Silicon template fabrication for imprint lithography with a very smooth side wall profile

A new fabrication process of silicon template for nanoimprint lithography is developed. A very fine and high aspect Si lines are fabricated by the combination of plasma process for Si deep etching and anisotropic wet etching by potassium hydroxide solution (KOH treatment). Improved switching process, which consists of sequentially alternating etching and deposition steps, is used as the Si deep etching. The side wall profile of the Si lines can be controlled by the deposition step time. Line pattern with vertical side wall is fabricated. The line width and height are 0.2 µm and 2.9 µm, respectively. By the KOH treatment the side wall corrugation can be reduced and the line width also decreases. Very fine line of 45 nm width at the line top with the pattern height of 3.0 µm can be fabricated by use of the line width shrink by the KOH treatment.     

Investigation on fabrication of nanoscale patterns using laser interference lithography

Nanoscale patterns are fabricated by laser interference lithography (LIL) using Lloyd's mirror interferometer. LIL provides a patterning technology with simple, quick process over a large area without the usage of a mask. Effects of various key parameters for LIL, with 257 nm wavelength laser, are investigated, such as the exposure dosage, the half angle of two incident beams at the intersection, and the power of the light source for generating one or two dimensional (line and dot) nanoscale structures. The uniform dot patterns over an area of 20 mm x 20 mm with the half pitch sizes of around 190, 250, and 370 nm are achieved and by increasing the beam power up to 0.600 mW/cm2, the exposure process time was reduced down to 12/12 sec for the positive photoresist DHK-BF424 (DongJin) over a bare silicon substrate. In addition, bottom anti-reflective coating (DUV-30J, Brewer Science) is applied to confirm improvements for line structures. The advantages and limitations of LIL are highlighted for generating nanoscale patterns.     

Painting with biomolecules at the nanoscale: biofunctionalization with tunable surface densities

We present a generic and flexible method to nanopattern biomolecules on surfaces. Carbon-containing nanofeatures are written at variable diameter and spacing by a focused electron beam on a poly(ethylene glycol) (PEG)-coated glass substrate. Proteins physisorb to the nanofeatures with remarkably high contrast factors of more than 1000 compared to the surrounding PEG surfaces. The biological activity of model proteins can be retained as shown by decorating avidin spots with biotinylated DNA, thereby underscoring the universality of the nano-biofunctionalized platform for the binding of other biotinylated ligands. In addition, biomolecule densities can be tuned over several orders of magnitude within the same array, as demonstrated by painting a microscale image with nanoscale pixels. We expect that these unique advantages open up entirely new ways to design biophysical experiments, for instance, on cells that respond to the nanoscale densities of activating molecules.     

Simulation of grafted polymers on nanopatterned surfaces

Structural properties of polymer brushes on nanopatterned surfaces in good solvent have been determined by computer simulations. Scaling relations for the brush height and brush width are proposed. The properties of the central part of the patterned brush remain constant as long as the pattern is wider than a few times the brush height. The results agree qualitatively with recent AFM experiments, but some quantitative differences call for a reassessment of experimental procedures.     

Separation of DNA with different configurations on flat and nanopatterned surfaces

We demonstrate that electrophoresis on a flat Si substrate is an effective method in separation of DNA with different configurations, e.g., linear, supercoiled, and relaxed or DNA of different length, e.g., supercoiled DNA ladder. The surface separation arises from the different number of contacts due to the conformational differences between adsorbed DNA chains. Imposing a Au nanopattern on the Si surface further improves the separation effect. The simulation of electric field on this patterned surface by the finite element method shows that Au nanodots act as local pinning points for DNA segments due to dielectrophoretic force. The results of molecular dynamics simulation showed that the conformational differences between adsorbed polymer chains were amplified on the patterned surface and enhanced separations were achieved, which are consistent with the experimental results.     

Use of nanopatterned surfaces to enhance immunoreaction efficiency

In this work, we compare the immunoreaction efficiency between uniformly functionalized surface and chemically nanopatterned surfaces when applied as platforms for antigen/antibody interactions with and without the use of protein A as orienting protein. On the nanopatterned platform, the immunoreaction efficiency is higher than all the other cases with no protein A pretreatment of the surface, providing evidence of the capability of the adhesive/antiadhesive nanopatterned surface to immobilize the molecules in a reactive state, increasing their possibility to form complexes.     

