US 3291650 A
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Description (OCR text may contain errors)
Dec. 13, 1966 DOHMEN ET AL 3,291,650
CONTROL OF CRYSTAL SIZE Filed Dec. 23, 1963 SEED PULLING MECHANISM H 7'TORNE Y United States Patent 3,291,650 CONTROL OF CRYSTAL SIZE Hubert G. Dohmen, Kokomo, and Herbert A. Sims,
Frankfort, Ind., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Dec. 23, 1963, Ser. No. 332,708 4 Claims. (Cl. 1481.6)
This invention relates to crystal growth and more particularly to an improved method and apparatus for growing monocrystalline semiconductor bodies of accurately controlled cross-sectional dimensions.
In greater particularity, the present invention concerns a process and apparatus for accurately monitoring the growth of a monocrystal in the manner set forth in United States patent application A-l903, entitled, Semiconductor Crystal Growth, filed simultaneously herewith in the name of Russell M. Pierson, and which is incorporated herein by reference.
It is, therefore, an objeect of the present invention to provide an improved method and apparatus for the production of high quality monocrystalline semiconductor bodies of accurately controlled cross-sectional dimensions.
Other objects, features and advantages of the invention will become more apparent from the following description of preferred examples thereof and from the drawing, in which:
FIGURE 1 shows a schematic sectional view of an apparatus for growing crystals in accordancewith the invention; and
FIGURE 2 shows an enlarged schematic fragmentary view of the solid-liquid interfacial area between a melt and the monocrystalline body being grown.
The invention concerns the growth of a monocrystal by forcing a domical projection of a melt for growing mono crystalline bodies through an aperture, contacting the domical projection with a seed crystal and pulling a single crystal from the domical projection while simultaneously monitoring and regulating the contact area between the single crystal and the domical projection. The single crystal is drawn from the upstanding domical projection of the melt. A beam of light irradiates the surface of the projection. Changes in the angle of reflection of the radiation from the surface of the projection are used to monitor changes in dimensions of the solid single crystal being grown from the domical projection. In the aforementioned United States patent application A-l903, the height of the projection is monitored to regulate crystal growth. However, we have found that even more effective monitoring can be achieved by irradiation techniques. A beam of light from a fixed source will reflect differently from the domical projection as the dimensions of the solid-liquid interface at the top of the projection changes. The interfacial dimensions determine crystal cross section dimensions. Hence, by observing changes in the angle of reflection, one can monitor changes in dimensions of the crystal being grown. By calibrating the changes in reflection of the light beam with known crystal sizes, one can accurately determine the cross-sectional dimensions of the crystal while it is being grown. Hence, one can grow crystals of predicted dimensions within very close tolerances.
Application of the invention can be more easily described by reference to the drawing. FIGURE 1 shows a cylindrical clear fused q-uartz furnace tube having end closures 12 and 14 forming a furnace chamber. End closures 12 and -14 have annular silicone rubber seals 16 and 18, respectively, for sealing the opposite end surfaces of tube 10. A graphite crucible 20 containing a germanium melt is supported on a quartz tube 22 which rests on end closure 14. A graphite melt cover 24 having an r0 Ice aperture 26 rests on the surface of the germanium melt. Weights 28 on the cover member 24 induce a projection 30 of the germanium melt up through cover aperture 26. A single crystal depending from a seed crystal is in contact with the projection 30. The seed crystal is supported by a chuck 32 which is connected to an appropriate p'ull ing mechanism for growing monocrystals. The furnace end closure 12 has a central opening 34 therein to allow entrance of the seed crystal chuck 32 into the furnace chamber. The aperture 34 is effectively shielded by means of a protective gas curtain produced by' diametrically opposed gas jets 36 and 38. A protective atmosphere is maintained within the furnace chamber by continuously introducing a protective gas, such as nitrogen, hydrogen, argon, helium or the like. Tube 40, located in the lower end of the furnace chamber, serves as a pro tective gas inlet. The crucible 20 in the furnace chamber can be heated by a radio frequency induction heating coil 42 located outside the furnace tube 10. Suitable temperature sensors (not shown) are appropriately located to precisely measure melt temperature. If it is desired to use resistance heating instead of induction heating, the crucible 20 need not be graphite but can be quartz, and if of graphite, the resistance heating unit should be located within, rather than outside the furnace tube 10. In such instance, the tube 10 can be stainless steel.
