US 3650703 A
Method of growing elongate essentially monocrystalline bodies of indefinite lengths and selected cross-sectional shapes from melts of selected high melting materials that melt congruently. Examples are filaments, ribbons and tubes of alpha -alumina.
Description (OCR text may contain errors)
11ttited States Patent Labelle, Jr. et :11
[451*Mar. 21, 1972 METHOD AND APPARATUS FOR GROWING INORGANIC FILAMENTS, RIBBON FROM THE MELT Inventors: Harold E. Labelle, Jr., Quincy; Abraham 1. Mlavsky, Lexington, both of Mass.
Assignee: Tyco Laboratories, Inc., Waltham, Mass.
sequent to Mar. 14, 1988, has been dis claimed.
Filed: Sept. 8, 1967 Appl. No.: 666,304
Related U.S. Application Data Continuation-impart of Ser. No. 621,731, Feb. 14, 1967, abandoned.
References Cited UNITED STATES PATENTS 4/1962 Bennett, Jr. ..23/301 The portion of the term of this patent sub- I 3,162,507 12/1964 Dermatis et al ..23/301 3,291,574 12/1966 Pierson ..23/301 3,370,927 2/1968 Faust, Jr. ..23/301 3,129,061 4/1964 Dermatis ..23/30l 3,206,286 9/1965 Bennett ..23/30l 3,265,469 8/1966 Hall ..23/301 3,291,650 12/1966 Dohmen ..23/301 3,337,303 8/1967 Lorenzini.... .....23/30l 3,340,016 9/1967 Wirth et al.. .....23/301 3,291,571 12/1966 Dohmen i ..23/30l 3,096,158 7/1963 Gaule et al ..23/30l 3,124,489 3/1964 Vogel, Jr. et al ..23/301 3,224,840 12/1965 Lefever ..23/301 Primary ExaminerNorman Yudkoff Assistant Examiner-N. Yudkoff AnomeyNicholas A. Pandiscio  ABSTRACT Method of growing elongate essentially monocrystalline bodies of indefinite lengths and selected cross-sectional shapes from melts of selected high melting materials that melt congruently. Examples are filaments, ribbons and tubes of a alumina.
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IN VENTORS HAROLD E4 LABELLE,JR ABRAHAM I MLAVSKY Mwm 111111;;
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iim-ij-j- 51 L 6 w 7 mm INVENTORS HAROLD E. LABELLE,JR. ABRAHAM l. MLAVSKY Has M yaw ATTORNEY Patented March 21, 1972 3 Sheets-Sheet 3 INVENTOR.
HAROLD E. LABELLE JR BYABRAHAM I. MLAvsKY ATTORNEY METHOD AND APPARATUS FOR GROWING INORGANIC F ILAMENTS, RIBBON FROM THE MELT This application is a continuation-in-part of our prior copending application Ser. No. 621,731 filed 2/14/67 (now abandoned) for Method And Apparatus For Growing Inor ganic Filaments.
The present invention relates to growing from the melt extended essentially monocrystalline bodies of alumina and other high melting point materials that melt congruently.
The primary object of this invention is to provide a method and apparatus by which selected high melting point inorganic materials that melt congruently can be pulled from the melt as elongate essentially monocrystalline bodies of indefinite length and predetermined cross-sectional shape.
A further primary object is to grow essentially monocrystalline bodies of indefinite length and selected cross-sectional shape from a melt of a selected high melting point material that melts congruently by the process of dendritic growth. As used herein dendritic growth denotes growth occurring or beginning below the surface of the melt rather than above the melt surface. Heretofore continuous dendritic growth has been applied with success to only a limited number of materials, notably germanium, silicon and gallium arsenide.
It is recognized that certain refractory materials exhibit improved mechanical properties when produced in fiber crystalline form and that solid matrices reinforced by said fibers are useful in the fabrication of a variety of mechanical components, such as parts for turbines, jet engines, rockets and other high performance equipment. Typical of the materials that have potential as reinforcing elements are metals such as beryllium, magnesium and boron and refractory oxides such as BeO, MgO, ZrO and A1 Certain of these materials have been grown in the form of small crystal fibers (also called whiskers) that exhibit very large tensile strengths. However, because of their relative small size, they are difficult to handle and the task of dispersing them in a suitable matrix is a formidable one at the present state of the art.
A specific object of this invention is to provide a method and apparatus by which selected high melting point inorganic crystalline materials can be grown in indefinitely long filaments with favorable mechanical properties that render them suitable for reinforcing selected metal or plastic matrices to provide new and useful composite materials.
a-alumina and other materials in the form of thin essentially monocrystalline plates may be used as substrates for epitaxially grown integrated circuit devices. It is recognized that these substrates may be produced by slicing a single crystal of substantial bulk into thin plates of appropriate thickness. However this procedure is costly, requires precision equipment, and its yield depends on the availability of bulk crystals that are relatively free of crystal defects and imperfections. The present invention overcomes these limitations by providing a simplified method of producing essentially monocrystalline substrates.
Accordingly another specific object of this invention is to grow a-alumina and other materials from the melt in the form of an essentially monocrystalline ribbon of extended length and relatively flat opposite side surfaces, that may be used as a substrate material for semiconductor circuit devices and the like. The ribbon also may be used as a reinforcement material for metal alloy and plastic matrices. It is noted that at temperatures close to the melting point, there appears to be a surface diffusion of atoms which results in the ribbon having smooth strain-free surfaces.
The invention is not restricted to filaments and ribbons, and it is possible to produce essentially monocrystalline bodies of extended length that are characterized by other cross-sectional configurations. Thus by way of example but not limitation, a further specific object is to grow from a melt of material such as a-alumina an elongate essentially monocrystalline body in the form ofa hollow tube that may be employed in the manufacture of high power optical pumping sources. The foregoing and still other cross-sectional shapes have possible utility in fiber optics and ultrasonic delay lines.
