Nanoimprint Lithography Templates Research Articles

Using Impedance Measurement to Characterize Cell-Electrode Adhesion with Customized 3D-Nanostructured Multielectrode Arrays

Multielectrode array (MEA) technology is used as an extracellular recording method to investigate the electrical activity of complex excitable cell networks and tissue slices. In this context, an array of single-contactable, sensing electrodes record the electrical field potentials induced by the ion flux across the cell membranes. Therefore, MEAs are an ideal tool to investigate network activities for prolonged periods. Still, MEA technology suffers from low-signal-to-noise ratios caused by a dissatisfied cell-electrode contact, thus revealing only limited measurements. To improve cell recording, a tight cell-electrode contact is inalienable. This can be achieved by three-dimensional micro- or nanostructures on top of the sensing electrodes. Beside the fact of increasing the electrochemically active surface area, thus leading to a decrease in the overall chip impedance, the nano-sized structures can also trigger an engulfment process of the cells. By minimizing the gap between cell and electrode, a high seal resistance can be achieved which in return results in recording of higher signal amplitudes. In this work, electrochemical concepts are used for fabrication as well as for characterizing the chips and to investigate the cell-electrode interaction.The developed process allows the fabrication of half-structured MEAs with different three-dimensional nanostructure shapes, geometries and pitch lengths and is mainly based on thermal nanoimprint lithography (NIL), gold electroplating and microstructuring techniques. During thermal NIL a customized, nanostructured master mold is pressed into a polymer resist of a wafer substrate, thus creating a negative copy of the master´s nanostructures. The subsequent electroplating step can be adjusted in a way, that it fills or overgrows the NIL templates, thereby creating different shapes. Photolithography, chemical vapor deposition and etching steps are used to fabricate the overall MEA geometry.Cyclic voltammetry as well as electrochemical impedance spectroscopy have been applied to characterize the effect of different nanostructure geometries on charge transfer, double-layer capacitance and impedance of the MEA electrodes. Compared to unstructured electrodes, charge transfer and double-layer capacitance of the nanostructured electrodes are increased, whereas the impedance of the nanostructured electrodes are decreased. These results can directly be related to an increase of the electrochemically active surface area and vary with the investigated nanostructure layout.To examine the effect of the nanostructures on cell-electrode coupling in detail and to quantify cell adhesion, impedance sensing at a fixed frequency of 1 kHz has been performed using the impedance-stable human embryonic kidney cell line HEK 293. Impedance values have been measured with and without cells. Due to the chip layout where non- and nanostructured electrodes have been included on the chip, the relative change of the impedance values could directly be correlated with cell-electrode adhesion. By this, a significant increase or decrease of cell-electrode coupling depending on the nanostructure layout could be demonstrated. For verification, action potential recordings with neural stem cells of the subventricular zone of postnatal mice have been performed, showing a distinct increase of the recorded signal amplitudes of the nanostructured electrodes compared to unstructured electrodes of the same chip. The latter might be correlated to an engulfment process of the cells. Figure: Left: Impedance measurement to quantify cell-electrode adhesion. Right: SEM picture of attached SVZ cells on top of overgrown pillar structures (Inset: Magnification, scale bar: 2 µm). Figure 1

The NanoTuFe — Fabrication of large area periodic nanopatterns with tunable feature sizes at low cost

