US 3206286 A
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Description (OCR text may contain errors)
Sept. 14, 1965 A. l. BENNET'l j JR., ETAL 3,206,285
APPARATUS FOR GRbWING CRYSTALS 2 Sheets-Sheet 2 Filed July 23, 1959 Twin Plane Fig.5.
llllll 78 I H T r i E M m R LmB mm n7 United States Patent 3,206,286 APPARATUS FOR GROWING CRYSTALS Allan I. Bennett, Jr., and Richard L. Longini, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed July 23, 1959, Ser. No. 829,069 4 Claims. (Cl. 23-273) This invention relates to a continuous process for producing crystals of solid materials, and, in particular, to intrinsic and suitably doped semi-conductor crystals.
This invention is closely related to U.S. patent application, Serial No. 757, 832, filed August 28, 1958 and now abandoned, of which one of the present inventors is the inventor, and assigned to the assignee of the present application.
At the present time, crystals of many solid materials are produced by preparing a melt of the solid material, contacting the surface of the melt with a previously prepared crystal of the material and slowly withdrawing the previously prepared crystal, usually at a rate of the order of an inch an hour to produce a desried grown crystal member. It has been the invariable practice in this procedure to maintain the melt during crystal growing at a temperature slightly above the melting point of solid material.
The nature and configuration of the withdrawn crystals produced by such prior art practices have generally been uncontrollable except within relatively broad limits. Thus, the thickness has not been radily maintained within precise dimensions. In many cases, surface and internal perfections such as dislocations and other crystal structure flaws have been present in the grown crystals.
In the semiconductor industry, crystals of silicon, germanium, and compounds of the Group III-Group V elements of the Periodic Table have been grown from melts in accordance with this prior art practice. In order to employ such grown crystals in semiconductor devices, it has been necessary to saw them into slices using, for example, diamond saws. Thereafter, dice of desired shape have been cut from these slices. The sawed surfaces of the dice have been lapped or otherwise mechanically polished to remove disturbances or otherwise unsatisfactory surface layers, which treatment is followed by an etch to remove microscopic surface imperfections. As a result of this working, which is performed on expensive precision machinery and requires highly-skilled labor, there may be a loss of as much as 90% of the original grown crystals in securing dice which have useful semi conductor shape and configuration.
An object of the present invention is to provide apparatus for continuously producing intrinsic or suitably doped crystals of any desired length, precisely controlled thickness and width from a suitable supercooled melt ,of a material.
Another object of the present invention is to provide apparatus for continuously producing intrinsic or suitably doped fiat dendritic crystals of materials having a diamond cubic lattice structure with flat faces having precise (111) surfaces.
Another object of the present invention is to provide a continuous process for producing intrinsic or suitably doped crystals of any desired length, precisely controllable thickness and width from supercooled melts of solid materials.
Another object of the invention is to provide a continuous process for growing intrinsic or suitably doped fiat dendritic crystals from a suitable supercooled melt while maintaining low temperature gradients in the crystals above the melt surface so that imperfection in the growing crystals are minimized.
Another object of the invention is to continuously produce intrinsic or suitably doped fiat dendritic crystals of materials having a diamond cubic lattice structure with flat faces having (111) surfaces.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.
For a better understanding of the nature and objects of the invention, reference should be had to the following detailed description and drawings, in which:
FIGURE 1 is a view in elevation, partly in cross section, of a continuous crystal growing apparatus in accordance with the teachings of this invention;
FIGURE 2 is a greatly enlarged fragmentary view of a dendritic crystal;
FIG. 3 is a side view partially in cross-section of two aligning rollers with a crystal disposed centrally thereon;
FIG. 4 is a side view of a drive system suitable for use in accordance with the teachings and apparatus of this invention; and,
FIGS. 5 and 6 are two top views of drive systems suitable for use in accordance with the teachings of this invention.
