Functionally decoupled soft lithography for patterning polymer brushes

Easy soft imprint nanolithography (ESINL) is employed in the patterning of multiple olymer brushes. This new approach to soft lithography is found to be uniquely effective at patterning brushes both prior to and subsequent to grafting of the polymer chains. Silicon substrates are grafted with polystyrene, polymethylmethacrylate, and polyhydroxyethylmethacrylate using surface-initiated atom transfer radical polymerization assisted by activators generated by electron transfer (ARGET-ATRP) and characterized by contact angle measurements, infrared spectroscopy, and ellipsometry. Line grating features of 3 cm × 3 cm with critical dimensions in the range of 410-480 nm are imprinted directly over grafted brush layers or over assembled monolayers of initiator molecules and transferred to the active layer by reactive ion etching. In all cases the grating pattern is accurately reproduced in the brush layer as confirmed by atomic force microscopy, demonstrating the capability of the technique to generate large-area nanoscale patterns on a range of surface types and functionalities.     

Spatial patterning of colloidal nanoparticle-based thin film by a combinative technique of layer-by-layer self-assembly and lithography

A novel approach to generating clear patterns of different types of nanoparticles is presented in this paper. Nanoassembly in the vertical direction was combined with planar micropatterning. This provides industrial applications of a popular layer-by-layer method to produce multilayers of polymers, nanoparticles, and proteins organized on the nanometer scale. A thin film of organic polystyrene spheres was first coated on the pretreated silicon wafer with layer-by-layer self-assembly. Then a layer of aluminium was deposited on the thin film. A layer of positive photoresist was spun on the surface of aluminum and then illuminated with UV light. The exposed parts of the resist were removed and windows were opened above the aluminum. The subsequent etching removed exposed aluminium and left a polystyrene thin film in the open windows. Oxygen plasma was employed to remove the polystyrene thin film on the bottom. Eventually, aluminum and photoresist were removed and only the desired pattern remained. This approach was also employed for the patterning of the silica nanoparticle thin film, a widely used material in various applications. In this case, wet etching was demonstrated to etch silica particles. A scanning electron microscope was used to produce the image of the pattern.     

Patterned self-assembly of gold nanoparticles on chemical templates fabricated by soft UV nanoimprint lithography

Chemical templates for the patterned immobilization of gold nanoparticles were fabricated by soft UV nanoimprint lithography. The template structures were fabricated by means of the consecutively performed process steps of nanoimprint lithography, reactive ion etching, chemical functionalization with amino groups, and lift-off of imprint resist. These chemical templates were used for the defined assembly of 20 nm diameter citrate stabilized gold nanoparticles from aqueous solution. By reducing the ionic strength of the solution, one- and zero-dimensional particle assemblies were generated on sub-100-nm template structures. By this means, the pattern resolution predefined by the lithography process could be easily enhanced by dilution of the nanoparticle solution.     

Formation of isolated carbon nanofibers with hot-wire CVD using nanosphere lithography as catalyst patterning technique

Recently the site-density control of carbon nanotubes (CNTs) has attracted much attention as this has become critical for its many applications. To obtain an ordered array of catalyst nanoparticles with good monodispersity nanosphere lithography (NSL) is used. These nanoparticles are tested as catalyst sites in hot-wire chemical vapor deposition (HWCVD) of carbon nanostructures. Aside from using NSL also nickel (Ni) nano-islands are made by thermal annealing of a thin Ni film and tested as catalyst sites. Multiwall CNTs, isolated carbon nanofibres, and other nanostructures have been deposited using HWCVD. Tungsten filaments held at ~ 2000 °C are used to decompose a mixture of ammonia, methane and hydrogen. The structures have been characterized with Scanning Electron Microscopy, High Resolution Transmission Electron Microscopy, Raman spectroscopy and Rutherford Backscattering Spectroscopy.     

