Ki-Don Kim

Ultraviolet nanoimprinted polymer nanostructure for organic light emitting diode application

Light extraction efficiency of a conventional organic light emitting diode (OLED) remains limited to approximately 20% as most of the emission is trapped in the waveguide and glass modes. An etchless simple method was developed to fabricate two-dimensional nanostructures on glass substrate directly by using ultraviolet (UV) curable polymer resin and UV nanoimprint lithography in order to improve output coupling efficiency of OLEDs. The enhancement of the light extraction was predicted by the three-dimensional finite difference time domain method. OLEDs integrated on nanoimprinted substrates enhanced electroluminance intensity by up to 50% compared to the conventional device.

Photo-induced hybrid nanopatterning of titanium dioxide via direct imprint lithography

A novel ultraviolet (UV)-assisted imprinting procedure that employs photosensitive titanium(IV) di-n-butoxide bis(2-ethylhexanoate) is presented for the fabrication of well-ordered titanium dioxide (TiO2) nanostructures at room temperature. The main novelty of this technique is the use of the photosensitive titanium organic compound, rather than a commonly used UV-curable resin, for direct UV-assisted nanoimprint lithography. Fourier transform infrared and X-ray photoelectron spectroscopy studies suggest that exposure to UV light resulted in the gradual removal of organic groups from films prepared from titanium(IV) di-n-butoxide bis(2-ethylhexanoate) photochemically and successively converted the films to TiO2 at room temperature. This approach allows direct fabrication of TiO2 nanopatterns with lines down to 35 nm in width, hole arrays of 265 nm in diameter, and three-dimensional TiO2 hybrid micro/nano-patterns without observable defects for use in applications where ordered surface nanostructures are required, such as photovoltaics, photonics, and optical waveguides.

An Antireflective Nanostructure Array Fabricated by Nanosilver Colloidal Lithography on a Silicon Substrate

An alternative method is presented for fabricating an antireflective nanostructure array using nanosilver colloidal lithography. Spin coating was used to produce the multilayered silver nanoparticles, which grew by self-assembly and were transformed into randomly distributed nanosilver islands through the thermodynamic action of dewetting and Oswald ripening. The average size and coverage rate of the islands increased with concentration in the range of 50–90 nm and 40–65%, respectively. The nanosilver islands were critically affected by concentration and spin speed. The effects of these two parameters were investigated, after etching and wet removal of nanosilver residues. The reflection nearly disappeared in the ultraviolet wavelength range and was 17% of the reflection of a bare silicon wafer in the visible range.

Effective fabrication of three-dimensional nano/microstructures in a single step using multilayered stamp

A technique in ultraviolet nanoimprint lithography (UV-NIL) for the creation of three-dimensional (3D) nanopatterns in a single step is proposed. The single-step fabrication of 3D or multilevel structures has a multitude of benefits. Inherent in this is the elimination of a need for alignment for multilevel fabrications as well as being a cost effective and simple process. For 3D UV-NIL, a trial in the fabrication of multilayered stamps has been conducted employing two-photon polymerization and diamondlike carbon (DLC) coating technique. The DLC coating layer enables the polymer patterns to be used effectively as a stamp without the need for an antiadhesion material. Additionally, O2-plasma ashing has the potential for an epoch-making improvement of the precision of polymer patterns with a linewidth of 60nm. Overall, several fine patterns are imprinted using the multilayered stamp onto a UV-curable resist via a single-step process without any identifiable damage.

Facile nanopatterning of zirconium dioxide films via direct ultraviolet-assisted nanoimprint lithography

A novel ultraviolet (UV)-assisted imprinting procedure that employs photosensitive zirconyl 2-ethylhexanoate is presented for the fabrication of both amorphous and crystalline zirconium dioxide (ZrO2) nanostructures. Upon annealing at 400 °C for 1 h, the lateral shrinkage and thickness shrinkage of ZrO2 nanostructures were 69.8 and 66.7%, respectively, indicating an isotropic volume loss. During UV irradiation and annealing treatment, the refractive index of UV-irradiated ZrO2 film is gradually increased by improvement in the packing density and crystallinity of the film. With increasing UV exposure time and annealing temperature, the optical band gap (Eg) of the UV-irradiated ZrO2 film is red-shifted from 5.745 to 5.265 eV, due to the removal of organic groups and the resultant densification of the film during the photochemical reaction and the heat-induced increase in the crystallinity of the film. These results suggest that the refractive index and optical Eg of ZrO2 nanostructures could be controlled by tuning the conditions of UV exposure time and annealing treatment. Nanopatterns of ZrO2, fabricated by direct UV-assisted nanoimprint lithography, are potential candidates for protective coatings for optical mirrors and filters, e.g. high-reflectivity mirrors and broadband interference filters, as well as active electro-optical devices where ordered surface nanostructures are necessary.

