US 3341376 A
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Sept. 12, 1967 E. SPENKE ETAL 3,341,315 METHOD OF PRODUCING CRYSTALLINE SEMICONDUCTOR MATERIAL ON A DENDRITIC SUBSTRATE Original Filed March 29, 1961 @mmm W U R v R. L. Longini in Phys.
United States Patent 3,341,376 METHOD OF PRODUCING CRYSTALLINE SEMI- CONDUCTOR MATERIAL ON A DENDRITIC SUBSTRATE Eberhard Spenke, Pretzfeld, and Heinrich Welker, Erlangen, Germany, assignors to Siemens-Schuckertwerke Aktiengesellschaft, Berlin and Erlangen, Germany, a corporation of Germany Continuation of application Ser. No. 382,691, July 8, 1964, which is a continuation of application Ser. No. 99,163, Mar. 29, 1961. This application Dec. 13, 1965, Ser. No. 523,486 Claims priority, application Germany, Apr. 2, 1960, S 67,895 3 Claims. (Cl. 148-175) This is a continuation of application Ser. No. 382,691, filed July 8, 1964, now abandoned which in turn is a continuation of application Ser. No. 99,163, filed Mar. 29, 1961, now abandoned and relates to the pyrolytic production of crystalline semiconductor material according to which the material is segregated by chemical reaction from a gaseous starting material and is precipitated upon a heated carrier crystal of semiconductor material having the same lattice structure. Such pyrolytic methods are described, for example, in US. application Ser. No. 86,389, filed Feb. 1, 1961, now Patent No. 3,145,447, and U8. application Ser. No. 665,086, filed June 11, 1957, now Patent 3,011,877. According to the first-mentioned application, individual monocrystals of semiconductor substance in the form of discs are used as carriers, and a thin coating of the same semiconductor material, likewise monocrystalline, is precipitated thereupon. The carrier discs can be severed, for example from semiconductor rods produced by a similar method. The production of such rods is described in the above-mentioned application Ser. No. 665,086. By additionally subjecting the rods thus produced to crucible-free zone-melting, the ultimate product is a rod-shaped monocrystal of extremely high degree of purity which can be sliced into discs suitable as fundamental body for semiconductor devices having several consecutive layers such as one or more p-n junctions, particularly for power rectifiers of high inverse blocking voltage, p-n junction transistors, controllable four-layer devices with thyratron or other gate charac teristics such as silicon controlled rectifiers. Due to the relatively large number of different processing steps required, the production of such semiconductor devices is rather intricate, expensive and results in a great deal of waste.
It is an object of our invention to afford a considerable simplification.
To this end, and in accordance with a feature of our invention, we employ as carrier for the pyrolytic precipitation of semiconductor substances, not individual discs or rods, but rather long and flat tapes or strips. More particularly, we employ as the carrier crystal in the pyrolytic precipitation method, a semiconductor crystal produced by dendritic growth from a melt of the semiconductor material by pulling the crystal out of the melt in form of a long tape.
The drawing illustrates an apparatus for continuous production according to the invention.
Production of tapes from semiconductor material is known, per se, for example from a paper by E. Billig, in Proc. Roy. Soc., London, A, Vol. 229 (1955), pages 346 to 363, and also from a paper by A. S. Benneth and Rev., Vol. 116, No. 1, of Oct. 1, 1959, pages 53 to 61.
Germanium, melting point 958 C., :is melted, for example, in a graphite crucible, which is preferably heated inductively. To pull a dentritic crystal, a corresponding placed upon the surface of the lower portion of the seed then will also melt. Thereafter a sudden supercooling of the melt at the seating location of the seed is effected, for example, by blowing a gaseous coolant, e.g. argon, onto the surface. The supercooling is to about 10 C. Simultaneously with the supercooling, the seed is pulled upwardly out of the melt at relatively high speed, e.g. at a pulling speed more than 50 mm./min. In this manner a tape-shaped dendrite is produced. The direction of growth is (211). The lateral face of tape-shaped twin exhibit (111) orientation. The width of the dendrite thus pulled may amount to 3 to 8 mm., the thickness of the tape to 500 microns, for example. The length mainly depends upon the size of the pulling equipment.
