Photoresist Patterns Research Articles

Interfacial capacitance of single layer graphene

The interfacial capacitance of large area, single layer graphene was directly measured with electrolyte accessing both sides of the graphene sheet. PMMA and photoresist patterns were used as supports to suspend the CVD grown graphene in electrolyte during electrochemical testing. Both one and two sides of single layer graphene films were measured and compared. The results show that the area normalized charge that can be stored simultaneously on both sides is significantly lower than could be stored on just one side of single layer graphene, consistent with charge storage having a quantum capacitance component. These measurements are also consistent with the specific capacitance of graphene materials as previously measured in supercapacitor cells and provide a basis for the further understanding and development of graphene based materials for electrical energy storage.

Supported pyrolysis for lithographically defined 3D carbon microstructures

We present a facile and effective route for the fabrication of carbon microstructures by direct carbonization of polymeric photoresist patterns without distortion or collapse of the microstructure due to high-temperature pyrolysis. The 3D porous structure was a woodpile structure prepared by five-beam interference lithography. Pyrolysis of the SU-8 woodpile structure resulted in shrinkage of the structure into the film with loss of the pore structure. The silica shell support on this SU-8 microstructure provided effective mechanical support to resist pattern distortion or collapse by melting of the photoresist and subsequent large mass loss in the heat treatment. The shrinkages in lattice distance and layer-by-layer distance of the silica-coated SU-8 were measured to be 3% and 60%, respectively. The composition of pyrolyzed SU-8 photoresists was characterized by Raman spectroscopy. Current-sensing AFM was used to measure the local electrical conductivity of a 3D pyrolyzed carbon structure. Our pyrolyzed 3D carbon structures were composed of a glassy carbon and possessed the local conductivity of about 103 S m−1. The fabrication of lithographically defined carbon structures with high fidelity will open new opportunities for carbon MEMS and various energy-related microdevices.

New Double-sided Patterning Process Using a Dilute Hydrochloric Acid Solution to Fabricate Stacked Intrinsic Josephson Junctions Inside Bi2Sr2CaCu2O8+x Single Crystals

We developed a new double-sided patterning (DSP) process using a dilute acid solution (pH=1.65) to fabricate Bi2Sr2CaCu2O8+x (Bi-2212) stacks of intrinsic Josephson junctions. The stacks, which were fabricated from a single crystal of Bi-2212, were surrounded by an acid-treated product. The critical aspect of this process is that the Bi-2212 surrounding the photo-resist pattern was changed to a transparent material, BiOCl, which was connected with the Bi-2212 crystal. As a result, this process provides an easy way to achieve the DSP of the surface of the Bi-2212 crystal. This new DSP process allows for remarkably improved reproducibility in fabricating the Bi-2212 stacks which show good current-voltage characteristics with large hysteresis and multiple branches at T=77 K.

Ultralarge-area block copolymer lithography via soft graphoepitaxy

We introduce soft-graphoepitaxy, a cost-effective, truly scalable, ultralarge-area block copolymer lithography. Soft graphoepitaxy employs a disposable photoresist prepattern attainable by conventional lithography, such as ArF lithography or I-line lithography, to direct lateral nanodomain ordering in block copolymer thin films. Since the organic photoresist prepattern is readily disposable, this approach provides laterally ordered self-assembled nanopatterns and corresponding pattern transferred functional nanostructures without any trace of the structure directing prepattern. Furthermore, the microscale photoresist pattern can be transformed into a sub-30 nm scale ultrafine lamellar pattern over an arbitrary large area via a scalable soft-graphoepitaxy principle.

Solution-processable π-conjugated dendrimers with hole-transporting, electroluminescent and fluorescent pattern properties

In this paper, four new π-conjugated dendrimers G1 and G2-1–G2-3, constructed by triphenylamine and carbazole moieties, have been successfully prepared via a simple synthetic route. This molecular design imparts the materials with good solution-processability, high thermal and morphological stabilities, and low oxidation potential, all of which are promising properties for optoelectronic materials. The double-layer OLEDs fabricated using these materials through solution processing demonstrate that they can exhibit dual functions, hole-transporting and light-emitting. Devices using G1 or G2-1 as the hole-transporting layer present good stability during the passage of current, with a maximum efficiency of 1.70 and 1.59 cd A−1, respectively. Moreover, devices using these dendrimers as the emitting layer show moderate performance, and the device of G2-2 gives a maximum luminance and efficiency of 1190 cd m−2 and 1.67 cd A−1, respectively, thanks to three-dimensional building of the dendritic system, which might suppress the inherent reductive or aggregation-caused quenching that usually happens for triphenylamine/carbazole derivatives to some degree. Also, the photo-cross-linking property of triple bonds enables ready fabrication of highly fluorescent photoresist patterns of these dendrimers.

