Ultraviolet Photolithography Research Articles

Controlled Growth of Organosilane Micropatterns on Hydrophilic and Hydrophobic Surfaces Templated by Vacuum Ultraviolet Photolithography.

In this report, micropatterns of (3-aminopropyl)trimethoxysilane (APTMS) were developed on hydrophilic and hydrophobic surfaces after patterning using 172 nm vacuum ultraviolet (VUV) photolithography. Self-assembled monolayers (SAMs) formed on Si substrates through UV hydrosilylation of 1-hexadecene (HD) and 10-undecenoic acid (UDA) were used as hydrophilic and hydrophobic surfaces, respectively. For templating the HD- and UDA-SAMs, the VUV light was exposed to HD- and UDA-SAMs from the slits of photomasks in atmospheric and evacuated environments, respectively. Various oxygenated groups were generated at the exposed domains of HD-SAM, while the COOH groups were trimmed from the irradiated domains of UDA-SAM. The APTMS molecules were immobilized on the domains that were terminated by oxygenated groups after chemical vapor deposition (CVD). The thicknesses of the developed APTMS micropatterns increased significantly by raising the CVD temperature and in the presence of ambient air in the CVD Teflon container as well. The increase in thicknesses was ascribed to the formation of APTMS multilayers, which were mediated by H3N+ ions. Also, the developed APTMS micropatterns on the UDA-SAM patterned by VUV light irradiation in a high-vacuum environment (HV-VUV) were thicker than those on the VUV/(O) patterned HD-SAM due to the presence of inactive oxygenated groups at the surface of VUV/(O)-terminated domains of HD-SAM such as COO-C and C-O-C groups. The presence of water or ambient air facilitated the silane coupling between the silyl groups with the oxygenated and amino groups The combination of VUV photolithography and the CVD method with control of the conditions would enable us to control the thicknesses and shapes of the developed APTMS micropatterns. These findings illustrate the applicability of VUV photolithography for templating hydrophobic and hydrophobic surfaces toward the development of organosilane architectures, which can be feasible for several applications.

Regulating the uptake of poly(N-(2-hydroxypropyl) methacrylamide)-based micelles in cells cultured on micropatterned surfaces.

Cellular uptake of nanoparticles plays a crucial role in cell-targeted biomedical applications. Despite abundant studies trying to understand the interaction between nanoparticles and cells, the influence of cell geometry traits such as cell spreading area and cell shape on the uptake of nanoparticles remains unclear. In this study, poly(vinyl alcohol) is micropatterned on polystyrene cell culture plates using ultraviolet photolithography to control the spreading area and shape of individual cells. The effects of these factors on the cellular uptake of poly(N-(2-hydroxypropyl)methacrylamide)-based micelles were investigated at a single-cell level. Human carcinoma MCF-7 and A549 cells as well as normal Hs-27 and MRC-5 fibroblasts were cultured on micropatterned surfaces. MCF-7 and A549 cells, both with larger sizes, had a higher total micelle uptake. However, the uptake of Hs-27 and MRC-5 cells decreased with increasing spreading area. In terms of cell shapes, MCF-7 and A549 cells with round shapes showed a higher micelle uptake, while those with a square shape had a lower cellular uptake. On the other hand, Hs-27 and MRC-5 cells showed opposite behaviors. The results indicate that the geometry of cells can influence the nanoparticle uptake and may shed light on the design of functional nanoparticles.

Single-Step Dual-Layer Photolithography for Tunable and Scalable Nanopatterning.