Fabrication of mid-infrared frequency-selective surfaces by soft lithography

We describe the fabrication of large areas (4 cm(2)) of metallic structures or aperture elements that have ~100-350-nm linewidths and act as frequency-selective surfaces. These structures are fabricated with a type of soft lithography-near-field contact-mode photolithography-that uses a thin elastomeric mask having topography on its surface and is in conformal contact with a layer of photoresist. The mask acts as an optical element to create minima in the intensity of light delivered to the photoresist. Depending on the type of photoresist used, lines of, or trenches in, photoresist are formed on the substrate by exposure, development, and lift-off. These surfaces act as bandpass or bandgap filters in the infrared.     

Programmable chemical gradient patterns by soft grayscale lithography

A method for fabricating chemical gradients on planar and nonplanar substrates using grayscale lithography is reported. Compliant grayscale amplitude masks are fabricated using a vacuum-assisted microfluidic filling protocol that employs dilutions of a carbon-black-containing polydimethylsiloxane emulsion (bPDMS) within traditional clear PDMS (cPDMS) to create planar, fully self-supporting mask elements. The mask is then placed over a surface functionalized with a hydrophobic coumarin-based photocleavable monolayer, which exposes a polar group upon irradiation. The mask serves to modulate the intensity of incident UV light, thereby controlling the density of molecules cleaved. The resulting molecular-level grayscale patterns are characterized by condensation microscopy and imaging mode time-of-flight secondary-ion mass spectrometry (ToF-SIMS). Due to the inherent flexibility of this technique, the photofuse as well as the gradient patterns can be designed for a wide range of applications; in this paper two proof-of-concept demonstrations are shown. The first utilizes the ability to control the resulting contact angle of the surface for the fabrication of a passive pressure-sensitive microfluidic gating system. The second is a model surface modification process that utilizes the functional groups deprotected during the photocleavage to pattern the deposition of moieties with complementary chemistry. The spatial layout, resolution, and concentration of these covalently linked molecules follow the gradient pattern created by the grayscale mask during exposure. The programmable chemical gradient fabrication scheme presented in this work allows explicit engineering of both surface properties that dictate nonspecific interactions (surface energy, charge, etc.) and functional chemistry necessary for covalent bonding.     

Application of scanning probe lithography to graphite patterning

We applied the scanning probe lithographic technique to a graphite patterning in air and analyzed the patterned sample with the lateral force microscopy and Raman spectroscopy. The local electric field generated from a tip caused either etching or oxidization depending on the electric field intensity in air. We have found that the frictional force between the tip and local oxidized graphite surface was increased remarkably from lateral force analysis. Also, it was found that the graphene layer was peeled from the graphite surface in the etching process, which could be a potential tool as a top-down nano-fabrication process for the graphene nano device without contamination.     

Novel DNA nano-patterning design method utilizing poly-L-lysine patterning by nanoimprint lithography

The DNA properties have been deliberated to comprehend new functional bio-devices. However there are few reports on DNA nano-patterning to make them. Therefore, we have tried DNA nano-patterning by using a nanoimprint process. A substrate coated with a poly-L-lysine was heated at 120 degrees C for 5 min in atmosphere, and nanoimprint carried out at 120 degrees C, 6 MPa, 5 min, and we applied 1 mg/ml DNA solution on the substrate and immobilized with the poly-L-lysine. Finally the substrate was washed at twice in water and at once in hot water intensely. DNA lines that consist of lines at about 700 nm as line width was obtain, and the very fine lines correspond to convex patterns of the mold surface. These results suggest that the imprint would make the poly-L-lysine reform by exposing it under high pressure and high temperature. Therefore since DNA is immobilized with the amino group in the poly-L-lysine, a lot of amino group would expose on the surface by the imprint from the balk, and would be patterned DNA.     

