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.
Development of simple power supplying system in multi-wire EDM slicing method
INTRODUCTIONS Monocrystalline and polycrystalline silicon are widely used in semiconductor and photovoltaic industries. In addition, silicon carbide and gallium nitride have been expected as a material of power semiconductor. These materials are hard and brittle, and it is required to slice these materials efficiently. A multi-wire saw method is widely applied to slicing of these ingots. However, this method has some problems such as cracks generated on sliced surface and a large kerf loss. On the other hand, a wire EDM method has a possibility to reduce a kerf loss, since a thin wire electrode can be used owing to its small machining force with no-mechanical contact. In this method, the removal of material can be performed regardless of material’s hardness. Therefore, a multi-wire EDM slicing method was newly developed to overcome these problems in order to increase the efficiency of slicing with wire EDM technologies [1]. Moreover, a wire electrode with a track-shaped section can be expected to reduce a kerf loss even in the case of thin wire electrode [2]. However, a multi-wire EDM slicing equipment becomes very complicated, when many power supplying units are arranged by corresponding the number of processing wire electrode. In this individual power supplying method, it is necessary to increase the number of conductivity piece with increasing the number of processing wire electrode as shown in Figure 1 (a). This complicated setup leads to a high initial cost, difficulties of the preparation and the heavy load on the maintenance for a multi-wire EDM slicing equipment. Therefore, a group power supplying method is newly proposed to satisfy both a simple equipment and a high efficiency slicing. In this method, a resistance of wire electrode between a conductivity piece and a processing point can play as a limiting resistance of EDM power supplying unit for controlling a peak current of a discharge pulse as shown in Figure 1 (b). In this study, a newly proposed group power supplying method was discussed, and its fundamental characteristics was experimentally investigated.
Submicron polymer structures with X-ray lithography and hot embossing
This article describes the process chain for replication of submicron structures with varying aspect ratios (AR) up to 6 in polymethylmethacrylate (PMMA) by hot embossing to show the capability of the entire LIGA process to fabricate structures with these dimensions. Therefore a 4.7 μm thick layer of MicroChem 950k PMMA A11 resist was spin-coated on a 2.3 μm Ti/TiOx membrane. It was patterned with X-ray lithography at the electron storage ring ANKA (2.5 GeV and λc ≈ 0.4 nm) at a dose of 4 kJ/cm3 using a Si3N4 membrane mask with 2 μm thick gold-absorbers. The samples were developed in GG/BDG and resulted in AR of 6–14. Subsequent nickel plating at 52°C resulted in a 200 μm thick nickel tool of 100 mm diameter, which was used to replicate slit-nozzles and columns in PMMA. Closely packed submicron cavities with AR 6 in the nickel shim were filled to 60% during hot embossing.