Conventional photolithography, due to its scalability, robustness, and straightforward processes, has been widely applied to micro- and nanostructure manufacturing in electronics, optics, and biology. However, optical diffraction limits the ultimate resolution of conventional photolithography, which hinders its potential in nanoscale patterning for broader applications. Here, we introduce a derivative of conventional photolithography for nanoscale patterning called dual-layer photolithography (DLPL), which is based on the controlled exposure and development of overlapping positive and negative photoresists. In a typical experiment, substrates are sequentially coated by two layers of photoresists (both positive and negative). Then, we purposefully control the exposure time to generate slightly larger features in the positive photoresist than those in the negative photoresist. After development, their overlapping areas become the final features, which outline the original features. We demonstrate line widths down to 300 nm here, which can be readily improved with more precise control. By adjusting the lithography parameters and material deposition, the feature sizes, shapes (e.g., rings, numbers, letters), line widths (300-900 nm), and materials (e.g., SiO2, Cr, and Ag) of these features can be independently controlled. Combined with anisotropic etching, more complex three-dimensional nanostructures can be fabricated as well, as we demonstrate here with Si. We further fabricate photodetectors as an example application to show that these nanostructures fabricated by DLPL can be used to promote light-trapping MAPbI3 perovskite films to achieve good photoelectric properties. This strategy is not limited to ultraviolet photolithography and may also be incorporated into other energetic beam-based lithographic approaches, including deep and extreme ultraviolet photolithographies and electron beam lithography, to enhance their resolution.

Kelvin probe force microscopy studies on the influence of hydrocarbon chain length on 1-alkene self-assembled monolayers on Si (111)

Surface potential contrasts were measured for n-type silicon (111) modified with 1-alkene self-assembled monolayers (SAMs) of varying hydrocarbon chain lengths (n = 10–20) using Kelvin probe force microscopy (KPFM). Micropatterned SAMs were used in KPFM analysis, with hexadecyl SAM acting as a reference to avoid possible variations due to different KPFM cantilever tips used for analysis. Micropatterning was performed via vacuum ultraviolet photolithography at λ = 172 nm. Individual samples were also analyzed by ellipsometry, X-ray photoelectron spectroscopy, static water contact angle tests and atomic force microscopy to determine SAM quality. Surface potential changes observed between the different SAMs were attributed to differences in the dipole moment of the precursor molecules, changes in SAM dielectric properties due to differences in molecular packing, and varying oxygen content at the surface.

Spatially-resolved insulator-metal transition for rewritable optical gratings

Optical relief gratings are usually composed of physical grooves with a constant periodicity, and typically suffer from light scattering, are mechanically fragile and are single function. Here, we develop WO3-based gratings by using a recently reported electron-proton synergistic doping route under ambient conditions. This doping strategy is compatible with conventional ultraviolet photolithography, and we show that it induces a selective insulator-metal phase transition and coloration in WO3, with spatial-resolution up to micron-scale. Due to the electrochromic-induced-contrast, a WO3 volume phase grating without grooves and a WO3 relief grating with tunable periodicity are demonstrated. Both gratings can be rewritten after a reset procedure by annealing in air. Our experiments demonstrate WO3–based gratings and an attractive technique for rewritable oxides.

Sumitomo expands advanced photoresists

Sumitomo Chemical will add production lines for argon fluoride and extreme ultraviolet photolithography resists at its plant in Osaka, Japan. The expansion builds on other advanced photoresist inve...

Patterned liquid crystal polymer C-plate retarder and color polarizer.

The patternability and guest-host interaction with dichroic dye and C-plate liquid crystal polymer (LCP) materials are investigated, and the optical properties of a thin film C-plate retarder and polarizer are studied and compared with theory. The C-plate retarder is a waveplate made of a uniaxial LCP where the optical axis of the LCP is parallel to the surface normal of the optic. No retardance is observed at a normal angle of incidence and retardance grows as the angle of incidence increases. The C-plate polarizer is a C-plate retarder with LCP as the host and a dichroic dye as the guest. The linear diattenuation (LD) of the linear polarizer is zero at a normal angle of incidence and grows with an increasing angle of incidence. Both the C-plate retarder and polarizer can be patterned with minimum feature size down to 2 µm by using ultraviolet photolithography and plasma etching. A planarization process is also developed to deposit a cover layer on top of the pattern to reduce optical loss and to serve as a barrier for subsequent layers.

