Making RCA Transistors in 1953

Production Techniques in Transistor Manufacture

Text from Electronics October 1953. See PDF download for full article

Complete details of the many steps involved, including pertinent timing data, temperatures, materials, dimensions, etching and other special processes. First technical article describing transistor manufacture at: RCA's Harrison plant

By JAMES D. FAHNESTOCK

Associate Editor, ELECTRONICS

This Edition of Electronics carried the following advertisement on the back cover. The earliest advertising for these transistors was in May 1953.

ALTHOUGH TRANSISTORS are curently available on a commercial basis, many of the production methods used in their manufacture are small-scale operations in comparison with modern mechanized tube-making techniques.

The processes to be described are those presently in use at RCA's commercial pilot-production setup at Harrison, New Jersey.

Industry needs for transistors are expected to exceed tens of thousands per day within a few years, and obviously automatic production methods will become mandatory to meet those demands.

Germanium Preparation

The first step in manufacturing 'transistors is the preparation of ultrapure germanium. The main source of germanium in this country is germanium dioxide, a byproduct of the zinc-refining process. Removing the oxygen from the white germanium dioxide powder is accomplished by placing a boat containing approximately 450 grams of the oxide in a long tube inside a reducing furnace.

Inside the furnace a constantly circulating stream of dried and purified hydrogen is passed over the oxide. A cam-operated timing mechanism heats the germanium dioxide to a temperature of 650 C for 3 hours, 850 C for 1 hour, and 1,050 C for 1 hour. During this time the powder becomes metallic as oxygen is removed (in the form of water vapor) by the hydrogen.

The metallic germanium resulting from this preliminary purification process has a volume resistivity of between 1½ and 3 ohm-cm, but is still much to variable in its characteristics for use in transistors. A second process, called zone refining, is used to further purify the metal to a volume resistivity as high as 50 ohm-cm. The intrinsic volume resistivity for pure germanium is 60 ohm-cm.

In RCA's zone refining process, the semi-pure germanium from the reduction furnace is pulled through one of two tubes in a boat similar to the one used in the reduction furnace. The boat must pass through six sets of induction heating coils each of which heats the metallic germanium under its turns to the melting point.

As the germanium leaves the influence of each set of coils, it again cools and solidifies and since most of the impurities commonly found in germanium prefer to be in molten metal rather than in solid metal, these impurities actually move toward the trailing end of the boat in each melting zone.

The result of the zone refining process is a bar of germanium, roughly 65 percent of the weight of germanium dioxide used, with resistivity ranging from 50 ohm-cm at one end and falling off to about 30 ohm-cm two thirds of the way toward the end that trailed in traversing the furnace. The six-foot tubes in the zone refining machine are traversed in two to three hours in an atmosphere of 90-10 forming gas.

Crystal, Growing

The proper amount of impurity is added and the bar of germanium formed into a single crystal for transistors, in the vertical "growing" machine. The zone-purified germanium is melted in a hydrogen-atmosphere furnace. By a precise mechanical linkage a single germanium crystal or seed is carefully lowered onto the top of the germanium puddle. The germanium immediately begins to crystallize around this seed. Complete and instantaneous crystallization is prevented by mechanically pulling the crystallized part away from the melt while imparting a spinning motion. Pulling speed is approximately 0.032 inch per minute, and the forming crystal is spun about its axis at 60 rpm.

This growing process takes several hours. The result is a single crystal of germanium (some impurities are added prior to the growing process to obtain the desired characteristics) about as big as a thumb and six or eight inches in length. These bars are subsequently sawed with diamond wheels into minute pieces for use in transistors. The dimensions for the pellets used in point-contact transistors are 0.045 in. square by 0.010in. thick and for the junction transistors the pellets are 0.090 X 0.130 X 0.005 inch.

Point-Contact Units

The 0.045-in. square pellet coming from the diamond saw contains mechanical surface irregularities that cannot be tolerated in transistors. An even surface is obtained by placing the pellets in a pre-etch bath. After the etch, the pellet is soldered to a support that has previously been tinned (with pure tin) . The tinned surface of the support is first coated with Divco No. 335 solder flux, the germanium pellet placed on this surface, and a small soldering iron is then applied underneath the support to form a bond between the pellet and the support without contamination.

The next process involves the squirting of a fine stream of hydrogen peroxide and hydrofluoric acid on the surface of the pellet as one of the final cleaning steps before applying the point contacts. This special etching process is carried on in a merry-go-round device in which the supports are fixed to the spokes of a rimless wheel in such a way that when the wheel rotates the pellets come under the nozzles of the acid streams and subsequent wash-water streams.

Used acid is collected beneath the wheel, and only fresh acid touches the germanium. When the supports reappear at the opening, the pellets have been completely processed and are ready for the locating of cat-whisker emitter and collector and the base lead.

A preformed point subassembly comprises three parallel wires 0.018 inch in diameter and held together by a glass bead. The center lead is shorter than the two outside leads which are formed to meet at one end for temporarily mechanical rigidity. This assembly is held firmly in the movable portion of a jig that also serves as one electrode of a precision resistance welder of the type commonly used in tube manufacture.

The first step in assembly is to weld the almost invisible preformed emitter and collector wires to the side rods. The latter operation is done with the aid of a 10-power microscope.

