Interferometric Lithography Research Articles

Nonlimiting Hydrogen Electrosorption Properties of Asymmetric Palladium Nanoparticle‐Modified Porous Carbon Electrodes

AbstractHydrogen electrosorption is studied on 3D porous carbon electrodes fabricated by interferometric lithography. Mixed acetonitrile/water solutions provided a route to electrodeposit Pd nanoparticles homogeneously throughout the structures. These monoliths exhibit hydrogen electrosorption profiles typical for limited volume electrodes and consequently are promising candidates for micro‐hydrogen based fuel cells. In contrast, aqueous solutions resulted in asymmetric Pd depositions due to the hydrophobic nature of the carbon electrode and its unique 3D porous structure. These electrodes exhibited unusual nonlimiting hydrogen electrosorption profiles.

Large area 3D helical photonic crystals

Three-dimensional helical photonic crystals are attractive for chiral metamaterial devices that mix electrical and magnetic responses. In chiral metamaterials, the refractive index for propagation of light along the axis of the helix is different as the handedness of the circular polarization is parallel or antiparallel to the chiral axis. The refractive index is increased for one circular polarization and reduced for the other, if the chirality is strong enough, negative refraction may occur. A novel interferometric lithography (IL) technique utilizing six separate two-beam exposures for fabricating three-dimensional helical photonic crystals is presented. In contrast to most previous demonstrations, which used a two-photon direct-write process, IL is a large-area process readily adaptable to realistic manufacturing constraints. This novel interferometric lithography uses only TE polarized light for maximum contrast and allows for independent dimensional control of the helix period along each lattice axis. Both mathematical models and experimentally realized three-dimensional helical photonic crystals (over a mm2 in area and up to 5 μm tall, with a helix spacing of 1.1 μm and a helix pitch of 1.9 μm on a hexagonal grid) are presented. The helical photonic crystals are formed as thick photoresist structures that can be subsequently used as mandrels for a sol-gel or metal electroforming processes, enabling a high index contrast chiral metamaterial. Optical transmission measurements of these helical photonic crystals are reported at the 1550 nm telecom wavelength.

Orienting the Demixion of a Diblock-copolymer Using 193 nm Interferometric Lithography for the Controlled Deposition of Nanoparticles

DUV interferometric lithography and diblock copolymer self-organization have successfully been combined to provide a simple and highly collective nanopatterning technique enabling the organization of nanoparticles over several orders of magnitude, from nanometre to millimetre. The nanostructural changes at the surface of the polymer film after thermal annealing have been monitored by AFM and the process parameters optimized for obtaining a long-range organization of the lamellar domains. In particular, the impact of the annealing conditions and geometric parameters of the substrate patterns have been investigated. The nanopatterns resulting from the lamellar demixion of (PS-b-MMA) were used for a controlled deposition of nanoparticles. The affinity of the hydrophobic particles for the PS block was demonstrated, opening new doors towards the preparation of high-density arrays of nanoparticles with potential applications in data storage.

Fabrication of biomimic GaAs subwavelength grating structures for broadband and angular-independent antireflection

We combine interferometric lithography and inductively coupled plasma etching to fabricate GaAs subwavelength grating (SWG) which mimics the moth eye structures. Through the modification of morphology parameters, including profile, height and packing fraction, tapered, high-aspect-ratio and closely-packed GaAs SWGs are obtained. The measurement of spectral reflectance of the fabricated SWGs shows that reflection has been dramatically reduced compared to the polished GaAs surface. Particularly, the optimized SWG structures exhibit an average reflection below 5% in the wavelengths ranging from 350 to 900 nm. Furthermore, the angular-independent property is demonstrated by measuring the reflectance versus varying angles of incidence at 532 and 632.8 nm wavelengths.

Micro- and Nanostructured Poly[oligo(ethylene glycol)methacrylate] Brushes Grown From Photopatterned Halogen Initiators by Atom Transfer Radical Polymerization

