Photoresist Layer Research Articles

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.

Investigation Into Photonic Jet Machining Using Shaped Optical Fibre Tips

This paper reviews the substantial body of work done by the Instrumentation and Photonic Processes (IPP) research team within the ICube laboratory on photonic jets emerging from shaped fibre tips. The concentration of the laser light in the vicinity of a shaped fibre tip allows us to focalise the light into a photonic jet. This propagative beam, weakly diverging, with a full width at half maximum smaller than the wavelength can reach power densities concentrated up to 50 times. Thanks to these properties, cheap low power, continuous wave lasers can now be used to process materials with high accuracy and resolution. Direct sub-wavelength machining, using low power laser, was studied on silicon. Our team also investigated the polymerisation of photoresist layer to achieve sub-wavelength structure. Spots with a diameter as small as 700 nm were successfully ablated on silicon wafer using a 1030 nm laser. The modelisation and physical understanding of the photonic jet are discussed in this paper.

Diffraction efficiency and noise analysis of hidden image holograms

The simplified approach for analysis of hidden image holograms is discussed in this paper. Diffraction efficiency and signal to noise ratio of reconstructed images were investigated using direct measurements technique and digitized image analysis employing “ImageJ” software. All holograms were recorded in a photoresist layer spin-coated on glass substrates applying laser interference lithography technique. Both analogue and digital analysis approaches showed the similar results thus confirming the appropriateness of the used analysis methods. It was found that holograms recorded at higher laser energy densities demonstrated improved diffraction efficiency and reduced signal to noise ratio of the reconstructed image. The best diffraction efficiency at sufficient signal to noise ratio was obtained using exposure energy density in the range from 150 to 200J/m2 during the hologram writing process.

Optimization and application of dry film photoresist for rapid fabrication of high-aspect-ratio microfluidic devices

Fabrication of high-aspect-ratio PDMS microfluidic devices with conventional SU-8 based soft photolithography is challenging, and often, the thickness of the master from which PDMS replicas are molded is non-uniform. Here, we present an optimized, low cost, fast prototyping microfabrication technique to make deep (up to 500 μm) and high-aspect-ratio (up to 10) microfluidic channels by producing masters by laminating a single or multiple layers of a thin dry film photoresist onto metal wafers. In particular, we explore the required exposure energy for different film thicknesses as well as the highest achievable channel depths and aspect ratios. The homogeneity of the depth of PDMS channels formed using these masters is quantified and found to be remarkably uniform over distances of 20 mm or more. The importance of the processing parameters, such as the exposure energy and development time on final feature size, wall angle, and channel aspect ratio, is investigated. In addition, we report some failure cases, the potential reasons, and strategies for making optimized devices. Potentially, deep microfluidic channels with a wide range of aspect ratios can be used to make long, homogenous separation devices that can be used in cell sorting, filtration, and flow cytometry. We believe the protocols we outline here will be of great utility to the microfluidics community.

Internalization of Auger electron-emitting isotopes into cancer cells: a method for spatial distribution determination of equivalent source terms

Purpose: A challenge for single-cell dosimetry of internalized Auger electron-emitting (AE) radiopharmaceuticals remains how best to elucidate their spatial distribution. To this end, a method, photoresist autoradiography (PAR), was previously developed to identify the lateral spatial distribution of AE-emitting radionuclides internalized in single cancer cells. In this paper, we present a simple mathematical model based on the radius and depth of radiation-induced patterns in photoresist material to identify the location in the z-plane of an 111In source capable of generating the pattern.Materials and methods: SQ20B cells, derived from a head and neck squamous cell carcinoma, were exposed to 111In-labeled epidermal growth factor (EGF) (8 MBq/μg). The integrated electron fluence after four half-lives from the internalized radionuclide-containing construct was detected by a photoresist layer that was placed in close proximity to the cells. The resultant latent patterns were chemically developed and analyzed by atomic force microscopy (AFM). The features in the patterns were matched to locations of electrons emitted from simulated point sources, thereby determining the likely locations of internalized radionuclides.Results: The modeling procedure was validated using simple patterns. The model relates the depth and radius (in the x-y plane) of a pattern to the location and fluence of the source giving rise to the pattern. This point source modeling method provided a good fit to experimental data and can be expanded to analyze more complex patterns.Conclusions: We have demonstrated the utility of the modelling technique to identify the location of internalized AE-emitting radionuclides. This methodology now needs to be extended to predict the source positions in more complex PAR patterns.

