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.
A New Slicing Method of Monocrystalline Silicon Ingot by Wire EDM
Monocrystalline silicon is one of the most important materials in the semiconductor industry because of its many excellent properties as a semiconductor. In the manufacturing process of silicon wafers, inner diameter (ID) blade and multi wire saw have conventionally been used for slicing silicon ingots. However, some problems in efficiency, accuracy, slurry treatment and contamination are experienced when applying this method to largescale wafers of 12 or 16 inch diameter expected to be used in the near future. Thus, the improvement of conventional methods or a new slicing method is strongly required. In this study, the possibility of slicing a silicon ingot by wire EDM was discussed and the machining properties were experimentally investigated. A silicon wafer used as substratum for epitaxial film growth has low resistivity in the order of 0.01Ω·cm, which makes it possible to cut silicon ingots by wire EDM. It was clarified that the new wire EDM has potential for application as a new slicing method, and that the surface roughness using this method is as small as that using the conventional multi wire saw method. Moreover, it was pointed out that the contamination due to the adhesion and diffusion of wire electrode material into the machined surface can be reduced by wire EDM under the condition of low current and long discharge duration.
Hygroscopic Expansion of some Casting Investments
CREDIT is generally given to Taggart1 for introducing the cast gold inlay to the dental profession. Although he realized that gold shrinks when cooled from molten state to solid state of room temperature, the investment available to him did not supply sufficient compensation. In 1910, Van Horn2 published a paper dealing with a method by which he obtained the compensation by means of wax expansion. The next major research in connection with this problem was the work of Coleman3 at the National Bureau of Standards. He determined the gold shrinkage to be 1.25 per cent and presented a casting technic with compensation values of quantitative nature. The total compensation for the gold shrinkage was obtained by the combination of the setting and thermal expansion of the mold. This procedure is the general practice followed at the present time for compensation. There are some disadvantages to this method. Skinner has indicated the difficulty of obtaining smooth castings and the improper compensation for the casting shrinkage.4 It has been found by Hollenback5 and Scheu6a that castings made at a high temperature are rougher than those made at a low temperature (450° C.), depending upon the great difference in thermal expansion between plaster of Paris and silica, which causes a poor union of the fine particles. This may also result in friability and there may be some warpage due to the differential expansion. A method which provides sufficient compensation at a low mold temperature is the hygroscopic casting technic.b Wilson7 showed that certain plasters expand as much as 0.0044 inch to the inch by taking up moisture from the air. With this information at hand, Scheula, b tested some commercial investment to determine what results the addition of moisture would have. He found6b: (1) All of the casting investments that were tested expanded. (2) The thicker the mix, the greater the expansion. (3) A mix with hot water expanded more than one with cold water. (4) The time at which the mix is placed in the water is of great importance. It was found that if the mix is placed in the water early, it would not expand so much as when placed in a little later. It was also found that if the mix was placed in the water after a certain time, it could not expand to the maximal amount. (5) The temperature of the water in which the mix is placed is important, for two reasons; (a) the temperature of the water controls slightly the expansion of the investment, and (b) water at a temperature of 1000 F. slightly softens the wax pattern so that the fine margins are not marred while expansion is taking place. The mix should remain in the water at least 20 minutes to obtain the full expansion, after which time no further expansion is obtained.