In accordance with the present invention, it has been discovered that either intrinsic or doped crystals of solid materials may be continuously prepared as flat dendritic crystals of continuous or infinite length having a closely controlled thickness with relatively precise fiat parallel faces. These flat dendritic crystals may be continuously pulled or grown from suitable melts at a relatively high rate of speed of pulling of the order of times greater than the linear pulling velocities previously employed in the art. The thickness of the crystals may be readily controlled and surface imperfections minimized or reduced by following the teachings of the present invention.
More particularly, in practicing the process of this invention in its preferred embodiment, a melt comprised of either the intrinsic or pure material to be grown into a flat dendritic crystal or the material combined with at least one selected doping material is prepared at a temperature slightly above the melting temperature thereof. The surface of the melt is contacted with a previously prepared crystal having a single twin plane at the interior thereof, the crystal being oriented with the 2ll direction vertical to the melt surface. Other necessary or desirable crystallographic and physical features of the seed crystal will be pointed out in detail hereinafter. The seed crystal is dipped into the surface of the melt a suf ficient period of time to cause wetting of the lower surface of the seed, usually a period of time of a few seconds to a minute is adequate and, then, the melt is supercooled rapidly, following which the seed crystal is withdrawn with respect to the melt at the speed of the order of from 1 to 10 inches a minute. Under some conditions, considerably slower pulling speeds can be employed, for example, 0.2 inch per minute. Pulling speeds of from 10 to 25 inches per minute have given good results. The degree of supercooling, the rate of pulling of the seed crystal from the melt can be so correlated so as to produce a thin strip of solidified melt material of continuous or infinite length having a precise desired thickness and with, doping concentration, and having the desired crystallographic orientation.
The present invention is particularly applicable to solid materials crystallizing in the diamond cubic lattice structure or zinc blend structure. Two examples of such materials are the elements silicon and germanium. Likewise, stoichiometric compounds having an average of 4 valence electrons per atom respond satisfactorily to the continuous crystal growing process of this invention. Such compounds, which may be continuously produced with excellent results, comprise substantially equal molar proportions of an element from Group III of the Periodic Table particularly gallium and indium. Combined with an element from Group V of the Periodic Table, particularly phosphorus, arsenic and antimony. Those materials crystallizing in the diamond cubic lattice structure are particularly satisfactory for various semiconductor or other applications.
Furthermore, the diamond cubic lattice structure materials may be intrinsic, or, as stated before, may be doped with one or more impurities to produce n-type or p-type semiconductor materials, For instance, the materials may be grown from a melt containing both nand p-type doping materials, whereby the pulled or grown crystal will have alternate layers of pand n-type semiconductivity. The melt may also contain both an nand p-type dopant but in such quantities that one will prevail over the other throughout the grown crystal. The continuous layer growing process and apparatus of the present invention may be applied to all of these different materials.
For a better understanding of the practice of this invention, reference should be had to FIG. 1 of the drawing, wherein there is illustrated apparatus for continuously producing dendritic crystals of indefinite length in accordance with the teachings of this invention. The apparatus comprises a base 12 carrying a support 14 for a crucible 16 of a suitable refractory material such as graphite to hold a melt 18 comprised of the material from which flat dendritic crystals are to be continuously drawn. Molten material 18, for example, germanium, is main tained within the crucible 16 in the molten state by a suitable heating means such, for example, as an induction heating coil 20 disposed about the crucible. Controls, not shown, are employed to supply alternating electrical current to the induction coil 20 to maintain a closely controllable temperature in the body of the melt 18. The temperature should be readily controllable to provide a temperature in the melt a few degrees above the melting point and also to reduce heat inputs so that the temperature drops in a few seconds, for example, in 5 to seconds, to a temperature at least one degree below the melting temperature and preferably to supercool the melt from 5 to 15 C. or lower. A cover 22 closely fitting the top of the crucible 16 may be provided in order to maintain a low thermal gradient above the top of the melt.