The foregoing and other objects are attained by providing a melt of a selected high melting point material that melts congruently (Le, a solid compound that melts to a liquid of the same composition at an invariant temperature), establishing within the melt a supercooled zone and a thermal distribution that is conducive to propagation vertically of crystal growth, introducing a seed of selected orientation into the supercooled zone for a period sufficient for dendritic growth to be initiated, and then withdrawing the seed at a rate approximately equal to, but not greater than, the rate at which the dendrite grows downward into the melt. The thermal distribution in the supercooled zone determines the shape of the elongate essentially monocrystalline body that grows onto the seed. Thus in growing relatively thin filaments, a small essentially round melt zone is established in which the thermal distribution is conducive to vertically propagated dendritic growth. To produce a flat ribbon a melt zone is established that is relatively elongate in one horizontal direction and relatively short in a second horizontal direction extending at a right angle to said one horizontal direction, the thermal gradients being such as to support dendritic growth propagated vertically throughout substantially all of said zone. Where the grown body is to be essentially tubular, a substantially annular melt zone is established. The desired thermal distribution may be achieved by various means including but not limited to the means illustrated in the drawings and described hereinafter in connection with the several embodiments of the invention. Whatever means are employed for this purpose must not interfere with withdrawal of the seed and the dendritically grown essentially monocrystalline body, and must be adapted to maintain the desired thermal distribution as the melt is depleted.
Other objects and many of the attendant advantages of our invention are believed to be apparent from the following detailed description which is to be considered together with the accompanying drawings, wherein:
FIG. 1 is an elevational sectional view, partly in schematic form, of one form of apparatus for growing crystalline filaments according to the invention;
FIG. 2 is a magnified sectional view of part of the apparatus of FIG. 1;
FIG. 3 is a fragmentary sectional view in elevation, partly in schematic form, of a second form of apparatus for carrying out the process of this invention;
FIG. 4 is a magnified sectional view similar to FIG. 2 of a portion of the apparatus illustrated in FIG, 3;
FIG. 5 is an enlarged sectional view in elevation of one form of means employed to grow a monocrystalline ribbon;
FIG. 6 is a cross-sectional view taken along line 33 of FIG. 5;
FIG. 7 is a sectional view in elevation of one form of means employed to grow an essentially monocrystalline tube; and
FIG. 8 is a fragmentary view in elevation of apparatus for pulling a monocrystalline filament or ribbon of indefinite length.
While the following description of preferred and alternative embodiments of the invention is directed to growth of a-alumina (corundum or sapphire) filaments, ribbons and tubes, it is to be understood that the invention is applicable to other refractory materials that l) melt congruently and (2) have a unique, i.e., single, c-axis, notably materials with a rhombohedral, hexagonal or tetragonal crystal structure such as BeO, Cr O and TiO It has been determined that from an open melt a-alumina tends to grow circumferentially, i.e., radially from the seed, rather than propagate down into the melt, so that dendritic growth when it occurs is generally parallel to the surface of the melt and often is characterized by branching of the dendrites. Variations in pulling speed do not change the direction of dendritic propagation. Our invention is based on the hypothesis that the temperature distribution within the melt is an extremely critical parameter affecting the direction of growth of monocrystalline body if the proper temperature distribution is provided in the melt. Autodendritic growth (without a seed having a preselected twin structure as is necessary for example for dendritic growth of zinc blende cubic materials) of alumina and similar congruently melting materials occurs preferentially along the c-axis. Nevertheless the disposition of the thermal gradients about and within the region of crystal growth is a critical parameter affecting the direction of growth and crystal growth can be made to propagate vertically in the melt in a manner permitting the pulling of a continuous filament, ribbon, tube etc. only if proper temperature distribution is achieved. More specifically, we have determined that establishing a thermal distribution conducive to vertical propagation in the region selected for crystal growth involves maintaining the average temperature of the melt above the melting point, shielding against radiative heat loss and adjusting the rate of heating to provide supercooling in the growth region surrounding the inserted seed. The means employed to prevent radiative heat loss should be so disposed with respect to the growth region that its coaction with such region will not vary substantially as the melt is depleted in order to prevent the thermal distribution from being upset to the extent that vertical crystal growth will cease. By proper adjustment of the rate of heating the desired degree ofsupercooling is achieved about and in the location of the inserted seed. In this connection it is to be noted that alumina can be supercooled substantially and we have been able to supercool molten alumina in a molybdenum crucible in excess of 100 C. measured with a W-7p Re, W3% Re thermocouple.
One aspect of the several embodiments of the method hereinafter described is the concept of growing from a relatively small diameter pool or body of melt which is continuously replenished from a larger body of melt. This small diameter pool provides a zone with a controlled thermal distribution conducive to continuous crystal growth. Since the growth is essentially dendritic, i.e., below the surface and in a supercooled region of the melt, the cross-sectional shape of the essentially monocrystalline body that is pulled depends upon the thermal distribution established in the growth zone. This relatively small diameter pool of melt may be formed in the orifice of a radiation shield disposed on the surface of the melt or may be formed in an elongate hollow member that is positioned within the larger melt body. Other means that will suggest themselves to persons skilled in the art also may be used to establish a zone characterized by a thermal distribution conducive to crystal growth permitting continuous pulling of an essentially monocrystalline body of desired cross-sectional configuration. In this connection it is to be appreciated that the small diameter pool may consist simply of a zone within the larger melt body that is characterized by the thermal distribution required to achieve crystal growth in the manner herein described. With respect to the use of a radiative shield with a growth orifice, an elongate hollow member, or like means to define the relatively small pool containing the growth zone, it is preferred to use a material of high conductivity that does not react with the melt, e.g. molybdenum or iridium. Because the radial thickness of such means is substantial, it acts substantially as an isothermal element that establishes in the relatively small melt pool isotherms which determine the shape of the growing essentially monocrystalline body. The cross-sectional size (and to some extent the shape) of the growing crystalline body may be varied by varying the pulling speed, so that by appropriately increasing the pulling speed it is possible to grow a monocrystalline body that is substantially smaller than the secondary melt pool in which growth occurs.