The fabrication of tunable nanoporous oxide surfaces by block copolymer lithography and atomic layer deposition

Patterned nanoscale materials with controllable characteristic feature sizes and periodicity are of considerable interest in a wide range of fields, with various possible applications ranging from biomedical to nanoelectronic devices. Block-copolymer (BC)-based lithography is a powerful tool for the fabrication of uniform, densely spaced nanometer-scale features over large areas. Following this bottom-up approach, nanoporous polymeric films can be deposited on any type of substrate. The nanoporous periodic template can be transferred to the underlying substrate by dry anisotropic etching. Nevertheless the physical sizes of the polymeric mask represent an important limitation in the implementation of suitable lithographic protocols based on BC technology, since the diameter and the center-to-center distance of the pores cannot be varied independently in this class of materials. This problem could be overcome by combining block copolymer technology with atomic layer deposition (ALD): by means of BC-based lithography a nanoporous SiO2 template, with well-reproducible characteristic dimensions, can be fabricated and subsequently used as a backbone for the growth of perfectly conformal thin oxide films by ALD. In this work polystyrene-b-poly(methylmethacrylate) (PS-b-PMMA) BC and reactive ion etching are used to fabricate hexagonally packed 23 nm wide nanopores in a 50 nm thick SiO2 matrix. By ALD deposition of Al2O3 thin films onto the nanoporous SiO2 templates, nanostructured Al2O3 surfaces are obtained. By properly adjusting the thickness of the Al2O3 film the dimension of the pores in the oxide films is progressively reduced, with nanometer precision, from the original size down to complete filling of the pores, thus providing a simple and fast strategy for the fabrication of nanoporous Al2O3 surfaces with well-controllable feature size.     

Conjugation of active iron superoxide dismutase to nanopatterned surfaces

Superoxide dismutase enzymes (SODs) are an essential part of the first line of cellular defense system against free radicals species. They catalyze the dismutation of superoxide radicals into oxygen and hydrogen peroxide. Although several studies have examined the attachment of superoxide dismutases to nanoparticles and nanostructures, never has been used a member of the Fe/MnSOD family. In this study, the behavior of plant origin FeSOD enzyme on three different nanopatterned surfaces was investigated as a function of covalent and electrostatic binding. Fluorescence microscopy was used to demonstrate that the protein is attached only to the gold layer. We also examined the activity of SOD by a colorimetric assay, and we have shown that the enzyme remains active after attachment to the three different surfaces under both kind of binding (electrostatic and covalent). This methodology could be useful for those who want to functionalize nanostructures with a SOD enzyme and test the activity. This process could be of great interest for the development of peroxynitrite and superoxide biosensors.     

Arrays of nanoscale lenses for subwavelength optical lithography

Poly(ethylene glycol) (PEG) polymer lens arrays are made by using dip-pen nanolithography to deposit nanoscale PEG features on hydrophobically modified quartz glass. The dimensions of the PEG lenses are controlled by tuning dwell time and polymer molecular weight. The PEG polymer lenses on the quartz substrate act as a phase-shift photomask for fabricating subwavelength scale features, ～ 100 nm in width. Depending upon UV irradiation time during the photolithography, the photoresist nanostructures can be transitioned from well-shaped (short time) to ring-shaped (long time) features. The technique can be used to pattern large areas through the use of cantilever arrays.     

Additive lithography for fabrication of diffractive optics

An innovative fabrication technique is introduced that is based on multiple-exposure techniques for micro-optics fabrication. This method utilizes various exposure times and combinations of binary and analog photo masks to sculpture complex photoresist profiles. It also demonstrates the fabrication of analog structures from the multilevel structures thus formed by using resist reflow.     

Nanoimprint lithography patterns with a vertically aligned nanoscale tubular carbon structure

A nanoscale tubular carbon structure array was demonstrated as a mold for nanoimprint lithography (NIL), in which a vertically formed and hexagonally aligned nanoscale tubular carbon array was fabricated through carbon growth inside an anodic aluminum oxide (AAO) nanotemplate, followed by controlled chemical etching of the AAO layer. High density (over 10(10)?cm(-2)) of the nanoscale carbon pillars with their controlled diameters and protruded lengths was inversely replicated onto a UV-curable resist for the first time using the imprinting lithography technique.     