A second crucible 26 of a suitable refractory material such as graphite is supported on base 12 by a support member 29. The crucible 26 contains an additional quantity of melt 18 which may be charged in the crucible 16 through a suitable conduit 28 by an actuating valve 3% operable by electromagnetic controls (not shown) or by other suitable means. The conduit 28 is preferably attached to crucible 16 to supply molten material at the bottom of the charge 18 therein so that it can attain the desired temperature before reaching the surface 19. The crucible 26 has a top member 32 and is surrounded by a heating means, such, for example, as an induction heating coil 34. Controls, not shown, are employed to supply alternating electrical current to the induction coil 34 to maintain a closely controllable temperature in the melt 18 within crucible 26. The temperature of the melt within crucible 26 should be a few degrees above the melting point. The melt within crucible 26 is used to replenish the melt in crucible 16. While crucible 26 has been illustrated in FIG. 1 in a particular relation to crucible 16, it will be understood that crucible 26 may be disposed anywhere within the apparatus 10. It will also be understood that depending upon the length of dendrite to be grown and the capacity of crucible 16, it may not be necessary to employ an auxiliary crucible such as crucible 26.
An aperture 24 is provided in the cover 22 of crucible 16 through which a seed crystal 44 may pass and through which a dendrite is to be pulled.
Referring to FIG. 2 of the drawing, there is illustrated in greatly enlarged view, a section of a preferred seed crystal 44 having a single twin plane. Such seed crystals may be obtained in various ways, for example, by supercooling a melt of the solid material to a temperature at which a portion thereof solidifies, at which time some dendritic crystals having a single internal twin plane will be formed and may be removed from the melt. While these crystals may not be uniform, they are suitable for seed purposes. Also, one can cut from a large twin crystal a section suitable for use as a seed crystal.
The seed crystal 44 comprises two relatively fiat parallel faces 45 and 47 with an intermediate interior twin plane 49. The twin plane ordinarily will be precisely between the faces 45 and 47. Examination will show that the crystallographic structure of the preferred seed on both faces 45 and 47 is that indicated by the crystallographic direction arrows at the right and left faces respectively of the figure. It will be noted that the horizontal direction perpendicular to the flat faces 45 and 47 and parallel to the melt surfaces are 111 The direction of growth of the dendritic crystal will be in 2l1 crystallographic direction. If the faces 45 and 47 of the dendritic crystal 44 were to be etched preferentially to the (111) planes, they will both exhibit equilateral triangular etch pits 51 which vertices 53 will point upwardly while their bases will be parallel to the surface of the melt. It is an important feature of the preferred embodiment of the present invention that the etch pits on both faces 45 and 47 of seed crystal 44 have their vertices 53 pointing upwardly. A non-twinned crystal or crystal containing two twin planes or any even number thereof will exhibit triangular etched pits on one face whose vertices will be pointing opposite to the direction of the vertices on the other face. The most satisfactory crystal growth can only be obtained by employing seed crystals of the type exhibited in FIGURE 2 wherein only a single twin plane is present interiorly.
Seed crystals having an odd number of twin planes containing the growth direction may be employed in practicing the process of this invention, due care being had to point the triangular etch pits on the outer face of the crystal with their vertices upwardly and the bases parallel to the surface of the melt. Further, seed crystals containing an even number of twin planes may be employed for crystal pulling, though as desirable pulled crystals will not be obtainable as with the preferred single twin plane seed crystal shown in FIG. 2.
The direction of withdrawal of seed crystal 44 having a single twin plane from the melt 1 8 must be with the direction of the vertices 53 of the etched pits being upward and bases being substantially parallel to the surface of the melt. When so withdrawn, the melt will solidify at the bottom of the crystal in a satisfactory prolongation thereof. If the crystal 44 were to be inserted into the melt so that the vertices 53 are pointed downwardly, very erratic grown crystals will be produced which are not only nonuniform in direction but grow at angles of to the longitudinal axis of the seed and produce very irregular spines and lateral projections, and generally are unsatisfactory.
Referring again to FIG. 1, a protective enclosure 36 of glass, quartz or other suitable material is disposed about the crucibles 16 and 26 with a cover 38 closing the top thereof.
