Seong-Mok Cho

Crystallographic orientations and electrical properties of Bi3.47La0.85Ti3O12 thin films on Pt/Ti/SiO2/Si and Pt/SiO2/Si substrates

We report on the crystallization and electrical properties of Bi3.47La0.85Ti3O12 (BLT) thin films for possible ferroelectric non-volatile memory applications. The film properties were found to be strongly dependent on process conditions especially on the intermediate heat treatment conditions. The crystallographic orientation of the films showed sharp changes at the intermediate rapid thermal annealing (RTA) temperature of 450°C. Below 450°C, BLT thin films have orientation while they have preffered c-axis orientation above 450°C. We found that RTA conditions of the first coating layer play a major role in determining the entire crystallographic orientation of the films. The films also showed of ferroelectric hysterisis behavior strongly dependent on RTA treatment. In fact, the remanent polarization of Bi3.465La0.85Ti3O12 thin films having preferred crystallographic orientation showed much smaller value compared to those having orientation. The BLT films also exhibited good fatigue characteristics under bipolar stress up to 1011 cycles regardless of their intermediate thermal treatments.

Development of high-resolution active matrix spatial light modulator

Abstract. We developed a high-resolution active matrix spatial light modulator on a glass substrate. To integrate a switching device on the glass substrate, we designed a high-performance oxide thin-film transistor with a minimum channel length of 1  μm and a maximum processing temperature of 380°C. To drive a large number of data lines, we used multiple source drivers and data drivers as well. For an optical modulation, we optimized a liquid crystal of a high anisotropic refractive index of 0.25 with a cell gap of 2.5  μm, which was effectively operated until pixel pitch is 1.6  μm. Hologram was successfully reconstructed by fabricated SLM with 7-μm pixel pitch. For the other approach for a high-resolution spatial light modulator, we tested a phase change material of Ge2Sb2Te5 [GST]. The variation of refractive index between a polycrystalline phase and an amorphous phase of the GST film is used for a hologram reconstruction. By optimizing the underlying oxide thickness, we can show a color hologram without color filters.

Material design schemes for single-transistor-type ferroelectric memory cells using Pt/(Bi, La)4Ti3O12/ONO/Si structures

We successfully fabricated metal-ferroelectric-insulator-semiconductor (MFIS) structures using Bi3.465La0.85Ti3O12 (BLT) ferroelectric thin films and SiO2/Si3N4/SiO2 (ONO) stacked buffer layers for single-transistor-type ferroelectric nonvolatile memory applications. The BLT films were deposited on the prepared Pt/Ti/SiO2/Si and ONO/Si substrates by a sol–gel spin-coating method. The dielectric constant and the leakage current density of the prepared ONO film were measured to be 5.6 and 1.0×10-9 A/cm2 at 3 MV/cm, respectively. We found that the material and electrical properties of the BLT films were effectively modulated by the crystallographic orientation control of the thin films, which were strongly affected by the first annealing process at relatively low temperature. While the fabricated MFIS capacitors using (117)-oriented BLT films showed a charge injection phenomenon in C–V properties at high operating voltage, c-axis-oriented BLT-loaded MFIS capacitors showed a memory window of 0.6 V even at a voltage sweep of ±8 V. We conclude from these results that the c-axis-oriented BLT films formed by the newly proposed fabrication method can be successfully applied to single-transistor-type ferroelectric memory cells.

Nitrogen Doping Effect for Improving Operation Reliability of Phase Modulator Using Ge2Sb2Te5 Thin Film for Hologram Image Implementation

A phase modulation device was proposed for the implementation of hologram image for display applications. A Ge2Sb2Te5 (GST) film as thin as 7 nm was prepared between the ITO films to form the cavities corresponding a unit pixel. Nitrogen was incorporated into the GST for improving the thermal stability of the GST active region. The effects of the nitrogen doping on the physical properties of GST was investigated with the variations in doping amounts. The nitrogen incorporation was found to reduce the surface micro-roughness and to improve the thermal stability of the GST even after the crystallization by effectively suppressing the excessive grain growth. As results, the number of repeatable operations for the fabricated phase modulation device was evidently improved from 10 to 69 cycles when a 2.7-at% nitrogen was doped into the GST.

Novel Process for the Electrodes of Microbolometer

The electrodes for microbolometer it has been required to have low thermal conductivity, low electrical resistivity, and low stress for small deflection of microbolometer membrane. In this work we introduced the nickel-chrome alloy as an electrode for our designed microbolometer. To define an electrode on the membrane the etching process is indispensable. Using a dry etching technique there is a selectivity problem with underlayer of nickel-chrome alloy layer during etching process. The other hand there is a topological deterioration, which causes a bad effect for further planarization process when we use wet process. We had fabricated the electrode of microbolometer using hybrid etching technique. Therefore we have successfully established the hybrid etching process for nickel-chrome alloy film which is used for the resistive microbolometer.

A portable electronic nose (E-Nose) system using PDA device

The electronic nose (e-nose) has been used in food industry and quality controls in plastic packaging. Recently it finds its applications in medical diagnosis, specifically on detection of diabetes, pulmonary or gastrointestinal problem, or infections by examining odors in the breath or tissues with its odor characterizing ability. Moreover, the use of portable e-nose enables the on-site measurements and analysis of vapors without extra gas-sampling units. This is expected to widen the application of the e-nose in various fields including point-of-care-test or e-health. In this study, a PDA-based portable e-nose was developed using micro-machined gas sensor array and miniaturized electronic interfaces. The rich capacities of the PDA in its computing power and various interfaces are expected to provide the rapid and application specific development of the diagnostic devices, and easy connection to other facilities through information technology (IT) infra. For performance verification of the developed portable e-nose system, Six different vapors were measured using the system. Seven different carbon-black polymer composites were used for the sensor array. The results showed the reproducibility of the measured data and the distinguishable patterns between the vapor species. Additionally, the application of two typical pattern recognition algorithms verified the possibility of the automatic vapor recognition from the portable measurements. These validated the portable e-nose based on PDA developed in this study.

Surface micromachining and characterization of pyroelectric infrared ray focal plane array utilizing Pb(Zr0.3Ti0.7)O3 thin films

We have developed surface micromachined Infrared ray (IR) focal plane array (FPA), in which single SiO2 layer works as an IR absorbing plate and Pb(Zr0.3Ti0.7)O3 thin film served as a thermally sensitive material. There are some advantages of applying SiO2 layer as an IR absorbing layer. First of all, the SiO2 has good IR absorbance within 8 ~ 12 μm spectrum range. Measured value showed about 60% absorbance of incident IR spectrum in the range. SiO2 layer has another important merit when applied to the top of Pt/PZT/Pt stack because it works also as a supporting membrane. Consequently, the IR absorbing layer forms one body with membrane structure, which simplifies the whole MEMS process and gives robustness to the structure.

Fabrication of Ferroelectric Gate Memory Device Using BLT/HfO2/Si Gate Structure

Ferroelectric gate FETs with BLT/HfO2 structure were fabricated on 5-inch-scale Si wafer using well-refined CMOS compatible 0.8 μm-based fabrication processes for the first time. We obtained excellent device characteristics and good memory operations of the fabricated n-ch and p-ch MFIS-FETs, in which the memory window and on/off drain current ratio of typical p-ch memory device were measured to be 1.5 V at VG of ±5 V and 8 orders-of-magnitude, respectively. We also confirmed by evaluating the gate voltage and gate size dependences of device properties that the fabricated devices showed quantitatively reasonable ferroelectric memory operations.