Thin Photoresist Layer Research Articles

Self-loading microfluidic platform with ultra-thin nanoporous membrane for organ-on-chip by wafer-level processing

Embedded porous membranes are a key element of various organ-on-chip systems. The widely used commercial polymer membranes impose limits with regard to chip integration and thinness. We report a microfluidic chip platform with the key element of a monolithically integrated, ultra-thin (700 nm) nanoporous membrane made of ultra-low-stress (<35 MPa) SixNy for culturing and testing reconstructed tissue. The membrane is designed to support various in vitro tissues including co-cultures and to allow passage of molecules but not of cells. A digital laser write method was used to produce nanopores with deterministic but highly flexible positioning within the membrane. A thin layer of photoresist was exposed by accumulation of femtosecond pulses for local two-photon polymerization, which allowed nanopores as small as 350 nm in diameter to be generated through the membranes in a subsequent plasma etch process. The fabricated membranes were used to separate a microfluidic chip into two compartments, which are connected to the outside by microchannel structures. With unique side inlets for fluids, all cells are exposed to identical flow velocities and shear stresses. With the hydrophilic nature of chip materials the self-loading seeding is controlled bottom-up by capillary forces, which makes the seeding procedure homogeneous and less dependent on the operator. The chip is designed to allow fabrication by wafer-level MEMS manufacturing technologies without critical assembly steps, thereby promoting reproducibility and scale-up of fabrication. In order to establish a fully functional test system to be used in a lab incubator, a holder for the bare chip was designed and 3D-printed with additional elements for gravity driven pumping. In order to mimic physiological conditions, the holder was designed to provide not only media delivery but also appropriate shear stress to the tissue. To prove usability, murine microvascular endothelial cells (muMEC) were seeded on the membrane within the chip. Cell compatibility was confirmed after 3 days of dynamic cultivation using fluorescence live/dead assays. Cultivation proved to be reproducible and led to confluent layers with cells preferentially grown on nanoporous areas. The system can in future be cost effectively manufactured in larger quantities in MEMS foundries and can be used for a wide variety of in vitro tissues and test scenarios including pumpless operation within cell incubator cabinets.

Improving Characteristic Parameters of Memristor Based on HfO2 Active Layer

The improvement of the characteristic parameters of the memristor depends on the factors such as thickness and surface area of the active layer. These parameters define leakage currents, which is the main disatvantige of the memory storage device and to improve the electrical features the leakage currents must be dropped to the zero in ideal case. In the presented work is described the electrical isolation of the active layer from the substrate by the thin layer of photoresist, which is an electrical insulator. For reducing area was used the new fotomask, which is able to reduce area 100 times. The memristor structures are designed in the form of “crossbars”, which allows us to individually investigate each memristor and create a database with the possibility of incorporating it into the microchip in the future. In this work is presented also research outcomes regarding to selection memristor’s contacts and active layers. As contacts are overviewed tungsten (W), titanium nitride (TiN) and aluminum (Al). Is considered metal and transition metal oxides as active layers WOx, HfO2, WOx +HfO2, HfO2 + HfOx. The oxide electrical and structural properties is defined from I-V, C-V, XRD and XPS characteristics.

Photoresist as a choice of molecularly thin gate dielectrics in graphene-based devices

Ultra-thin polymeric dielectrics are of great interest for the ever-increasing development of high-performance novel electronics. Up to date, the fabrication of polymer layers as thin as few nanometers is still an extremely demanding process. Here, we report a facile method to fabricate molecularly thin (4 nm–5 nm) plasma-hardened photoresist (PHPR) layers by applying O2 plasma to treat the surface of the photoresist (SPR 220) to cross-link the constituent novolac resin. It is found that such ultra-thin PHPR layers also possess molecular-scale smoothness, superior chemical resistance, and thermal endurance. Furthermore, we develop an in situ transfer technique that is compatible with the planar process to stabilize the patterning of the PHPR layers. By using PHPR layers as the gate dielectric and tunneling barrier (breakdown strength up to 500 kV/mm), a graphene-PHPR-graphene (G-PHPR-G) sandwich-like structure is demonstrated, exhibiting a high photo-responsivity (>13 A/W) under low operating voltages (<1 V), which enables the ultra-thin PHPR layers to be a very promising candidate for the dielectrics in low-power, flexible electronic applications.