Photolithographic techniques have been used to fabricate polymer brush micro- and nanostructures. On exposure to UV light with a wavelength of 244 nm, halogens were selectively removed from films of chloromethylphenyltrichlorosilane and 3-(2-bromoisobutyramido)propyl-triethoxysilane on silicon dioxide. Patterning was achieved at the micrometer scale, by using a mask in conjunction with the incident laser beam, and at the nanometer scale, by utilizing interferometric lithography (IL). Friction force microscopy images of patterned surfaces exhibited frictional contrast due to removal of the halogen but no topographical contrast. In both cases the halogenated surface was used as an initiator for surface atom-transfer radical polymerization. Patterning of the surface by UV lithography enabled the definition of patterns of initiator from which micro- and nanostructured poly[oligo(ethylene glycol)methacrylate] bottle brushes were grown. Micropatterned brushes formed on both surfaces exhibited excellent resistance to protein adsorption, enabling the formation of protein patterns. Using IL, brush structures were formed that covered macroscopic areas (approximately 0.5 cm²) but exhibited a full width at half maximum height as small as 78 nm, with a period of 225 nm. Spatially selective photolytic removal of halogens that are immobilized on a surface thus appears to be a simple, rapid, and versatile method for the formation of micro- and nanostructured polymer brushes and for the control of protein adsorption.

EUV Interferometric Lithography

EUV Interferometric Lithography

EUV interferometric lithography and structural characterization of an EUV diffraction grating with nondestructive spectroscopic ellipsometry

In this study, we constructed an exposure platform to perform extreme ultraviolet interferometric lithography (EUVIL) exposure. This platform can be used to expose 1D and 2D gratings, simulating the line/space and contact/hole patterns, respectively. A transmission grating and an achromatic setup were utilized in the exposure. A simple and nondestructive optical characterization approach based on spectroscopic ellipsometry was proposed to characterize the structure of an EUV transmission grating. The optical behavior of a grating was analyzed by rigorous coupled-wave analysis (RCWA). The preliminary exposure results were obtained using the constructed exposure platform.

Large area nanopatterning of alkylphosphonate self-assembled monolayers on titanium oxide surfaces by interferometric lithography

We demonstrate that interferometric lithography offers a fast, simple route to nanostructured self-assembled monolayers of alkylphosphonates on the native oxide of titanium. Exposure at 244 nm using a Lloyd's mirror interferometer caused the spatially periodic photocatalytic degradation of the adsorbates, yielding nanopatterns that extended over square centimetre areas. Exposed regions were re-functionalised by a second, contrasting alkylphosphonate, and the resulting patterns were used as templates for the assembly of molecular nanostructures; we demonstrate the fabrication of lines of polymer nanoparticles 46 nm wide. Nanopatterned monolayers were also employed as resists for etching of the metal film. Wires were formed with widths that could be varied between 46 and 126 nm simply by changing the exposure time. Square arrays of Ti dots as small as 35 nm (λ/7) were fabricated using two orthogonal exposures followed by wet etching.

Pattern definition employing prism-based deep ultraviolet lithography

Interferometric lithography provides a simple, cost-effective technique for the fabrication of large-area submicrometre periodic features. Biprism-based interferometric lithography employing a deep ultraviolet (DUV) light source is investigated for defining periodic patterns in large area with sub-500 nm pitch and sub-200 nm resolution. Collimated DUV light along with biprism with an angle of 30.4° was employed to fabricate grating lines using single exposure and square lattice features using double-exposure technique. Pyramidal prism was also employed to fabricate holes in square lattice using single exposure. Pitch size of around 410 nm and minimum feature size of 190 nm were realised employing this technique.

Interferometric lithography for nanoscale feature patterning: a comparative analysis between laser interference, evanescent wave interference, and surface plasmon interference

In this paper, we experimentally demonstrate and compare single-exposure multiple-beam interference lithography based on conventional laser interference, evanescent wave interference, and surface plasmon interference. The proposed two-beam and four-beam interference approaches are carried out theoretically and verified experimentally, employing the proposed configurations so as to realize the patterning of one- and two-dimensional periodic features on photoresists. A custom-fabricated grating is employed in the configuration in order to achieve two- and four-beam interference.

Large area three-dimensional photonic crystals with embedded waveguides

Three-dimensional photonic crystals are attractive for very compact waveguide devices. A novel interferometric lithography technique for fabricating three-dimensional photonic crystals is presented, which allows for independent dimensional control of each axis of the crystal. Previous interferometric approaches using 3, 4, 5, or more beams have inherent constraints between the lattice constants and the exposure wavelength. With this new technique, it is possible to control each individual crystal lattice constant largely independent of the exposure wavelength, vastly increasing the available parameter space. Both mathematical models and experimentally realized three-dimensional photonic crystals, over 2 cm2 in size and up to 12 μm, are presented. Photonic crystals with integrated waveguides are of particular significance. A new approach to fabricating waveguides embedded in a three-dimensional photonic crystal is also presented. This approach uses multiple-exposure wavelengths, with one longer wavelength propagating throughout the photoresist for the photonic crystal fabrication and another shorter highly absorptive wavelength for the waveguide fabrication. This new approach to waveguide fabrication leads itself to scalable manufacturing using standard semiconductor lithography equipment.