As used herein the term essentially monocrystalline is intended to embrace a filament, ribbon, tube and any other crystalline body of indefinite length grown from the melt which over any given portion of its length exceeding the maximum cross-section dimension is comprised ofa single crystal or two or more single crystals growing together longitudinally but separated by a relatively small angle (i.e. less than about 4) grain boundary.
The drawings illustrate several forms of apparatus for practicing the invention, but it is to be understood that other forms may also be employed successfully for the same purpose providing they are capable of establishing and maintaining the requisite thermal distribution within a given melt region where the seed is introduced and growth is to occur.
Turning now to FIG. 1, the illustrated apparatus comprises a vertically moveable horizontal bed 2 on which is supported a furnace enclosure consisting of two concentric-spaced quartz tubes 4 and 6. At its bottom end the inner tube 4 is positioned in a L-gasket 5 in the bed. Surrounding tube 4 is a sleeve 8 that screws into a collar 10. Between sleeve 8 and collar 10 is an O- ring 12 and a spacer 13. The O-ring 12 is compressed against tube 4 to form a seal. The upper end of sleeve 8 is spaced from tube 4 so as to accommodate the bottom end of tube 6. The bottom end of tube 6 is secured in place by an O-ring l4 and a spacer 15 compressed between a collar 16 that screws onto sleeve 8. Sleeve 8 is provided with an inlet port fitted with a flexible pipe 20. The upper ends of tubes 4 and 6 are secured in a head 22 so that they remain stationary when the bed is lowered. Head 22 has an outlet port with a flexible pipe 24. Although not shown, it is to be understood that head 22 includes means similar to sleeve 8, O-rings l2 and 14, and collars 10 and 16 for holding the two tubes in concentric sealed relation. Pipes 20 and 24 are connected to a pump (not shown) that continuously circulates cooling water through the space between the two quartz tubes. The interior of the furnace enclosure is connected by a pipe 28 to a vacuum pump or to a regulated source ofinert gas such as argon or helium. The furnace enclosure also is surrounded by an RF. heating coil 30 that is coupled to a controllable 500 kc. power supply (not shown) of conventional construction. The heating coil may be moved up or down along the length of the furnace enclosure and means (not shown) are provided for supporting the coil in any selected elevation. At this point it is to be noted that the circulating water not only keeps the inner quartz tube at a safe temperature but also absorbs most of the infrared energy and thereby makes visual observation of crystal growth more comfortable to the observer.
The head 22 is adapted to provide entry into the furnace enclosure of an elongate pulling rod 32 that is connected to and forms part of a conventional crystal pulling mechanism represented schematically at 34. It is to be noted that the type of crystal-pulling mechanism is not critical to the invention and that the construction thereof may be varied substantially. Preferably, however, we prefer to employ a crystal pulling mechanism that is hydraulically controlled since it offers the advantages of being vibration-free and providing a uniform pulling speed. Regardless ofits exact construction which is not required to be described in detail, it is to be understood that the pulling mechanism 34 is adapted to move pulling rod 32 axially at a controlled rate. Pulling rod 32 is disposed coaxially with the quartz tubes 4 and 6 and its lower end has an extension in the form ofa metal rod 36 that is adapted to function as a holder for a seed crystal 38.
Located within the furnace enclosure is a cylindrical heat susceptor 40 made of carbon. The top end of susceptor 40 is open but its bottom end is closed off by an end wall. The susceptor is supported on a tungsten rod 42 that is mounted in bed 2. Supported within susceptor 40 on a short tungsten rod 44 is a crucible 46 adapted to contain a suitable supply of alumina. The crucible is made of a material that will withstand the operating temperatures and will not react with or dissolve in the molten alumina. In the illustrated embodiment the crucible is made of molybdenum, but it also may be made of iridium or some other material with similar properties with respect to molten alumina. The molybdenum crucible must be spaced from the susceptor since there is a eutectic reaction between carbon and molybdenum at about 2200 C. The inside of the crucible is of constant diameter and may have a hemi-spherical bottom. In order to obtain the high operating temperatures necessary for the process, a cylindrical radiation shield 50 made of carbon cloth is wrapped around the carbon susceptor. The carbon cloth does not appear to couple directly to the RF field but greatly reduces the heat loss from the carbon susceptor. At a given RF power setting the shield 50 increases the susceptor temperature by as much as 500 C.
In addition to the supply of material from whose melt 48 a filament is to be pulled, the crucible contains a heat shield in the form of an orifice plate 52 that is adapted to float on the surface of the melt. In growing a-alumina filaments, the orifice plate 52 is made of molybdenum or other suitable material. The underside of the orifice plate, and preferably also the upperside that is exposed to the furnace atmosphere, is polished smooth. The plate preferably has a thickness in the range of 0.025 to 1 mm. and is shaped to conform to the interior of the crucible. However, as shown in FIG. 2, the plate is made slightly smaller than the interior of the crucible so as to provide clearance sufficient for it to float evenly on the melt and to move down in the crucible if the melt is depleted. In the usual case a clearance of about 0.010 inch suffices. As illustrated in FIG. 2, the orifice plate has a centrally located circular orifice 54. Preferably the orifice has a diameter of about one-sixteenth inch, but equally satisfactory results have been obtained with orifices measuring one thirty-second inch and smaller. An orifice larger than one-sixteenth also may be used provided its cross-sectional area is not so large with respect to the overall size of the plate to prevent the latter from establishing the proper thermal distribution in the melt.