The fabrication of integrated and three-layer SU-8 nozzles for electrohydrodynamic printing

Electrohydrodynamic (EHD) jet printing can fabricate high resolution micro- and nano-scale lines with low cost and simple procedure. However, less attention has been paid to the fabrication of integrated nozzles with low cost and volume production capacity. In this work, three-layer SU-8 nozzles with slot dimension ranging from 10 to 50 μm were fabricated by ultraviolet photolithography and thermal bonding methods. The influence of the thicknesses (bottom and top layers) on the deformation of SU-8 nozzle was studied by numerical simulation. The results show that the suitable thicknesses should be 60 and 10 μm for bottom and top layer, to ensure the fabrication quality of SU-8 nozzles. Based on the established EHD printing platform, PEO (Polyethylene Oxide) lines with line-width from 750 nm to 8.2 μm can be printed by using the fabricated SU-8 nozzles, indicating the indicating the practicality of developed nozzle fabrication method.

Direct UV laser writing system to photolithographically fabricate optimal microfluidic geometries: Experimental investigations

Ultraviolet (UV) photolithography is utilised to obtain required geometric features on a substrate coated with photosensitive polymer by impinging beam of UV light. Using this fabrication technique, dimensions in the order of sub-micron range can be achieved effectively. However, the conventional photolithography is quite complex, tardy and inefficient approach, specially to achieve higher dimensions (>50 µm). In this work, optimization for fabrication of microchannels using a custom PC based UV (405 nm) Direct Laser Writing (DLW) system for photolithography has been performed. Liquid photoresist and dry film photoresist of various thicknesses are used on the substrate and the patterns are fabricated using the DLW system. The geometry of developed pattern is observed using profilometry and imaging techniques to understand how the width and depth of pattern is being affected by the varying parameters (laser intensity and stage speed), and their optimal values for desired width and depth have been identified. Parameters for developing (developer solution and time) are analysed for both types of photoresists and their effect of mould dimensions have also been studied.

Effect of rubidium incorporation on the optical properties and intermixing in Mo/Si multilayer mirrors for EUV lithography applications

High reflectivity and long-term stability in multilayer mirrors (MLMs) are crucial for extreme ultraviolet (EUV) photolithography. The conventional base stack to reflect 13.5 nm light is a Mo/Si multilayer, which offers a maximum theoretical reflectivity of 75%. In practice, however, the efficiency of the mirror is strongly affected by intermixing between Mo and Si. Diffusion barriers have therefore been adopted, which nevertheless do not provide a perfect solution. In this work, we propose to suppress intermixing in Mo/Si MLMs by substituting the pure Si with a Si compound that can ensure higher thermodynamic stability, while simultaneously providing comparably high EUV theoretical reflectivity, with the net effect of increasing both reflectivity and lifetime. Our theoretical calculations show that rubidium silicide is the most promising material for this purpose. We estimate the optical and thermodynamic properties for each phase of rubidium silicide, and we show that Mo/Rb12Si17 provides the highest theoretical reflectivity, while Mo/RbSi is the most thermodynamically stable. The suppression of intermixing in Mo/RbSi MLMs should lead to a maximum reflectivity at least 2% higher than the best Mo/Si MLMs, integrated with diffusion barriers. The proposed Mo/RbSi MLM solution has the potential to increase the total EUV lithography throughput by ~50%.

Atypical quantized resistances in millimeter-scale epitaxial graphene p-n junctions

We have demonstrated the millimeter-scale fabrication of monolayer epitaxial graphene p-n junction devices using simple ultraviolet photolithography, thereby significantly reducing device processing time compared to that of electron beam lithography typically used for obtaining sharp junctions. This work presents measurements yielding nonconventional, fractional multiples of the typical quantized Hall resistance at ν=2 (RH≈ 12906 Ω) that take the form: abRH. Here, a and b have been observed to take on values such 1, 2, 3, and 5 to form various coefficients of RH. Additionally, we provide a framework for exploring future device configurations using the LTspice circuit simulator as a guide to understand the abundance of available fractions one may be able to measure. These results support the potential for drastically simplifying device processing time and may be used for many other two-dimensional materials.