An optical monitoring device 44 is located in the furnace tube 10 above the level of the crucible 20'. It includes a light source 46 and a light sensor 48. The light sensor can be a shrouded ground glass screen 50. The light source preferably provides a beam of light having a maximum cross-sectional dimension less than the height of projection 30. The more concentrated the light is, the more accurate the monitoring can be. The light source 46 is appropriately spaced from the reflected light sensor 48 by means of the interconnecting element 52. As shown in FIGURE 2, the ground glass screen 50 has a graduated scale which can be calibrated to indicate precise crystal diameters.
Other radiation sensors can also be used to pickup the reflected radiation, including photoelectric cells and the like. In such instance, it is possible to automatically regulate crystal growth. The photoelectric cell can be used to automatically alter pulling speed to maintain crystal size within desired tolerances.
In order to grow a monocrystalline germanium rod of accurately controlled cross-sectional dimensions in accordance with the invention, the crucible 20 is appropriately charged with germanium prepared in the normal and accepted manner. For example, 40 ohm-centimeter germanium ingot is etched in an equimolar mixture ofnitric acid, acetic acid and hydrofluoric acid. It is then rinsed with deionized water, rinsed with methanol and dried. The germanium ingot is then placed in the cmcible, along with sufficient impurity to suitably dope the melt, if concurrent doping is desired. An impurity content on the order of approximately 1 part per million is frequently employed. The crucible cover20 is then placed over the ingot and the weights 28 placed on the crucible cover 20. The germanium charge, of course, is prepared in such a way that the cover and weights retain their proper disposition after the charge is melted. For example, a single ingot of germanium having a broad, flat upper surface can be used. On the other hand, the arrangement shown in United States patent application A-298 8, entitled CrystalGroWth, filed simultaneously herewith in the name of Hubert G. Dohmen, can be used.
The furnace is then closed and purged; The usual furnace gases for crystal growing can be employed including inert gases, nitrogenhydrogen mixtures, substantially pure hydrogen or the like. In some instances it may be desired to use a vacuum. The charge is then melted. It is initially held at a temperature somewhat above the usual growing temperature. If appropriately weighted, the crucible cover 24, with its weights 28 thereon, will be supported on the surface of the melt forcing a domical projection 30 of the melt up through the cover aperture For a one and one-half degree inward tapered crucible having a maximum inner diameter at its mouth of tap proximately 2.156 inches, a crucible cover having an outer diameter of approximately 2.152 inches and a 0.4 inch circular central aperture can be used. The lower surface of the cover member is inwardly tapered at about six degrees to an edge thickness of approximately 0.01 inch around the central aperture, forming a generally convex or frustoc-onical surface on the underside of the cover. Weights comprising about 4-6 times the weight of germanium displaced, e.g., 50 grams, are generally preferred.
A seed crystal having a 1,1,1 plane parallel to the surface of the melt is brought into close proximity of the domical projection for preheating. The melt temperature is retained at 5 C.- C. above its melting point. After approximately five minutes, the seed is lowered into the domical projection for a slight melt-back. The temperature is then dropped in small increments until pricipitation of the melt commences on the seed crystal. The pull mechanism is then initiated to slowly withdraw the seed crystal from the melt as the crystal grows without breaking contact between the solid monocrystalline body and the melt projection. If desired, the speed of the pull mechanism is then increased slightly to reduce the contact area between the domical projection and the single crystal. This is used to reduce crystal diameter as much as practical before growth at the desired diameter, to improve crystal quality. The projection 30 is then irradiated by source 46. The speed of pull is then gradually increased, along with the appropriate temperature adjustments until the desired rate of pull has been obtained. Temperature is continually adjusted, gradually dropped, as the speed of pull is increased. It is usually preferred to maintain the crystal cross-sectional dimension during this period only fairly close to that desired.
The projection should be in the center of the heat field or the crystal will not grow symmetrically. Hence, during these preliminary steps, the heat field is adjusted to center it on the projection 30. If not, an elliptical crystal may result. A similar result is attained if the direction in which the crystal is pulled is not substantially vertical. The melt temperature is then adjusted, lowered, to register the reflected light appropriately on screen 50.
When the image on the screen is located properly according to a previously calibrated scale, at the desired speed of pull, the temperature and speed of pull are attempted to be held constant. Minor adjustment of temperature may thereafter be needed to-maintain the light beam image properly located on the screen. When this location has been substantially stabilized, both temperature and speed of pull are preferably held constant for substantially the balance of crystal growth. To terminate crystal growth, the speed of pull can be increased to break contact between the crystal and the domical projection. Once this contact is broken, crystal withdrawal is terminated so as to avoid rapid quench of the freshly grown end portion of the crystal. The furnace is then shutdown in the normal and established procedure for furnace shutdown in germanium crystal growth.