The effect of the orifice plate on the growth process is confirmed by experiments to propagate a-alumina filaments continuously using the apparatus of FIG. 1 in the manner described below but with the plate omitted from the crucible. In these experiments a mass of alumina was melted, slowly cooled until solidification proceeded (this is to establish the melting point), remelted, and slowly cooled again to within a few degrees above the melting point. A seed with its c-axis parallel to the axis of the crystal holder was then inserted into the melt and rapidly withdrawn at a predetermined rate. These experiments succeeded in propagating dendrites parallel to the surface of the melt but failed to grow any dendrites down into the melt. This propagation behavior suggested an improper thermal distribution within the melt. Accordingly additional experiments were conducted using the same procedure but supplying more heat to the bottom of the crucible (relative to its sides). This was achieved by shifting the relative positions of the susceptor and the RF. coil. These additional experiments succeeded in dendritic growth clearly propagated vertically into the melt, but the propagation was not continuous. Dendrites grown in this manner always grew downward from a much larger mass of solidified a-alumina and would stop growing vertically as the larger mass was withdrawn. This behavior suggested that although the increased heat applied to the bottom of the crucible appeared to improve the thermal distribution within the melt enough to promote vertical dendritic growth, removal of the larger mass of solidified aalumina from the melt caused a thermal fluctuation sufficiently larger to prevent further vertical propagation.
Introduction of a heat shield, e.g. plate 52, near or on the surface of the melt solved the problem of continuous growth of a-alumina and confirmed that in order to obtain continuous dendritic propagation vertically into the melt, it is necessary not only to establish a critical thermal distribution but also to maintain this distribution as the dendrite is withdrawn from the melt.
Operation of the above-identified apparatus and an example of the method of growing a-alumina filaments according to our invention will now be described. An a-alumina seed crystal 38 is mounted in holder 36 with its c-axis aligned parallel to the holders path of movement and perpendicular to the top surface of the melt formed in the crucible. At the same time a quantity of substantially pure a-alumina is placed in crucible 46, orifice plate 52 is placed on top of the alumina, and then the crucible is placed within susceptor 40 on tungsten rod d4v Access to the seed holder and the susceptor is achieved by lowering bed 2 away from the furnace enclosure and lowering the seed holder to below the bottom end of tube 4. With the bed restored to the position of FIG. 1, cooling water is introduced between the wall of the two quartz tubes, and the enclosure is evacuated and then filled with helium. The latter is kept at a pressure of about 1 atmosphere thereafter. Then the RF. coil is energized and operated so that the alumina is brought to a molten condition. It is to be noted that the orifice plate does not sink into the melt but floats at its surface, even through the plates density is greater than that of the melt. One possible explanation for this is that the melt does not wet molybdenum. The alumina is brought to a temperature slightly above its melting point which is in the vicinity of 2000 C. In this molten condition the height of the meniscus 56 of the alumina in orifice 54 is an inverse function of the diameter of the orifice, but typically is almost flush with the top surface of the orifice plate. Once temperature equilibrium is established, the pulling mechanism is actuated and operated so as to bring the seed crystal 38 into contact with the meniscus. Almost immediately thereafter the pulling mechanism is operated so as to pull the crystal at a predetermined rate of speed. The melt temperature is critical and often initial withdrawal of the seed is unaccompanied by continuous growth. At this point it is to be appreciated that if the surface of the melt in the orifice 54 is too cold, it will solidify and no growth will occur on the seed; on the other hand, if the surface of the melt is too hot, the seed will melt. The temperature of the melt is adjusted accordingly and the seed again is brought into contact with the melt. At the proper melt temperature, dendritic growth will occur on the end of the seed. Thereafter the seed is withdrawn at a speed corresponding to the rate at which the dendrite growth propagates down into the melt. If the seed continues to be withdrawn at the proper speed, the growth will be continuous until the melt is depleted. The orifice plate will settle within the crucible as the alumina is depleted. The maximum length has been limited only by the maximum pulling distance afforded by pulling mechanism 34.
In this connection it appears from observation that the molybdenum orifice plate has a lower total emissivity than alumina at temperatures in the order of 2000 C. Accordingly it is believed that this property enables the floating plate to act as a heat shield which limits the heat loss from the melt surface and thereby controls the radial and longitudinal temperature gradients in the immediate vicinity of the small surface of the melt exposed within the orifice. The heat shielding effect of the floating plate not only establishes the correct temperature distribution required to promote propagation vertically but also permits the melt to be supercooled in the region where the seed is introduced, an essential condition for dendritic growth. In summary the floating molybdenum plate serves the dual function of providing an effective heat shield and an exposed central growth orifice of any chosen diameter.
In the growth of corundum filaments by the present invention as described above, it has been found that the habit of the crystalline filaments is not always the same. In some cases the crystal habit is characterized by a substantially uniform outer surface and a cross section that is circular or has a rounded triangular shape. In other cases the surfaces of the filaments have irregular undulations or are stepped longitudinally. A few filaments even appear to be twisted longitudinally. After a particular habit has been nucleated and propagated, it is possible to separate the grown filament by fast retraction, reinsert it into the melt and then resume normal growth procedure. When this is done the dendrite filament grown afterward usually displays the same habit as that propagated before the filament was separated from the melt. The lack of close conformity of the cross-sectional shape and size of the filament with the shape and size of the orifice of the metal orifice plate 52 is believed attributable to the fact that the crystal grows dendritically down into the melt, i.e., the solid-liquid interface of the filament is below the melt surface. The orifice plate does not shape the filament directly, as in an extrusion process, but does affect the temperature gradients of the melt in the orifice, and such gradients together with the average temperature of the melt and the orientation of the seed crystal, influences the filament shape.
Laue X-ray back reflection photographs of a-alumina (corundum or sapphire) filaments made according to the foregoing method reveal that the filament is comprised of one or two, and in some cases, three or four crystals growing together longitudinally separated by a low angle (within 34 of the c-direction)grain boundary. In other words, the several crystals have their c-axes approximately parallel to the longitudinal axis of the filament but slightly misoriented with respect to each other. The essential aspect of the invention is that the seed crystal should be mounted so that growth occurs along its c-axis 000l i,e., with its c-axis extending parallel to the axis of movement of the crystal holder and normal to the surface of the melt, While growth will occur on a seed mounted so that its c-axis is at an angle to the seed holder, the larger the angle the more difficult it is to sustain continuous growth and the more inferior the product. Filaments grown in the c-direction have smoother surfaces and superior strength while filaments grown off the c-axis by angles as little as 10 have irregular surfaces and exhibit substantially lower tensile strengths.