Pyrolysis of two-dimensional and three-dimensional interferometrically patterned resist structures

Interferometric lithography was used to create a wide variety of two-dimensional and three-dimensional patterns in standard photoresist. The patterns were then converted to amorphous carbon structures through pyrolysis in a reducing atmosphere. The structures maintain their fundamental in-plane morphology despite undergoing significant shrinkage. As an indication of their functionality, the authors highlight their use in two diverse applications: (1) as a defect reduction mask in metal-organic chemical vapor deposition growth of gallium nitride (GaN) on sapphire and (2) as a nanoparticle decorated electrode for catalytic oxidation of methanol.

Nanostructures and Functional Materials Fabricated by Interferometric Lithography

Interferometric lithography (IL) is a powerful technique for the definition of large-area, nanometer-scale, periodically patterned structures. Patterns are recorded in a light-sensitive medium, such as a photoresist, that responds nonlinearly to the intensity distribution associated with the interference of two or more coherent beams of light. The photoresist patterns produced with IL are a platform for further fabrication of nanostructures and growth of functional materials and are building blocks for devices. This article provides a brief review of IL technologies and focuses on various applications for nanostructures and functional materials based on IL including directed self-assembly of colloidal nanoparticles, nanophotonics, semiconductor materials growth, and nanofluidic devices. Perspectives on future directions for IL and emerging applications in other fields are presented.

Increased Mass Transport at Lithographically Defined 3-D Porous Carbon Electrodes

Increased mass transport due to hemispherical diffusion is observed to occur in 3D porous carbon electrodes defined by interferometric lithography. Enhanced catalytic methanol oxidation, after modifying the porous carbon with palladium nanoparticles, and uncharacteristically uniform conducting polymer deposition into the structures are demonstrated. Both examples result in two regions of hierarchical porosity that can be created to maximize surface area, via nanostructuring, within the extended porous network, while taking advantage of hemispherical diffusion through the open pores.

Large-Area Nanopatterning of Self-Assembled Monolayers of Alkanethiolates by Interferometric Lithography

We demonstrate that interferometric lithography provides a fast, simple approach to the production of patterns in self-assembled monolayers (SAMs) with high resolution over square centimeter areas. As a proof of principle, two-beam interference patterns, formed using light from a frequency-doubled argon ion laser (244 nm), were used to pattern methyl-terminated SAMs on gold, facilitating the introduction of hydroxyl-terminated adsorbates and yielding patterns of surface free energy with a pitch of ca. 200 nm. The photopatterning of SAMs on Pd has been demonstrated for the first time, with interferometric exposure yielding patterns of surface free energy with similar features sizes to those obtained on gold. Gold nanostructures were formed by exposing SAMs to UV interference patterns and then immersing the samples in an ethanolic solution of mercaptoethylamine, which etched the metal substrate in exposed areas while unoxidized thiols acted as a resist and protected the metal from dissolution. Macroscopically extended gold nanowires were fabricated using single exposures and arrays of 66 nm gold dots at 180 nm centers were formed using orthogonal exposures in a fast, simple process. Exposure of oligo(ethylene glycol)-terminated SAMs to UV light caused photodegradation of the protein-resistant tail groups in a substrate-independent process. In contrast to many protein patterning methods, which utilize multiple steps to control surface binding, this single step process introduced aldehyde functional groups to the SAM surface at exposures as low as 0.3 J cm(-2), significantly less than the exposure required for oxidation of the thiol headgroup. Although interferometric methods rely upon a continuous gradient of exposure, it was possible to fabricate well-defined protein nanostructures by the introduction of aldehyde groups and removal of protein resistance in nanoscopic regions. Macroscopically extended, nanostructured assemblies of streptavidin were formed. Retention of functionality in the patterned materials was demonstrated by binding of biotinylated proteins.

Disturbance Effects in Extreme Ultraviolet Interferometric Lithography Affecting Interference Fringes

We set up an extreme ultraviolet (EUV) interferometer lithography tool dedicated to the study of the photoresist properties at the EUV wavelength. Two coherent beams are recombined to generate a standing wave, which can be recorded in a photoresist. However, other physical phenomena are generated by the interferometer structure which can disturb the quality of interference fringes. The Fresnel effect is one of these phenomena which is fully detailed in this work. Accurate numerical models have been compared to experimental studies to quantify the reduction of the interference area by the Fresnel effect. Depending on the Fresnel number (NF), several advantageous conditions are presented in order to reduce the Fresnel disturbance on resist.

