Yong-Kyu Yoon

Multidirectional UV Lithography for Complex 3-D MEMS Structures

Various three-dimensionally (3-D) complex MEMS structures are fabricated using multidirectional ultraviolet (UV) lithography, which includes reverse-side exposure through a UV-transparent substrate, inclined exposure with or without simultaneous substrate rotation, and the combination of these processes. A reverse-side exposure scheme through UV-transparent substrates (e.g., glass, sapphire, or quartz) has been exploited for implementing high-aspect-ratio structures (greater than 20:1), repeatable self-alignment photoresist patterning with subsequent metallization on a BST/sapphire substrate, and unconventional patterning using substrate optics such as proximity patterning or integrated lens techniques. Inclined exposure has been applied to a SU-8 substrate with differing inclination angles and incidence directions. The refractive index of SU-8 is experimentally determined to be 1.68 by means of test structures fabricated using this approach. Implemented structures using the inclined exposure include vertical screen structures, inclined tubes, and conical shape structures. Dynamic mode operation, in which the substrate is continuously rotated and tilted during exposure is also discussed. Examples of achievable 3-D structures using dynamic mode operation are presented

Subwavelength direct laser patterning of conductive gold nanostructures by simultaneous photopolymerization and photoreduction.

This article presents a new method for fabricating highly conductive gold nanostructures within a polymeric matrix with subwavelength resolution. The nanostructures are directly written in a gold precursor-doped photoresist using a femtosecond pulsed laser. The laser energy is absorbed by a two-photon dye, which induces simultaneous reduction of gold in the precursor and polymerization of the negative photoresist. This results in gold nanoparticle-doped polymeric lines that exhibit both plasmonic effects, due to the constituent gold nanoparticles, and relatively high conductivity (within an order of magnitude of the bulk metal), due to the high density of particles within these lines. Line widths from 150 to 1000 nm have been achieved with this method. Various optically functional structures have been prepared, and their structural and optical properties have been characterized. The influence of laser intensity and scan speed on feature size have been studied and found to be in agreement with predictions of a mathematical model of the process.

An electrically active microneedle array for electroporation

We have designed and fabricated a microneedle array with electrical functionality with the final goal of electroporating skin’s epidermal cells to increase their transfection by DNA vaccines. The microneedle array was made of polymethylmethacrylate (PMMA) by micromolding technology from a polydimethylsiloxane (PDMS) mold, followed by metal deposition, patterning using laser ablation, and electrodeposition. This microneedle array possessed sufficient mechanical strength to penetrate human skin in vivo and was also able to electroporate both red blood cells and human prostate cancer cells as an in vitro model to demonstrate cell membrane permeabilization. A computational model to predict the effective volume for electroporation with respect to applied voltages was constructed from finite element simulation. This study demonstrates the mechanical and electrical functionalities of the first MEMS-fabricated microneedle array for electroporation, designed for DNA vaccine delivery.

Polymer-core conductor approaches for RF MEMS

In many radio frequency (RF) microelectromechanical systems (MEMS) applications, currents are confined to the outermost portions of conductors due to the skin effect. Conductors consisting of polymer cores coated with metal, the so-called polymer-core conductor, are appropriate to consider for these applications, and in many instances are easier to fabricate than their solid-metal-core counterparts. Implementation of polymer-core conductors using an SU-8 epoxy-core patterning and subsequent metal electrodeposition is reported. The SU-8 core approach allows for relatively simple formation of extremely high-aspect-ratio columns for inductor sidewalls. In addition, an SU-8 bridge fabrication technique has been realized using a double exposure and single develop scheme. The bridge thickness has been characterized as a function of the optical dose and the post bake time in an oven. Three-dimensional, high-aspect-ratio, high Q-factor, solenoid-type RF inductors are fabricated and tested to demonstrate the feasibility of the polymer-core conductor approach for RF applications. A single, vialess metallization over SU-8 back-bone structure provides the complete conducting paths of the inductor. A single turn inductor that is 900 /spl mu/m in height and 600 /spl mu/m in lateral extension shows a maximum Q-factor of 84 and an inductance of 1.17 nH at 2.6 GHz.

Integrated vertical screen microfilter system using inclined SU-8 structures

A multidimensional, vertical screen filter system is developed in which structures analogous to the mesh of a window screen are extended vertically through the cross-sectional area of a simultaneously-formed flow channel. Vertical screens with heights of up to 400 microns and aperture sizes of 10 microns have been achieved in a simple, multiexposure fabrication approach without the need for stacking or lamination. A triple filtering system with multiple inlet and outlet streams and three different mesh sizes of 57.3 /spl mu/m, 27.3 /spl mu/m, and 10.0 /spl mu/m in its horizontal diagonal has been simultaneously fabricated with flow channels and tested. Each mesh filters out microparticles larger than 35 /spl mu/m, 18 /spl mu/m, and 6 /spl mu/m, accordingly. In addition to microfiltering, the utility of the vertical screen filter structure as a passive micromixer has been demonstrated.

