Optical control of crystal growth

Abstract

The invention is a method of improving production of shaped crystalline bodies according to the processes described in U.S. Pat. Nos. 3,591,348 and 3,471,266. It consists of optically monitoring the height of a vertical liquid meniscus in the region of the growth zone and adjusting the rate of heating or the rate of crystal pulling to maintain the height of the liquid meniscus between selected limits, with the result that outside dimensions of the crystalline product, e.g. the outside diameter of a sapphire tube, will be substantially unvarying and will meet preset tolerances.

Description

This invention pertains to growth of crystalline bodies having a predetermined cross-section and more particularly to improvements in the processes described in my U.S. Pat. No. 3,591,348, issued July 6, 1971 for Method of Growing Crystalline Materials and my U.S. Pat. No. 3,471,266, issued Oct. 7, 1969 for Growth of Inorganic Filaments.

My U.S. Pat. No. 3,,591,348 describes how to grow crystalline bodies according to what is called the edge-defined, film-fed growth technique (also now known as the EFG process). In the EPG process the shape of the crystalline body is determined by the external or edge configuration of the end surface of a forming member which for want of a better name is called a die. The process involves growth on a seed from a liquid film of feed material sandwiched between the growing body and the end surface of the die, with the liquid in the film being continuously replenished from a suitable melt reservoir via one or more capillaries in the die member. By appropriately controlling the pulling speed of the growing body and the temperature of the liquid film, the film can be made to spread (under the influence of the surface tension at its periphery) across the full expanse of the end surface of the die until it reaches the perimeter or perimeters thereof formed by intersection of that surface with the side surface or surfaces of the die. The angle of intersection of the aforesaid surfaces of the die is such relative to the contact angle of the liquid film that the liquid's surface tension will prevent it from overrunning the edge or edges of the die's end surface. Preferably the angle of intersection is a right angle whch is simplest to achieve and thus most practical to have. The growing body grows to the shape of the film which conforms to the edge configuration of the die's end surface. Since the liquid film has no way of discriminating between an outside edge and an inside edge of the die's end surface, a continuous hole may be grown in the crystalline body by providing in that surface a blind hole of the same shape as the hole desired in the growing body, provided however, that any such hole in the die's end surface is made large enough so that surface tension will not cause the film around the hole to fill in over the hole.

The process described in said U.S. Pat. No. 3,471,266 employs a forming member that defines a capillary which contains a column of melt from which a crystal body is grown and pulled. Depending upon the cross-sectional configuration of the capillary and by appropriate control of the thermal conditions in the upper end of the column of melt contained in the capillary, it is possible to grow crystal bodies of selected materials having arbitrary selected cross-sectional shapes. Thus, by employing a forming member having a capillary in the shape of an annulus, it is possible to grow a hollow tube. Similarly, if the capillary is circular in cross-section, it is possible to grow a round rod or filament. The forming member is mounted so that the capillary is connected to a reservoir pool of melt, whereby the capillary is self-filling. Accordingly, the process is sometimes known as the SFT method, the term SFT being an abbreviation of the phrase "self-filling tube" used to denote the forming member used to grow rods or filaments.

Both of the foregoing processes are useful in growing hollow tubes of alpha-alumina for use as envelopes in the manufacture of high temperature sodium vapor lamps. For the latter purpose it is essential (for several reasons, including minimizing problems in sealing off the ends of the tubes) that the outer diameters of the tubes be held to relatively close tolerances. Thus, for example, a typical requirement is for the tubes to have an outer diameter (o.d.) of 0.375 .+-. 0.003 inch.