Production and Mechanical Characterization of Graphene Micro-Ribbons

Patterning of graphene into micro- and nano-ribbons allows for tunability in emerging fields such as flexible electronic and optoelectronic devices, and is gaining interest for the production of more efficient reinforcement for composite materials. In this work we fabricate micro-ribbons from graphene synthesized via chemical vapor deposition (CVD) by combining ultraviolet (UV) photolithography and dry etching oxygen plasma treatments. We used Raman spectral imaging to confirm the effectiveness of the patterning procedure, which is suitable for large-area patterning of graphene on wafer-scale, and confirms that the quality of graphene remains unaltered. The produced micro-ribbons were finally transferred and embedded into a polymeric matrix and the mechanical response was investigated by in-situ mechanical investigation combining Raman spectroscopy and tensile/compressive tests.

Ambient-Pressure X-ray Photoelectron Spectroscopy Characterization of Radiation-Induced Chemistries of Organotin Clusters.

Advances in extreme ultraviolet (EUV) photolithography require the development of next-generation resists that allow high-volume nanomanufacturing with a single nanometer patterning resolution. Organotin-based photoresists have demonstrated nanopatterning with high resolution, high sensitivity, and low-line edge roughness. However, very little is known regarding the detailed reaction mechanisms that lead to radiation-induced solubility transitions. In this study, we investigate the interaction of soft X-ray radiation with organotin clusters to better understand radiation-induced chemistries associated with EUV lithography. Butyltin Keggin clusters (β-NaSn13) were used as a model organotin photoresist, and characterization was performed using ambient-pressure X-ray photoelectron spectroscopy. The changes in relative atomic concentrations and associated chemical states in β-NaSn13 resists were evaluated after exposure to radiation for a range of ambient conditions and photon energies. A significant reduction in the C 1s signal versus exposure time was observed, which corresponds to the radiation-induced homolytic cleavage of the butyltin bond in the β-NaSn13 clusters. To improve the resist sensitivity, we evaluated the effect of oxygen partial pressure during radiation exposures. We found that both photon energy and oxygen partial pressure had a strong influence on the butyl group desorption rate. These studies advance the understanding of radiation-induced processes in β-NaSn13 photoresists and provide mechanistic insights for EUV photolithography.

Wafer-Scale Multilayer Fabrication for Silk Fibroin-Based Microelectronics.

Silk fibroin is an excellent candidate for biomedical implantable devices because of its biocompatibility, controllable biodegradability, solution processability, flexibility, and transparency. Thus, fibroin has been widely explored in biomedical applications as biodegradable films as well as functional microstructures. Although there exists a large number of patterning methods for fibroin thin films, multilayer micropatterning of fibroin films interleaved with metal layers still remains a challenge. Herein, we report a new wafer-scale multilayer microfabrication process named aluminum hard mask on silk fibroin (AMoS), which is capable of micropatterning multiple layers composed of both fibroin and inorganic materials (e.g., metal and dielectrics) with high-precision microscale alignment. To the best of our knowledge, our AMoS process is the first demonstration of wafer-scale multilayer processing of both silk fibroin and metal micropatterns. In the AMoS process, aluminum deposited on fibroin is first micropatterned using conventional ultraviolet (UV) photolithography, and the patterned aluminum layer is then used as a mask to pattern fibroin underneath. We demonstrate the versatility of our fabrication process by fabricating fibroin microstructures with different dimensions, passive electronic components composed of both fibroin and metal layers, and functional fibroin microstructures for drug delivery. Furthermore, because one of the crucial advantages of fibroin is biocompatibility, we assess the biocompatibility of our fabrication process through the culture of highly susceptible primary neurons. Because the AMoS process utilizes conventional UV photolithography, the principal advantages of our process are multilayer fabrication with high-precision alignment, high resolution, wafer-scale large area processing, no requirement for chemical modification of the protein, and high throughput and thus low cost, all of which have not been feasible with silk fibroin. Therefore, the proposed fabrication method is a promising candidate for batch fabrication of functional fibroin microelectronics (e.g., memristors and organic thin film transistors) for next-generation implantable biomedical applications.