Similar conditions apply to silicon. The melting temperature of silicon is 1420 C. The supercooling in this case may also amount to about 10 C. The pulling speed must be greater than 40 mm./min. Since silicon cools better than germanium, the pulling speed can be less than with germanium. The width of the pulled dendrites may be 3 to 8 mm., and their thickness 80 to 500 microns.
The melt, from which these tapes are pulled, may consist of doped or undoped semiconductor material. Further semiconductor material can be precipitated either upon one flat side only, or upon both flat sides of the tape. Since with ordinary dendritically grown monocrystals, one of the two flat sides has a more perfect structure than the dendritic crystal seed is germanium melt. The
other, it is preferable to precipitate the additional semiconductor material only upon the smoother side because this affords the assurance that the precipitation also grows monocrystalline. For the same reason, when precipitating material onto both fiat sides of the .tape, the use of a dendritically grown twin crystal, as described above, is preferable as the carrier.
The dendritically grown tape to be used as the carrier crystal may consist of the same semiconductor material as that to be precipitated thereupon. However, the carrier crystal may also consist of a different semiconductor material, provided it possesses the same lattice structure. Particularly useful, for the purposes of the invention, are the known semiconductor materials having a diamond lattice structure, such as germanium, silicon, and the A B intermetallic semiconductor compounds of elements from the third and fifth groups of the periodic system or intermetallic semiconductor compounds of elements of the second and sixth groups of the periodic system (ZnS). A definition and list of A B semiconductor compounds is found in Welker Patent No. 2,798,989. For example, a dendritically grown tape-shaped germanium carrier may be provided with a coating of gallium arsenide (GaAs) or another of the above-mentioned semiconducting intermetallic compounds. Analogously, a germanium layer may be precipitated upon a dendritic tape of monocrystalline silicon. A condition to be observed in each case is that the reaction temperature required for the pyrolytic production and precipitation of the coating material is less than the melting temperature of the carrier material.
The lattice constant of the semiconductor material to be precipitated thereon can differ from each other only up to about 5%. Consequently, for example, germanium can be precipitated upon silicon, gallium arsenide (GaAs) upon germanium, aluminum arsenide (AlAs) upon germanium as well as upon silicon, gallium arsenide (GaAs) upon aluminum arsenide and vice versa, aluminum phosphide ('AlP) upon silicon, gallium-phosphide (GaP) upon silicon, indiumphosphide (InP) upon germanium.
The transition from one element or compound to another may also include mixed crystals. For example, when carrier crystal and of the germanium is to be precipitated upon a silicon dendrite, the process may be commenced by precipitating silicon, from a corresponding gaseous silicon compound such as silicon tetrachloride (SiCl or silico-chloroforrn (SiHCl By gradually admixing the corresponding germanium compounds to the gas flow and correspondingly reducing the silicon compounds simultaneously, the process can be ultimately transferred to pure germanium. This method offers the possibility of joining with each other, two semiconductor substances which exhibit a greater difference in the lattice constants than 5%, without excessive crystal-lattice disturbances and without interfering with monocrystalline glow.
The semiconductor material pyrolytically produced by precipitation fromthe gaseous phase may be given an addition of doping substance during the reaction. The doping concentration can be varied during the processing. Furthermore, by changing the doping substances, different layers of respectively different conductance type can be precipitated in order to thereby produce p-n junctions. In view of this possibility of variation, in conjunction with the abovementioned possibility of using different semiconductor materials of the same lattice structure, the invention affords the production of novel semiconductor devices of heretofore unknown composition with particular properties. This affords in the technique of micro-circuits or molecular electronic circuits, a considerable increase in the available possibilities of combining into a single semiconductor component, a number of difienent elements of an electric circuit, such as rectifiers, transistors, capacitors and resistors, in a desired interconnection, for example in form of a complete amplifier unit, oscillator, or trigger circuit.
The method, of our invention permits the production of layers of extremely slight thickness with extreme uniformity. It permits observing minimum tolerances for any prescribed or desired layer thickness, accurate dosing of the doping concentration, and varying of that concentration to any degree over the layer thickness. The method further affords producing any desired number of sequential layers differing from each other with respect to their height or/ and the type of conductance. In this manner, the novel method permits, among other things, the production of semiconductor structures or stratifications which can neither be obtained by the diffusion principle, nor by the alloying principle, nor by a combination of these two known types of methods.