Biological Applications of Short Wavelength Microscopy Based on Compact, Laser-Produced Gas-Puff Plasma Source

Over the last decades, remarkable efforts have been made to improve the resolution in photon-based microscopes. The employment of compact sources based on table-top laser-produced soft X-ray (SXR) in the “water window” spectral range (λ = 2.3–4.4 nm) and extreme ultraviolet (EUV) plasma allowed to overcome the limitations imposed by large facilities, such as synchrotrons and X-ray free electron lasers (XFEL), because of their high complexity, costs, and limited user access. A laser-plasma double stream gas-puff target source represents a powerful tool for microscopy operating in transmission mode, significantly improving the spatial resolution into the nanometric scale, comparing to the traditional visible light (optical) microscopes. Such an approach allows generating the plasma efficiently, without debris, providing a high flux of EUV and SXR photons. In this review, we present the development and optimization of desktop imaging systems: a EUV and an SXR full field microscope, allowing to achieve a sub-50 nm spatial resolution with short exposure time and an SXR contact microscope, capable to resolve internal structures in a thin layer of sensitive photoresist. Details about the source, as well as imaging results for biological applications, will be presented and discussed.

Detection and spatial mapping of conductive filaments in metal/oxide/metal cross-point devices using a thin photoresist layer

A simple means of detecting and spatially mapping volatile and nonvolatile conductive filaments in metal/oxide/metal cross-point devices is introduced, and its application demonstrated. The technique is based on thermal discolouration of a thin photoresist layer deposited on the top electrode (TE) of the cross-point device and relies on the increase in temperature produced by local Joule heating of an underlying conductive filament. Finite element modelling of the temperature distribution and its dependencies shows that the maximum temperature at the TE/photoresist interface is particularly sensitive to the TE thickness. The technique is demonstrated on NbOx-based metal-oxide-metal cross-point devices with a 25 nm thick top (Pt) electrode, where it is used to undertake a statistical analysis of the filament location as a function of device area. This shows that filament formation is heterogeneous; the majority of filaments form preferentially along the TE edge, and the fraction of these increases with the decreasing device area. Transmission electron microscopy of the top and bottom electrode edges is used to explain this observation and suggests that it is due to a reduction in the effective oxide thickness in this region.

Optically processed microlens array for single-beam lithography of plasmonic structures

Abstract A 2D dielectric grating produced by photolithography was employed as a microlens array (MLA), which was based on a smooth surface and a shape that may be defined roughly as a hemisphere of each lattice. Such a 2D MLA can focus an incident ultraviolet light beam into a matrix of light spots, which were estimated to be as small as 500 nm in diameter. Using a thin layer of photoresist (PR) to record the pattern of the focusing spots, we achieved an approximately inversed structure of periodically arranged holes in PR in submicron sizes. Filling these holes with gold using chemically synthesized colloidal gold nanoparticles produced a plasmonic grating consisting of gold nanoparticles larger than 580 nm in average diameter. Localized surface plasmon resonance in both first and second orders was observed, which was verified by the spectroscopic response and theoretical simulations. MLA can be thus repeatedly used as a master to produce plasmonic photonic structures with high reproducibility.

Nanoimaging using soft X-ray and EUV laser-plasma sources

In this work we present three experimental, compact desk-top imaging systems: SXR and EUV full field microscopes and the SXR contact microscope. The systems are based on laser-plasma EUV and SXR sources based on a double stream gas puff target. The EUV and SXR full field microscopes, operating at 13.8 nm and 2.88 nm wavelengths are capable of imaging nanostructures with a sub-50 nm spatial resolution and short (seconds) exposure times. The SXR contact microscope operates in the “water-window” spectral range and produces an imprint of the internal structure of the imaged sample in a thin layer of SXR sensitive photoresist. Applications of such desk-top EUV and SXR microscopes, mostly for biological samples (CT26 fibroblast cells and Keratinocytes) are also presented. Details about the sources, the microscopes as well as the imaging results for various objects will be presented and discussed. The development of such compact imaging systems may be important to the new research related to biological, material science and nanotechnology applications.

Perforation Does Not Compromise Patterned Two-Dimensional Substrates for Cell Attachment and Aligned Spreading.