A definite indication of the fact that growth is dendritic is the speed at which the filament may be pulled from the melt. In practice we have pulled filaments at speeds up to about 150 mm./min. through l/32-inch-diameter orifice. It is believed that growth rates substantially faster than 150 mm./min. can be achieved if the heat loss from the filament (primarily radiative in the apparatus and process described above) is augmented by forced convection. It also is to be appreciated that with proper control, simultaneous growth ofa plurality of filaments from a common melt may be achieved using an orifice plate with a number of orifices located so that each meniscus is at approximately the same temperature. In practice we have pulled filaments measuring 0.05 to 0.50 mm. in diameter at pulling rates up to I50 mm./min. We have grown filaments up to 12 inches long at this speed. The length of these filaments was not limited by the growth process per se but by the limited capability of our pulling mechanism. Longer lengths are possible with apparatus of greater pulling capacity. Sample sapphire filaments produced in the manner described above have been found to have an elastic modulus of 40-70 l0 p.s.i. (as measured by various techniques), and a tensile strength of as much as about 300,000500,000 p.s.i., when grown onto a seed oriented so that its c-axis is perpendicular to the melt surface. Substantial reductions in elastic modulus, flexure modulus and tensile strength occur if the seed crystal is mounted so that its c-axis is not parallel to the axis of movement of the crystal holder and perpendicular to the melt surface.
FIGS. 3 and 4 illustrate a second form of apparatus for growing filaments according to this invention. FIG. 3 illustrates the bottom half of the apparatus, it being understood that the upper part is essentially the same as the corresponding section ofthe apparatus shown in FIG. 1 and includes a pulling mechanism positioned above and coupled to the upper end of the furnace enclosure. For the sake of convenience, like parts are identified by the same numerals employed in the description ofthe apparatus ofFIG. 1.
In the alternative embodiment of the invention, the orifice plate 52 is attached to and supported by a plurality of rigid, small diameter rods 60 that are anchored in and depend from a quartz plate 62 that is located within the inner quartz tube 4 of the furnace enclosure. Plate 62 is fixed to the furnace tube 4 so that it cannot move. The rods 60 support the orifice plate 52 concentric with the furnace enclosure. The orifice plate 52 sits within the crucible 46 in contact with or spaced slightly above the surface ofthe melt in the crucible. The tungsten rod 42 that supports the susceptor 40 is not fixed but is mounted so that it can move vertically. The rod 42 extends through a suitable hole formed in the base 2, the latter being provided with a rubber seal 66 that allows reciprocal movement of the rod while sealing the base so as to permit a selected gas environment to be maintained within the furnace. Rod 42 is keyed to base 2 as shown at 68, so that it can slide up or down but cannot rotate. Mounted on the underside of base 2 is a bearing support 70 in which is mounted a bearing 72. Slidably and rotatably supported by bearing 72 is a shaft 74 whose upper end is rotatably connected by a suitable coupling (not shown) to the bottom end of rod 42. For convenience of illustration the upper end of shaft 76 and the lower end of rod 42 are omitted from the drawings. The upper end portion of shaft 74 has a smooth cylindrical configuration so that it can rotate and also move axially within bearing 72. The bottom end portion of shaft 74 is of similar construction and is mounted in a second bearing 76 supported in a fixed support plate 78. Shaft 76 also has a threaded portion 80 that permits it to function as a worm gear. Mating with this threaded portion 80 is a gear 82 that is rotatably mounted on a shaft 84 which also carries another gear 86. Although not shown, it is to be understood that means are provided for rotatably supporting shaft 84. Gear 86 mates with still another gear 88 that is mounted on the output shaft of speed reducer 90 that is driven by a motor 92. The power circuit of motor 92 comprises a programmable motor speed controller 94. Operation of motor 92 causes shaft 74 to rotate and, depending upon its direction of rotation, shaft 74 will move up or down and cause corresponding movement of rod 42. This movement of rod 42 causes the assembly mounted on its upper end, i.e., susceptor 40, crucible 46, and shield 50, to move within the furnace enclosure. The motor speed controller 94 permits the motor speed to be adjusted so that rod 42 and crucible 46 will be moved upward at a speed proportional to the growth of filament and, more specifically, at a speed equal to the speed at which the level of the melt drops in the crucible during filament growth. In this way the spatial relationship of the orifice plate with respect to the melts surface is maintained at all times. The rods 60 are vertical and are spaced so as not to interfere with upward movement ofthe crucible.
The method of operating the apparatus of FIGS. 3 and 4 is substantially the same as the method described above, except for programming operation of the drive means that controls the position of the crucible with respect to the orifice plate. Assume that an a-alumina filament is to be produced. In such a case, an a-alumina seed crystal would be mounted in the holder of the pulling mechanism in the manner previously described and a quantity of substantially pure aalumina would be placed in the crucible 46 which has been withdrawn away from the orifice plate 52. Access to the crucible is achieved by lowering the bed 2 away from the furnace enclosure. It is to be understood that the mechanism for moving rod 42 is supported by the bed 2 and is movable therewith. With the crucible restored to the position shown in FIGS. 3 and 4, cooling water is introduced between the walls of the furnace and the furnace enclosure is evacuated and then filled with inert gas such as helium. Then the RF. heating coil 30 is energized (by means not shown) to bring the contents ofthe crucible to a molten condition. The alumina is brought up to a temperature slightly above its melting point. After temperature equilibrium is established, the pulling mechanism is operated to bring the seed crystal into contact with the meniscus of the melt exposed in the orifice 56. Thereafter the pulling mechanism is operated to draw the crystal away from the melt. If growth does not occur on the seed as it is withdrawn, the temperature of the melt is adjusted. When the correct temperature is achieved, the seed is withdrawn at a continuous rate and simultaneously the motor 92 is energized to initiate movement of rod 42. The motor speed is adjusted so that as the melt is depleted the crucible moves up at a rate sufficient to maintain the surface of the melt in touch with or slightly below the orifice plate.