The germanium pellet and its support are next placed in the fixed portion of the jig, which is arranged to slide the points into contact with the germanium pellet. Again, using a microscope, the operator determines the position at which both emitter and collector points come in contact with the germanium plus 0.002 in., causing the cat-whiskers to exert slight pressure on the surface of the germanium. The base connection is then welded to the center lead of the glass bead assembly.

A third step in this operation, using a modified optical comparator, consists of bending the side rods closer together or farther apart to adjust spacing between the bevelled points. This process sets the spacing to within 0.00025 in. of the desired value, which is 0.0015 to 0.002 in. depending on the resistivity of the germanium.

The transistor is now assembled, but mechanical, thermal, moisture, and light protection must be added. First, a coating of polyisobutylene is applied to the contact area by an operator using a toothpick and a magnifying glass. Since this material is resilient, a protective covering such as quick-hardening Amphenol 912 polystyrene is applied over it for mechanical protection. The latter hardens in air in about 3 minutes.

The point-contact transistor is finished by surrounding it with an ordinary medicinal capsule. A small paint brush and an ordinary eye dropper are used to seal and fill the capsule with Araldite, an opaque white plastic chosen for its low shrinkage. The transistors are then baked in electric ovens for 72 hours at 105 C to polymerize the Araldite- by this treatment shrinkage is held to about 0.5 percent. The electrical forming process consists of discharging a charged capacitor across the collector base terminals of the transistor a number of times to obtain certain specified electrical properties. This process has been discussed in the literature and will not be explained here.

Alloyed-Junction Transistors

The alloyed-junction transistor uses the same germanium base material as the point-contact transistor, but the dimensions are different as mentioned previously. A small slab 0.090 X 0.130 in. by 0.005 in. thick is desired. These slabs are saw cut slightly over-size, so that an initial etching process to obtain surface smoothness may be applied.

After this etching process, the individual slabs are gaged and divided, according to thickness, into ten groups. Those having thicknesses less than 0.0045 in. are scrapped. Those between 0.00.475 and 0.00525 are suitable for use. Those thicker' than 0.00525 , in. are re-etched in batches for a length of time depending on the amount of germanium to be removed. After the second etch, thickness is again checked. The slabs falling into the 0.0045 to 0.00475 in. category are held aside for a special subsequent processing.

Next comes the process in which emitter and collector pellets are fixed on opposite sides of the germanium slab. To make pnp junctions, indium pellets are used; for npn junctions, lead-antimony pellets are used. The pellets are cylindrical in shape, with an axial length thickness of 0.015 in. The emitter pellet has a diameter of 0.015 in., while the collector pellet is 0.045 in. in diameter.

Base Contact

Twenty-five of the germanium slabs are placed in precision slots in a jig. At the same time a tinned nickel tab is placed in contact with the germanium slab to serve as a contact point for the base connection. First, the collector pellets are brought into contact with one side of the wafers. The-jig is then inserted in a close-fitting nichrome tube through which 100 amperes of current flows. The resulting heat penetrates the stainless-steel jig and fuses the tinned nickel tab and the collector pellet to the germanium slab.

The atmosphere in the furnace is one of purified and dehumidified hydrogen. The temperature is controlled in the range of 520 to 550 C to secure proper penetration of the pellet material into the germanium slab. After 1½ minutes in the furnace, the jig is removed, turned over, and the emitter pellets are inserted in the smaller holes on the opposite side of the germanium. The jig is then returned to the oven for a 4-minute firing. Shorter times are used for the undersize germanium slabs.

The individual transistors are then immersed in an etch solution of nitric and hydrofluoric acids for 25 seconds to remove any contamination on the exposed junction surface, taking care not to dip the nickel tab in the solution with the accompanying possibility of cross deposition of the metal. The acid action is stopped by washing the transistor in a circulating bath of hot water and a blast of purified air removes water particles for the Short-circuit test that follows.

Resistance Testing

To facilitate rapid testing for low barrier resistance, a special ohmmeter is adjacent to the etching position. The nickel tab is held in a pair of metal tweezers. The tweezers are placed across a metal bar which forms one side of a circuit containing a 3-volt battery and a 0 to 100 micro-ammeter. The circuit is completed by bringing the collector pellet in contact with a spring contact.

A current reading of 6µa represents a collector barrier resistance of 500,000 ohms which is the low limit for acceptance on an initial etch. If more than 6 µa is indicated, the resistance is too low and the transistor is re-etched. If re-etching the second or third time is successful the transistor is accepted-if not, it is scrapped. A reversing switch on the tester permits testing of npn as well as pnp junctions.

The next step in the fabrication of RCA alloyed-junction transistors involves the connection of 0.005·inch copper-plated tungsten wires to the collector and emitter pellets and the base lead. This is accomplished by first tinning the ends of the wires with Cerrobend, a solder having a melting point of 90 C. The base wire is welded to the nickel tab by means of a small resistance welder. The emitter and collector wires, which have previously been welded to the base assembly, are brought into contact with the indium pellets in a stream of hot hydrogen. This operation is observed through a microscope to ensure a thorough bond. No flux is used in this operation. An inspector pulls on the soldered wires to insure firm mechanical bond.

The finished junction is then dipped in Amphenol 912 polystyrene. Because of the photosensitivity of the junction a coat of black paint is applied over the polystyrene and allowed to dry. The unit is then cast in room-setting Araldite using flexible molds similar to plastic ice-cube trays, to facilitate easy removal after the plastic hardens.

October, 1953 - ELECTRONICS