Polymeric sheets were perforated by laser ablation and were uncompromised by a debris field when first treated with a thin layer of photoresist. Polymer sheets perforated with holes comprising 5, 10, and 20% of the nominal surface area were then patterned in stripes by photolithography, which was followed by synthesis in exposed regions of a cell-attractive zirconium oxide-1,4-butanediphosphonic acid interface. Microscopic and scanning electron microscopy analyses following removal of unexposed photoresist show well-aligned stripes for all levels of these perforations. NIH 3T3 fibroblasts plated on each of these perforated surfaces attached to the interface and spread in alignment with pattern fidelity in every case that is as high as that measured on a nonperforated, patterned substrate.

Direct bonding of polymer/glass-based microfluidic chips with dry film photoresist

Dry film has been widely used as a low-cost photoresist in the print circuit board industry which consists of a thin layer of photoresist sandwiched between two protective polymer layers. In this research, a simple, cleanroom-free, low-cost and highly adaptable bonding method for various polymer and glass-based microfluidic systems was proposed using the cross-linked dry film photoresist. In this proposed approach, the uncross-linked dry film photoresist was sandwiched between substrates and cover plate, then using UV exposure for the crosslinking of the photoresist to reach a secured bonding, after bonding, a cleaning process for the removal of photoresist residuals trapped inside the microchannels was also applied. The proposed bonding method is highly adaptable for different kinds of polymer or glass-based microfluidic devices, even the hybrid bonding between polymer and glass substrates could be achieved, which is usually very challenging using the conventional bonding technologies. Comparing with the traditional adhesive bonding method, the proposed method is simple, low-cost and without the requirement for toxic organic solvents, in addition, the cleaning procedure proposed in this study could effectively remove the residual of the adhesives trapped in the microchannels.

Xurography for microfluidics on a reactive solid.

In this paper, we propose a simple method to embed transparent reactive materials in a microfluidic cell, and to observe in situ the dissolution of the material. As an example, we show how to obtain the dissolution rate of a calcite window of optical quality, dissolved in water and hydrochloric acid (HCl). These fluids circulate at controlled flowrates in a channel which is obtained by xurography: double sided tape is cut out with a cutter plotter and placed between the calcite window and a non-reactive support. While the calcite window reacts in contact with the acid, its topography is measured in situ every 10 s using an interference microscope, with a pixel resolution of 4.9 μm and a vertical resolution of 50 nm. In order to avoid inlet influence on the reaction, a thin layer of photoresist is added on the calcite surface at the inlet and outlet. This layer is also used as a non reactive reference surface.

Deastigmatism, circularization, and focusing of a laser diode beam using a single biconvex microlens

A single biconvex microlens is proposed to correct the astigmatism and ellipticity of a laser diode (LD) beam and focus it to a smallest circular spot. The microlens has three different profiles in which one cylindrical input surface is to collimate the beam in the fast-axis (y-axis) direction. Output surface, on the other hand, holds two different parabolic profiles in fast- and slow-axis (x-axis) directions to correct the astigmatism and focus the beam into a smallest circular spot. A simulation software is used to design and optimize those lens profiles. Theoretically, the smallest focused spot size is around 4.24 μm in diameter. The three profiles are then transferred to photo-masks to fabricate the microlens on polycarbonate material using an excimer laser dragging method with alignment accuracy of 1 μm. The machined microlens is assembled with the LD using double-sided optically clear adhesive tape. The experimental focused spot is found to be 16.75 μm in diameter. Circularity of the focused spot is demonstrated by a single-shot exposure test on thin photoresist layer that shows a circular-dot diameter of 7.32 μm. The proposed technique has great potential in applications such as beam pen lithography, fiber coupling, and optical read-write head.

Photonic devices prepared by embossing in PDMS

In this paper, we present useful technique for fabrication of novel photonic devices created in the polydimethylsiloxane (PDMS). We use combination of direct laser writing in thin photoresist layer with embossing process of liquid PDMS. We prepared ring resonator and Mach-Zehnder interferometer in PDMS. The shape of prepared PDMS photonic devices was analyzed by confocal laser microscope and atomic force microscope. Optical characterization of these devices reveals extinction ratios of up to 20dB.

