Hwan-Soo Lee

Influences of film microstructure and defects on magnetization reversal in bit patterned Co/Pt multilayer thin film media

Quasisingle crystalline and polycrystalline Co/Pt multilayered films were prepared via sputtering technique. The polycrystalline Co/Pt multilayers exhibited an appreciable number of planar defects such as twin boundaries and stacking faults whereas few defects were present for the quasisingle crystalline films. The polycrystalline films had smoother surface, and as patterned into arrays of small islands, a smaller critical size for single domain was unexpectedly observed. The corresponding magnetic domain images revealed that nucleation interestingly occurred at any locations of a patterned element, which was attributed to the observed defects. Moreover, micromagnetic modeling was utilized to further quantitatively study influences of an anisotropically soft region (which can represent existing defects) in the patterned element on nucleation field in terms of exchange coupling strength.

Thermal Imprint Lithography onto Filler Incorporated Composite Resin

Thermal imprint lithography was investigated for application to printed circuit boards. Features in size ranging from 6 to 100 µm were successfully transferred onto an epoxy-based thermoset resin with a filler (SiO2) content of approximately 38 vol %. An apparent increase in imprinted pattern width was observed and attributed to an increase in the filler content inside the pattern. Resin-filler interaction was shown to be an important factor in the observed pattern dimension when a filler incorporated composite resin was used for thermal imprint lithography. Control of the squeezing flow during imprinting was crucial for the pattern transfer fidelity.

Microcup Array Fabricated by Thermal Imprint Lithography for E-Paper Application

Thermal imprinting method was developed to provide a microcup array for the application to e-paper, and the imprinted feature was investigated as functions of time and temperature. Even in a short period of time of 2 second, the pattern was imprinted onto a resin to the full depth at a temperature higher than T/Tg = 0.64 (where the Tg is 156°C), however, the hardening process as long as 30 minutes appears to be necessary to achieve the full mechanical strength. We propose a roller imprint process that suffices the two experimental observations and is put to practical use. A large array of microcup having typical dimension in the range of 60–100 µm in width or length, 20–40 µm in height, and 10–20 µm in width of partition walls was able to be fabricated via the roller imprint method.

A Study on Phase Separation in InGaN/GaN Multi-quantum Wells

Phase separation in InGaN/GaN multi-quatum wells (MQWs) grown with various annealing time is studied for obtaining the high brightness green LED. For this purpose, transmission electron microscopy (TEM), and photoluminescence (PL) are used. InGaN/GaN MQWs for the Green LED with high Indium composition up to 30% are grown at 800-900°C by metal organic chemical vapor deposition (MOCVD) and annealed during 0, 5, 12.5 minutes at 1000°C. It is found that the phase separation in InGaN/GaN multi-quantum wells under the condition of annealing for 5minutes is severe according to PL spectrum and indium ratio map. It is directly obtained indium concentration map images with energy filtered transmission electron microscopy (EFTEM) which can be interpreted as indium compositional fluctuation in Fig 1. The PL intensities of MQWs with annealing for 5 minutes increased when compared to MQWs prepared without annealing at 1000°C and The PL spectra separated blue and green emission peaks as the result of indium phase separation in Fig. 2. As the annealing time increases, the emission peak shows a blueshift in Fig 2. The presence of indium clustering with high indium composition results in high luminescence efficiency. It is likely to see the recombination center of excitons localized in indium clustering regions.