Highquality crystals of about 0.358 inch, plus or minus 0.005 inch, can be consistently grown in this manner at rates up to inches per hour. However, at rates in excess of 12 inches per hour, quality begins to diminish. For the faster rates of growth, it is preferred to grow at a constanttemperature and final regulationof diameter control effected by slight'changes in pull speed, just the opposite from that described in the preceding example- Thus, either, or both, diameter control techniques can be used to arrive 'at the constant speed-constant temperature condition that produces constant diameter in the crystal being grown.
The aperture in the crucible cover need not be circular to practice the invention. However, with other than a circular aperture, best crystal conformity is attained by a small projection height. Analogously, the crucible configuration can vary to some extent. As shown in FIGURE 1, the crucible may have a slight inward taper from top to bottom. Ofcourse, in such a crucible, the cover 24 will have a sufficiently small diameter to allow the cover to drop into the crucible to a sufficient depth.
The height of the domical projection 30 is increased in a plurality of ways. The lower surface of the cover member is of a generally concave or frusto-conical configuration. Hence, the cover edge adjacent the aperture is extremely thin. Moreover, if the lower corner of this thin edge is rounded, additional projection height can be increased so that the projection extends substantially from the upper surface of the cover, not the lower surface. However, of greatest importance in forming the projection is the impression of a force on the genmanium to push a domical projection through the aperture to useful height. As described, this force can be applied by weights 28 placed on the upper surface of the cover. Since the surface tension forces of germanium are extremely strong, much more weight can be placed on the cover than one would ordinarily expect. We prefer to employ weights equal to about 4-6 times the weight of germanium displaced by the cover. In general, an empirical formula which can be used to approximate the weight to be employed for any given crucible-cover combination is determined by the following formulae:
(I) P'=4 t/r, where t=surface tension and r==radius of the cover aperture.
(II) P"=weight of cover assembly/area of cover surface in contact with germanium.
P should be larger than P". As a safety factor, P" of Equation II is generally held to be approximately twothirds of P. For greater accuracy, the buoyancy elfect of the germanium displaced can also be included in P. However, it is preferred to experimentally optimize the weight used to obtain the safest maximum projection height for any crucible-melt cover combination used. It is understood, of course, that while the melt proection 30 is formed in the described embodiment of the nvention by means of a weighted melt cover, the proect1on can be forced through the aperture in a variety of ways. For example, the hydrostatic pressure of a molten head of germanium in a reservoir can be used to induce a projection of germanium through an aperture in the end of a closed tube. Analogously, a germanium melt can be contained in a cylinder containing a piston that is moved to urge the melt through an aperture in the end wall of the cylinder. Similarly, other means can be used to apply a force to the upper surface of the cover on the surface of the melt.
Other modifications of the invention will be obvious to those skilled in the art and, for this reason, it is understood that the preceding description is intended only to serve as an illustration of the main features of the invention. There is no intention to be limited thereby except as defined 1n the appended claims.
1. The method of growing a monocrystalline body of accurately controlled cross section which comprises the steps of forming a melt for growing monocrystals, forcs ing a domical projection of the melt through an aperture in a member contacting a surface of the melt, contacting said domical projection with a seed crystal to establish a melt zone having a surface extending between the seed crystal and the upper surface of said apertured member, directing a light beam onto the surface of said melt zone to produce a reflected beam of light having an angle of reflection therebetween, pulling a single crystal from said melt by progressive unicrystalline solidification from said melt onto said seed crystal at its contact area with said melt zone and, after initial crystal growth, maintaining said angle of reflection at a predetermined value during crystal growth by adjusting any one or more crystal growth conditions such as melt temperature, rate of pull, inclination of pull axis, location of pull axis in said melt zone, location of the melt zone in the melt heat field, and melt temperatures 2. The method as recited in claim 1 wherein the References Cited by the Examiner UNITED STATES PATENTS 2,890,139 6/1959 Shockley 23-301 2,916,593 12/1959 Herrick 23-30 1 2,927,008 3/1960 Shockley 1481.6 2,979,386 4/1961 Shockley et al. 148-1.6 3,002,824 10/1961 Francois 1481.6 3,078,151 2/1963 Kappelmeyer 23-273 DAVID L. RECK, Primary Examiner.
N. F, MARKVA, Assistant Examiner.