Microelectromechanical Parallel Scanning Nanoprobes for Nanolithography

This paper presents a new approach and preliminary results for MEMS-enabled low-cost and high throughput parallel scanning probe nanolithography. The proposed approach is based on integrated microelectromechanical systems capable of simultaneous generation of a large number of identical nanoprecision patterns on a substrate. Integration of two-degree-of-freedom MEMS actuators with two-dimensional nanoprobe arrays as well as preliminary results for simultaneous generation of multiple submicrometer patterns using such devices is reported. Electrical cross talk between the fully silicon fabricated actuators along X - and Y -axes has been eliminated by optimizing the design. Scratch marks as narrow as 250 nm and as long as 42 μm have been generated on a thin photoresist layer, which was spin-coated on a silicon substrate using the electrothermal MEMS structure carrying up to 25 nanotips. Furthermore, the submicrometer patterns have been successfully transferred to the underlying silicon substrate via reactive ion etching. Patterns as narrow as 250 nm (∼60 nm in depth) were obtained on the underlying silicon substrate.

ZnO nanowires strips growth: Template reliability and morphology study

In this work we report on hydrothermal growth of ZnO nanostructures obtained by using strip-shaped resist templates made by electron beam lithography, with particular focus on: (i) the effects of materials used and processing on the template adhesion and its stability during wet growth; (ii) the influence of the template pattern dimensions on the nanowires morphology. Templates made of patterned films with methylmethacrylate–methacrylic acid copolymer showed greater adhesion and stability with respect to the commonly used poly (methylmethacrylate) electron resist or to the use of thin adhesion-promoter photoresist layers. The growth of nanowires in strip-shaped methyl methacrylate–methacrylic acid copolymer templates has been investigated using typical hydrothermal growth parameters and a marked dependence of nanowires length and diameter has been found on the template size for strips narrower than about 2μm.

Fabrication of a seamless roller mold with wavy microstructures using mask-less curved surface beam pen lithography

Roller imprinting is one of the most commonly used methods for the fabrication of continuous functional structures over large areas. However, the fabrication of roller molds with seamless and complex patterns poses a significant challenge. This paper presents an innovative technique for fabricating a seamless roller mold with wavy microstructures using a novel mask-less curved surface beam pen lithography technique. The major steps in the proposed technique include spray coating a thin photo-resist (PR) layer on the roller, exposing the PR layer though a translating micro-lens array (MLA), etching the patterned PR layer, and electro-polishing the etched microstructures. The proposed method is used to pattern roller molds with different wavy microstructures by varying the rotation speed of the roller, the translation speed of the MLA holder, and the translation distance of the MLA holder. The patterned metal roller molds are then used to replicate wavy microstructures on a thin polyethylene terephthalate substrate by means of continuous UV-type roller imprinting methods. The line-width and height of the wavy microstructure are 84.5 µm and 25.5 µm, respectively.