Fabricating nanopatterns for large area and in small feature sizes is in general cost-intensive. In this paper, a cost-efficient process chain is demonstrated to fabricate periodic nanopatterns with tunable feature sizes (NanoTuFe) utilizing a series of well-known technologies such as soft UV Nanoimprint Lithography (NIL), dry and wet etching. An intermediate template featuring nanocone and nanopyramid is introduced to bridge the original and the final patterns. The feature size on the final fabricated substrate can be widely tuned and controlled. Wafers in diameter of 100mm full of square nanocavities with 130/160/190/220nm feature sizes, respectively, have been fabricated from a single master wafer containing nanocavities with 350nm diameter. Nanopillar patterns with diameters of 150/200/250nm, respectively, have been produced on wafer level from a single master with 450nm feature size. Cryogenic silicon etching based on SF6/O2 chemistry is employed to create the final shrunk nanopatterns with smooth and vertical profiles. The fabrication processes cope without costly electron beam or laser beam direct writings, achieving the nanopatterns with tunable feature sizes in a cost-efficient manner. In addition, the process chain grants the wafer-level production at a constant pitch, which is of importance for the stacked alignment of layers with varying feature sizes. The generated periodic nanopatterns can be applied for NIL templates, photonic crystals, optics, energy conversion and storage, sensing elements, etc.

Low-cost fabrication of nanoimprint templates with tunable feature sizes at a constant pitch

Nanoimprint lithography (NIL) templates are in general costly, especially for large area and small feature sizes. With a simple shrinking technique using a serial of well-known technologies, it has been feasibly realized to produce high-quality soft NIL templates with widely tunable feature sizes at a constant pitch from a single master at very low-cost and in a short processing cycle. The master featuring at sub-micron range has been replicated to new silicon NIL templates with feature sizes at, but not limited to, sub-200nm. soft UV-NIL combined with a dry etch process, were employed to fabricate an intermediate template of nanocone patterns that is further used for an imprint on the final substrate. The feature size of the SiO2 etch mask on the final substrate can be adjusted by varying the duration of mask formation etching. The final patterning of the template is realized by cryogenic etching based on SF6/O2 chemistry. Silicon NIL templates featuring nanopillar patterns with diameters of 150nm, 200nm and 250nm, respectively, have been fabricated on wafer level from the same master with 450nm feature size. The presented process flow avoids the time-consuming and cost-intensive electron beam writings and gives more flexibility in the fabrication of nanopatterns. The fabrication cycle for such NIL working templates with tunable feature sizes is maintained short and at a low cost. Moreover, the technique allows the fabrication of wafer level products at a constant pitch, which is of importance as well for the stacked large-area imprintings with varying feature sizes.

Overlay improvement in nanoimprint lithography for 1×-nm patterning

Nanoimprint lithography (NIL) is becoming a promising technique for fine-patterning with a lower cost than other lithography techniques. High overlay accuracy is one of the issues in NIL. Using die-by-die alignment with moiré fringe detection, an NIL alignment measurement accuracy below 1 nm and an overlay accuracy below 5 nm have been reported. On the other hand, the requirement for overlay in 2020 is estimated to be 3–4 nm for dynamic random access memory, flash and logic devices. In order to make the overlay accuracy requirement qualify from the semiconductor industry, a lot of technology enhancements, such as the improvement of overlay control accuracy for NIL-tools, image placement accuracy improvement for NIL templates, mix and match technique of NIL, and other lithography tools such as immersion exposure ones, are needed. In this paper, the authors describe the evaluation of the NIL overlay performance using up-to-date NIL tools, and discuss the potentials of NIL overlay in the future. Alignment accuracy, precision, and overlay correction performance of NIL tools, NIL-to-NIL and NIL-to-optical lithography tool distortion matching performance and overlay error structure analysis, are discussed. From these analyses based on NIL overlay data, the authors discuss the possibility of NIL overlay evolution to realize the adaptation to mass production for the 1×-nm device.

Development of nanoimprint lithography templates toward high-volume manufacturing

Development of nanoimprint lithography (NIL) templates is discussed. The template fabrication process and its performance are presented with consideration of the requirements of NIL for high-volume manufacturing. Defectivity, image placement, and critical dimension uniformity are the three major performance parameters of the templates, and their current status is shown.