Within the interior of enclosure 36 there is provided a suitable protective atmosphere entering through a conduit 40 and, if necessary, a vent 42 may be provided for circulating a current of such protective atmosphere. Depending on the crystal material being processed in the apparatus, the protective atmosphere may comprise a noble gas, such as helium or argon, or a reducing gas such as hydrogen or mixtures of hydrogen and nitrogen, or nitrogen or the like or mixtures of two or more gases. In some cases, the interior of apparatus 10 may be evacuated to a high vacuum in order to produce crystals of materials free from any gases.
In the event that the process is applied to compounds having one component with a high vapor pressure at the temperature of the melt, a separately heated vessel containing the component may be disposed in the enclosure 36 to maintain therein a vapor of such compound at a partial pressure suflicient to prevent impoverishing the melt or the grown crystals with respect to the component. Thus, an atmosphere of arsenic may be provided when doped or intrinsic crystals of gallium arsenide are being pulled. The enclosure 36 may be suitably heated, for example, by an electrically heated blanket or wrapping to maintain the walls thereof at a temperature of the separately heated vessel containing the arsenic in order to prevent condensation of arsenic therein.
In operation of the apparatus 10, the seed crystal 44 is passed through the aperture 24 in the cover 22 of crucible 16. At the beginning of the operation, the seed crystal 44 is butt soldered or otherwise joined to one end of a leader tape 46. The tape 46 may be of steel or any other suitable material or may be comprised of a previously grown dendrite. It is preferable but not necessary that the tape 46 have the same width and thickness as that of the dendrite to be grown. The other end of the tape 46 is passed between guide rollers 48, 50, 52 and 54, and is fastened to a winding drum 68. The seed crystal 44 is lowered into crucible 16 until it contacts the surface 19 of the melt 18. The melt 18, which is slightly above its melting point, dissolves the tip of the seed crystal. A meniscus-like contact between the molten tip of the seed crystal and the melt is formed.
The power input to the heating coil 20 is reduced in order to supercool the melt (or reducing the applied heat if other modes of heat application than inductive heating are employed). There will be observed in a period of time of the order of 5 seconds after the heat input is reduced to a crucible of about two inches in diameter and two inches in length, the supercooling being about 8 C., an initial elongated hexagon-a1 growth or enlargement on the surface of the melt adjoining the tip of the seed crystal. The hexagonal surface growth increases in an area so that in approximately 10 seconds after heat input is reduced, its area is approximately three times that of the cross section of the seed crystal. At this stage, there will be evident spikes growing out of the two opposite hex-- 'agonal vertioes lying in the plane of the seed. These spikes appear to grow at a rate of approximately two millimeters per second. When the spikes are from two to three millimeters in length, the seed crystal pulling mechanism, which will be discussed in detail hereinafter, is energized to pull the crystal from the melt at the desired rate. The initiation of pulling is timed to the appearance and growth of the spikes for best results.
After pulling the seed crystal upwardly from the supercooled melt, it will be observed that the fiat solid diamond shaped area portion is attached to the seed crystal and that a downwardly extending dendritic crystal has formed at each end of the elongated diamond area adjacent to the spike. Accordingly, two dendritic crystals can be readily pulled from the melt at one time from a single seed crystal. By continued pulling, the two dendritic crystals may be extended to any desired length.
If the seed crystal is disposed so that one edge is nearer the thermal center of the melt crucible than the other edge, it is possible to increase briefly either the pulling rate or the temperature of the melt, and under these variations the dendritic crystal furthest away from the thermal center or any hotter region will usually stop growing and thereafter only a single dendritic crystal will be attached to and grow from the seed. Also, one of the dendrites can be mechanically severed from the seed.
For the purpose of this invention, it is preferred to pull only a single dendrite from the melt. The control of the width and thickness pose problems that can be best handled with a single dendrite.
If the double dendritic crystal attached to the original seed crystal is introduced into the same or another melt slightly above the melting temperature and after supercooling the melt, on pulling the double dendritic crystal from the surface, there will be formed two diamond shaped areas attached to the double dendrite and four dendritic crystals will be pulledtwo attached to each of the original dendrites. Thus, in one instance four germanium dendrites in length were pulled from a melt. While more than four dendritic crystals can be pulled from the melt, there may arise interference and other factors which will render such growth difficult. However, it is to be understood that the pulling of more than one dendrite from one seed may be extremely diflicult and ordinarily is not desired.
