Melting Silicon for Semiconductors - Cover Story
May 1959 Electronics World

May 1959 Electronics World Table of Contents  Wax nostalgic about and learn from the history of early electronics. See articles from Electronics World, published May 1959 - December 1971. All copyrights hereby acknowledged.

This feature appeared at the end of a larger article titled, "RF Induction Heating." A notable difference between the type of induction heating in the other article and the type described here is that rather than directly heating a metallic substance to be treated (melted, bent, tempered, etc.), a "susceptor" (graphite crucible) is used to absorb the field and heat up to melt by conduction (via a quartz liner) the silicon material within. Pure silicon cannot absorb the RF energy sufficiently to be heated directly. Interestingly, if you go to the Wikipedia susceptor page, it has an image of Hot Pockets, which are wrapped in a type of susceptor that produces a crispy exterior while heating the interior. As you are probably aware from personal experience, the outside dough would be soft and/or soggy without the susceptor sleeve. A company called Allstar Innovations makes a microwave oven crisper called Reheatza.

Melting Silicon for Semiconductors - Cover Story

The cover photograph shows D. R. Ginter, a technician in the Semiconductor Laboratory of the Chemical and Metallurgical Division of Sylvania Electric Products Inc., at Towanda, Pennsylvania, operating equipment for the melting and casting of silicon. The silicon is being melted under a protective cover by power supplied from a radio-frequency oscillator through a fourteen-turn coil. The resultant eddy currents cause intense heat to be produced.

Molten silicon at 2600 degrees Fahrenheit is an extremely reactive material and will attack and dissolve nearly all substances. The silicon is being melted, prior to casting, in a quartz crucible in an argon atmosphere. The argon prevents oxidation of the silicon while quartz is the only refractory material with which the molten material does not react appreciably. The radio-frequency field of the coil causes a graphite susceptor, into which the crucible fits, to become hot enough to melt the silicon.

The power for the operation is supplied by a 10-kilowatt radio-frequency unit with an output frequency of 450 kilocycles. The unit is manufactured by the Lindberg Engineering Co., Chicago, Ill. Silicon, which has a high melting point and is an extremely reactive material in the molten state, must be kept out of contact with all metals and with almost all nonmetals. The use of radio-frequency power, rather than some other method of heating, allows the melting chamber to be kept free of resistance units and other heater elements which would contaminate the silicon. This is extremely important because much of today's electronic-grade silicon has a total impurity content of one atom or less in one billion atoms of silicon.

The molten silicon will be cast into rods to permit further processing in one of the many and exacting steps in the manufacture of diodes, transistors, and rectifiers.

Silicon in rod form is necessary for "floating zone" purification required for special types of transistors. In this process a long uniform rod of silicon has a transverse zone melted through it by induction heating. This "floating" molten zone receives its only support from silicon above and below, and comes in contact with no other substance. The zone is made to travel the length of the rod a number of times. The impurities, which remain in the liquid state, move to one end. They are subsequently removed simply by cutting off the end of the rod. If the operation is carried on in a vacuum, some impurities will also be lost by sublimation. In addition, silicon rods are made in various diameters which are then cut into ingots for crucible melting and drawing of doped single crystals.

Sylvania is a leading producer of silicon that is used in the fabrication of single crystals which are cut into wafers, shaped, and processed into transistors, diodes, and other semiconductor devices. The performance of all semiconductor devices is dependent ultimately on the purity of the basic material used. Extremely minute quantities of any contaminant can seriously hamper the performance of a semiconductor device. Therefore, every possible precaution is exercised during production to maintain the highest purity.

(Cover photo by John Miller)

Posted July 20, 2018