Nanoscale patterning on insulating substrates by critical energy electron beam lithography

This Letter describes a method to generate nanometer scale patterns on insulating substrates and wide band gap materials using critical energy electron beam lithography. By operating at the critical energy (E2) where a charge balance between incoming and outgoing electrons leaves the surface neutral, charge-induced pattern distortions typically seen in e-beam lithography on insulators were practically eliminated. This removes the need for conductive dissipation layers or differentially pumped e-beam columns with sophisticated gas delivery systems to control charging effects. Using a "scan square" method to find the critical energy, sub-100 nm features in 65 nm thick poly(methyl methacrylate) on glass were achieved at area doses as low as 10 microC/cm2 at E2 = 1.3 keV. This method has potential applications in high-density biochips, flexible electronics, and optoelectronics and may improve the fidelity of low voltage e-beam lithography for parallel microcolumn arrays.     

Focused ion beam lithography and anodization combined nanopore patterning

In this study, focused ion beam lithography and anodization are combined to create different nanopore patterns. Uniform-, alternating-, and gradient-sized shallow nanopore arrays are first made on high purity aluminum by focused ion beam lithography. These shallow pore arrays are then used as pore initiation sites during anodization by different electrolytes. Depending on the nature of the anodization electrolyte, the nanopore patterns by focused ion beam lithography play different roles in further pore development during anodization. The pore-to-pore distance by focused ion beam lithography should match with that by anodization for guided pore development to be effective. Ordered and heterogeneous nanopore arrays are obtained by the focused ion beam lithography and anodization combined approach.     

A new chemically amplified resist for high resolution patterning by E-beam lithography

In this work, we have undertaken evaluation of the lithography property of a recently available chemically amplified resist (CAR) resist, UV1116 supplied by Rohm and Haas Company. Systematic study of the EBL property such as sensitivity, contrast, high resolution limit and dense capability, as well as resistance to plasma dry etching has been carried out. In comparison with the performance of UVIII, we conclude that the UV1116 can be a good alternative with better lithography quality.     

Polymer nanocomposites processed via self-assembly through introduction of nanoparticles,nanosheets, and nanofibers

Nanocomposites offer the theoretical potential to achieve mechanical properties surpassing those of conventional (micro-scale) composites. The underlying reasons for the high potential of nanocomposites include the uniquely high mechanical attributes of nano-scale reinforcement, effective control of defect size and growth by nano-spaced interfaces, and interactions between the polymer matrix and the large surface areas of nanomaterials. Attempts to produce nanocomposites via conventional processing techniques have encountered challenges associated with thorough dispersion and effective interfacial interactions of nano-scale reinforcement with the polymer matrix. In order to address these challenges, materials were processed into polymer nanocomposites via electrostatically driven layer-by-layer self-assembly. Electrostatically dispersed nanomaterials and oppositely charged polyelectrolytes were sequentially built upon a substrate (cellular scaffold). The self-assembled nanocomposites, after complementary cross-linking, provided a unique balance of strength and ductility, which surpassed those of conventional (micro-scale) composites. Self-assembly was found to be an effective approach to producing nanocomposites embodying uniformly dispersed nanomaterials with controlled interfacial interactions. This approach is highly versatile and enables introduction of diverse nanomaterials into polymer nanocomposites. The work reported herein evaluated introduction of diverse categories of nanomaterials incorporating nanoparticles, nanosheets, nanotubes, and nanofibers. This investigation also evaluated the potential for a biomimetic approach to processing of light-weight structural systems by self-assembly of polymer nanocomposites onto cellular scaffolds.     

Hierarchical, interface-induced self-assembly of diphenylalanine: formation of peptide nanofibers and microvesicles

To gain insight into the hierarchical self-assembly of peptides and the surface effect on assembly formation, an aromatic peptide of diphenylalanine (FF) was used in this study as the model peptide. We found that the diphenylalanine peptide could self-assemble into a core-branched nanostructure through non-covalent interactions in aqueous solution. The pre-assemblies further assembled into nanofibers and microvesicles on the glass surface and microporous membrane, respectively, showing a significant dependence on surface characteristics. The structural and morphological differences between nanofibers and microvesicles were investigated directly using several spectroscopy and microscopy techniques. Our results revealed a hierarchical and interface-induced assembly behavior of diphenylalanine peptide. The novel strategy based on the surface effect allows one to controllably fabricate various peptide-based nanostructures.     