Alternative nano-structured thin-film materials used as durable thermal nanoimprint lithography templates

Nanoimprint templates made of diamond-like carbon (DLC) and amorphous silicon carbide (SiC) thin films and fluorine-doped associated materials, i.e. F–DLC and F–SiC were investigated in the context of thermal nanoimprint lithography (NIL) with respect to their release properties. Their performances in terms of durability and stability were evaluated and compared to those of conventional silicon or silica molds coated with antisticking molecules applied as a self-assembled monolayer. Plasma-enhanced chemical vapor deposition parameters were firstly tuned to optimize mechanical and structural properties of the DLC and SiC thin films. The impact of the amount of fluorine dopant on the deposited thin films properties was then analyzed. A comparative analysis of DLC, F–DLC as well as SiC and F–SiC molds was then carried out over multiple imprints, performed into poly (methyl methacrylate) (PMMA) thermo-plastic resist. The release properties of un-patterned films were evaluated by the measurement of demolding energies and surface energies, associated with a systematic analysis of the mold surface contamination. These analyses showed that the developed materials behave as intrinsically easy-demolding and contamination-free molds over series of up to 40 imprints. To our knowledge, it is the first time that such a large number of imprints has been considered within an exhaustive comparative study of materials for NIL. Finally, the developed materials went through standard e-beam lithography and plasma etching processes to obtain nanoscale-patterned templates. The replicas of those patterned molds, imprinted into PMMA, were shown to be of high fidelity and good stability after several imprints.

Generation of 3D nanopatterns with smooth surfaces

Ga implantation into Si and reactive ion etching has been previously identified as candidate techniques for the generation of 3D nanopatterns. However, the structures manufactured using these techniques exhibited impedingly high surface roughness. In this work, we investigate the source of roughness and introduce a new patterning process to solve this issue. The novel patterning process introduces an additional layer absorbing the implanted Ga, thus preventing the clustering of the implanted Ga observed with uncoated Si substrates. This process enables 3D nanopatterning with sub-100 nm lateral resolution in conjunction with smooth height transitions and surface roughness down to 4 nm root mean square. Such patterns are ideally suited for optical applications and enable the manufacturing of nanoimprint lithography templates for low-profile Fresnel lenses.

Planar-localized surface plasmon resonance device by block-copolymer and nanoimprint lithography fabrication methods

The authors report on the integration of delocalized surface plasmon resonances (SPRs) and localized surface plasmon resonances (LSPRs) on a single device. The submicron SPR device was fabricated with nanoimprint lithography (NIL). Gold nanoparticles for LSPR generation were created and deposited via three methods and analyzed with rhodamine 6 G and surface-enhanced Raman spectroscopy (SERS). Compared to drop-cast and thin film annealing methods, gold nanoparticles fabricated from a diblock-copolymer NIL template produced the most significant effect on the charge-transfer component of the SERS enhancement mechanism due to near-field interactions at the 10 nm inter-particle separation region. The authors also report a 26% enhancement of optical resonance with an integrated SPR-LSPR plasmonic device consisting of a two-dimensional submicron aluminum grating fully coupled with gold nanoparticles measuring 20.4 nm in diameter in a water medium. If the 2D aluminum grating were coupled to an optimized nanoparticle SERS device fabricated from a DBCP NIL template, the coupled nanoparticle-grating device could exhibit an even higher enhancement and optical resonance performance.

Mask Cleaning in EUV and Nano-Imprint Lithography

Extreme Ultra-Violet Lithography (EUVL) and Nano-Imprint Lithography (NIL) are the two leading candidates for next generation lithography (NGL). New image projection methods and materials have led to inherent changes in the way EUVL masks and NIL templates are manufactured, handled, stored and processed. Where a pellicle is used to protect the patterned area of a conventional optical mask, both EUV masks and NIL templates are without surface protection and thus more susceptible to contamination during transportation, storage and the photolithography process. Zero tolerance levels for impurities, particles, pattern defects and/or surface layer damage is driving the development of new cleaning technology that can eliminate contamination and preserve the integrity of the mask/ template. In this paper, key cleaning challenges for EUV masks and NIL templates are addressed and novel cleaning techniques are demonstrated that remove contamination without pattern damage or surface properties changes.