Cellular unbinding forces of initial adhesion processes on nanopatterned surfaces probed with magnetic tweezers

To study the dependence of unbinding forces on the distance of molecularly defined and integrin specific c(-RGDfK-) ligand patches in initial cellular adhesion processes, we developed a magnetic tweezers setup for applying vertical forces of up to 200 pN to rat embryonic fibroblasts. The ligand patch distance is controlled with a hexagonally close packed pattern of biofunctionalized gold nanoparticles prepared by block-copolymer micelle nanolithography. Each gold nanoparticle potentially activates up to one alpha(v)beta(3)-integrin. The distances between the gold nanoparticles determine the separation of individual integrins and thus the assembly of integrin clusters. The results show an increase in cellular unbinding forces from approximately 6 to more than 200 pN for a decreasing ligand distance of 145 to 58 nm after 5 min of cell adhesion. Furthermore, we observe a strong dependence on adhesion time during the first 10 min of cell surface contact suggesting an active, cooperative cell response that is controlled by the spacing between individually activated integrins.     

Transforming the fabrication and biofunctionalization of gold nanoelectrode arrays into versatile electrochemical glucose biosensors

High-density arrays of conducting nanoelectrodes (i.e., nanoelectrode arrays [NEAs]) have been developed on the surface of a single electrode for numerous electrochemical sensing paradigms. However, a scalable fabrication technique and robust biofunctionalization protocol are oftentimes lacking and thus many NEA designs have limited efficacy and overall commercial viability in biosensing applications. In this report, we develop a lithography-free nanofabrication protocol to create large arrays of Au nanoelectrodes on a silicon wafer via a porous anodic alumina template. To demonstrate their effectiveness as electrochemical glucose biosensors, alkanethiol self-assembled monolayers (SAMs) are used to covalently attach the enzyme glucose oxidase to the Au NEA surface for subsequent glucose sensing. The sensitivity and linear sensing range of the biosensor is controlled by introducing higher concentrations of long-chain SAMs (11-mercaptoundecanoic acid: MUA) with short-chain SAMs (3-mercaptopropionic acid: MPA) into the enzyme immobilization scheme. This facile NEA fabrication protocol (that is well-suited for integration into electronic devices) and biosensor performance controllability (via the mixed-length enzyme-conjugated SAMs) transforms the Au NEAs into versatile glucose biosensors. Thus these Au NEAs could potentially be used in important real-word applications such as in health-care and bioenergy where biosensors with very distinct sensing capabilities are needed.     

Fabrication of TiO2 memristive arrays by step and flash imprint lithography

Identical patterns and characteristics of sub-100 nm TiO2-based memristive systems on 4 inch silicon substrates were demonstrated using Step and flash imprint lithography (SFIL). SFIL is a nanoimprint lithography technique that offers the advantagess of a high aspect-ratio, reliable nano-patterns, and a transparent stamp that can be used to facilitate overlay techniques. The overlay process from the alignment system in IMPRIO 100 was appropriate for the fabrication of nanoscale crossbar arrays in this study. High-density crossbar arrays that consisted of TiO2 resistive switching material that was sandwiched between Pt electrodes with a width of 80 nm and a half-pitch of 100 nm were in turn replicated through successive imprinting and etching processes. The use of the direct metal etching process enhanced the uniformity of the TiO2/Pt interface. The electrical property of the crossbar arrays showed the bipolar switching behavior that resulted in the application of the nonvolatile resistive memory.     

Molding and replication of ceramic surfaces with nanoscale resolution

The design of reproducible and more efficient nanofabrication routes has become a very active research field in recent years. In particular, the development of new methods for micro- and nanopatterning materials surfaces has attracted the attention of many researchers in industry and academia as a consequence of the growing relevance of patterned surfaces in many technological fields, ranging from optoelectronics to biotechnology. In this work we explore, discuss, and demonstrate the possibility of extending the well-known molding and replication strategy for patterning ceramic materials with nanoscale resolution. To achieve this goal we have combined physical deposition methods, molecule-thick anti-sticking coatings, and nanostructured substrates as master surfaces. This new perspective on an "old technology", as molding is, provides an interesting alternative for high-resolution, direct surface-relief patterning of materials that currently requires expensive and time-consuming lithographic approaches.     