According to a further feature of our invention, the deposition of one or more coatings on a tape-shaped carrier crystal is made particularly economical by employing a continuous process. For this purpose, the tape-shaped carrier crystal is sequentially passed through one or more spacially sequential furnaces or furnace portions which contain respective reaction chambers with gas inlet and outlet conduits and the required heating devices, and which are separated from each other and from the ambient atmosphere by gas locks.
The apparatus as illustrated in the drawing comprises a series of five interconnected chambers 1 to 5. The dendrite 6 entering at 15 and exiting at 16 sequentially passes through chambers 1 to 5, for example at a rate of 45 mm./min. The chamber 1 serves as a gas lock and is traversed by a current of protective gas for example argon or helium. The protective gas enters at 8 and exits at 9. The dendrite may have p-type conductance. In this case, a reaction gas which imparts n-type conductance to the semiconductor material being precipitated is introduced at 10; the reacted gas exits at 11. The gas may be hydrogen mixed with a corresponding silicon or germanium compound and an addition of a gaseous donor compound, for example a halide, particularly chloride, bromide or hydride of phosphorus or arsenic. The chamber 3 serves as a gas lock and, like chamber 1, is traversed by a fiow of protective gas from 8 to 9. Chamber 4 again serves precipitation purposes, in this case of p-type material. The reaction gas mixture for chamber 4, which en: ters the chamber at 10, comprises an admixture of corresponding gaseous compounds of elements from the third group of the peroidic system. The reacted gas exits at 11. The chamber 5 again serves as a gas lock and is operated like chambers 1 and 3. The introduction of the reaction gas into chambers 2 and 4 is preferably effected by nozzle means 14 in order to produce a forceful whirling of the reaction gas in the reaction space. The quantities of reaction gas are similar to that described for the batch process below.
The heating of the entire equipment can be effected by radiation or induction. In the latter case, an induction winding 7 may be wound on the outside of the entire equipment in the direction of the travelling semiconductor tape. The induction heating winding 7 may form a single circuit traversed by alternating current. However, the winding may also be separated at individual places and be supplied with different heating currents.
Heating of the dendrite tape by passing current directly therethrough is not feasible because, due to the progressing precipitation the tape possesses different cross section and different conductivity at different localities and hence does not have a uniform electric resistance over its entire length. If current were passed directly through the dendrite tape to heat the tape, it would be subjected to different degrees of heating at different localities with the result of obtaining differing rates of precipitation.
The inductive heating can be readily adapted to the different cross sectional and conductance conditions of the tape at different localities. This can be done in the above-described manner by subdividing the heater coil and applying different current intensities to the respective coil portions. However, the same effect can also be obtained by serially passing a current through all winding turns but giving the winding a greater number of turns per unit of length at some locations as compared with others.
A tape thus provided with one or more coatings, can be cut into pieces having an area of any particular size desired, and these pieces need then only be provided with terminal contacts and a protective enclosure. The protective enclosure may consist of a metallic housing or an insulating embedment produced, for example, by embedding the semiconductor device in synthetic resin.
The dendritically grown tape-shaped carrier crystals, since they are produced by pulling them out of a crucible containing the semiconductor melt, are-not, as a rule, of such an extremely high degree of purity as carrier crystals produced without the use of a crucible. This, however, is not objectionable for many semiconductor devices because they must anyhow contain at least one highly doped layer. In many cases it is possible to have such a highly doped layer, which often constitutes an outer layer to be provided with a terminal contact, formed by the original carrier crystal. Layers of extremely high purity can be precipitated by chemical or pyrolytic reaction from gaseous mixtures that are purified to a correspondingly great extent, and can thus be precipitated as coatings upon a carrier crystal of lesser purity. Such extremely pure precipitated coatings are often applicable as base layers in transistors or other gating devices.
As an example of the production of a complete circuit component, we shall first describe the production of a p-n-p transistor, by a pyrolytic process using apparatus similar to that used in the above-mentioned application Ser. No. 665,086. For this purpose, the process is started by pulling a tape-shaped germanium dendrite of n-type conductance and a specific resistance of 20 ohm/cm, out of the melt with a thickness of microns, this being done in the manner described above. Precipitated upon both sides of the dendrite tape is a p-type layer of 20 microns thickness and a specific resistance of 0.20 ohm/cm. This is pyrolytically precipitated from the gaseous phase also as described above.