Adsorption Behavior of Gold Nanoparticles on Amino-terminated Micro Domains Fabricated on Quartz Substrate by Vacuum Ultra-violet Photolithography

Immobilization of gold nanoparticles (AuNPs) onto quartz is of considerable interest in optoelectronic applications. In this paper, we demonstrated a linear arrangement of AuNPs by controlling the function and optical properties at the microstructure of a quartz surface through formation of a 3-aminopropyltriethoxysilane (APS) self-assembled monolayer and the subsequent vacuum ultraviolet lithography. Citrate-capped AuNPs have been arranged on an APS-terminated surface by electrostatic attraction and in a VUV irradiated area by electrostatic repulsion. In addition, the density of AuNPs on along the boundary of the linear feature was higher than that in the center during the initial stage of immersion. The density of AuNPs along the linear feature became uniform over the entire surface with increasing immobilization time. These results induced the electrostatic interaction of negatively charged AuNPs on the interface between differently charged surfaces.

Experience in the Development of a Configurable Laboratory UV Projection Photolithography System of Micron Resolution

The possibility of creating a laboratory UV photolithography setup with the help of commercially available components, such as optical-mechanical positioners and UV (ultraviolet) objective lenses, is discussed. Existing technical solutions concerning the optical systems of optical lithography, which rely on object‒image reduction, are considered. The main trends in the design of such systems based on lens optics are analyzed. The theoretical and practical aspects underlying the design of similar systems are examined taking into account the basic conditions of image obtainment: congruence to the initial object, ray-path telecentricity, and achievement of the required parameters by linear fields and resolution.

Homogeneous transfer of graphene oxide into photoresist: Fabrication of high surface area three-dimensional micro-arrays by modified photolithography

Herein, we studied the homogeneous transfer of graphene oxide (GO) into photocurable SU-8 photoresist for fabricating GO/SU-8 three-dimensional (3D) composite micropillar arrays by modified photolithography. SU-8 is a negative toned epoxy based photoresist, which includes 8 epoxy groups in each SU-8 monomer. The effects of the concentration of GO flakes on the thermal, mechanical and specific surface area (SSA) properties of the resultant composites were investigated. The large amount of oxygen functional groups on the GO plane and at the edges allowed uniform distribution of GO sheets within the SU-8 resin. The GO/SU-8 micropillar arrays were fabricated by a versatile ultraviolet (UV) photolithography technique that allowed the synthesis of various 3D micro–nano integrated carbon microelectrode arrays. The integrated GO flakes were bonded to the surface or embedded within the primary structure of SU-8 micro-pillars. These SU-8 micropillars having surface-bonded or embedded GO flakes showed superior thermal stability, strength, and SSA characteristics than bare SU-8 micropillars. Thus, this new synthesis approach provides a novel route for developing high performance catalysts, sensors, and adsorbents, among other materials.

(Invited) Orientation Dependent GaAs Nanowire Schottky Solar Cells with 16% Efficiency