Quantitative characterizations of a nanopatterned bonded wafer: force determination for nanoimprint lithography stamp removal

The nanoimprint lithography process consists of two mechanical steps: molding and stamp removal. While many publications dealing with anti-sticking layer properties or the understanding of polymer flow during imprinting have recently been published, only a few studies have been carried out to deeply characterize the demolding step. Regarding the small amount of theoretical work dedicated to this issue, in this paper both experimental and first theoretical approaches are proposed to characterize the demolding process in a peeling scheme. Full 200?mm stamp and imprinted wafers were used to identify the experimental limitation of such a full wafer peeling demolding scheme. A rectangular stamp and substrate samples with or without nanoscale features combined with an augmented beam theory are proposed to extract quantitative data for the required demolding force as well as the friction stress along the feature sidewall. Therefore both adhesion and friction forces were characterized on single stamp structures.     

Flip-chip fabrication of nanoscale co-planar embedded electrodes with controlled exposed areas

We discuss the fabrication of closely spaced nanoscale embedded co-planar electrodes with concealed contact wires, using a GaAs-based flip-chip technology. The co-planarity of the electrodes with the substrate and the low roughness of the exposed surface are achieved by templating the deposition of both the dielectric and electrode metal onto a smooth GaAs substrate. The resulting electrodes, with sizes of around 300 nm and separations as low as 25 nm, have RMS roughnesses of less than 0.2 nm and a co-planarity of around 1 nm.     

Pattern-definable and low cost fabrication of nanopatterned conducting polymer film on flexible substrates

This study reports the pattern definable and low cost fabrication of nanopatterned conducting polymer film on flexible substrates. Noble nanopatterned polymer hard template was fabricated by using nanoimprint lithography (NIL) and used for electropolymerization of conducting polymer. Conducting polymer was electrochemically deposited on the template and transferred over to flexible substrates. Eventually conducting polymer films with various nanopatterns were fabricated on flexible substrates. High pattern definability was achieved by nanoimprinted polymer template, which was molded from lithographically fabricated stamp. Low cost fabrication was accomplished due to low cost NIL, reusable polymer templates, and low material consumption of electrodeposition. The electrodeposited films were transferred using double sided tape. Because the templates are made of flexible polymer, the transfer bonding method applied in this study is adaptable to both wafers and flexible polymer substrates. The fabricated nanopatterned conducting polymer film can be applied to gas sensors, super capacitors, super wetting films, and neuron interfaces due to its characteristic of high surface to volume. For an illustrative application, the gas sensing properties of films were tested. The result showed enhanced sensing characteristic with nanopatterned film, which are attributed to the high surface to volume ratio of nanopatterned films.     

Pattern-integrated interference lithography: single-exposure fabrication of photonic-crystal structures

Multibeam interference represents an approach for producing one-, two-, and three-dimensional periodic optical-intensity distributions with submicrometer features and periodicities. Accordingly, interference lithography (IL) has been used in a wide variety of applications, typically requiring additional lithographic steps to modify the periodic interference pattern and create integrated functional elements. In the present work, pattern-integrated interference lithography (PIIL) is introduced. PIIL is the integration of superposed pattern imaging with IL. Then a pattern-integrated interference exposure system (PIIES) is presented that implements PIIL by incorporating a projection imaging capability in a novel three-beam interference configuration. The purpose of this system is to fabricate, in a single-exposure step, a two-dimensional periodic photonic-crystal lattice with nonperiodic functional elements integrated into the periodic pattern. The design of the basic system is presented along with a model that simulates the resulting optical-intensity distribution at the system sample plane where the three beams simultaneously interfere and integrate a superposed image of the projected mask pattern. Appropriate performance metrics are defined in order to quantify the characteristics of the resulting photonic-crystal structure. These intensity and lattice-vector metrics differ markedly from the metrics used to evaluate traditional photolithographic imaging systems. Simulation and experimental results are presented that demonstrate the fabrication of example photonic-crystal structures in a single-exposure step. Example well-defined photonic-crystal structures exhibiting favorable intensity and lattice-vector metrics demonstrate the potential of PIIL for fabricating dense integrated optical circuits.