Growth of essentially monocrystalline a-alumina ribbon may be achieved using the same furnace apparatus shown in FIG. 1 but replacing the crucible 46 and orifice plate 52 with the arrangement shown in FIGS. 5 and 6.
Referring now to FIGS. 5 and 6, there is shown a molybdenum crucible 46A adapted to be supported on the susceptor 40 of the apparatus of FIG. 1 in the same manner as crucible 46. Crucible 46A is provided with a molybdenum cover 102 that serves as a heat shield for the melt. This cover has a centrally located opening 104 and a second smaller opening 106 to one side of opening 104. A thermocouple 108 is disposed in the crucible below the surface of the melt 109 formed therein, the lead wires of the thermocouple passing through the opening 106 and being connected to a responsive temperature indicating unit (not shown) of conventional design.
Situated in crucible 46A is a molybdenum member 112 comprising an annular plate 114 and an elongate bar 116 of rectangular cross-section which projects up beyond the upper end of the crucible. The bar 116 is slotted as shown at 118 with the slot extending from its top end down to about the level of plate 114. By way of example but not limitation, the slot may have a width (measured from top to bottom in FIG. 6) of about one-eighth inch and a depth (measured from left to right in FIG. 6) of 8-12 mils. Its length, the vertical dimensions in FIG. 5, depends upon the size of the crucible. Preferably the thickness of the bar 116 on each side of the slot is substantially greater than the slot depth so as to assure that these portions of the bar function as isothermal elements with respect to the melt in the slot. As further illustrated in FIGS. 2 and 3, the upper end of the bar is tapered to a knife edge. Preferably the tapered end surfaces shown at 120 and 122 are disposed at an angle no greater than about 4560 with respect to the longitudinal axis of the slot. It has been found that with shallower end surfaces, notably end surfaces disposed at angles greater than 60, the growth process sometimes has resulted in ribbons having a thickness slightly larger than slot depth. This appeared to be due to growth occurring from the melt that overflowed onto and accumulated on the tapered end surfaces as an extension of the pool of melt in the slot. This phenomenon does not occur if the end surfaces are sufficiently tapered to produce a thin knife edge at each side ofthe upper end of slots 118. It is to be noted that the depth of the slot may be varied substantially and is limited only to the extent that it permits the upward flow of melt by capillary action. If desired the slot may have a stepped shape, with a smaller depth at its top end and a greater depth at its bottom end.
In using the apparatus of FIGS. and 6, a melt 106 of alu mina is provided in the crucible. That portion of slot 118 which is not immersed in the melt will nevertheless be filled by a column of molten alumina resulting from capillary action (capillary action also is involved in filling the orifice of plate 52 described above). In this connection it is to be noted that the height to which a column of molten alumina will rise in an elongate hollow member is an inverse function of its cross-sectional area. Relatively long columns of alumina can be achieved by capillary action. It is to be noted also that the column will rise to the top of the slot despite the fact that the sides of the slot are open. As a consequence of growth and withdrawal of a-alumina ribbon, the column of molten alumina tends to be depleted; however, the solid-liquid interface remains at the same level due to continued replenishment of the column by inflow of material from the crucible. Continued growth will cause the level of the melt in the crucible to drop, but the column will continue to exist until no further reservoir of melt exists in the crucible.
It is to be noted that it is not necessary for the sides of the slot to be open as shown in FIGS. 5 and 6. The illustrated construction is preferred merely because it is easier to fabricate. Thus if desired, bar 116 could be replaced by a tube of rectangular cross section fitted with one or more holes at its bottom end to permit inflow of melt from the crucible.
Operation of the apparatus of FIG. 1 using the crucible arrangement shown in FIGS. 5 and 6 and an example of growing a-alumina ribbon according to our invention will now be described.
An a-alumina seed crystal 38 is mounted in holder 36 with its c-axis aligned parallel to the holders path of movement. At the same time a quantity of substantially pure alumina is placed in crucible 46A, cover 102 is set in place, and then crucible 46 is placed within susceptor 40 on tungsten rod 44. Access to the seed holder and the susceptor is achieved by lowering bed 2 away from the furnace enclosure and lowering the seed holder to below the bottom end of quartz tube 4. With the bed restored to the position of FIG. 1, cooling water is introduced between the walls of the two quartz tubes, and the enclosure is evacuated and then filled with helium. The latter is kept at a pressure of about 1 atmosphere thereafter. Then the RF coil is energized and operated so that alumina is brought to a molten condition. The alumina is brought to a temperature slightly above its melting point which is in the vicinity of 2000 C. In this molten condition the molten alumina will rise in slot 118 so that its meniscus is substantially flush with the upper end of the slot. Once temperature equilibrium is established (as determined by the temperature indication unit to which the thermocouple is connected), the pulling mechanism is actuated and operated so as to bring seed crystal 38 into slot 118 to a depth of about 0.5 mm. below the meniscus of the melt column. It is allowed to rest there for about five seconds. Then the pulling mechanism is operated so as to withdraw the seed crystal at a rate of about 2-4 inches per minute. It should be noted that the parameters of seed depth in the melt, rest time of the seed in the melt column, and the pulling rate are related and are determined by trial and error for the particular apparatus employed and the temperature of the melt column surrounding the seed. As indicated above with respect to filaments, the melt temperature is critical and initial withdrawal of the seed may be unaccompanied by continuous growth. The temperature of the melt is adjusted accordingly and the seed again is brought into contact with the melt. Attainment of the proper melt temperature is indicated by commencement of dendritic growth on the end of the seed. Thereafter the seed is withdrawn at a speed corresponding to the rate at which the dendrite growth propagated down into the melt. However, the operator may have to vary the melt temperature and the pulling rate to some degree to optimize the growth process. If the seed continues to be withdrawn at the proper speed the growth will be continuous until the melt is depleted. The maximum length of the ribbon is limited only by the maximum pulling distance afforded by the pulling mechanism 34 and the supply of melt in the crucible.