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.

Design and fabrication of field‐emission tips with self‐aligned gates

A novel approach to the design and fabrication of field-emission tips with self-aligned gates intended for electric propulsion micro-thruster applications is presented. Their micro-electromechanical systems fabrication process is derived from the recent proliferation of research toward developing field emitter arrays, which are used primarily for field-emission flat-panel display applications. An array of micron-sized tips for electric field enhancement via wet isotropic etching of silicon, using silicon nitride as a hard mask is fabricated. The wet etching is accomplished using a combination of hydrofluoric, nitric, and acetic acids. The tips were then coated with a metal layer to enhance wetting by indium, the proposed propellant. Next, a layer of SU-8 photoresist was applied by spin coating and patterned to serve as a dielectric spacer. A second layer of metal was then applied to serve as a gate electrode. In addition, the results of electrostatic simulations of the prototype is described.

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.

High-resolution conductive patterns fabricated by inkjet printing and spin coating on wettability-controlled surfaces

In this study, we develop a simple and low-cost surface wettability patterning based on soft lithography processes such as nano-imprint lithography (NIL) and micro-contact printing (μCP) to fabricate high-resolution conductive patterns using two different solution processes: inkjet printing and spin coating. An epoxy-based photoresist layer was imprinted by an elastomeric stamp during the NIL process to produce negative micro-structures in the epoxy-based photoresist layer. To form surface wettability contrast, a hydrophobic fluorocarbon film was transferred onto the top surface of the imprinted epoxy-based photoresist layer using μCP. The epoxy-based photoresist layer was UV-treated before the μCP process in order to increase surface wettability contrast; the hydrophilic imprinted patterns surrounded by hydrophobic surfaces were fabricated in the epoxy-based photoresist layer. After printing Ag ink on the imprinted epoxy-based photoresist layer, high-resolution printed line array and square spiral patterns with line width and gap distance of several micrometers can be fabricated with an aid of high surface wettability contrast. Even though well-defined high-resolution conductive patterns with electrical resistivity lower than 6μΩcm can be obtained regardless of the solution processes, inkjet printing seems more efficient from the viewpoint of the amount of ink used in each solution process. The surface wettability patterning suggested in this study is expected to be used in the fabrication of high-resolution conductive patterns in printed electronics.

Plasma chemical etching of photoresist layers based on diazonaphthoquinones in an installation with remote oxygen plasma

Specific features of remote plasma chemical etching of photoresist layers based on diazonaphthoquinones in oxygen at reduced pressure was studied. The possibility of performing “soft” etching at rates of 4–10 nm min–1 to obtain a photoresist layer surface with the root-mean-square roughness no higher than 0.2 nm was demonstrated.

Photolithography-Based Patterning of Liquid Metal Interconnects for Monolithically Integrated Stretchable Circuits.

We demonstrate a new patterning technique for gallium-based liquid metals on flat substrates, which can provide both high pattern resolution (∼20 μm) and alignment precision as required for highly integrated circuits. In a very similar manner as in the patterning of solid metal films by photolithography and lift-off processes, the liquid metal layer painted over the whole substrate area can be selectively removed by dissolving the underlying photoresist layer, leaving behind robust liquid patterns as defined by the photolithography. This quick and simple method makes it possible to integrate fine-scale interconnects with preformed devices precisely, which is indispensable for realizing monolithically integrated stretchable circuits. As a way for constructing stretchable integrated circuits, we propose a hybrid configuration composed of rigid device regions and liquid interconnects, which is constructed on a rigid substrate first but highly stretchable after being transferred onto an elastomeric substrate. This new method can be useful in various applications requiring both high-resolution and precisely aligned patterning of gallium-based liquid metals.