In both of the above-described processes, substantial changes, within limits, in the pulling speed and the temperature of the growth interface are possible without terminating growth or causing any substantial change in the cross-sectional configuration of the growing body. However, changes in pulling speed and growth interface temperature can affect the cross-sectional size of the growing body. It is a relatively easy matter to hold the pulling speed constant. Thus the usual practice, once the crystalline body is growing to the desired shape, is to fix the pulling speed at a suitable rate and to adjust the growth interface temperature (by adjusting the rate of heating) so that the body will grow to the desired size. However, there exists a problem in monitoring the growing body and keeping it within the prescribed tolerance limits. In growing an alpha-alumina tube for use as a lamp envelope, there is also a tendency for the outside diameter to go slightly out of round, so that the tube is slightly oval and is characterized by a maximum and a minimum diameter. Although the differential between the maximum and minimum diameter is relatively small, typically no more than about 1 mil for a tube with a 0.375 inch o.d., it increases the need to precisely control the operating parameters -- notably adjusting the growth interface temperature -- to minimize variations in tube size. However, obtaining a precise measurement of the temperature at the solid-liquid interface by means which do not interfere with the growth process is difficult to achieve. An obvious choice of temperature measuring instrument is an optical pyrometer, but because of the disposition and relatively small size of the growth interface, the emissivity of the forming member and any radiation shields that may be associated with the forming member tends to cause the pyrometer to yield an erroneous reading. However, even if the pyrometer accurately measures the temperature of the melt in the region of the growth interface, a change in temperature does not necessarily means that the o.d. of a growing tube has changed (a change in pulling speed may offset a change in temperature enough so that no change in tube o.d. will result). Furthermore, even knowing the exact temperature of the growth interface and the pulling speed, it is impossible for the operator to as certain precisely whether the o.d. of the growing tube is within the tolerance limits or exceeds the lower or upper limit, and hence the operator does not know whether the temperature needs to be adjusted, or in what direction and by how much. If the growth interface temperature is too hot, the tube will grow to a smaller diameter, while if it is too cold, stress and grain boundaries may occur in the product and the melt may start to solidify to the forming member. The problem is complicated when several like crystalline bodies are being pulled simultaneously at the same speed from different die members fed by a common melt supply. Measuring the growth interface temperature at one die member, even if it could be done accurately, does not suffice as a measurement of the growth temperatures at all of the dies since one die may be at a higher or lower temperature than the others.

Accordingly, the primary object of this invention is to provide a method of monitoring and controlling crystal growth so that the outside dimensions of the growing body, notably the o.d. of a tubular body, is within prescribed limits.

A further object is to improve upon the methods described and claimed in said U.S. Pat. Nos. 3,591,348 and 3,471,266 by providing a method of monitoring the crystal growth and maintaining the cross-sectional size of the growing body substantially constant.

These and other objects of the invention are achieved by optically monitoring the height (thickness) of a selected portion of the melt in the region of the growth zone and adjusting the operating conditions, notably the rate of heating and thereby the growth interface temperature so as to maintain said height substantially constant. Other features and specific details of this invention are set forth in the following description which is to be considered together with the drawings wherein:

FIG. 1 is a sectional view in elevation of a crucible and die assembly and illustrates growth of a crystalline tube according to the EFG process;

FIG. 2 is a view like FIG. 1 but on a reduced scale of a crucible and die assembly for growing a crystalline tube according to the SFT process;

FIG. 3 is a fragmentary enlargement of the apparatus of FIG. 2 and shows growth of a crystalline tube according to the SFT process;

FIG. 4 is a fragmentary sectional view in elevation showing an optical apparatus associated with a crystal-growing furnace for monitoring crystal growth according to this invention; and

FIG. 5 is a plan view of the crucible and die used in arrangement embodying four dies of the type shown in FIG. 1, as used in the apparatus of FIG. 4.

Like numerals are used to indicate like parts in the several figures.

The processes described in said U.S. Pat. Nos. 3,591,348 and 3,471,266 are both characterized by the presence of a meniscus of melt extending between an edge of the die or forming member and the growth interface. If a solid body such as a rod or filament is being grown, the process is characterized by a single meniscus. If a hollow body such as a round tube is being grown the process is characterized by two menisci; one at the outside and one at the inside as hereinafter described. I have determined that the height (and also the degree of concavity) of the meniscus can change with changes in the operating conditions. More importantly, the height of the meniscus is affected by the temperature of the melt in the region of the growth interface and the pulling speed, and I have determined that, within limits, the outer diameter of a hollow tube or a solid rod will decrease as the outer meniscus height increases (an increase in the same diameters occurs if the outer meniscus height decreases). It is to be noted also that the inner diameter of a hollow tube will increase or decrease as the outer meniscus height increases or decreases respectively. I have also determined that a relatively small change in the outer diameter of a tube or rod is reflected by a relatively large percentage change in meniscus height. Thus the essence of the invention is to directly measure the height of the meniscus, rather than the melt temperature at the growth interface, and to use that measurement as a basis for determining how to vary the rate of heating to achieve a substantially constant outside diameter for a hollow tube or a solid rod. The same technique may be used to grow other shapes, e.g., flat ribbons, to prescribed sizes. Other features of the invention are described below.