Water-inducing molecular self-assembly of amphiphilic molecules into nanofibers

We present investigations on the microcosmic self-assembly process of new synthesized amphiphilic TPDP molecules. It can be seen that pure TPDP nanofibers with smooth surfaces can be obtained by reprecipitation method using ethanol as good solvent and water as poor solvent. In the self-assembly process, during the water adding to the amphiphilic molecules’ saturated solution, the amphiphilic molecules firstly assembled into needle-like small rods. With an increase in the self-assembled time, a large number of the nanofibers are produced. The assembly behavior was revealed in the course of direct in situ monitoring of its growth with optical microscopy. Field emission scanning electron microscopy was adopted to characterize the morphologies of the products.     

Directed self-assembly lithography and its application based on simulation approach

The directed self-assembly (DSA) technology of block copolymers is a candidate for next-generation lithography. The computation model of this lithography can assist in overcoming its challenges. In this paper, a template-assisted self-assembly (DSA graphoepitaxy) is modeled and simulated in a molecular scale to reduce the process complexity. For a full simulation, the template fabrication and DSA process are modeled and simulated to evaluate the impact of the process parameters on the pattern profile. Simulation results similar to the experimental results allow the prediction of the self-assembled patterns of the confined block polymers.     

Bond-detach lithography: a method for micro/nanolithography by precision PDMS patterning

We have discovered a micro/nanopatterning technique based on the patterning of a PDMS membrane/film, which involves bonding a PDMS structure/stamp (that has the desired patterns) to a PDMS film. The technique, which we call "bond-detach lithography", was demonstrated (in conjunction with other microfabrication techniques) by transferring several micro- and nanoscale patterns onto a variety of substrates. Bond-detach lithography is a parallel process technique in which a master mold can be used many times, and is particularly simple and inexpensive.     

Thin films of two functional oxides patterned laterally by soft lithography

Thin films of two laterally patterned functional oxides of uniform thickness were obtained in a two-step soft-lithographic micromolding process. CoFe(2)O(4)/ZnO and CoFe(2)O(4)/BaTiO(3) dual-phase patterns were fabricated. The films showed good replication of the pattern that was defined in the first patterning step. X-ray diffraction showed that the films consisted of two distinct phases, and magnetic force microscopy showed that the compounds were laterally separated, the separation pattern being the same as that of the initial soft-lithographic process. The films exhibited slight height variations near the edges of the phases, which were introduced in the first deposition step and were not fully compensated in the second deposition step. The films are sufficiently smooth to allow fabrication of multilayer structures.     

Self-organized meso-patterning of soft solids by controlled adhesion: elastic contact lithography

The surface of a soft elastic film becomes unstable and deforms when a rigid flat plate is brought into its contact proximity, without application of any external pressure. These isotropic undulations have a characteristic wavelength, lambda approximately 3H, where H is the film thickness. The wavelength is independent of the adhesive interactions and the mechanical properties of the film. We present here a mini-review of our recent work on techniques of aligning, modulating, and ordering the instability structures by the use of simple 1-D patterned stamps, by changing the stamp-surface separation, by slow shearing of a flat stamp and by confining the instability in soft narrow channels. The generality of the technique for different soft materials is illustrated by patterning cross-linked polydimethylsiloxane (PDMS), aluminum coated PDMS and hydrogels films. Use of a flexible stamp such as a metal foil provides enhanced conformal contact by adhesive forces, which aids large area patterning without critically maintaining a parallel configuration and uniform pressure between the stamp and the film. The technique has the potential to develop into a new soft lithography tool--"Elastic Contact Lithography" suitable for rapid, large area micron and sub-micron self-organized patterning of a variety of soft materials without any special equipments.