Joon-Soo Kim

Directional Solidification Behaviors of Polycrystalline Silicon by Electron-Beam Melting

The advanced electron beam melting (EBM) system with the combination of vacuum refining and directional solidification (DS) performed the purification of large amounts of metallurgical grade silicon (MG-Si). In order to increase grain size or to align columnar grains being parallel to DS pulling direction in Si ingots, non-irradiated inner diameters in an EB pattern in the DS process were varied at a range of 5–35 mm. Average grain size increased with increasing non-irradiated inner diameter due to a smaller temperature gradient during the solidification of Si melts. However, the slope of the grain boundary inclined towards the ingot axis, which led to the formation of a triple junction in the ingot center in the case of large non-irradiated inner diameter. This happened despite there being a large temperature gradient due to the turbulent flow in the pool. This work reported that a purity of 99.8% for MG-Si was improved to above 99.999% with an ingot yield of 90% for 1 h.

Polycrystalline silicon wafer with columnar grain structure grown directly on silicon carbide coated graphite substrate

Concerning horizontally direct growth technology, graphite material with excellent thermal and chemical resistance is frequently used as both the crucible and the substrate in equipment operated at high temperatures. However, this can cause a major problem, namely, contamination due to the carbon and metallic impurities derived from graphite components. This study presents the experimental findings on a method of preventing the incorporation of unintended impurities into the silicon wafer by modifying the surface of the graphite. Coating the crucible and substrate with silicon carbide (SiC) could significantly reduce carbon (1.5u2009×u20091017 atom/cm3) and metallic (undetected under analysis resolution) contamination by suppressing physical contact between the silicon melt and the graphite. It is expected that the purity of the silicon wafer will be improved to meet the demand for next-generation solar cell production with the optimum growth conditions required for obtaining the desired property of the silicon wafer.

Wafer bumping technology for LDI application by electroless nickel plating

Electroless nickel layer has been used to provide an under bump metallization on the aluminum bond pads, as part of low cost wafer bumping process, prior to solder deposition. Recently, it has also found increasing use in wafer bumping for LDI(LCD for Drive IC) type device. However, the application of electroless nickel to the LDI wafer bumping is limited by the fact that bath stability of nickel is closely related with passivation opening size. In the present work, the effects of nickel bath parameters of stabilizer concentration, temperature and pH on the formation of electroless nickel bump have been investigated and optimum process conditions for LDI bumping were suggested. The measurements of bump surface and height distribution have been performed for the bump quality estimation by optical microscope and thickness profiler.

Study of Europium-activated Calcium Aluminium Silicate Phosphors

Europium-activated calcium aluminium silicate phosphors were synthesized for the first time and the structures and luminescence characteristics of these phosphors were investigated. The phosphors in this study emitted blue, green, and even red light depending on the starting milterials and annealing conditions for synthesis. In addition, the structure was also changed when the different starting materials were used. When was used as a starting material, tetragonal was formed. However, pure green light was emitted when the annealing was conducted in reduced atmosphere and red one was emitted by annealing in air. In the case of as a starting material, triclinic was formed and only pure blue emission was observed. Moreover, this blue phosphor exhibited higher intensity than that of commercial YAG:Ce phosphor, which showed the possibility of application on the phosphor for new light source such as a UV-LED.

Synthesis of Nanoporous Carbon as a Gas Adsorbent by Reverse Replication Process of Silica Template

Porous carbon with high surface area and pore volume was prepared by a reverse replication process and its toluene equilibrium adsorption behavior was investigated. The preparation process of the porous carbon was composed of following sub-processes in series: synthesis and template preparation of silica gel, impregnation and polymerization of DVB monomer in silica template, carbonization of DVB polymer in a silica-polymer composite, and HF-assisted selective etching of silica in carbon-silica composite. The prepared porous carbon was nano porous and had ultrahigh specific surface area (2007 ㎡/g) and large pore volume (3.07 ㎤/g). The nanoporous carbon showed rapid toluene adsorption rate and good toluene adsorption capacity, compared with a commercial Y-type zeolite. In the present study, a reverse replication process to prepare nanoporous carbons will be introduced and its application potential as a gas adsorbent will be discussed.