Bong-Hwan Kim

Release of ciprofloxacin from poloxamer-graft-hyaluronic acid hydrogels in vitro

Recently, in situ gel formation has extensively been studied to enhance ocular bioavailability and duration of the drug activity. In this study, we report grafting of poloxamer onto the hyaluronic acid for application of tissue engineering oriented ophthalmic drug delivery system. Graft copolymers were prepared by coupling mono amine-terminated poloxamer (MATP) with hyaluronic acid (HA) backbone using 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) and N-hydroxylsuccinimide (NHS) as coupling agents. The coupling of MATP with HA was clarified by 1H NMR and FT-IR spectroscopy. The gelation temperature of graft copolymers was dependent on the content of HA and the concentration of poloxamer. From drug release studies in vitro, ciprofloxacin was sustainedly released from the poloxamer-g-hyaluronic acid hydrogel due to the in situ gel formation of the copolymer and viscous properties of HA.

Fabrication of an electrostatic track-following micro actuator for hard disk drives using SOI wafer

This paper presents track-following control using an electrostatic microactuator for super-high density hard disk drives (HDDs). The electrostatic microactuator, a high aspect ratio track-following microactuator (TFMA) which is capable of driving 0.3??g magnetic head for HDDs, is designed and fabricated by a microelectromechanical systems process. It was fabricated on a silicon on insulator wafer with a 20??m thick active silicon layer and a 2??m thick thermally grown silicon dioxide layer; a piggyback electrostatic principle was used for driving the TFMA. The first vibration mode frequency of the TFMA was 18.5?kHz, which is enough for a recording density of higher than 10?Gb?in-2. Its displacement was 1.4??m when a 15?V dc bias plus a 15?V ac sinusoidal driving input was applied and its electrostatic force was 50.4??N when the input voltage was 30.7?V. A track-following feedback controller is designed using a feedback nonlinear compensator, which is derived from the feedforward nonlinear compensator. The fabricated actuator shows 7.51?dB of gain margin and 50.98? of phase margin for a 2.21?kHz servo bandwidth.

A novel 3D process for single-crystal silicon micro-probe structures

A new fabrication method for a three-dimensional (3D), single-crystal silicon micro-probe structure is developed. A probe card structure requires tips that are at least 50 μm tall on cantilevers thick enough to withstand a few mN of force as well as 50 μm of tip bending. The cantilever structure also must be able to move at least 50 μm of vertical motion, requiring a large sacrificial gap. The developed 3D fabrication method is based on the surface/bulk micromachining technology, which can fabricate released, high aspect ratio, single-crystal silicon microstructures with high yield using (111) silicon.

Cantilever-Type Microelectromechanical Systems Probe Card with Through-Wafer Interconnects for Fine Pitch and High-Speed Testing

A new microelectromechanical systems (MEMS) probe card made of electroplated nickel cantilevers, which has compliant structure, with through-wafer interconnections for IC testing is developed. The measured contact resistance was less than 0.5 ohm and leakage current was approximately 0.13 nA between tips and pads. In addition, planarity of tips and the alignment of x- and y-axes were satisfied with conventional needle probe card performance. Therefore, this probe card is suitable for wafer-level burn-in testing and function testing of memory and RF devices. In particular, since through-wafer interconnection can offer the shortest signal line path and reduce signal delay, it is possible to implement fine pitch and high-speed devices. The probe was fabricated with the 4-inch silicon substrate.

A new class of surface modifiers for stiction reduction

In order to achieve stiction-free polysilicon surfaces, we have suggested a new class of chemical grafting precursors and confirmed their excellent characteristics. The strategy is to adopt dialkyldichlorosilanes (DDS, R/sub 2/SiCl/sub 2/) instead of monoalkyltrichlorosilanes (MTS, RSiCl/sub 3/) such as octadecyltrichlorosilane (OTS, C/sub 18/H/sub 37/SiCl/sub 3/) or 1 H,1 H,2 H,-2 H-perfluorodecyltrichlorosilane (FDTS, C/sub 10/H/sub 4/F/sub 17/SiCl/sub 3/). DDS has two short chains and compactly deposits on the hydrophilic polysilicon surface. DDS coated surface is even superior to those of conventional compounds in some respects. Since interactions among precursor molecules are reduced, conglomeration both in homogeneous solution and on surface can be effectively avoided. No difference was found between results from the processes in ambient environment and in dry box. And the final quality of the modified surfaces is much less dependent on temperature. In addition, DDS has some advantages of remarkably reduced process time and acceptably low cost. Even the cantilevers with 3 mm in length can be protected successfully from the stiction.

