TECHNOLOGY
Si Yields to Horizontal Zone Refining Sylvania uses water cooled, nonreactive vessels to produce semiconductor silicon by zone purification It's now possible to purify silicon by horizontal zone refining. Although the method has been used successfully with germanium, it had been impractical for silicon because, at the temperature of the molten metal, most container materials react with silicon and contaminate the product. Now, two scientists at Sylvania Electric Products have designed horizontal zone refining equipment with receptacles that don't react with molten silicon. Duncan M. Lamb of the chemical and metallurgical division, To wan da, Pa., and John L. Porter of the semiconductor division, Woburn, Mass., use cold-wall containers. Water in direct contact with the container walls keeps them below the melting point of silicon (1420° C ) . Under this condition, molten silicon doesn't attack or wet the walls. Nor does the silicon adhere to the container when solidifying. Sylvania has used the process to make silicon commercially. Product quality compares favorably with that of silicon refined by the floating zone
method. But process costs are high. Further work will be required to lower costs of the cold quartz system and put it on a competitive basis with other methods. Speaking in Boston, Mass., before the Conference on Ultrapurification of Semiconductor Materials, sponsored by the Air Force's Cambridge Research Laboratories, Hanscom Field, Mass., Mr. Lamb described Sylvania's work. He indicated that resistivity of silicon bars purified by horizontal zone refining compares to that of single crystals grown from the same starting material. Resistivity, he adds, is very close to that obtained with poly- and single-crystal germanium in the higher resistivity ranges (about 40 ohm-cm.). Work on horizontal zone purification has been aimed chiefly at finding suitable materials for the zoning vessel. Although the floating zone method for purifying silicon doesn't require a crucible, it is plagued by other difficulties, such as poor zone length stability and small cross sectional area. Also, it allows only one zone per pass.
PURIFIES SILICON. High purity silicon is produced by this laboratory model of Sylvania's cold quartz, horizontal zone refiner. Water flowing in jacket cools inner quartz tube, keeps it from reacting with molten silicon 46
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In their studies, Mr. Lamb and Mr. Porter found that the usual crucible materials, such as quartz, refractory carbides, and nitrides, are unsuitable because they react with molten silicon. The obvious answer was to make the vessel nonreactive. Cold Wall Bars Reaction. The,first such vessel was simply a quartz test tube, immersed in a container of water. The silicon charge was melted by an R.F. (radio-frequency) coil, which was also immersed in the water. When the melt froze, the silicon did not wet, adhere to, or react with the cold quartz. Sylvania's experiments have led to a prototype production model using water cooled quartz tubes. In this model, a double-pipe heat exchanger arrangement is used. With the prototype equipment, it's possible to operate in a vacuum or in atmospheres such as argon or helium.
Metal Containers Also. Mr. Lamb and Mr. Porter have extended the idea to metal containers—copper, steel, and tantalum. In another prototype production model, a cylindrical tube of clear quartz encircles the container. Silver plated copper seals the ends of the tube, allows use of an inert gas as a protective atmosphere. The metal container is moved through an induction heating coil, which encircles both the container and the quartz tube. But metal is a possible source of contaminants, thus is generally used when making low purity product. To zone-purify a silicon ingot, a quartz-enclosed graphite block is used to initiate the R.F. pickup. Radiation from the graphite block heats the silicon. As the silicon heats, its resistivity decreases, finally becomes low enough to allow direct coupling. However, says Mr. Lamb, if the silicon's resistivity is initially low (in the 0.5-ohm-cm. range), it will couple directly and the graphite block isn't needed. To eliminate the chance of melting unrefined silicon on subsequent passes, the Sylvania scientists zone-refine from the center of the ingots toward each end. W7ith large diameter bars, unmelted silicon forms a dam at each end, prevents the silicon from "running out," or becoming longer and thinner. After zone refining, the only significant impurity is boron (in the partsper-billion range). All other impurities have favorable segregation coefficients and are swept out by zone purification.
BRIEFS A $786,897.50 contract for 1739 fuel elements for the Experimental Gas Cooled Reactor (EGCR) has been awarded to Westinghouse's atomic fuel division by Union Carbide. The fuel elements, about 5 in. in diameter and 29 in. long, consist of ceramic uranium oxide pellets clad in stainless steel. The fuel will be 2.46'/c enriched in uranium-235. Delivery of the 1739 elements, sufficient for the initial loading of EGCR plus 300 spares, is expected to start in March 1962. EGCR, a $40 million prototype reactor of 22,300 ekw. located near Oak Ridge National Laboratory, is scheduled to be completed in late J 962.
BROOKFIELD VISCOSITY H e l p s (Armstrong usher in the golden a g e of Rotovinyl Floorings
A patent on interlocking lead brick for constructing gamma radiation shielding has been issued to American Smelting and Refining (U.S. 2,970,218). Asarco claims its design gives packless joints that are at least as impenetrable to gamma radiation as the body of the bricks themselves. With the bricks, shielding can be constructed easily and rapidly, the company says. The bricks arc available in standard shapes and sizes.
A system for electronically blending petroleum products has started up at Union Oil's Los Angeles refinery. A $1.5 million computer-controller system automatically controls valves at storage tanks a mile away, blends stocks in desired ratios. According to Union Oil, its blender can blend as many as 10 different gasoline stocks with five additives plus tetraethyllead to produce a continuous stream of finished gasoline at rates up to 5000 bbl. per hr. A companion unit blends turbine and diesel fuel at rates up to 3500 bbl. per hr., the company adds.
FLOOR COVERINGS A g a i n , recognition of viscosity as a vital product dimension has helped to score an impressive product breakthrough! In this case, it's Armstrong's new Vinyl Accolon . . . rotovinyl floor coverings with a brand new range of spectacular colors and designs, reproduced in almost photographic detail ! At the bottom of the breakthrough is color control. Low-investment Brookfield instrumentation helped solve that problem through precise, automatic control of the viscosity of roto inks used by the Armstrong Cork Company. If there's a fluid in your process, Brookfield instrumentation can turn it into a profitable product dimension
Tedlar replaces Teslar as the trademark for Du Pont's polyvinyl fluoride film. The company took the action to avoid possible confusion with an already existing mark. Tedlar is now in pilot plant production. Du Pont says it expects to start producing the material commercially in a new Buffalo, N.Y., plant in mid-1962.
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