FIG. 1 shows a crucible-die assembly for use in growing a tubular body according to the process of my U.S. Pat. No. 3,591,348. The assembly comprises a crucible 2 containing a die assembly or forming member 4 which consists of a round rod 6 that has a coaxial cavity or blind hole 8 of circular cross-section at its top end so that its substantially flat top and surface 10 is annular. Rod 6 is made of a material that is wetted by and will not react with or dissolve in the melt material. Hole 8 must have a diameter large enough so that the film 20 (described below) will not close over its upper end. As an alternative measure, hole 8 may be made so that it extends for the full length of rod 6, i.e., so that its bottom end is open to the melt in the crucible. If such is the case, its diameter must be large enough so that it cannot fill with melt by action of capillary rise.

The round rod 6 also has a plurality of small diameter of longitudinally extending bores 12 (only two of which are visible in FIG. 1) that are spaced substantially uniformly about its axis, bores 12, are sized to function as capillaries for the melt 14 contained in the crucible. Rod 6 is affixed to a plate 16 that rests on a shoulder 18 formed at the top end of the crucible. Rod 6 is mounted in a center hole in plate 16 and projects slightly above its top surface. The bottom end of rod 6 is spaced from the bottom of the crucible.

The apparatus of FIG. 1 is mounted in a suitable crystal growing furnace, e.g. of the type disclosed in my aforesaid U.S. patents, and a charge of material to be grown is placed in the crucible and melted. The molten liquid will flow up into the capillaries 12 by action of capillary rise. Essentially each capillary contains a column of melt. The cross-sectional sizes of the capillaries 12 and the length of rod 6 are preferably set so that for a given melt material, e.g. alumina, the capillary action is sufficient to cause the melt to fully fill the capillaries so long as the level of the melt supply in the crucible is high enough to trap the bottom end of the rod, i.e., high enough for the bottom end of the rod to be immersed in the melt supply.

The choice of materials used to form the crucible and die assembly depends upon the composition of the melt. By way of example, if the melt is alumina, the crucible and die assembly are preferably made of molybdenum or tungsten.

In practicing the process described in my U.S. Pat. No. 3,591,348, a film of melt is established on the upper end surface 10 of the forming member, as shown at 20. The film 20 overlies and conforms to the configuration of the end surface 10 (the orifices of capillaries 12 are ignored in determining the configuration of surface 10). The film may be formed by bringing a seed crystal into contact with melt in one of the capillaries and adjusting the temperature of the melt in the capillaries and also the pulling speed of the crystal so that crystal growth will occur on the seed and so that, as the seed is withdrawn, surface tension will cause the melt in contact with the seed to move up out of the capillary and spread out onto the upper surface 10 to form the film 20. As this occurs the crystal growth will spread laterally so that after the surface is fully covered by film 20, crystal growth will occur at all points along the horizontal expanse of the film and the growing body will have a crosssectional shape conforming to the shape of surface 10. An alternative and preferred mode of establishing film 20 is to bring a seed (preferably one whose cross-sectional shape corresponds to the shape of surface 10) into contact with the end surface 10, and hold the seed there long enough for some of it to melt and cover the end surface 10 as well as connect with the melt in the capillary or capillaries. Then the seed is withdrawn at a rate and with the melt film at a temperature such that crystal growth will occur on the seed at all points along the interface of the seed and the film 20. With the apparatus of FIG. 1, the growing body will be a hollow tube 22.

As seen in FIG. 1, the melt film 20 is characterized by a meniscus 24 at its outer edge and a meniscus 26 at its inner edge. Each of the menisci extends between an edge of the die's upper surface 10 and the growth interface and is concave, i.e. the two menisci are bowed inwardly toward one another, as shown. It is to be understood that as a practical matter it is not possible to observe the inner meniscus 26 during crystal growth and for this reason only the outer meniscus is measured and used as a basis for monitoring and controlling the outer diameter of the growing tube 22. As previously noted, the height h (and the degree of bowing) of meniscus 24 will vary if a change is made in pulling speed and/or the temperature off film 20, and the inner and outer diameters of the growing tube 22 will change as the height of the meniscus changes. More specifically, the inner and outer diameters of the growing body will increase and decrease respectively as the height of meniscus 24 increases and will decrease and increase respectively when the meniscus height decreases. However, the minimum size of the inner diameter and the maximum size of the outer diameter of the growing body are determined by the corresponding diameters of end surface 10 since the melt film 20 cannot expand beyond the inner and outer edges of end surface 10.

