Fabrication of high aspect ratio nanostructures using capillary force lithography

A new ultraviolet (UV) curable mold consisting of functionalized polyurethane with acrylate group (MINS101m, Minuta Tech.) has recently been introduced as an alternative to replace polydimethylsiloxane (PDMS) mold for sub-100-nm lithography. Here, we demonstrate that this mold allows for fabrication of various high aspect ratio nanostructures with an aspect ratio as high as 4.4 for 80 nm nanopillars. For the patterning method, we used capillary force lithography (CFL) involving direct placement of a polyurethane acrylate mold onto a spin-coated polymer film followed by raising the temperature above the glass transition temperature of the polymer (T_g). For the patterning materials, thermoplastic resins such as polystyrene (PS) and poly(methyl methacrylate) (PMMA) and a zinc oxide (ZnO) precursor were used. For the polymer, micro/nanoscale hierarchical structures were fabricated by using sequential application of the same method, which is potentially useful for mimicking functional surfaces such as lotus leaf.     

Simultaneous fabrication of line defects-embedded periodic lattice by topographically assisted holographic lithography

We have demonstrated simultaneous fabrication of designed defects within a periodic structure. For rapid fabrication of periodic structures incorporating nanoscale line-defects at large area, topographically assisted holographic lithography (TAHL) technique, combining the strength of hologram lithography and phase-shift interference, was proposed. Hot-embossing method generated the photoresist patterns with vertical side walls which enabled phase-shift mask effect at the edge of patterns. Embossing temperature and relief height were crucial parameters for the successful TAHL process. Periodic holes with a diameter of 600 nm at a 1 μm-pitch incorporating 250 nm wide line-defects were obtained simultaneously.     

毛细作用力法制备二维有序聚(苯乙烯-N-异丙基丙烯酰胺)膜

微波辐射条件下,苯乙烯(St)和N-异丙基丙烯酰胺(NIPAAm)经无皂乳液聚合,制得单分散聚(苯乙烯-N-异丙基丙烯酰胺)微球;用扫描镜(SEM)观察了微球表面形态和大小;将一定浓度的微球分散液通过毛细力作用,在玻璃基片上制备二维有序聚合物膜,并用原子力显微镜(AFM)探针表征二维薄膜的形态。     

Patterned conductive polyaniline films fabricated using lithography and in situ polymerization

In this article, we describe a novel bottom‐up technique for the preparation of transparent conductive films of polyaniline (PANI). A UV‐curable photoresist was formulated containing an acrylate‐endcapped urethane oligomer [UA(PPG400)], acrylic acid, a photoinitiator, and a reactive diluent (tripropylene glycol diacrylate), and the lithography techniques were used to pattern the structure with line widths/spaces of 100 μm/100 μm, 10 μm/10 μm, and 5 μm/5 μm on a polyethylene terephthalate substrate. The carboxylic acid units on the surface of the patterned photoresist interacted with the aniline monomer units to form anilinium complexes; using ammonium persulfate as a chemical oxidant, we then synthesized a layer of conductive PANI on the surface of the patterned resist through in situ polymerization. The optimal conductivity of the PANI conductive film was ca. 10 S/cm. The thin film was characterized, and its physical properties investigated using Fourier transform infrared spectroscopy, UV–Vis spectroscopy, differential scanning calorimetry, optical and atomic force microscopy, and four‐point probe conductivity measurements. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Optimization of the electron‐beam‐lithography parameters for the moth‐eye effects of an antireflection matrix structure

In this study, we mainly used the characteristics of electron‐beam lithography in measurement control and direct‐write technology to improve the physical restrictions and production processes of optical lithography and other nanopattern production methods. We did this by using a silicon wafer as a substrate, coating a negative‐tone photoresist, and using scattering and the reflection produced by the collision of an electron beam with the wafer lattice and the proximity effect of a secondary electron inside the electron‐beam photoresist to produce an antireflection matrix structure with a moth‐eye effect. In addition, we used the Taguchi quality method with an orthogonal array to plan the experiment and the signal‐to‐noise ratio to analyze the experimental data, and in the experimental process, we produced a full factorial equivalent experiment, using very few experiment repetitions and deriving optimum conditions. Also, we used back‐propagation neural networks to fine‐tune significant factors, allowing the production of the deepest process control parameters and thereby imparting to the antireflection matrix structure the best effect. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5303–5313, 2006     

