5% higher than that of ordinary ITO glass.

Protein patterning on the micro- and nanoscale by thermal nanoimprint lithography on a new functionalized copolymer

The localized depositions such as DNA or proteins at the micron or submicron scale on solids supports are a crucial step in the fabrication of advanced biochips and laboratory on chip devices. The present work shows a new approach: A new biofunctionalized copolymer based on 80% benzyl methacrylate and 20% succinimydil methacrylate is used as a printable polymer with a great affinity to proteins, leading to combined areas covered by proteins and areas with resistance to protein adsorption in the submicron scale. Based on this approximation, an immunoassay is proposed as a proof of concept to detect the presence of the rabbit IgG protein up to concentrations of 200 ng/ml.

Numerical Simulation on Deformation During Molding Process for Thermal Nano Imprint Lithography

Numerical Simulation on Deformation During Molding Process for Thermal Nano Imprint Lithography

Easy mask-mold fabrication for combined nanoimprint and photolithography

Hybrid transparent working stamps with both a surface relief and absorbing mask pattern were fabricated by replicating nanostructures in an Ormostamp film on prepatterned glass substrates. By using a combined nanoimprint and photolithography process, self-aligned mixed patterns of nano- and microstructures can be generated within one single resist layer. In this article the authors present a simple process based on the organic-inorganic hybrid polymer Ormostamp by replication on prepatterned Borofloat substrates. Using these working stamps, grating arrays with 35 nm half pitch were replicated in thermoplastic UV-curable resist. The method is easy to employ for generating mesas with nanostructures on top or the backfilling with microstructures in thermal nanoimprint lithography applications.

Nanoimprinted Polyethyleneimine: A Multimodal Template for Nanoparticle Assembly and Immobilization

AbstractPolyethyleneimine (PEI) is used as a scaffold for integrated top‐down/bottom‐up fabrication. In this synergistic strategy, patterned PEI surfaces are created using thermal nanoimprint lithography (NIL) using a sacrificial polystyrene (PS) overlayer. These imprinted surfaces act as versatile templates for assembling nanoparticles and dyes, with the amine groups of the PEI enabling electrostatic assembly, carbodiimide coupling, and dithiocarbamate attachment to the nanoimprinted features. The efficient assembly of particles and dyes is confirmed through fluorescence and atomic force microscopy. In these studies the PS overlayer serves two roles. First, the PS layer protects the PEI surface during the plasma‐etch removal of the residual layer of the NIL process. Second, the PS overlayer serves as a mask, enabling sequential functionalization of the sides and the tops of the PEI features.