Plasma-polymer interactions: A review of progress in understanding polymer resist mask durability during plasma etching for nanoscale fabrication

Photolithographic patterning of organic materials and plasma-based transfer of photoresist patterns into other materials have been remarkably successful in enabling the production of nanometer scale devices in various industries. These processes involve exposure of highly sensitive polymeric nanostructures to energetic particle fluxes that can greatly alter surface and near-surface properties of polymers. The extension of lithographic approaches to nanoscale technology also increasingly involves organic mask patterns produced using soft lithography, block copolymer self-assembly, and extreme ultraviolet lithographic techniques. In each case, an organic film-based image is produced, which is subsequently transferred by plasma etching techniques into underlying films/substrates to produce nanoscale materials templates. The demand for nanometer scale resolution of image transfer protocols requires understanding and control of plasma/organic mask interactions to a degree that has not been achieved. For manufacturing of below 30 nm scale devices, controlling introduction of surface and line edge roughness in organic mask features has become a key challenge. In this article, the authors examine published observations and the scientific understanding that is available in the literature, on factors that control etching resistance and stability of resist templates in plasma etching environments. The survey of the available literature highlights that while overall resist composition can provide a first estimate of etching resistance in a plasma etch environment, the molecular structure for the resist polymer plays a critical role in changes of the morphology of resist patterns, i.e., introduction of surface roughness. Our own recent results are consistent with literature data that transfer of resist surface roughness into the resist sidewalls followed by roughness extension into feature sidewalls during plasma etch is a formation mechanism of rough sidewalls. The authors next summarize the results of studies on chemical and morphological changes induced in selected model polymers and advanced photoresist materials as a result of interaction with fluorocarbon/Ar plasma, and combinations of energetic ion beam/vacuum ultraviolet (UV) irradiation in an ultrahigh vacuum system, which are aimed at the fundamental origins of polymer surface roughness, and on establishing the respective roles of (a) polymer structure/chemistry and (b) plasma-process parameters on the consequences of the plasma-polymer interactions. Plasma induced resist polymer modifications include formation of a thin (∼1–3 nm) dense graphitic layer at the polymer surface due to ion bombardment and deeper-lying modifications produced by plasma-generated vacuum ultraviolet (VUV) irradiation. The relative importance of the latter depends strongly on initial polymer structure, whereas the ion bombardment induced modified layers are similar for various hydrocarbon polymers. The formation of surface roughness is found to be highly polymer structure specific. Beam studies have revealed a strong ion/UV synergistic effect where the polymer modifications introduced at various depths by ions or ultraviolet/UV photons can interact. A possible fundamental mechanism of initial plasma-induced polymer surface roughness formation has been proposed by Bruce et al. [J. Appl. Phys. 107, 084310 (2010)]. In their work, they measured properties of the ion-modified surface layer formed on polystyrene (PS) polymer surfaces, and by considering the properties of the undamaged PS underlayer, they were able to evaluate the stressed bilayer using elastic buckling theory. Their approach was remarkably successful in reproducing the wavelength and amplitude of measured surface roughness introduced for various ion bombardment conditions, and other variations of experimental parameters. Polymer material-dependent VUV modifications introduced to a depth of about 100 nm can either soften (scission) or stiffen (cross-linking) this region, which produce enhanced or reduced surface roughness.

ZnO Modified High Aspect Ratio Carbon Electrodes for Hydrogen Sensing Applications

Design, fabrication, and testing results of a hydrogen gas sensor based on a surface modified high density and high aspect ratio 3-D carbon post microarray are described in this paper. After the conversion of photoresist patterns into carbon electrodes, the increased surface was conformally coated with a metal oxide semiconductor film by atomic layer deposition (ALD). A maximum sensitivity as high as 100 (Rair/ Rhydrogen) has been observed in the fabricated sensor even in the presence of moisture at low temperature. A fast initial response (90% resistance drop in 30sec at 3000ppm hydrogen) of the sensor could be utilized for early leak detection. The methodology of surface modification as well as the test result shows good promise for various chemical sensing devices based on the proposed strategy.

Simple fabrication of patterned gold nanoparticle arrays on functionalized block copolymer thin films

Patterned arrays of gold nanoparticles were fabricated using a simple dipping method that makes use of their specific interactions with nano-domains of carboxylic acid on a block copolymer template. Polystyrene-block-poly(tert-butyl acrylate) on the SU-8 photoresist pattern was selectively transformed to polystyrene-block-poly(acrylic acid). Au nanoparticles are selectively immobilized on the resulting carboxylic acid patterns to produce well-defined patterned Au nanoparticle arrays. This stable and robust template can be used to obtain any patterned nonaggregated metal or inorganic nanoparticle arrays.

