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.
Improving -wire electro-discharge machining operation of polished silicon wafer by conductive coating ICOMM 2014 No.72
Micro-Wire Electro-discharge machining (-WEDM) is a nonconventional machining technology which is extensively used for metal based micro fabrication process. This is a non-contact machining process where material removal is taken place by electro-thermal action. -WEDM process is difficult to be applied for semiconductor material like Silicon (Si). In this paper a new approach is proposed for machining polished Si (p-type, resistivity 1-50 -cm) wafer. In this method, initially Si workpiece is coated with a conductive material (gold for this study) and then -WEDM operation is carried out. Finally, after WEDM operation, the conductive layer is removed from the polished Si substrate without damaging the substrate. WEDM process stability was found to be improved (up to 60 times for certain machining condition) if coated Si wafer is used as compared to uncoated Si workpiece. Material removal rate was also found to be increased by a good margin (~ 100% maximum) for coated Si wafer. Overall this new method of -WEDM operation of polished Si wafer has been found to be more efficient and useful.
Transient Workpiece Temperature Analysis in the EDM Processes of High Electric Resistance Materials Considering Joule Heating.
This paper investigates the removal mechanism of high electric resistance materials in EDM processing by calculation of the distribution of the workpiece temperature taking the Joule heating caused by voltage drop in the workpiece into account. The voltage drop in the workpiece is extensive in the EDM processing of high electric resistance materials such as conductive ceramics. The voltage drop was therefore estimated by a numerical calculation, and its results were used to calculate the distribution of Joule heating inside the workpiece. Then considering both the Joule heating and the heat flux from the arc column, the time dependent workpiece temperature was computed by the finite-difference method. The expansion of arc column with time lapse which affects greatly the result of the above calculation was obtained experimentally and taken into account in the calculation. The calculated results show that the voltage drop mostly occurs in the vicinity of the discharge spot, and the calculated total voltage drop coincides well with the measured one. From the computed temperature distribution it is found that the Joule heating causes a temperature rise nearby the discharge spot which is comparable to what is given by the heat flux from the arc column, which was confirmed by the measured material removal by a single pulse which is greater than the removal of steel. Consequently, sufficient cooling of the discharge spot may assure the stability of machining and increase the material removal rate of conductive ceramics rather than metallic materials.