A number of methods for pulling only a single dendrite from a seed are available and may be employed in practicing the invention.
The seed crystal 44 is pulled from the melt 18 by activation of rollers 48, 50, 52 and 54. The rollers 48, 50, 52 and 54 are comprised of a material that will not scratch the dendrite, but will grip the material tightly enough to prevent slippage, for example neoprene rubbers, nylon, polytetrafluoroethylene, polytrifluoromonochloroethylene and the like.
During growth, unless the dendrite pull and growth direction is exactly perpendicular to the axis of the drum 60, the growing dendrite will not remain centered upon the drum but will move axially along it. The contemplated length of pull, which may be several hundreds of feet, is enough that a very minute misalignment will eventually result in an intolerable axial displacement of the dendrite upon the drum. Such misalignment will result also from errors in the initial alignment of the seed in the melt. Small differences in the physical shape and size of the dendrite may result in axial displacements of undesirable magnitude. It is therefore necessary that the lateral position of the dendrite be controllably aligned in some manner. The rollers 48, 50, 52 and 54 serve to align the dendrite. The rollers are so designed that the only stable position of the dendrite is at the center of the rollers.
With reference to FIG. 3, there is illustrated a sectional view of the rollers 48 and 50 with a dendrite 44 aligned centrally thereon. Each roller has a riged shaft disposed therethrough and connected to a driving means which will be described hereinafter. If the shafts were coaxial, the sensing characteristic of the rollers would be lost. The axes of the rollers 48 and 50 and S2 and 54 are slightly angularly displaced with respect to each other, for example, each axis is inclined approximately one-half degree above the horizontal. The rollers 48 and 50 and 52 and 54 are butted together with sufficient axial pressure to maintain contact over the entire end faces 55 and 58 respectively of the rollers. As a consequence of this arrangement, a dendrite passed upward between rollers 48, 5t) and 52 and 54 will be stable in a lateral position only at the center division line thereof at 55 and 58.
The pulling of the dendrite 44 from the melt 18 may be accomplished by applying a driving force to both the rollers 48, 50, 52 and 54 and to drum 60 or by allowing the rollers, or one set of rollers, to idle and applying the main pulling force to only one set of rollers and/or only to the drum 60. The rollers and drum are maintained at the controlled speed necessary to maintain the desired crystal pull rate.
With reference to FIG. 4 there is illustrated one suitable system for pulling the dendrite by driving only one set of rollers. A separate means is used to drive the drum. The system will be described in terms of driving rollers 52 and 54 with rollers 48 and 50 idling. It will be understood of course that rollers 48 and 50 may be driven with 52 and 54 idling. In the system illustrated in FIG. 4, power is transmitted from an electric motor 62 to a shaft 64. Two gears 66 and 68 are disposed upon shaft 64. The gears 66 and 68 drive two hypoid gears 7 70 and 72 which in turn drive shafts 63 and 61 to which are connected rollers 54. and 52. A duplicate identical system could be used to drive rollers 48 and 50 at the same time.
With reference to FIG. 5, there is illustrated one suitable drive system for driving all four rollers. In this system, the angular speed of the four rollers will be equal but the torque upon the different rollers may vary. An electric motor 74 is connected to a gear 76 through shaft 78. The gear 76, is meshed with and drives gear 80 which in turn meshes with and drives gear 82. Gear 80 through shaft 84 and gears 86 and 88 drives shaft 59 which is connected to roller 50 (not shown). Gear 80 through shaft 84 and gears 90 and 92 drives shaft 63 connected to roller 54. Gear 82 through shaft 94 drives gears 96 and 98 which drive shaft 57 connected to roller 48 and; through gears 100 and 102 drives shaft 61 which is connected to roller 52.