The preferred pyrolytic precipitation temperature for producing germanium from the corresponding germanium compounds is about 700 to 850 C. That is, the carrier crystal must be heated to this temperature. It is advisable to maintain the walls of the reaction vessel at a much lower temperature so that no precipitation will occur at these walls.
The production of a n-p-n transistor is carried out in the following manner. The production is preferably started from a p-type twin having specific resistance from 80 to 240 ohm/cm. Suitable, for example, is a silicon crystal exhibiting a specific resistance of 200 to 240 ohm/ cm. and a thickness of 100 microns. Precipitated upon both sides of the tWin crystal is a layer of n-type silicon with a thickness of 20 microns and a specific resistance of 0.01 ohm/cm. This can be done, for example, as follows:
Two silicon tapes, each of 20 cm. length and 8 mm. width, are mounted in a reaction chamber, for example within a quartz vessel, and are heated to a temperature between about 1100 and about 1250 C. The heating is preferably effected by electric inductance heating. However the tapes may also be heated by heat radiation. Now, a gaseous mixture is passed through the reaction chamber. The mixture consists of hydrogen, which serves both as a carrier and as a reaction gas, and one or more of the abve-mentioned silicon compounds (SiCl SiHCl The quantity of the gas mixture passing through the reaction chamber is approximately 0.5 to 30 liter per minute. The molar ratio of the silicon compound to hydrogen, when using silicochloroform, is approximately 0.1, and when using silicon tetrachloride is about 0.05.
To produce the required 20 micron thick n-type layer having the required specific conductance, the corresponding silicochloroform-hydrogen mixture, containing 2.10 grams of phosphorus trichloride (PCl per gram of silicochloroform, is passed through the reaction chamber for approximately 5 minutes in a quantity of 8 liters per minute. The carrier gas (hydrogen) as well as the silicon compound are extremely purified prior to commencing the method.
Another example is the production of a four-layer semiconductor device of the p-n-p-n type. It is preferable to start with an n-type silicon twin dendrite having a specific resistance of 20 ohm/cm. and a thickness of 75 to 80 microns. At first, a p-type layer is precipitated upon both flat sides of the dendrite tape, with a layer thickness of 15 microns and a specific resistance of 2 ohm/cm. Thereafter a n-type layer, with a thickness of 15 microns and a specific resistance of 0.05 ohm/cm., is precipitated upon each of the p-type layers. As in the preceding example, the pyrolytic precipitation can be effected from the corresponding silicon compounds. For obtaining the required p-type conductance, the gas mixture may be given an addition of boron chloride (BCl For producing the desired n-type conductance, and admixture of phosphorus trichloride (PCl may be used, for example, 1.1-10 gram PCl per gram SiHCl The electric connecting terminals, of the semiconductor circuit components produced in the above-described manner, can be eifected, for example by precipitating nickel from a bath of a corresponding nickel salt. The attachment of the terminals may also be eifected by vapordeposition of metals, for example by placing metal foils, such as a gold foil, onto the circuit components device and alloying the materials together.
In the above-described example of the four-layer semiconductor device, one of the two outer n-type layers is eliminated by overdoping, thereby obtaining the fourlayer crystal. For example, the overdoping may be effected by placing a boron-containing gold foil (with about 0.5% boron, the remainder being gold) upon the outer layer, the foil having a thickness of about 30 microns. Thereafter the foil is alloyed into the surface layer at a temperature of about 700 C. As a result, the n-type layer is overdoped and now possesses p-type conductance and undisturbed surface proper, employing as the carrier crystal a semiconductor body yielded by dendritic growth from a melt of the last-mentioned semiconductor material by pulling a tape-shaped crystal out of a supercooled region of the melt, and removing some semiconductor material from the dendrite prior to the precipitation step so as to secure an undisturbed surface upon which monocrystalline growth can ensue.
3. A method for producing a fiat semiconductor body of single crystal structure and of a uniform thickness including several zones of different semiconducting properties according to claim 2, wherein each fiat side of said dendritically grown twin crystal is provided with a DAVID L. RECK, Primary Examiner. N. F. MARKVA, Assistant Examiner.