Nanoimprint lithography stamp modification utilizing focused ion beams

Nanoimprint lithography (NIL) has been established as a high-throughput technique to fabricate sub-25-nm patterns at a low cost. The fabrication of NIL templates with features in the submicrometer range is currently a bottleneck of the NIL technology. The replication of errors on NIL templates places a major challenge on the reusability of templates. Focused ion beam (FIB) technology is employed to modify prestructured NIL templates. In this work, repair strategies for NIL stamps are discussed. Excess material from stamps has been removed by ion milling. Nanoscale trenches and ultrathin lamellas fabricated with a focused ion beam and their corresponding imprints are presented. It has been confirmed that commercial UV-NIL stamps can be modified by FIB milling and imprinted line patterns were successfully replicated by UV-NIL using the repaired templates. Furthermore, the potential of three-dimensional NIL templates structured by FIB was evaluated. Three-dimensional imprints with features down to 80nm with good structure conformity to the template were demonstrated. The capabilities and limitations of FIB as repair technology for NIL stamps are discussed.

Fabrication and validation of fused silica NIL templates incorporating different length scale features

A cost effective methodology for pattering of Nano Imprint Lithography (NIL) templates incorporating different length scale features is proposed. The approach relies on selecting the optimum processing window of different technologies for cost effective micro- and nano-patterning. Very promising results were obtained when first fused silica templates were structured by F2 laser ablation at 157 nm. It was demonstrated that nano-scale features and complex 3D micro-scale features could be machined with a Focused Ion Beam (FIB) over the existing topography produced by laser ablation. Thus, a large area (up to several square centimeters) of the NIL templates is easily patterned with micro- and even meso-scale features by laser ablation while nano- and micro-scale features could be introduced locally by FIB machining. Through S-FIL process such templates were validated and the nano- and micro-structures were accurately replicated in one imprinting step.

Review of Template Fabrication for Nanoimprint Lithography

Nanoimprint lithography (NIL) is attracting attention as a low-cost and high-throughput method for printing nanometer-scale geometries, and has been included on the ITRS lithography roadmap at the 32 and 22 nm nodes. The template making is one of the most critical issues for the realization of NIL. The ultimate resolution of the patterns fabricated by NIL is primarily determined by the resolution of the features on the surface of the mold. Because of the 1X nature of NIL compared with 4X for photolithography, the 1X template fabrication is considered as the greatest challenge for NIL. Advances in NIL template fabrication processes are reviewed. The fabrication methods of 3D, large area and high resolution templates are currently critical requirements and challenges. As a result, new mold making methods should be further developed to meet the requirements from emerging market applications such as patterned media, photonic crystals and wire grid polarizers.

Sub-30-nm patterning on quartz for imprint lithography templates

A parallel and economical method for obtaining nanoscale features on large-area quartz substrates has been developed for use in nanoimprint lithography template fabrication. Self-assembled multilayer films (molecular rulers) are used in conjunction with photolithographically defined metallic features to generate precise nanogaps with sub-30-nm resolution on quartz substrates. These nanopatterns are then transferred to the quartz substrates using the metallic thin films as etch masks.

Close-Packed Noncircular Nanodevice Pattern Generation by Self-Limiting Ion-Mill Process

We describe a self-limiting, low-energy argon-ion-milling process that enables noncircular device patterns, such as squares or hexagons, to be formed using precursor arrays of uniform circular openings in poly(methyl methacrylate) defined using electron beam lithography. The proposed patterning technique is of particular interest for bit-patterned magnetic recording medium fabrication, where square magnetic bits result in improved recording system performance. Bit-patterned magnetic medium is among the primary candidates for the next generation magnetic recording technologies and is expected to extend the areal bit density limits far beyond 1 Tbit/in(2). The proposed patterning technology can be applied either for direct medium prototyping or for manufacturing of nanoimprint lithography templates or ion beam lithography stencil masks that can be utilized in mass production.