With reference to FIG. 6, there is illustrated a second satisfactory method for driving all four rollers. The system illustrated in FIG. 6 will drive the rollers in such a manner that the torque will be the same on all four rollers but the speed of the rollers may differ. In the system of FIG. 6 an electric motor 104 drives a shaft 106 which in turn drives a gear 108. Gear 108 drives a gear 110 in a plane perpendicular to shaft 106. Gear 110 drives gear 112 which drives gears 114 and 116. Gear 114 drives gear 118 which in turn drives shaft 59 which is connected to roller 50. Roller 54 is driven in a like manner through shaft 63 and gear 120. A duplicate system is used to drive rollers 48 and 52.
As explained above herein, the rollers 48, 50, 52 and 54 may be idle rollers and all the pulling force on the dendrite is supplied by the drum 60. Since the radius of the drum 60 is changing constantly as the dendrite is wound thereon, the motor driving the drum should be a constant torque variable speed motor.
Even when the crystal pulling force is supplied by the rollers a constant torque force must be applied to the drum to maintain a tension on the segment of the dendrite between the rollers and the drum. If this tension is not maintained the dendrite may sag into a loop and break.
Referring again to FIG. 1, in addition to aligning the dendrite by the use of rollers such as described hereinabove, the position of the dendrite may be sensed by photoelectrical or optical means 5. The dendrites have essentially perfect, optically flat surfaces that are parallel to each other within a few angstroms. Consequently, optical means based on interference techniques may be employed to determine the relative thickness of the dendrites and to sense any changes in thickness. Thus, a beam of monochromatic light may be split and one part directed on one surface and the other part directed on the other surface. The beams are reflected from the surfaces and brought together to indicate by the interference fringe changes in relative thickness of the dendrite. Known control devices (not shown) operating on interference measurements are correlated with the optical device to indicate the thickness as well as changes in thickness, and further to vary the pull rate of the dendrite and the heat input to the melt through electrical leads 7 and 9 respectively.
In addition, guide arms straddling the dendrite can be activated by a motor responsive to the photoelectrical. or optical sensing means to shift the dendrite and thus it may be kept aligned in a desired position.
After passing between rollers 48 and 50 and 52 and 54, the dendrite 44 is wound around the drum 60. To prevent the dendrite from being scratched as it is'rolled onto roller 60, a plastic interlayer, for example, a film, foil or tape of polyethylene, polytetrafluoroethylene, polytrifluoromonochloroethylene, nylon and the like may be disposed about or between each succeeding layer of dendrite. The plastic strip may be formed with a flat depression in the middle of either or both sides to act as a spacer. The
8 plastic strip will also serve to keep the dendrite in place if there should be a break during the drawing process.
Because of the thermodynamic relationship between the supercooled melt and the growing crystal, the pull rate and the thickness of the grown crystal and other factors when a dendrite of considerable length is being pulled there is a tendency for the grown crystal to progressively grow thicker. This condition can be overcome with a rather sudden displacement or jerking of the dendrite crystal with respect to the melt in a vertical upward direction. The sudden displacement causes the dendrite to decrease in thickness a fraction of a mil. Repeating the sudden displacement after a minute or so, another decrease in thickness will be effected. This may be repeated as often as is necessary to produce or maintain a desired thickness of dendrite.
Such vertical displacement or jerking can be accomplished for example by imparting a saw-tooth displacing movement to the crucible. The crucible may be dropped rapidly, for example, 2 or 3 mm., and then raised over a period of about 30 seconds back to its normal position. The procedure is repeated as necessary. The same result may be accomplished by suddenly displacing the entire pulling mechanism in an upward direction and then gradually lowering it to its initial position over a period of 30 seconds or so. The procedure is repeated as necessary. Another alternative would be to jerk the winding drum in an upward direction and then slowly lower it. The mentioned range of 2 to 3 mm. is intended only as an illustrative example, as the actual distance will depend upon the thermal gradient between melt and crystal, rate of pulling, and degree of supercooling of the surface of the melt. Likewise, the 30 second period for returning the displaced member of the system to its normal position is intended merely as an example, the important factor being that the return rate to the original position is lower than the pull rate of the crystal from the melt.