A reduced intermodulation distortion tunable ferroelectric capacitor-architecture and demonstration

A ferroelectric tunable capacitor device architecture is presented that allows for a reduction of intermodulation distortion (IMD), while maintaining high tunability at low bias voltages. The tunable capacitor is fabricated from epitaxial thin-film barium-strontium-titanate deposited on a sapphire substrate. The RF portion of the capacitor is a conventional planar gap capacitor with a 12-14-/spl mu/m gap. However, rather than superimposing the dc bias on the RF pads, a separate bias structure is fabricated within the RF gap. The interdigital bias structure has narrowly spaced high resistance (2-3/spl times/10/sup 4/ /spl Omega//sq) oxide conductor electrodes, such as indium-tin-oxide electrodes or lanthanum-strontium-cobalt-oxide electrodes spaced 1-2 /spl mu/m apart. The high resistivity of the bias electrodes decouples the dc bias from the RF signal path. This bias structure allows high dc fields to be developed with less than 30 V applied to tune the material permittivity (1 : 1.4), but is sufficiently resistive to avoid affecting the Q factor of the RF capacitor. Since the RF gap is wide, the IMD performance remains good, even at modest tuning voltages. The following three classes of gap capacitor have been fabricated for concept verification: 1) a conventional gap structure (without additional bias structure); 2) the proposed RF gap capacitor with the dc-bias structure; and 3) a narrower conventional RF gap-capacitor structure used as an IMD reference. The proposed RF gap capacitor with dc-bias structure has been fabricated in two versions: one in which the highly resistive bias electrodes are electrically connected to the RF electrodes (the attached-bias-electrode (ABE) scheme) and one in which the highly resistive electrodes are provided with a separate port for further control (the isolated-bias-electrode (IBE) scheme). In addition, parallel and perpendicular orientation of the bias electrodes relative to the RF field is investigated. The frequency response of the proposed gap capacitor with the dc-bias structure is characterized and its analysis shows that the highly resistive bias lines are serving as a dc-bias path for high tunability, but are not attenuating the RF signal. While the IBE structure has more degrees of freedom for biasing as compared to the ABE structure, the overall tunability at 30 V and IMD performance of both the ABE and IBE structures are similar. Two-tone IMD tests show that the IMD performance for the gap capacitor with the bias structure is improved by 6 dB over the conventional reference structure at the same tunability.

A Compact Omnidirectional Self-Packaged Patch Antenna With Complementary Split-Ring Resonator Loading for Wireless Endoscope Applications

A patch loaded with a complementary split-ring resonator (CSRR) is fabricated on a flexible substrate and folded in a cylindrical shape, forming a self-packaged folded patch antenna with a quasi-omnidirectional radiation pattern. The space inside the cylindrical cavity is electromagnetically shielded by the ground plane of the patch, and therefore electronic circuits can be accommodated in it with little electromagnetic interference (EMI) from the antenna or other external electronics. The CSRR contributes to size reduction. As a test vehicle, a 2.4-GHz ISM-band folded patch antenna is designed, fabricated, and characterized for a wireless capsule endoscope application, where the implemented antenna has a patch length of 10.5 mm (0.11λ ) and a folded cylinder diameter of 10 mm. A 74% size reduction is achieved after CSRR loading. The antenna located at the outermost surface not only functions as an electromagnetic radiator and an EMI shield, but also serve as a mechanical packaging structure.

Fabrication and Characterization of Gold−Polymer Nanocomposite Plasmonic Nanoarrays in a Porous Alumina Template

A facile, cost-effective, and manufacturable method to produce gold-polymer nanocomposite plasmonic nanorod arrays in high-aspect-ratio nanoporous alumina templates is reported, where the formation of gold nanoparticles and the polymerization of a photosensitive polymer by ultraviolet light are simultaneously performed. Transverse mode coupling within a two-dimensional array of the nanocomposite rods results in a progression of resonant modes in the visible and infrared spectral regions when illuminated at normal incidence, a phenomenon previously observed in nanoarrays of solid gold rods in an alumina template. Finite element full-wave analysis in a three-dimensional computational domain confirms our hypothesis that nanoparticles, arranged in a columnar structure, will show a response similar to that of solid gold rods. These studies demonstrate a new simple method of plasmonic nanoarray fabrication, apparently obviating the need for a cumbersome electrochemical process to grow nanoarrays.

Micromachined biodegradable microstructures

We present a fabrication approach for the production of micromachined biodegradable microstructures, and illustrate its application in two areas: biodegradable microneedles for transdermal drug delivery, and biodegradable ratcheting surgical ties for blood vessel surgery. We fabricated solid polymer microneedles out of polyglycolide, polylactide and their copolymer using a micromolding technique that created needles with beveled tips. Polymer microneedles were strong enough to be inserted into cadaver skin without breaking. Polymer microneedles impregnated with both low- and high- molecular weight model compounds to simulate drug release were fabricated and inserted into full thickness cadaver skin. Quantitative measurement of model compound release as a function of time was obtained. The fabrication technology was also utilized to produce more mechanically complex biodegradable microstructures: cable ties for surgical ratcheting. These devices were successfully integrated with blood vessel tissue. The change in the mechanical properties of these devices under physiological conditions was investigated and shown to depend on the chemical and physical properties of polymer, implant temperature, and chemical environment.

Analysis and Characterization of a High-Performance Ka-Band Surface Micromachined Elevated Patch Antenna

A novel Ka-band surface micromachined air-elevated patch antenna is designed, fabricated, and characterized. It demonstrates a 10.5%¿10-dB fractional bandwidth and 9.5-dBi directivity for an elevation of 600