Two-step photolithography to fabricate multilevel microchannels

In this paper, we study the variation of the thickness of patterned microchannel features in photoresist (PR) by two-step photolithography. The final PR thickness is determined by the thickness and width of the predefined PR pattern in the first-step lithography and the thickness of the spin-coated PR film in the second-step lithography. Thickness variation is demonstrated to be an important consideration within the critical pattern width that the capillary pressure is still dominant for forming the PR film thickness. With this mechanism of two-step photolithography, we demonstrate its ability to form multilevel channel features and its use to fabricate a cross-flow filter with two different pore sizes as a microfluidic application.

Two-spherical-wave ultraviolet interferometer for making an antireflective subwavelength periodic pattern on a curved surface

An ultraviolet two-spherical-wave interferometer was developed in order to make a subwavelength structured surface on a curved surface. The change in fringe period on the curved surface was significantly suppressed compared with the two-plane-wave interferometer. The optical setup method for suppressing the change in fringe period is described. The effect of the two-spherical-wave interferometer was investigated by numerical simulations. In an experimental demonstration for a concave spherical surface with 11.1 mm radius of curvature and 10 mm diameter, the change in period of the photoresist pattern was reduced to 12 nm for the target period of 250 nm.

Microfluidic immobilization of physiologically active Caenorhabditis elegans

We present a protocol for building and operating a microfluidic device for mechanical immobilization of Caenorhabditis elegans in its physiologically active state. The system can be used for in vivo imaging of dynamic cellular processes such as cell division and migration, degeneration, aging and regeneration, as well as for laser microsurgery, Ca2+ imaging and three-dimensional microscopy. The device linearly orients C. elegans, and then completely restrains its motion by pressing a flexible membrane against the animal. This technique does not involve any potentially harmful anesthetics, gases or cooling procedures. The system can be installed on any microscope and operated using only one syringe and one external valve, making it accessible to most laboratories. The device fabrication begins by patterning photoresist structures on silicon wafers, which are then used to mold features in elastomeric layers that are thermally bonded to form the device. The system can be assembled within 3 d.

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.

Through-Mask Electroetching for Fabrication of Metal Bipolar Plate Gas Flow Field Channels

Transition of PEM fuel cells to the commercial market requires low-cost components, materials and manufacturing processes. Considering that bipolar plate manufacturing currently contributes to a significant percentage of the total fuel cell manufacturing cost, an inexpensive bipolar plate manufacturing process must be realized for fuel cells to achieve prominence in the commercial energy sector. A through-mask electro-etching process for high volume manufacturing of stainless steel bipolar plate gas flow field channels is being developed using pulse and pulse reverse electric fields. It has been demonstrated that the process can achieve a uniform channel depth, exhibits good repeatability for multiple experiments performed under the same conditions and the undercut beneath the photoresist mask varies linearly with channel depth indicating that pattern design compensation prior to photoresist patterning of the substrate to achieve the dimensional accuracy of the gas flow field channels and ribs is possible.

Synthesis of well‐defined and end‐polymerizable star‐shaped polysulfides and their application to negative photoresist

AbstractVarious star‐shaped poly(phenoxy propylene sulfide)s (PPSs) bearing curable end groups were synthesized by the functionalization of the propagating ends of star‐shaped poly(PPS) with various electrophilies. The functionalization with chloromethyl styrene proceeded quantitatively, and afforded polymers with Mn almost agreed with theoretical value and narrow Mw/Mn. The photocuring conditions were optimized, and the addition of 10 wt % of poly(ethylene glycol) diacrylate was effective to attain sufficient crosslinking. The photocuring reaction of the end‐functionalized poly(PPS) films cast on silicon wafers was conducted by UV irradiation. The cured poly (PPS)s became insoluble in THF, supporting the sufficient crosslinking. Developing of a cured polymer yielded a negative photoresist pattern. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010

Ultralarge-Area Block Copolymer Lithography Enabled by Disposable Photoresist Prepatterning

We accomplished truly scalable, low cost, arbitrarily large-area block copolymer lithography, synergistically integrating the two principles of graphoepitaxy and epitaxial self-assembly. Graphoepitaxy morphology composed of highly aligned lamellar block copolymer film that self-assembled within a disposable photoresist trench pattern was prepared by conventional I-line lithography and utilized as a chemical nanopatterning mask for the underlying substrate. After the block copolymer film and disposable photoresist layer were removed, the same lamellar block copolymer film was epitaxially assembled on the exposed chemically patterned substrate. Highly oriented lamellar morphology was attained without any trace of structure directing the photoresist pattern over an arbitrarily large area.