Enabling Resist Processing Technologies for Advanced Packaging

The increasing adoption of advanced wafer-level packaging (WLP) technologies and high density interposer concepts clearly reflect the permanent need for form factor reduction, smaller process geometries and higher-count I/O on ICs. Currently, several strategies are being pursued to achieve these goals. The most promising approaches are summarized under the concept of three-dimensional integrated circuits (3D-IC) and three-dimensional wafer level packaging (3D-WLP) technology. A key component for 3D device integration schemes is the requirement of vertical through-silicon-via (TSV) interconnections that enables electrical through-chip communication through stacks of vertically integrated layers on the wafer scale. Ultimately, the use of TSVs also enables higher performance and smaller package sizes. In order to realize TSV connections, a series of process steps is required such as the thinning and bonding of the wafer to a carrier prior to the formation of through-wafer vias, followed by the passivation and metallization of the vias. Despite the potential benefits associated with the integration of TSVs also significant challenges have to be overcome. One of the greatest challenges for present and even more for upcoming TSV design strategies still remains the processing of photoresist and other functional polymers at and within TSV geometries. To this day, it is still very difficult to achieve a conformal polymer coating in deep cavities, along steep side walls and especially within the extreme aspect ratios of TSV. Mainly this is due to the fact that standard surface coating methods such as spin coating were just not developed to meet the requirements posed by these high aspect ratio microstructures. New and innovative approaches are needed to meet these new challenges. Spray coating is one of the most promising technologies to overcome current barriers. However, even most of the available spray deposition equipment is facing its limits with steadily decreasing via diameters and increasing aspect ratios on the other hand. Successively, the multitude of these challenging technological developments in the 3D-IC and wafer-level packaging area has created the demand for innovative manufacturing approaches, new equipment and related tools. Herein we present our new EVG ®NanoSprayTM coating technology with unique capabilities to overcome the present limits of conformal resist coating over extreme topography. We demonstrate one particularly promising application for conformal polymer coatings; as an annular lining at the interface between the conducting metal filling in the TSV and the silicon wafer. The intrinsic properties of the polymer allow a TSV design solution that is more forgiving on coefficient of thermal expansion (CTE) mismatch-induced stress between the silicon substrate and the interfacing metal. Consequently, this new type of polymer buffered TSV interconnect design promises to significantly reduce thermal stress-induced TSV delamination as one of the dominant failure modes for 3-D interconnects. We further demonstrate the application of EVG ®NanoSprayTM as enabling coating technology for llithographic processing of conformal coated TSVs. The patterning of thin photoresist layers at the bottom of vias and along the steep sidewalls of deep cavities allows for more degrees of freedom in electrical contact formation. The presented EVG ®NanoSprayTM coating technology opens new dimensions in advanced wafer level packaging and provokes reconsidering prevailing limitations in interconnect design.

Fabrication of metal roller mold with submicrometer feature size using contact printing photolithography technique

This paper presents an innovative approach for directly forming surface microstructures at submicrometer scale on the cylindrical surface of a metal roller. This roller can then serve as a roller mold in roller imprinting processes for large-area micro/nanofabrication. In the proposed approach, a novel contact printing method is used to transfer a patterned metal film from a planar soft mold to a thin photoresist (PR) layer coated on the roller surface. The pattern definition capability can easily reach submicrometer scale over a large patterning area. Subsequent ultraviolet light exposure and PR developing processes then form a layer of patterned PR microstructures on the cylindrical surface. Finally, patterned microfeatures are directly formed on the roller's surface by an additive (metal deposition) approach. The overall system design, experimental procedures, and experimental results are systematically discussed.

Development of High Accuracy Spray Coating Method using Multi-layer Coat

In this paper, we propose a high-accuracy spray coating method for obtaining the thick photoresist over 200μm. A multi-layered coating method with thin photoresist layers is applied in order to achieve a thick photoresist. We evaluated the waviness and the reproducibility of the thickness of the photoresist coated on a flat substrate. The multi-layered spray coating method with the edge cover achieves high accuracy thickness of the photoresist with covering wide-range of the target thickness.

Spatial resolution of apertureless metal-coated fiber tip for NSOM lithography determined by tip-to-tip scan

Spatial resolution of the near-field scanning optical microscope (NSOM) as an effective tool for high resolution optical measurements strongly depends on the aperture properties of the fiber tip. A novel tip-to-tip scanning technique for characterization of the optical near field produced at the apex of apertureless metal-coated fiber tip is described here. Analysis of the scanned near-field intensity profile directly shows the spatial resolution of the investigated fiber tip that is finally verified by using NSOM in illumination mode, where two-dimensional structures in thin photoresist layer were prepared by NSOM lithography.

Regular ZnO nanopillar arrays by nanosphere photolithography

Highly regular vertical ZnO nanopillar arrays were hydrothermally grown through a nucleation window pattern generated by nanosphere photolithography. The in-plane intensity modulation of the exposing ultraviolet light in the photoresist was performed by Stöber silica or polystyrene nanospheres in the masking Langmuir–Blodgett monolayer. By comparing six different nanosphere diameters in the 180–700nm range only those with diameter above the exposure wavelength of 405nm generate a pattern in the thin photoresist layer. The pattern quality is improving with increasing diameter, therefore, the masking for nanopillar growth was demonstrated with 700nm polystyrene nanospheres. The results of the nanosphere photolithography were supported by finite-difference time-domain calculations. This growth approach was shown to have the potential for low-cost, low-temperature, large area fabrication of ZnO pillars or nanowires enabling a precise engineering of geometry.