Fabrication of platinum nanoparticles and nanowires by electron beam lithography (EBL) and nanoimprint lithography (NIL): comparison of ethylene hydrogenation kinetics

Electron beam lithography (EBL), size reduction lithography (SRL), and nanoimprint lithography (NIL) have been utilized to produce platinum nanoparticles and nanowires in the 20–60-nm size range on oxide films (SiO₂ and Al₂O₃) deposited onto silicon wafers. A combination of characterization techniques (SEM, AFM, XPS, AES) has been used to determine size, spatial arrangement and cleanliness of these fabricated catalysts. Ethylene hydrogenation reaction studies have been carried out over these fabricated catalysts and have revealed major differences in turnover rates and activation energies of the different nanostructures when clean and when poisoned with carbon monoxide. The oxide-metal interfaces are implicated as important reaction sites that remain active when the metal sites are poisoned by adsorbed carbon monoxide.     

基于天然高分子的可再生光刻材料及其光刻机理的研究进展

基于天然高分子的新型光刻材料与石油原料的传统光刻材料相比,不仅保留了高分辨率的光刻性能,还具有绿色可再生和无毒显影等优点。本文综述了天然高分子光刻材料中的蛋白类光刻材料和多糖类光刻材料,系统的总结了各种天然高分子光刻材料的优缺点。通过对不同材料光刻机理的深入研究,得出蛋白质光刻材料的光刻机理主要依靠辐射改变蛋白结构,使其溶解度在显影液中发生改变实现光刻;而天然多糖类光刻材料则主要依赖引入光响应基团实现光刻。最后本文对基于天然高分子的可再生光刻材料的现存问题进行了分析并对发展前景做出了展望。     

Meniscus behaviors and capillary pressures in capillary channels having various cross-sectional geometries

A numerical study has been conducted to simulate the liquid/gas interface (meniscus) behaviors and capillary pressures in various capillary channels using the volume of fluid (VOF) method. Calculations are performed for four channels whose cross-sectional shapes are circle, regular hexagon, square and equilateral triangle and for four solid/liquid contact angles of 30°, 60°, 120° and 150°. No calculation is needed for the contact angle of 90° because the liquid/gas interface in this case can be thought to be a plane surface. In the calculations, the liquid/gas interface in each channel is assumed to have a flat surface at the initial time, it changes towards its due shape thereafter, which is induced by the combined action of the surface tension and contact angle. After experiencing a period of damped oscillation, it stabilizes at a certain geometry. The interface dynamics and capillary pressures are compared among different channels under three categories including the equal inscribed circle radius, equal area, and equal circumscribed circle radius. The capillary pressure in the circular channel obtained from the simulation agrees well with that given by the Young–Laplace equation, supporting the reliability of the numerical model. The channels with equal inscribed circle radius yield the closest capillary pressures, while those with equal circumscribed circle radius give the most scattered capillary pressures, with those with equal area living in between. A correlation is developed to calculate the equivalent radius of a polygonal channel, which can be used to compute the capillary pressure in such a channel by combination with the Young–Laplace equation.     

Computational simulation of cold spray process assisted by electrostatic force

The integrated model of compressible thermofluid, splat formation and coating formation for the cold spray process has been established. In-flight behavior of nano-micro particles and the interaction between the shock wave and the particles in a supersonic jet flow impinging onto the substrate and further effect of electrostatic force on the particle acceleration are clarified in detail by carrying out a real-time computational simulation. The optimal particle diameters for an impinging particle velocity exceeding critical velocity exist. Particles with the diameter of submicron interact with shock wave and particles are decelerated prior to the impact. However, the particles can be accelerated considerably by utilizing electrostatic forces even in the presence of unavoidable shock waves. Finally, based on the integrated model, the coating thickness in an electrostatic assisted cold spray process is evaluated.     

Solvent effects on the elasticity of polysaccharide molecules in disordered and ordered states by single-molecule force spectroscopy

Single-molecule force spectroscopy, especially as implemented on an atomic force microscopy (AFM) platform is unique in its ability to apply small (F<10 pN) and large (F>1000 pN) stretching forces to individual polymer chains and in this way examines their elasticity and also reports force-induced conformational transitions in whole polymers and in their building blocks. In this paper, we briefly review recent applications of single-molecule force spectroscopy to the study of polysaccharides elasticity. We provide examples illustrating AFM measurements of solvent effects on the hydrogen bonding and the elasticity of individual polysaccharides and how molecular dynamics simulation can aid the interpretation of AFM results. We also discuss the use of single-molecule force spectroscopy in exploring ordered secondary structures of individual polysaccharide chains and their multi-strand complexes.     