Nowadays, high efficiency and low cost solar cell is still technological challenging. In this study, crystalline GaAs NWs with the uniform, pure <110>, <111> orientations and other different mixture ratios can be successfully prepared by tuning the catalyst thickness, nucleation and growth temperatures in the two-step chemical vapor deposition. The single GaAs NWs are fabricated into Au-Al asymmetric Schottky contact solar cells 1 by a simple ultraviolet photolithography process. The low-cost NW solar cell shows a record high efficiency of ~16% under air mass 1.5 global (AM 1.5G) illumination, which produces another record in addition to the vertically aligned p-i-n GaAs NW solar cells previously reported (40%), where the superior photovoltaic performance is attributed to the light concentrating effect of NWs with diameter in the order of light wavelength 2. Employing lift-off resists, three-layer NW parallel arrays can be easily attained for X-ray diffraction in order to evaluate their growth orientation along with the fabrication of NW parallel arrays based Schottky photovoltaic devices for the subsequent performance assessment3. Notably, the open circuit voltage of purely <111>-oriented NW arrayed cells is far higher than that of <110>-oriented NW arrayed counterparts, which can be interpreted by the different surface Fermi level pinning existed on various NW crystal surface planes due to the different As dangling bond densities.Fig1. (a) Output IV curve of a single GaAs NW solar cell and (b) the orientation dependent PV property of printed GaAs NW array solar cellsRef. 1. Han, N.; Yang, Z.; Wang, F.; Dong, G.; Yip, S.; Liang, X.; Hung, T. F.; Chen, Y.; Ho, J. C., High performance GaAs nanowire solar cells for flexible and transparent photovoltaics. ACS Appl. Mater. Interfaces 2015, 7, 20454-20459. 2. Krogstrup, P.; Jorgensen, H. I.; Heiss, M.; Demichel, O.; Holm, J. V.; Aagesen, M.; Nygard, J.; Morral, A. F. I., Single-nanowire solar cells beyond the Shockley-Queisser limit. Nat. Photonics 2013, 7 (4), 306-310. 3. Han, N.; Yang, Z. X.; Wang, F. Y.; Yip, S.; Li, D. P.; Hung, T. F.; Chen, Y. F.; Ho, J. C., Crystal Orientation Controlled Photovoltaic Properties of Multilayer GaAs Nanowire Arrays. ACS Nano 2016, 10 (6), 6283-6290. Figure 1

Morphology of Nanostructured Polymer Brushes Dependent on Production and Treatment

The morphology of top-down and bottom-up nanostructured neutral brushes is studied as a function of the production process and the quality of solvent to which the brushes have been exposed. Neutral brushes of poly(tert-butyl methacrylate) (PtBuMA) were grafted from the substrate by surface-initiated radical-chain polymerization (SI-RCP). Brush patterning was conducted using deep ultraviolet (DUV) photolithography following either top-down (TD) or bottom-up (BU) approaches with patterned lines as small as 200 nm. The resulting brushes were immersed in toluene or methanol, and the change of morphology of the dried brushes was monitored using tapping mode atomic force microscopy (AFM). TD nanopatterned brushes (NPBs) exhibit a box-like and BU generated brushes a lens-like shape directly after the production process. We show that the pattern width for the BU brushed depends not only on the wetting properties of the areas adjacent to the pattern but also on the process history to which the sample has been exposed, especially the solvent quality. Furthermore, we show that TD NPBs are cross-linked on the edges of the pattern because of the high-energy process conditions during brush patterning and study the behavior of the thus-obtained structures.

A novel SU-8 nanofluidic chip fabrication technique based on traditional UV photolithography

Nanofluidic chips are becoming increasingly important for biological and chemical applications due to the special phenomena only occur in nanochannels. In this study, a simple and low-cost method for fabricating SU-8 nanofluidic chips is proposed based on traditional ultraviolet (UV) photolithography. The thermal-induced cracks (nanochannels) can be automatically formed by thermal stress release during the postbake process. The influence of pattern-shape, pattern-angle and pattern-distance on the maximum stress of the SU-8 patterns was investigated. And the effect of postbake temperature on the maximum stress of the SU-8 patterns was also analyzed. The numerical and experimental results show that when the triangle SU-8 patterns with pattern-angle of 60° is postbaked at 125 °C, the nanocracks can be easily formed between two patterns. With this newly developed technology, simple, low-cost and large scale nanofluidic chips can be fabricated only by traditional UV photolighography, which allows a commercially manufacturing of nano-components.