Reverse-absorbance-modulation-optical lithography for optical nanopatterning at low light levels

Absorbance-Modulation-Optical Lithography (AMOL) has been previously demonstrated to be able to confine light to deep sub-wavelength dimensions and thereby, enable patterning of features beyond the diffraction limit. In AMOL, a thin photochromic layer that converts between two states via light exposure is placed on top of the photoresist layer. The long wavelength photons render the photochromic layer opaque, while the short-wavelength photons render it transparent. By simultaneously illuminating a ring-shaped spot at the long wavelength and a round spot at the short wavelength, the photochromic layer transmits only a highly confined beam at the short wavelength, which then exposes the underlying photoresist. Many photochromic molecules suffer from a giant mismatch in quantum yields for the opposing reactions such that the reaction initiated by the absorption of the short-wavelength photon is orders of magnitude more efficient than that initiated by the absorption of the long-wavelength photon. As a result, large intensities in the ring-shaped spot are required for deep sub-wavelength nanopatterning. In this article, we overcome this problem by using the long-wavelength photons to expose the photoresist, and the short-wavelength photons to confine the “exposing” beam. Thereby, we demonstrate the patterning of features as thin as λ/4.7 (137nm for λ = 647nm) using extremely low intensities (4-30 W/m2, which is 34 times lower than that required in conventional AMOL). We further apply a rigorous model to explain our experiments and discuss the scope of the reverse-AMOL process.

Quick and easy microfabrication of T-shaped cantilevers to generate arrays of microtissues

Over the past decade, a major effort was made to miniaturize engineered tissues, as to further improve the throughput of such approach. Most existing methods for generating microtissues thus rely on T-shaped cantilevers made by soft lithography and based on the use of negative SU-8 photoresist. However, photopatterning T-shaped microstructures with these negative photoresists is fastidious and time-consuming. Here we introduce a novel method to quickly generate T-shaped cantilevers dedicated to generation of cellular microtissues, based on the use of positive photoresist. With only two layers of photoresist and one photomask, we were able to fabricate arrays of microwells in less than 3h, each containing two T-shaped cantilevers presenting either a rectangular or a circular geometry. As a proof of concept, these arrays were then replicated in poly(dimethylsiloxane) and microtissues composed of NIH 3T3 fibroblasts encapsulated in collagen I were generated, while the two cantilevers simultaneously constrain and report forces generated by the microtissues. Immunostainings showed longitudinally aligned and elongated fibroblasts over the whole microtissue after 8days of culture. The method described here opens the potential to quick prototyping platforms for high-throughput, low-volume screening applications.

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.

Formation of silver and zinc selenide relief patterns by the lift-off photolithography method

Conducting the lift-off photolithography on silicon wafers with thicknesses of the FP-383 photoresist layers varying from 1.60 ± 0.20 to 4.20 ± 0.20 μm is considered. As a lift-off layer, either a silver layer or zinc selenide layer with a thickness of 80–90 nm was deposited. The edge roughness of the image elements after the lift-off is 5.00 ± 0.20 μm.

Multiplexed electrochemical immunosensor for label-free detection of cardiac markers using a carbon nanofiber array chip

We present an electrochemical multianalyte or multiplexed immunosensor for simultaneous label free detection of cardiac markers panel, comprising of C-reactive protein, cardiac troponin-I and myoglobin. The multi-electrode biosensor chip contains nine identical but electrically isolated microelectrodes arranged in a 3×3 array configuration. Each electrode contains carbon nanofiber nanoelectrodes grown vertically using plasma enhanced chemical vapor deposition. A hydrophobic photoresist layer, lithographically etched on the chip, exposes the electrodes and helps to selectively immobilize the antibody probes for the three target cardiac biomarkers using carbodiimide chemistry. The real-time label free detection of the three cardiac markers from a mixture is demonstrated with high sensitivity and selectivity. Detection in complex protein mixtures in human blood serum does not show any false positives from non-specific protein adsorption. The results show that the present sensor can serve as a miniaturized, low cost lab-on-a-chip system for the detection of various biomarkers in healthcare, environmental monitoring and security applications.

Formation of self-assembled domain structures in single crystals of lithium tantalate with artificial dielectric layer

ABSTRACTThe polarization reversal was studied in single crystals of lithium tantalate with intermediate deviation from stoichiometric composition (([Li]/([Li]+[Ta]) = 48.9%) with and without surface photoresist layer. The switching current was analyzed according to the stages of domain structure evolution revealed during in situ visualization of domain kinetics. The characteristic times of the switching process were extracted by fitting of the experimental data using modified Kolmogorov-Avrami model. The formation of the self-assembled domain chains was observed during polarization reversal with photoresist dielectric layer on one polar surface.

Large-Area High Aspect Ratio Plasmonic Interference Lithography Utilizing a Single High-k Mode.