Design and synthesis of new photoresist materials for ArF lithography

A new class of photoresist matrix polymers based on vinyl ether–maleic anhydride (VEMA) alternating copolymers was developed for ArF single‐layer lithography. These polymers were synthesized by copolymerization of alkyl vinyl ether and maleic anhydride alternating copolymers with acrylate derivatives containing bulky alicyclic acid‐labile protecting groups. The resulting polymers showed good control of polymerization and high transmittance. Also, these resists exhibited good adhesion to the substrate, high dry‐etching resistance against CF₄ mixture gas (1.02 times the etching rate of deep UV resist), and high selectivity to silicon oxide etching. Using an ArF excimer laser exposure system with 0.6 NA, 120‐nm L/S patterns were resolved under conventional illumination. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 165–170, 2004     

Superhard transparent hybrid nanocomposites for high fidelity UV-nanoimprint lithography

Transparent hyperbranched acrylate nanocomposites were produced using different combinations of silica nanoparticles and silicon-based sol–gel precursors. The nanocomposites were processed using a dual-cure UV polymerization and condensation scheme. The viscosity of hybrid suspensions was found to be one to two orders of magnitude lower than that of particulate composites with the same equivalent silica fraction. The Vickers microhardness of the polymer was 112 MPa. It was equal to 190 MPa and 148 MPa for the hybrid composites and particulate composites with 20 vol% SiO₂, respectively, and it was equal to 287 MPa for the hybrid material with 30 vol% SiO₂. Light-trapping textures in the form of random sub-micron pyramidal features were replicated in the hybrid composites from a nickel template using UV-nanoimprint lithography. After optimization of the dual-cure process sequence, a very high replication fidelity was obtained for all investigated compositions, leading to a haze above 99% over the visible light spectrum and a very effective light scattering performance in a broad angular exposure.     

Electron beam lithography with feedback using in situ self-developed resist

Due to the lack of feedback, conventional electron beam lithography (EBL) is a ‘blind’ open-loop process where the exposed pattern is examined only after ex situ resist development, which is too late for any improvement. Here, we report that self-developing nitrocellulose resist, for which the pattern shows up right after exposure without ex situ development, can be used as in situ feedback on the e-beam distortion and enlargement. We first exposed identical test pattern in nitrocellulose at different locations within the writing field; then, we examined in situ at high magnification the exposed patterns and adjusted the beam (notably working distance) accordingly. The process was repeated until we achieved a relatively uniform shape/size distribution of the exposed pattern across the entire writing field. Once the beam was optimized using nitrocellulose resist, under the same optimal condition, we exposed the common resist PMMA. We achieved approximately 80-nm resolution across the entire writing field of 1 mm × 1 mm, as compared to 210 nm without the beam optimization process.     

Solvent assisted post-polymerization of PET

We have examined the influence of the solvent mixture diphenyl ether-biphenyl (DPE-BP) on the post-polymerization of poly(ethylene terephthalate) (PET) of intrinsic viscosity (IV=0.42 dL/g) in the swollen state and in solution, by following the IV increase and the end-group depletion. During the swollen state polymerization (SwSP) of thin disks (180 μm) at 195 °C, the initially rapid step growth polymerization slows dramatically beyond IV=1.2 dL/g in 5 h, and is unable to proceed beyond 1.4 dL/g. This appears to be related to the temporarily restricted mobility of the end groups due to the observed solvent induced crystallization, because sufficient reactive end groups can be directly detected, and further post-polymerization in melt state is possible. When limitations due to crystallization are eliminated by carrying out post-polymerization in solution at 250 °C, it proceeds to IV=1.8 dL/g in a single step. Since solution polymerization eliminates the requirement of handling fine PET particles, it offers an attractive route to high molecular weight PET, particularly when the solution can be directly used for further processing, e.g. into fibers.     

Triphenylsulfonium salt methacrylate bound polymer resist for electron beam lithography

A new photoacid generator (PAG) bound polymer containing triphenylsulfonium salt methacrylate (TPSMA) was synthesized and characterized. The PAG bound polymer was employed to improve electron beam lithographic performance, including sensitivity and resolution. The PAG bound polymer resist exhibited a higher sensitivity (120 μC/cm²) than the PAG blend polymer resist (300 μC/cm²). Eliminating the post exposure baking process during development improved the resolution due to decreased acid diffusion. A high-resolution pattern fabricated by electron beam lithography had a line width of 15 nm and a high aspect ratio. The newly developed patterns functioned well as masks for transferring patterns on Si substrates by reactive ion etching.     