Tailoring Anisotropic Wetting Properties on Submicrometer-Scale Periodic Grooved Surfaces

The use of simple plasma treatments and polymer deposition to tailor the anisotropic wetting properties of one-dimensional (1D) submicrometer-scale grooved surfaces, fabricated using interferometric lithography in photoresist polymer films, is reported. Strongly anisotropic wetting phenomena are observed for as-prepared 1D grooved surfaces for both positive and negative photoresists. Low-pressure plasma treatments with different gas compositions (e.g., CHF(3), CF(4), O(2)) are employed to tailor the anisotropic wetting properties from strongly anisotropic and hydrophobic to hydrophobic with very high contact angle and superhydrophilic with a smaller degree of wetting anisotropy and without changing the structural anisotropy. The change of the surface wetting properties for these 1D patterned surfaces is attributed to a change in surface chemical composition, monitored using XPS. In addition, the initial anisotropic wetting properties on 1D patterned samples could be modified by coating plasma treated samples with a thin layer of polymer. We also demonstrated that the wetting properties of 1D grooved surfaces in a Si substrate could be tuned with similar plasma treatments. The ability to tailor anisotropic wetting on 1D patterned surfaces will find many applications in microfluidic devices, lab-on-a-chip systems, microreactors, and self-cleaning surfaces.

Large‐Scale Protein Arrays Generated with Interferometric Lithography for Spatial Control of Cell‐Material Interactions

Understanding cellular interactions with material surfaces at the micro‐ and nanometer scale is essential for the development of the next generation of biomaterials. Several techniques have been used to create micro‐ and nanopatterned surfaces as a means of studying cellular interactions with a surface. Herein, we report the novel use of interference lithography to create a large (4 cm2) array of 33 nm deep channels in a gold surface, to expose an antireflective coating on a silicon wafer at the bottom of the gold channels. The fabricated pores had a diameter of 140–350 nm separated by an average pitch of 304–750 nm, depending on the fabrication conditions. The gold surface was treated with 2‐(2‐(2‐(11‐mercaptoundecyloxy)ethoxy)ethoxy)ethanol to create protein‐resistant areas. Fibronectin was selectively adsorbed onto the exposed antireflective coating creating nanometer‐scale cell adhesive domains. A murine osteoblast cell line (MC3T3‐E1) was seeded onto the surfaces and was shown to attach to the fibronectin domains and spread across the material surface.

Super permeable nano-channel membranes defined with laser interferometric lithography

We report the design, fabrication, and testing of super permeable nano-channel membranes, characterized by the absolute control in the pore size at the nano-scale dimensions, large surface area, very high permeability, mechanical stability and durability. The membranes were fabricated using a unique nanotechnology process that combines laser interferometric lithography to define nano-channels (pores) and micro-machining to produce free-standing amorphous silicon membranes, allowing rapid and cost-effective mass production. The suspended membranes were defined as 50 nm thick a-Si, characterized by a very high porosity of approximately 20%, achieved by definition of large arrays of nano-channels. The dimensions of each individual nano-channel was 65 nm wide, 250 nm long. The measured apparent permeability was 0.14 ± 0.05 cm/min for each individual 70 μm × 70 μm membrane, representing one of the highest permeability values ever reported for this scale.

Investigation of Resolution Enhancement by Using Interferometric Immersion Lithography with a Lloyd Mirror

The minimum feature size of semiconductor devices will be smaller and smaller because of the increasing demand for high integration of such devices. The resolution limit of an optical lithography system is described by Rayleigh's criterion. While it is hard to achieve a small critical dimension by reducing the illumination wavelength and the processing-dependent factors, the critical dimension can be reduced by increasing the refractive index of the material between the lens and the wafer. We have used an interferometric immersion lithography system. This system uses a Lloyd mirror for interference between the 1st and the 1st di raction order and deionized water with a refractive index of 1.437 at 193 nm. The experimental result showed that immersion lithography, compared to conventional dry lithography, improved not only the resolution but also the depth of focus. The simulation result con rmed this behavior, showing a larger depth of focus for immersion compared to dry lithography. Also, the linewidth with immersion lithography was smaller than the linewidth with dry lithography.