If the temperature of the film 20 is held substantially constant, an increase in pulling speed will cause the meniscus height to increase (the meniscus height decreases when the pulling speed is lowered). If the pulling speed is held constant, an increase in the temperature of film 20 will cause the meniscus height to increase, while the reverse occurs if the film temperature is lowered. Since it is a relatively easy matter to hold the pulling speed substantially constant, i.e., to within 1% of the desired speed, and since the meniscus height is relatively unaffected by small changes in pulling speed (e.g., a change of 1% in pulling speed at constant film temperature produces substantially no change in meniscus height), it is preferred to hold the pulling speed constant and to control meniscus height by adjusting the rate of heat input which, assuming a constant rate of heat loss by radiation, conduction, etc., in turn controls the temperature of film 20.

FIG. 2 shows a crucible 30 containing a die or forming member assembly 32 designed for growing a tubular body according to the process disclosed in my U.S. Pat. No. 3,471,266. The forming member 32 consists of a plate 34 that rests on the bottom of the crucible and a capillary unit that comprises a round tube 36 surrounding and concentric with a solid rod 38. The elements of the capillary unit are made of a material that is wetted by and will not react with or dissolve in the melt material.

Tube 36 and rod 38 are disposed in depressions in plate 34 and are welded thereto. The bottom end of tube 36 is slotted or has holes as at 40 so as to provide inlet ports whereby melt can flow into the annular space 42 between it and rod 38. The radial distance or gap between rod 38 and the inside surface of tube 36 is such as to allow the space 42 to function as a capillary for the melt material 44 contained in the crucible. The upper end of tube 36 is bevelled as shown at 46 to provide a sharp top edge. The upper end of rod 38 is formed with a conical cavity 48 as shown so that it also has a sharp top edge. The upper edges of tube 36 and rod 38 are level with each other as shown and the height of the capillary unit is set so that, for the given radial distance or gap between the tube and rod, capillary action will cause melt to rise within and fully fill the capillary so long as there is enough melt in the crucible to trap the ports 40. Completing the assembly is a cover 50 for the crucible. The latter has a center hole to accommodate the upper end of the capillary unit which projects a short distance above the cover as shown. Cover 50 functions as a radiation shield for the melt.

The choice of crucible, die and cover material depends upon the composition of the melt. By way of example, if the melt is alpha-alumina, these components are preferably made of molybdenum or tungsten.

The apparatus of FIG. 2 is mounted in a suitable crystal growing furnace, e.g. of the type and in the manner disclosed in my U.S. Pat. No. 3,471,266. As taught in said patent, a charge of material to be grown is placed in the crucible and melted, and when this occurs, the molten liquid will fill capillary 42. Crystal growth is initiated by inserting an appropriate seed into the column of melt 52 in the capillary, and adjusting the thermal distribution in the upper end of the melt column 52 so that crystal growth will occur and be sustained as the seed crystal is withdrawn at an appropriate speed. Assuming that the seed is not of the same annular cross-sectional shape as the capillary, the crystal growth will spread horizontally to the annular cross-sectional shape of the melt column so that the growth product will assume the shape of a hollow tube as shown at 54 in FIG. 3. Of course, a previously grown tube, of a size suitable for introduction into the column of melt in the capillary, may be used as a seed with the apparatus of FIG. 2.

FIG. 3 illustrates the growth interface using the apparatus of FIG. 2 and how the growth interface may be monitored in accordance with this invention. As the growing crystal body 54 (or the seed) is withdrawn, surface tension causes the column of melt 52 to adhere to it and to rise above the level of the top edges of the capillary unit. Crystal growth occurs at all points along the top end of the column of melt, due to the surface tension effect noted above, and the melt will form a vertical meniscus at each top edge as shown at 56 and 58. Each meniscus extends between an upper edge of the capillary unit and the growth interface. The shape of the menisci is similar to that of the menisci 24 and 26 of FIG. 1.