Electron irradiation of polymerisable langmuir-blodgett multilayers as model resists for electron-beam lithography

Amphiphilic molecules containing a polymerisable oxiran (epoxide) group have been assembled into Langmuir-Blodgett multilayers. Lines were drawn on the multilayers by exposure to an electron beam and subsequent development by rinsing in toluene. Line widths and heights were examined for various doses of electrons of energies 10, 5 and 2 kV. Generally, for the higher energies and shorter exposures, narrow well defined lines were obtained of height similar to the thickness of the original multilayers. Conversely, at lower energies and exposure times, troughs rather than lines were observed following incomplete development. The implications of these results are compared with theoretical predictions of bbnd forming and bond scission processes during electron irradiation of organic molecules.     

Wall slip and melt-fracture of polystyrene melts in capillary flow

We investigated slip and unstable flow phenomena of polystyrene melts in capillaries from the view of the effects of temperature and molecular weight by using three polystyrene samples with different molecular weights (M_w = 192,000, M_w = 258,000, and M_w = 321,000). The slip velocities are estimated by the Mooney method and the modified Mooney method. We found that the slip velocity increases and the critical slip stress above which a slip starts to occur decreases with the temperature. We also observed the melt-fracture at above a critical melt-fracture stress higher than . We found that the onset of melt fracture is affected by the extensional stress near the entry region to the capillary in the barrel and the melt-fracture tends to easily occur with increase of the molecular weight, but is not sensitive to the temperature.     

Scalable and number-controlled synthesis of carbon nanotubes by nanostencil lithography

Controlled synthesis and integration of carbon nanotubes (CNTs) remain important areas of study to develop practical carbon-based nanodevices. A method of controlling the number of CNTs synthesized depending on the size of the catalyst was characterized using nanostencil lithography, and the critical dimension for the nanoaperture produced on a stencil mask used for growing individual CNTs was studied. The stencil mask was fabricated as a nanoaperture array down to 40 nm in diameter on a low-stress silicon nitride membrane. An iron catalyst used to synthesize CNTs was deposited through submicron patterns in the stencil mask onto a silicon substrate, and the profile of the patterned iron catalyst was analyzed using atomic force microscopy. The feasibility toward a scalable, number-, and location-controlled synthesis of CNTs was experimentally demonstrated based on the diameter and geometry of the apertures in the stencil mask.     

Reconfigurable surface patterns on covalent adaptive network polymers using nanoimprint lithography

Covalent adaptable networks (CANs) are cross-linked polymers capable of relieving internal stresses by reconfiguring their network topology. Specifically, reversible addition-fragmentation chain transfer (RAFT) is a mechanism by which covalent bonds are exchanged to relieve internal stresses. While stress relaxation in such systems has been studied in bulk materials, here, we characterize the ability of RAFT-based CANs to undergo direct mechano-patterning of sub-micron surface topologies via nanoimprint lithography. The effects that different imprinting parameters and material properties have on the achievable surface pattern geometries are investigated, and we provide the first demonstration of permanent sub-micron surface reconfiguration on crosslinked polymers, which exhibit features orders of magnitude smaller than any previously published.     

Formation mechanism of SiGe nanorod arrays by combining nanosphere lithography and Au-assisted chemical etching

ABSTRACT: The formation mechanism of SiGe nanorod (NR) arrays fabricated by combining nanosphere lithography and Au-assisted chemical etching has been investigated. By precisely controlling the etching rate and time, the lengths of SiGe NRs can be tuned from 300 nm to 1 μm. The morphologies of SiGe NRs were found to change dramatically by varying the etching temperatures. We propose a mechanism involving a locally temperature-sensitive redox reaction to explain this strong temperature dependence of the morphologies of SiGe NRs. At a lower etching temperature, both corrosion reaction and Au-assisted etching process were kinetically impeded, whereas at a higher temperature, Au-assisted anisotropic etching dominated the formation of SiGe NRs. With transmission electron microscopy and scanning electron microscopy analyses, this study provides a beneficial scheme to design and fabricate low-dimensional SiGe-based nanostructures for possible applications.