Resist Flow Behavior in Ultraviolet Nanoimprint Lithography as a Function of Contact Angle with Stamp and Substrate

This study was performed to investigate dynamic resist filling in ultraviolet nanoimprint lithography. The effects of surface condition on flow behavior during the filling process, including the contact angle and the aspect ratio of recessed features, are presented through numerical simulations. The optimized surface conditions for good imprinting results were determined through numerical experiments covering a range of test conditions. A decrease in the feature aspect ratio of up to 0.5 resulted in a contact angle twice the original value. Complete filling occurred when the contact angle of the vertical side wall was as low as that of the substrate.

Adaptive bonding technique for precise assembly of three-dimensional microstructures

Precise fabrication of three-dimensional (3D) self-standing microstructures on thin glass plates via two-photon induced polymerization (TPP) has been an important issue for innovative 3D nanodevices and microdevices. However, there are still issues remaining to be solved, such as building 3D microstructures on opaque materials via TPP and being able to implant them as functional parts onto practical systems. To settle these issues simply and effectively, the authors propose a contact print lithography (CPL) method using an ultraviolet-curable polymer layer. They report some of the possibilities and potential of CPL by presenting their results for transplanting 3D microstructures onto large-area substrates and also their examination of some of the effects of the process parameters on CPL.

UV-nanoimprint lithography using a diamond-like carbon stamp

Two-dimensional (2-D) and three-dimensional (3-D) diamond-like carbon (DLC) stamps for ultraviolet nanoimprint lithography (UV-NIL) were fabricated with two methods: namely, two-photon polymerization (TPP) patterning, followed by nanoscale-thick DLC coating; and a fluorine-doped (F-DLC) coating process, followed by O2 plasma etching. The DLC layer on top of polymer pattern or flat quartz substrate was formed using radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) process or Direct current (DC) and radio frequency (RF) magnetron sputtering process. It was also demonstrated that the DLC stamp with no anti-adhesion layer could be used for imprinting wafers on UV-NIL and the dimensions of the stamps features correlated well with the corresponding imprinted features.

Mass fabrication of resistive random access crossbar arrays by step and flash imprint lithography

Step and flash imprint lithography (SFIL) is a promising method recently used for next generation lithographic technology because it is a high-speed process that can be carried out at room temperature and low pressures. Improvements made to SFIL enable the replication of crossbar patterns with a high resolution and the development of suitable materials and techniques to achieve high resolution capability. In this study, SFIL is used to fabricate high-density random access crossbar arrays based on a NiO resistive switching system. The bottom and top electrodes are transferred onto silicon wafers perpendicular to each electrode using the inductively coupled plasma reactive ion etching (ICP-RIE) technique. Direct metal etching without a wet-based process minimizes damage to the electrode surface. The I-V curves of individual active cells (70 x 70 nm(2)) for crossbar arrays reveal the unipolar resistive switching (RS) behaviour of the fabricated device. A high off/on resistance ratio (>10(4)) and reproducible resistance switching characteristics for each active cell were found in different fields and for different wafers. The experimental data indicate that high-density crossbar arrays can be well replicated and that the electrical performance of these arrays is reliable.

Plasma-assisted quartz-to-quartz direct bonding for the fabrication of a multilayered quartz template for nanoimprint lithography

In this paper, a low-temperature plasma-assisted process is developed to realize a uniform, ultraviolet (UV) transparent and chemically inert quartz-to-quartz direct bonding. Two sets of pretests are performed in order to understand how the bond surface energy changes with the plasma exposure time and the wet etching of quartz, respectively. The developed technique is used to fabricate a multilayered quartz template for UV nanoimprint lithography (UV-NIL). The multilayered quartz template is fabricated by bonding a square piece of a standard quartz wafer, which is about 625 µm in thickness, to a wet-etched 6.35 mm thick quartz photomask plate. A fabricated multilayered template is loaded to the commercial UV-NIL tool Imprio™ 100, and NIL was performed successfully. The developed direct bonding technique makes it possible for standard quartz wafers, which are compatible with high-resolution semiconductor fabrication processes, to be utilized as the templates in commercial UV-NIL machines with enhanced mechanical stability.