In a modification of the present invention, instead of rolling the drawn dendrite about a drum such as drum 60 illustrated in FIG. 1, the grown dendrite may be passed directly into an assembly line whereby it is processed into a semiconductor device or series of devices by being acted upon by suitable doping materials and having electrical contacts or leads attached thereto. The contacts, leads and doping materials may be applied in accordance with the teachings of US. application Serial No. 807,570, of A. I. Bennett, Jr., R. L. Longini and H. F. John (W. E. Case 32,096), filed April 20, 1959, US. Patent No. 3,106,764, the assignee of which is the same as that of the present invention.
In a further modification, the grown dendrite strip may be cut into sections after passing through rollers 48, 50, 52 and 54, rather than being deposited upon a drum. If it is desired to cut the grown dendrite into strips of a predetermined length within the apparatus 10, it is important that the cutting be done in such a manner that no fragments may fall back into the melt 18 disposed Within crucible 16.
In accordance with another modification of this present invention, the dendrite strip 44 may be grown with predetermined p-n-p or n-p-n regions therein in accordance with the teachings of application Serial No. 824,355, of A. I. Bennett, Jr. (W. E. Case 31,877), filed July 1, 1959, and now abandoned, the assignee of which is the same as in the present invention.
The following examples are illustrative of the practice of this invention.
Example I In apparatus similar to FIG. 1, a graphite crucible containing a quantity of intrinsic germanium is heated by the induction coil to a temperature several degrees above the melting point of germanium, the temperature being about 938 C., until the entire quantity forms a molten pool. A dendritic seed crystal having a single interior plane and oriented as in FIG. 2 of the drawing is soldered to a thin steel strip. The seed crystal is passed throughthe aperture in the top of the crucible until its lower end touched the surface of the molten germanium. The contact with the molten germanium is maintained until a small portion of the end of the dendritic seed crystal had melted. Thereafter, the temperature of the melt is lowered rapidly in a matter of five seconds by reducing current to the coil 20, to a temperature 8 below the melting point of the germanium so that the melt is supercooled (about 928 C.). After an interval of approximately ten seconds, at this temperature, the germanium seed crystal is pulled upwardly at a rate of approximately seven inches per minute. The pulling is accomplished by two sets of rollers as illustrated in FIG. 1. The two sets of /2 inch diameter rollers turning in opposite directions at a speed of approximately 4 r.p.m. The steel strip and the dendritic crystal is wound on a drum. A thin film of polyethylene is disposed between each winding of the dendritic on the drum. The dendrite can be of indefinite length providing the germanium in the crucible is replenished.
The dendritic crystal thus prepared will have a thickness of 7 mils and a width of approximately 2 millimeters. The grown dendritic crystal will have substantially fiat and highly parallel faces from end to end with (111) orientation. The germanium dendritic crystal so grown will be found to have no surface imperfections except for a number of microscopic steps difiering by about 50 angstroms and will be of a quality suitable for semiconductor applications.
Example II The process of Example I is repeated except for increasing the pull rate to 12 inches per minute. The dendritic crystal is approximately 3.5 mils in thickness and of a width of about 30 mils. The surface perfection and flatness is exceptional.
Example 111 Example IV A melt of indium antimonide is prepared following the procedure of Example I employing apparatus as illustrated in FIG. 1 of the drawing. The indium antimonide is withdrawn at a rate of inches per minute from a melt supercooled 5 C. The resultant flat dendritic crystal is tested and found to be suitable for semiconductor applications. The surface had (111) orientation.
Example V The procedure of Example I is repeated evcept that the melt is comprised of a quantity of germanium and 1 10 by weight, antimony and 1 10 by weight, boron. The crystal is pulled at a rate of 7 inches per minute. The resultant dendritic crystal has substantially fiat highly parallel faces from end to end with (111) orientation. The germanium dendritic crystal so grown is found to have n-p-n alternate regions therein. The crystal so prepared is suitable for fabrication into a semiconductor device merely by attaching of leads thereto.
