Development of multi-wire EDM slicing method for silicon ingot
INSTRUCTIONS The diameter of silicon wafer becomes larger in order to increase the number of chips per one wafer. For such a larger diameter silicon ingot is mainly sliced by using multi-wire method, in which a long thin wire is used with slurry. In this method, several hundreds of wafers could be sliced at the same time. However there are still problems remained, such as large cracks, slurry treatment, contamination and so on. Therefore, we proposed a new slicing method of monocrystalline silicon ingot using wire electrical discharge machining (wire-EDM), and showed the possibility of wire EDM slicing for monocrystalline silicon ingot [1]. However, the efficiency of wafer slicing by the commerciallyavailable wire-EDM compared to multi-wire saw method is very low, because the number of sliced wafer was only one per one process in the conventional wire-EDM method. Therefore, a multi-wire EDM slicing equipment was newly developed with a single wire and a multi-EDM power supply unit in order to increase the efficiency of wafer slicing [2-4]. In this report, machining characteristics for mono-crystalline silicon ingot using this newly developed multiwire EDM slicing equipment was discussed. Besides, the possibility of applying this technique to silicon ingot for solar cell was also investigated.
Experimental study on improving μ-WEDM and μ-EDM of doped silicon by temporary metallic coating
Wire electro-discharges machining (WEDM) is a variant of electro-discharge machining (EDM) based method for non-contact machining that uses wire as the machining tool. Micro-EDMing/micro-WEDMing (where discharge energy is very low and tool size in micrometer range) operation of silicon (Si) can achieve features with very complex shapes, which have many useful applications in microelectro-mechanical system (MEMS). However, Si is a semiconductor material with high resistivity, and it is difficult to create electrical sparks during μ-WEDM/μ-EDM process. Thus, the machining of Si can be enhanced if it is possible to increase the conductivity of this semiconductor. This study aims to develop and characterize the process of improved μ-WEDM and μ-EDM of Si by temporarily coating Si with a high conductive metal (gold in this study). Micro-WEDM process stability was found to be improved (~100× for different machining condition) if coated Si wafer is used as compared to uncoated Si workpiece. Material removal rate (MRR) was also found to be increased by a good margin (~30 % average) for coated Si wafer. Machined slots were found to be more uniform though kerf width was slightly larger for coated Si wafer. In case of μ-EDM operation, gold-coated Si also enhanced the machining as compared to uncoated Si. In this case, MRR was increased by up to seven times for coated samples. Overall, this new method of μ-WEDM and μ-EDM technique of polished Si wafer has been found to be more efficient and useful. Removal of the conductive coating without damaging the substrate is a challenge for this process, which was carried out successfully by selective etching method.
Development of high precise and efficient slicing technology by using unidirectional multi-wire-saw.
A slicing technology based on new concept of Multi-Wire-Saw has been developed for high precision and efficiency. This technology is composed of new driving system and new temperature control system. First, to achieve high efficiency, unidirectional wire driving system was adopted. In this system, a very long wire is driven in only one direction at high speed (max 600 m/min), while a wire is driven reciprocally in coventional Multi-Wire-Saw. Second, to achieve high slicing precision, temperature control system has been newly developed, in which cooling oil is circulated to keep the temperature constant on the grooved roller surface during slicing. This system has made it possible to stabilize positions of wire and to minimize wafer bow caused by thermal expansion of grooved rollers during slicing.