Crystal growth occurs at all points along the crystal-melt interface which tends to be both within and above the capillary as shown. As noted in my U.S. Pat. No. 3,471,266, the shape of the growing body is determined by the temperature and the temperature gradients of the upper end of the column of melt, and the temperature gradients are shaped by the capillary unit. Also the cross-sectional size of the growing body is affected by the pulling rate and the temperature of the melt column.

As with the process illustrated in FIG. 1, I have determined that the height of the outer meniscus 56 (the dimension h in FIG. 3) will depend upon the pulling speed and the temperature at the upper end of the melt column, i.e. at the growth interface, and that the greater the height h, the smaller the o.d. and the larger the i.d. of the growing tube. Similarly, the less the height h, the larger the o.d. and the smaller the i.d. of the growing tube.

With both of the illustrated processes, a relatively small change in the outer diameter of the growing tubular body is reflected by a relatively large change in meniscus height. Thus, for example, if the capillary unit of FIGS. 2 and 3 (or the forming member of FIG. 1) is designed and the growth process parameters set so that a sapphire tube having an o.d. of 0.375 inch .+-. 0.003 inch can be grown, a change of about 30.degree.C (assuming a constant pulling speed) at growth interface, i.e. at the outside meniscus, will cause the o.d. of the tube to change about .+-. 0.003 inch, depending upon whehter the temperature is raised or lowered, and will cause the meniscus height (typically in the order of 0.007 inch) to change from 60-100% (depending on the pulling speed). Since the meniscus height can be measured very accurately, i.e. to within about 0.0005 inch, the effect of a change in heating rate on the meniscus height can be readily determined and the power input to the crucible heating means of the furnace can be adjusted so as to produce relatively precise incremental changes in meniscus height, and thereby provide close control over the o.d. of the growing tube.

By way of example but not limitation, the meniscus height may be precisely measured using a microscope with a recticle in its eyepiece focus. Other suitable commercially-available optical devices are known to persons skilled in the art and may be used to measure meniscus height.

In the preferred mode of practicing this invention, i.e. as used in growing tubes for lamps, the operator adjusts the power input to the heating means of the furnace so that the observed meniscus is maintained at a height which, as determined from prior runs, using the same constant pulling speed, will cause the crystal body to grow with an o.d. that is within the prescribed limits. In contrast to when controlling the heating rate according to direct temperature measurements, the operator can determine by measuring the meniscus height whether the o.d. of the growing tube is on the high or low limits side of the desired o.d. size and can, by appropriately adjusting the heating rate, adjust the meniscus, if necessary, to bring or maintain the o.d. within the prescribed tolerance limits. Since a tube may tend to grow slightly oval, the preferred procedure is to maintain the observed meniscus height at a value which assures that the maximum and minimum outer diameters of the tube are within the high and low tolerance limits respectively.

Both of the processes described in my U.S. Pat. Nos. 3,591,348 and 3,471,266 have the advantage that a plurality of crystal bodies of like (or different) cross-sectional shape and size may be grown simultaneously using a plurality of like (or different) forming members or dies mounted in a common crucible and a common pulling mechanism. The present invention facilitates growth of several like bodies, e.g. tubes, simultaneously so that the o.d. of each body is within prescribed tolerance limits. In such case, only one of the several growing bodies is optically monitored to determine meniscus height, and the power input to the heating means of the furnace is adjusted so that the meniscus height of the monitored growth zone is kept at a value that assures that the body growing at that zone will have an o.d. substantially half-way between the prescribed upper and lower tolerance limits, e.g. an o.d. of 0.375 inch where the tolerable maximum and minimum o.d. values are 0.372 and 0.378 inch respectively. It has been found that when this mode of monitoring is practiced, the other ones of the several growing bodies also will have outside dimensions that comply with the prescribed tolerances.

FIG. 4 illustrates how a furnace of the type shown in my aforesaid U.S. Patents is modified to permit optical monitoring and measurement of meniscus height by means of a microscope system. Although in this case the furnace contains a crucible-multiple die arrangement for growing a plurality of crystalline bodies simultaneously according to the EFG process, it is to be understood that the illustrated crucible-die arrangement may be replaced by one suitable for growing bodies according to the SET process.