Measurement of temperature and refractive index based on surface long-period gratings deposited onto a D-shaped photonic crystal fiber

We propose a surface long-period grating (LPG) based on a D-shaped photonic crystal fiber (PCF). The D-shaped PCF is fabricated by a side-polishing technique. The surface LPG based on periodic patterns of photoresist (PR) is formed by using the spin-coating and the standard contact lithography methods. Resonant coupling is created by the surface PR-LPG in the D-shaped PCF. The resonant peak shifts to longer wavelength as the ambient index is increased and shifts to shorter wavelength as the temperature is increased. The total wavelength shift is measured to be 122 nm in the refractive index range from 1 to 1.45 and the temperature sensitivity is measured to be −0.3 nm/°C in the temperature range from 30 to 100°C.

Protein patterning on polycrystalline silicon–germanium via standard UV lithography for bioMEMS applications

Polycrystalline silicon–germanium (poly-SiGe) is a promising structural material for the post-processing of micro electro-mechanical systems (MEMS) on top of complementary metal-oxide-semiconductor (CMOS) substrates. Combining MEMS and CMOS allows for the development of high-performance devices. We present for the first time selective protein immobilization on top of poly-SiGe surfaces, an enabling technique for the development of novel poly-SiGe based MEMS biosensors. Active regions made of 3-aminopropyl-triethoxysilane (APTES) were defined using silane deposition onto photoresist patterns followed by lift-off in organic solvents. Subsequently, proteins were covalently bound on the created APTES patterns. Fluorescein-labeled human serum albumin (HSA) was used to verify the immobilization procedure while the binding capability of the protein layer was tested by an antigen-labeled antibody pair. Inspection by fluorescence microscopy showed protein immobilization inside the desired bioactive areas and low non-specific adsorption outside the APTES pattern. Furthermore, the quality of the silane patches was investigated by treatment with 30 nm-diameter gold nanoparticles and scanning electron microscope observation. The developed technique is therefore a promising first step towards the realization of poly-SiGe based biosensors.

Self-aligned ALD AlO x T-gate insulator for gate leakage current suppression in SiN x-passivated AlGaN/GaN HEMTs

A proof-of-concept metal–insulator–semiconductor (MIS) AlGaN/GaN high-electron mobility transistor (HEMT) that uses a self-aligned 10 nm AlO x gate insulator and SiN x passivation in the device access regions was investigated. Self-alignment of the gate insulator to metal was achieved by utilizing a submicron tri-layer photoresist pattern to lift-off sequentially-deposited AlO x dielectric and Ni/Au gate metal layers. By keeping the temperature low (100 °C) during the atomic-layer deposition (ALD) of AlO x , reflow of the photoresist pattern was prevented, which maintained the integrity of its re-entrant profile. After lift-off, the resulting transistor gate had a T-shaped profile with AlO x directly under the gate metal only. In a split wafer comparison, this experimental structure reduced reverse-bias gate leakage current after passivation by one to two orders of magnitude over Schottky gate devices. Plasma-enhanced chemical vapor deposition (PECVD) SiN x passivation of the exposed AlGaN surface access regions of AlO x -insulated gate devices was found to produce pulsed I– V improvements that are similar to those observed in passivated Schottky gate devices. This fabrication technique has been successfully used to demonstrate insulated gate devices with gate lengths ( L G) as short as 160 nm with f T = 35 GHz and f max = 77 GHz small-signal performance. Substantial output conductance and f T · L G product roll-off were observed at short gate lengths for both AlO x -insulated and Schottky gate devices.

1.55‐µm DFB lasers with narrow ridge stripe and second‐order metal surface gratings by holographic lithography

AbstractWe report a high‐performance operation of laterally coupled distributed feedback (LC DFB) lasers with a narrow ridge width of 2 µm, based on second‐order metal surface gratings by holographic lithography. Through theoretical analysis, the laser structure was optimized in terms of ridge width of laser, order of grating, and isolation layer around the ridge for more stable and improved device operation. By carefully controlling the photoresist patterning and etching, the second‐order metal surface gratings of Λ = 489 nm with a duty factor of 0.25 could be achieved. The 300‐µm long laser exhibits an output power of 9 mW and a threshold current of 24.8 mA at 25 °C and 100 mA in a continuous‐wave mode, with still 2 mW at 60 °C. Stable single‐mode operation with a sidemode suppression ratio up to 41 dB, a temperature‐tuning coefficient of 0.08 nm/K, an electrical power‐tuning coefficient of 14.97 nm/W was observed for the LC DFB laser with a self‐aligned SiO2 isolation layer, estimating a gain coupling coefficient of ∼27 cm−1. The beam full‐width at half‐maximum of far‐field patterns are approximately 23.1 and 42.2° for the horizontal and the vertical directions, respectively.