The pulling speed for a flat ribbon is less than that used in pulling filaments. Nevertheless the growth process is the same. In this connection it is to be noted that in one instance where a ribbon pulled clear of the melt, it was observed to have three or four small vertically elongated dendrites projecting from its end.
We have determined that the temperature in the growth region need not be held constant in order to achieve continuous growth. Instead the temperature may vary over a narrow range with the extent of the range depending upon the pulling speed. The operative temperature range narrows with higher pulling speeds and widens with lower pulling speeds.
While the molybdenum cover plate 102 is not essential, its use does improve the thermal distribution of the melt both in the crucible and in the capillary member 112. It limits heat loss from the melt and helps control the radial and longitudinal temperature gradients in the small melt pool defined by the capillary member as well as in the larger melt body. Of course the capillary member itself functions as a heat shield as well as providing an exposed central growth orifice of any chosen size.
Inspection of a-alumina ribbon made according to the foregoing procedure reveals that the ribbon is similar to the filaments described above in the sense that they also comprise one or two, and in some cases, three or four crystals growing together longitudinally separated by a low angle (within 34 of the c-direction) grain boundary. Under consistent growth conditions the ribbons often have surfaces that are sufficiently smooth and flat to permit their use as substrates for semiconductor circuit elements without intermediate chemical or mechanical surface treatment. This is believed attributable to surface diffusion of atoms as the ribbon is pulled clear of the growth orifice.
As indicated above, the invention also is applicable to growth of essentially monocrystalline hollow tubing. This is achieved using the same apparatus employed in growing ribbon but substituting in place of the capillary member 112 a member 124 preferably constructed as shown in FIG. 7. The latter comprises a flat plate 126 having a diameter just small enough to fit within crucible 46A. Plate 126 has a vertical extension in the form ofa circular rod 127. Secured to plate 126 in surrounding concentric spaced relation to rod 126 is a round tube 128. The upper end of rod 127 is enlarged as shown at 129 so as to define a narrow annular space measuring about 0.015 inch, and its end face has a conical depression as shown at 130 so that the end of the rod presents a circular knife edge. The upper end of tube 128 is bevelled exteriorly as shown at 131 so that it also presents a circular knife edge. Tube 128 also has one or more holes 132 at its bottom end to permit the molten alumina to fill the intervening annular space. It is to be noted that the width of the intervening annular space below the enlarged section 129 of rod 127 is not critical except that it be such as to permit a column of melt to rise readily and at least partially fill the narrower annular space defined by the enlarged section 129 and tube 128; in the illustrated embodiment this space is about 0.030 inch. The width ofthe aforesaid narrower annular space measured from the outer surface of rod section 129 and the inner surface of tube 128 also is not critical and may be greater or less than 0.015 inch e.g., as little as 0.001 inch. The method of growing an extended tube of essentially monocrystalline a alumina is essentially the same as that described above for ribbon. In both embodiments the seed may be a crystal that fills only a small portion of the growth orifice or may be a previously grown crystalline body having a shape corresponding to what is to be grown. lf the seed crystal fills only a portion of the growth orifice, it initially must be allowed to dwell in the growth orifice for a period of time sufficient to grow laterally to a shape corresponding to that ofthe growth orifice. By way of example, an extended tube of a-alumina may be grown according to the procedure now to be described. Assuming that the melt is at the correct temperature, a seed crystal that is essentially round and has a diameter less than the distance between parts 128 and 130 is introduced into the melt filling the annular space to a vertical depth of about 0.5 mm. This seed is allowed to dwell in the melt for about a minute, during which time crystal growth occurs both vertically and horizontally. Then the seed is withdrawn at a rate of about 1 inch per minute. The initial growth of crystal on the seed may extend only part way around the annular space between the two parts. However, as pulling continues, the crystal growth progresses laterally as well as vertically until the growing assumes the shape of a tube. Thereafter the pulling speed is adjusted to correspond to the rate at which growth of crystal occurs vertically. The product has essentially smooth inner and outer surfaces and will have a wall thickness almost the same as or less than the corresponding dimension of the top part of the space between parts 128 and 130, depending upon the pulling speed. It is to be noted that in growing an extended tube, the seed crystal must be oriented with its c-axis normal to the surface of the melt. The capillary member establishes the temperature gradients necessary for vertical dendritic growth. Because the capillary member is essentially isothermal, the same thermal distribution exists around the annular growth orifice, i.e., the thermal distribution is consistent for the full 360 of the growth orifice. This is necessary in order to have growth occur simultaneously at different points around the growth orifice. The same thing is true in growing extended crystalline bodies ofother shapes, e.g., triangular, etc.
It is to be appreciated that the length of the product pulled from the melt is determined by the pulling mechanism and the supply of melt in the crucible. For continuous growth it is contemplated that the pulling mechanism should comprise an arrangement such as shown in H6. 8, comprising one or more pairs of cylindrical pulling rolls 134 disposed to grip and pull the growing crystalline strip from the melt at the desired speed. With such arrangement the furnace would comprise inner and outer quartz tubes 4A and 6A whose upper ends are secured in a top member 136 having a removable plate 138 permitting access to the crucible. The plate 138 has an aperture 140 for initially introducing the seed crystal to the melt and for subsequently withdrawing the seed crystal with the growing essentially monocrystalline product. Periodic replenishment of the melt in the crucible could be made through a supply tube 142 provided in plate 136 and communicating with a feed tube 144 that is mounted in cover 102 and extends into the crucible 46A. A screw cap 146 covers the supply tube 142 when not in use.