A robust MEMS probe card with vertical guide for a fine pitch test

A vertically guided MEMS probe card was designed to deflect 50 ?m at a force of 1.5 g and achieve less than 50 ?m of pad pitch. Based on our experimental results, the measured average contact resistance of a device under test (DUT) was approximately 0.2 ? at 1.44 g of force and the leakage current between two tips in the distance of one pitch was about 10 pA. In addition, tip planarity was about ?6 ?m with x?y alignment errors within ?8 ?m. A reliability test showed that the average contact resistance was 0.34 ? and the probe tip wear was less than 1 ?m after the 10?000 timed touchdowns. To be capable of fine pitch probing, a cantilever beam was constructed by dry etching using a positive photoresist. After the cantilever beam was formed by silicon etching using a deep RIE etcher inside a deep-recessed trench, a vertically guided structure was created from the cantilever beam. Furthermore, to make a horizontally stopped structure, the cantilever beam was designed to have a pyramid tip with a width bigger than that of the beam itself. This kind of structure is mechanically stable when the tip is applied with an oblique force. Because the probe card can be guided vertically and horizontally, it can be neither broken nor deformed by any directional force. In that respect, this newly proposed probe card is suitable for wafer-level testing and fine pitch device testing.

A novel MEMS silicon probe card

We have developed a novel cantilever-type probe which is capable of less than 70 of pitch and 12g of force. This probe is suitable for wafer-level burn-in testing, function testing and circuit-board O/S testing including memory and RF devices. The probe was fabricated with epitaxial polysilicon on silicon substrate. The through via hole interconnection was formed in silicon wafer by nickel electroless plating and copper electroplating. The electroless plating is easy method and can deposit film uniformly for deep trench and through hole. Especially, it can deposit film on any substrates without seed layer and allow it to be electroplated. The aspect ratio of through via hole was larger than 10:1 and the contact resistance was less than 1 ohm.

MEMS fabrication of high aspect ratio track-following microactuator for hard disk drive using silicon on insulator

We have achieved a high aspect ratio track-following microactuator (TFMA) which is capable of driving 2 /spl mu/g magnetic head for hard disk drive (HDD). It was fabricated on silicon on insulator (SOI) wafer with 20 /spl mu/m thick active silicon and 2 /spl mu/m thick thermally grown oxide and piggyback electrostatic principle was used for driving TFMA. The first vibration mode frequency of TFMA was 18.5 kHz which is enough for a recording density of higher than 10 Gb/in/sup 2/. Its displacement was 1.4 /spl mu/m when 15 V DC bias plus 15 V AC sinusoidal driving input was applied and its electrostatic force was 50 /spl mu/N when input voltage was 30 V.

LabVIEW Based Study for PV Module Characteristics and Their Maximum Power Point Tracking

In this paper, we examined the solar module characteristics at standard test condition and different temperature and irradiance using LabVIEW software. For full utilization of photovoltaic(PV) system, it can be controlled by maximum power point tracking(MPPT) algorithm. The LabVIEW programming can easily implement perturb and observe (P&O) algorithm with various step sizes and their output characteristics. The results proved that the step size influence on the performance of the MPPT system. The bigger the step size, the less the tracking time but the more oscillation occurs in the steady state. Therefore, improved control system is required to solve that problem. The scheme that we devised can adjust the step size automatically to improve the MPPT speed and reduce the oscillation under steady state as well.

Design and Fabrication of Microelectromechanical Systems Probe Card with Vertical Trench Guide for Fine Pitch Probing

In this study, we developed a new microelectromechanical systems (MEMS) probe card with a vertically guided and electroplated cantilever in order to overcome pitch limitation, which has a compliant structure and is capable of a 35 µm pad pitch, with through-wafer interconnections for IC testing. We also investigated the reliability of the MEMS probe card using tribology and scaling of the MEMS probe card for fine pitch probing. The measured contact resistance was less than 0.5 Ω at 1.5 gf applied force and the leakage current was approximately 10 pA between one tip and an adjacent tip. In addition, the planarity of tips and the alignment of x- and y-axes were satisfied with a conventional needle probe card performance. Therefore, this probe card is suitable for wafer-level burn-in testing and function testing of memory and RF devices.