Plasmonic lithography, which utilizes subwavelength confinement of surface plasmon polartion (SPP) waves, has the capability of breaking the diffraction limit and delivering high resolution. However, all previously reported results suffer from critical issues, such as shallow pattern depth and pattern nonuniformity even over small exposure areas, which limit the application of the technology. In this work, periodic patterns with high aspect ratios and a half-pitch of about 1/6 of the wavelength were achieved with pattern uniformity in square centimeter areas. This was accomplished by designing a special mask and photoresist (PR) system to select a single high spatial frequency mode and incorporating the PR into a waveguide configuration to ensure uniform light exposure over the entire depth of the photoresist layer. In addition to the experimental progress toward large-scale applications of plasmonic interference lithography, the general criteria of designing such an exposure system is also discussed, which can be used for nanoscale fabrication in this fashion for various applications with different requirements for wavelength, pitch, aspect ratio, and structure.

Design principle of super resolution near-field structure using thermally responsive optical phase change materials for nanolithography applications

The super resolution near-field structure (Super-RENS) which is composed of a thin layer of a thermally responsive optical phase change material (PCM) between two dielectric layers, can be a means of resolving the limited resolution of the laser beam for direct laser lithography. In Super-RENS, incident laser irradiation induces the direct, reversible opening and closing of a nanoaperture in the PCM layer, and a nanoscale pattern is realized in the lithography system. Here, we first introduce the complete modeling procedures and optimization methodology for Super-RENS in nanolithography based on a rigorous analysis of near-field structure, thermal analysis in the finite-element method, and analysis of the corresponding feature size on the photoresist (PR) layer. Multiple combinations of the PCM layer and the two dielectric layers with varying dimensions are considered as design parameters to achieve the required resolution in the nanolithography system. The feasible line profiles are investigated at the general operating conditions of the pulsed laser beam, based on varying dimensions of the PCM layer (5–30nm) and the two dielectric layers (10–200nm). This work will provide a detailed methodology for the design and optimization of the Super-RENS for applications in the nanolithography system.

Height Uniformity of Micro-Bumps Electroplated on Thin Cu Seed Layers

The emerging 3-dimensional integration relies on components such as through-silicon vias (TSVs) and micro-bumps for vertical stacking [1, 2]. In a typical micro-bump fabrication process, a thin Cu seed layer is coated with a thick layer of photoresist. After exposure and development, cylindrical openings are formed in the photoresist layer. Depending on the application, bumps with single or multiple metal layers, one of them typically being Cu, are deposited. The removal of photoresist is followed by wet etch of the Cu seed, revealing individual micro-bumps. While Cu in the ‘body’ of the micro-bump is deposited electrochemically, the Cu seed layer is typically fabricated using physical vapor deposition (PVD). As the plated Cu etches up to 30% faster than PVD Cu [3], the Cu part of the bumps may suffer from significant radial etch before the seed is removed completely. Figure 1 shows the post-etch SEM image of a micro-bump with Cu-Ni-Sn metal stack. For 4 µm micro-bumps plated on a 200 nm Cu seed, the contact area with the pad will be reduced by about 25 % after seed etch. In order to minimize the ‘under-etch’ of the micro-bumps, the thickness of the Cu seed layer should be reduced. However, the reduction of Cu seed layer thickness leads to the well-known ‘terminal effect’ on the wafer scale [4]. Figure 2 illustrates the cross-section of a plating cell. Due to the potential drop along the radial direction, the plating current density decreases from the wafer edge to the wafer center, resulting in the micro-bump height decreasing from the wafer edge to the wafer center. In this study a quick and cost-effective approach was developed to evaluate the micro-bump height uniformity on wafer level. We studied dependency of micro-bump height uniformity on Cu seed thickness using 300 mm wafers and demonstrated the impact of terminal effect on height variation along the radial direction. By using only the experimentally measured i-ηcurve for the kinetics, numerical simulation of the wafer-scale plating uniformity was significantly simplified. As the proposed methodology required no explicit knowledge of the plating bath composition and only involved coupon-scale experiments, it provided a fast and cost-effective way to evaluate the effectiveness of proprietary plating chemistries with respect to wafer-scale height uniformity of micro-bumps. The usefulness of the proposed approach was supported by the good match between simulation results and the corresponding experimental measurement. Although the simulation capability was only demonstrated for Cu, the same approach can be applied to other layers deposited for micro-bumps. The overall uniformity can be readily calculated through summation of all the layers involved. The simulation program can also be modified to address different profile control approaches such as additional current collectors or anodes with modified geometry.