The products of this invention, notably a-alumina filaments and ribbon, have a certain degree of flexibility which permits them to be bent around wide diameter rolls to effect a change of direction, e.g., when it is desired to move them horizontally in continuous fashion through apparatus designed to perform a selected operation, e.g., edge trimming, severing, surface polishing etc. For most purposes it is preferred to sever them into pieces ofsuitable length, e.g., l-3 feet.
It is to be recognized that the product may be doped with other materials to form solid solutions. The doping material is added to the melt in amounts suitable for providing the desired concentration in the product. Thus where the melt is alumina, it is possible to grow ruby filaments, ribbon, tubing etc., by adding chromium oxide to the melt. This is particularly advantageous in the case of producing tubing since elongate ruby tubes have application in the construction of laser devices. The present method of providing ruby tubes is substantially cheaper than forming same from a solid rod of ruby.
A further advantage of the invention is that it is applicable to growth of extended essentially monocrystalline bodies of other materials that melt congruently and have a rhombohedral, hexagonal or tetragonal crystal structure, notably aalumina, BeO, Cr 0 TiO as well as other materials which have a unique c-axis. The process for these materials is essentially the same as the process described above for a-alumina, except that it requires different operating temperatures because of different melting points. Additionally certain minor changes may be required in the apparatus, e.g., different crucible materials in order to avoid reaction between the melt and the crucible.
Certain other variations and extensions also may be made. Thus the process need not be carried out in an argon or helium atmosphere, but instead the furnace enclosure may be partially evacuated to a suitable pressure level. Furthermore two or more essentially monocrystalline bodies may be pulled simultaneously, using either an orifice plate 52 having two or more growth orifices or two or more capillary members 112 or 124 in the crucible. The several growth orifices or capillary members should be located so that the several growth zones will have substantially the same thermal distribution. The holders for the several seeds required to grow several bodies simultaneously may be mounted on the same or different pulling mechanisms.
The invention offers several important advantages. The apparatus required is relatively simple. The process is essentially continuous so that the products may be grown in any suitable length. By properly controlling the thermal distribution in the growth zone, it is possible to pull extended essentially monocrystalline bodies of different cross-sectional configurations and sizes according to end use requirements. The pull rates are relatively fast since the growth is dendritic.
It is to be understood that the invention is not limited in its application to the details of apparatus and method specifically described or illustrated, and that within the scope of the appended claims, it may be practiced otherwise than as specifcally described or illustrated.
What is claimed is:
1. Method of growing from a melt of alumina an extended monocrystalline body of predetermined cross-sectional configuration, comprising the steps of providing a reservoir body of melt of alumina, establishing in said reservoir body a pool of All said melt having a thermal distribution conducive to dendritic growth propagated vertically throughout a zone thereof conforming generally to said predetermined configuration, inserting into said pool a seed crystal of alumina oriented so that its c-axis is substantially normal to the surface of said pool, holding said seed crystal in said pool for a period of time sufficient for growth to occur thereon in a substantial portion of said zone, and withdrawing said seed at a rate permitting successive continuous accretions on said seed to form an extended essentially monocrystalline alumina body having said predetermined cross-sectional configuration.
2. A method of producing a filament of alumina comprising, providing in a crucible a melt of alumina having a thermal distribution conducive to dendritic growth propagates vertically, adjusting the temperature of said melt so as to establish a supercooled zone near the surface of said melt, inserting a seed crystal of alumina oriented so that its c-axis is normal to said surface into said zone for a period sufficient for dendritic growth to occur thereon, and pulling said seed away from said melt at a speed not exceeding the rate at which said growth occurs so that successive accretions of grown crystal form an extended filament.
3. Method of growing an extended essentially monocrystalline ribbon from a melt of alumina, comprising the steps of providing a reservoir body of melt of alumina, establishing in said reservoir body a pool of said belt having a thermal distribution conducive to dendritic growth propagated vertically in said pool throughout a zone that is relatively wide in a first horizontal direction and relatively narrow is a second horizontal direction at a right angle to said first horizontal direction, inserting an alumina seed into said pool so that crystal growth can commence thereon, holding said seed in said pool for a period of time sufficient to permit said crystal growth to occur in a substantial portion of said zone, and withdrawing said seed at a rate not exceeding the rate at which growth is propagated vertically in said zone so that successive crystalline accretions of said material from an extended ribbon.
4. Method of growing from a melt of a selected material that is an oxide of a member of the class consisting of beryllium, chromium, titanium and aluminum an extended monocrystalline body of predetermined cross-sectional configuration, comprising the steps of providing a reservoir body of melt of said selected material, establishing in said reservoir body a pool of said melt having a thermal distribution conducive to dendritic growth propagated vertically throughout a zone thereof conforming generally to said predetermined configuration, inserting into said pool a seed crystal of said material oriented so that its c-axis is substantially normal to the surface of said pool, holding said seed crystal in said pool for a period of time sufficient for growth to occur thereon in a substantial portion of said zone, and withdrawing said seed at a rate permitting successive continuous accretions on said seed to form an extended essentially monocrystalline body of said material having said predetermined cross-sectional configuration.
UNITED STATES PATENT @FHQE cirimir or crew Patent No. 3 650 703 Dated March 21 1972 Invent0r(s) Harold E... LaBelle, Jr. et a1 7 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
Column 13, Line 14, Claim 2, change -propagates-- to "propagated";
Column 13, Line 26 Claim 3 change -belt-- to "melt";
Column 14, Line 1, Claim 3, change -isto "in"; and
Column 14 Line 9, Claim 3, change fromto "form".
Signed and sealed this 1 8th day of July 1972..
ROBERT GOTTSGHALK a LETCHER JR EDWARD M F 9 Commissio ner of Paten Attesting Officer FORM PC4050 (10459) uscoMM-Dc 60376-P69 1.5, GOVERNMENT PRINTING OFFICE Z 1959 0"366'334