While emphasis has been made herein with respect to semiconductor materials, it will be understood that the apparatus and process can be applied to materials otherwise meeting the requirements set forth herein but are not considered to be semiconductors.
While the invention has been described with reference to particular embodiments and examples, it will be understood, of course, that modifications, substitutions and the like may be made therein without departing from its scope.
We claim as our invention:
1. In apparatus for growing thin fiat dendritic crystals of any desired length, in combination, a crucible containing a confined melt of a material from which the dendrite is to be grown, electrical heating means associated with the crucible for maintaining a predetermined temperature Within the melt, and roller means disposed above the crucible, said roller means being comprised of two pairs of rollers, each pair consisting of two rollers having an axis, and a rigid shaft passing through said axis, the axes of each roller being angularly displaced with respect to each other and inclined above the horizontal, the rollers of each pair having end faces butted together with suflicient axial pressure to maintain contact over the entire end faces of two adjacent rollers, said pairs of rollers disposed in a generally horizontal plane and in a relationship to engage a withdrawn dendrite, said roller means being capable of pulling the dendritic crystal from the melt and maintaining said crystal in a relatively fixed position relative to said melt.
2. In apparatus for growing thin flat dendritic crystals of any desired length, in combination, a crucible containing a confined melt of a material from which the dendrite is to be grown, electrical heating means associated with the crucible for maintaining a predetermined temperature within the melt, roller means disposed above the crucible, said roller means being comprised of two pairs of rollers, each pair consisting of two rollers having an axis, and a rigid shaft passing through said axis, the axes of each roller being angularly displaced with respect to each other and inclined above the horizontal, the rollers of each pair having end faces butted together with sufiicient axial pressure to maintain contact over the entire end faces of two adjacent rollers, said pairs of rollers disposed in a generally horizontal plane and in a relationship to engage a withdrawn dendrite, said roller means being capable of pulling the dendritic crystal from the melt and maintaining said crystal in a relatively fixed position relative to said melt, winding means capable of receiving said dendritic crystal disposed above the melt, and means for replenishing the melt as dendritic crystals are withdrawn from the melt.
3. In apparatus for growing thin fiat dendritic crystals of any desired length, in combination, a crucible containing a confined melt of a material from which the dendrite is to be grown, electrical heating means associated with the crucible for maintaining a predetermined temperature within the melt contained within the crucible, roller means disposed above the melt, said roller means including aligning means so that it is capable of pulling the dendritic crystal from the melt and maintaining said crystal in a relatively fixed position relative to said melt, means responsive to the dimensions of the dendritic crystal to control the temperature of the melt and rate of pulling, and means for replenishing the melt as dendritic crystals are withdrawn from the melt.
4. In apparatus for growing thin flat dendritic crystals of any desired length, in combination, a crucible containing a confined melt of a material from which the denrite is to be grown, electrical heating means associated with the crucible for maintaining a predetermined temperature within the melt contained within the crucible, roller means disposed above the melt, said roller means being capable of pulling the dendritic crystal from the melt and maintaining said crystal in a relatively fixed position relative to said melt and means for imparting a temporary sudden separation of the order of 1 mm. to 5 mm. between the melt and the dendritic crystal, whereby, the thickness of the dendrite is decreased, means for returning the dendritic crystal with respect to the melt at a slow rate, and
means for replenishing the melt as dendritic crystals are withdrawn from the melt.
References Cited by the Examiner UNITED STATES PATENTS Strong. Ritzmann. Himmelheber et al. Koury 148-15 Kniepkamp 23301 Shockley 148-1.5 Schweickert et a1. 23273 12 2,907,643 10/59 Reynolds et a1. 23273 2,916,593 12/59 Herrick. 2,960,418 11/60 Zierdt 1481.5 2,993,301 7/61 Muller 4983.1
OTHER REFERENCES Proceedings of the Royal Society; vol. 229, 1955, by Billig; pages 346-363.
10 NORMAN YUDKOFF, Primary Examiner.
ANTHONY SCIAMANNA, MAURICE A. BRINDISI,
RAY K. WINDHAM, Examiners.