With reference to FIGS. 4 and 5 and also to my aforesaid U.S. patents, a crucible 2 is mounted within the furnace enclosure which consists of two spaced quartz tubes 60 and 62 that define an annular space (which is closed off at its top and bottom ends) through which cooling water is circulated for the purpose of keeping the quartz at a safe temperature and also to absorb infra-red energy so as to make it easier for the operator to comfortably observe growth of the product. The plate 16 supported by the crucible carries three die assemblies 4 (a, b and c) constructed as shown in FIG. 1, plus a hollow filler tube 59 (FIG. 4) that is made of the same material as the die assemblies. The bottom end of tube 59 terminates near but is spaced from the bottom of the crucible while its upper end terminates above plate 16. A delivery tube 61 made of quartz or other suitable heat-resistant material extends through and is sealed to the furnace tubes 60 and 62 as shown. The bottom end of tube 61 is located in line with and terminataes close to but does not engage the upper end of filler tube 59. The purpose of filler tube 59 and delivery tube 61 is to permit replenishing the melt in the crucible without interrupting crystal growth. A tubular body is grown from a melt film supported on the upper end surface of each die assembly as described above in connection with FIG. 1. Only die assembly 4c and filler tube 59 are visible in FIG. 4.

In accordance with this invention, a short section of transparent quartz pipe 64 is inserted into aligned holes formed in the furnace tubes 60 and 62 and sealed thereto so as to preserve the integrity of the cooling water jacket. The inner end of pipe 64 is open but its outer end is closed off by an end wall 66 so as to prevent escape of the inert gas or vacuum that customarily is provided within the furnace. As shown the pipe 64 is inclined outwardly and is located so that its axis is directed at the upper end of one of the three die assemblies, e.g. die assembly 4c.

The pulling mechanism (not shown) associated with the furnace has a pulling rod 68 that corresponds to pulling rod 32 shown in FIG. 1 of my U.S. Pat. No. 3,471,266. Attached to the pulling rod is a seed holder 70 which is adapted to hold a selected number, in this case three, seeds 72. Each of the three seeds 72 (of which only one is shown) is held by holder 70 in vertical alignment with a different one of the three die assemblies 4a-c. Seed holder 70 has a slot 73 large enough for it to clear delivery tube 61, whereby to prevent the latter from obstructing the up and down movement.

Viewing and measurement of the meniscus associated with the selected die assembly during crystal growth is accomplished by a microscope 74 attached to a holder 76 that is adjustably mounted to a suitable fixed support 78 which preferably but not necessarily is part of or secured to a stationary portion of the furnace apparatus. By way of example but not limitation, the microscope may be a stereomicroscope. An essential requirement of the microscope is that it be adapted with a recticle device for precisely measuring the meniscus height as herein described. In the practice of this invention, it is preferred to employ a Model 562B-LI Stereostar Zoom microscope manufactured by American Optical Company, Instrumental Division, Buffalo, N.Y., that is fitted with 10X eyepieces and has a suitable linear division recticle or eyepiece disc mounted in one of its eyepieces, e.g., American Optical eyepiece disc Catalog No. 1428, which has 200 divisions each 0.001 inch at 2X magnification. The eyepiece disc is oriented so that the graduated scale is a vertical image and the microscope is aimed along pipe 64 so that the scale is focused on the meniscus to be monitored and measured.

It is to be noted that in both of the processes illustrated in FIGS. 1 and 3, the crystalline body grows from a pool of melt that is a continuation of a column of melt in the capillary, and that the pool of melt is characterized by at least one meniscus that extends from the growth interface down to a top edge of the capillary forming member. In FIG. 1, the growth pool of melt is the film 20 that overlies the top end surface 10 and the meniscus 24 extends from the outer edge of the surface 10 up to the edge of the growth interface. Although the height of the meniscus can be controlled by adjusting the film temperature and/or the pulling speed, surface tension will cause the bottom end of the meniscus to remain substantially at the edge of the surface 10 despite changes (within limits) of its height h. However, the curvature or bowing of the meniscus will change, its radius of curvature becoming shorter as the height h increases. The same is true of the meniscus 26. The growth pool of melt is less sharply defined in the process illustrated in FIG. 3, but it is to be understood as being the upper portion of the column of melt that forms the interface with the growing body and notably includes that portion of the column of melt that is bounded by the two menisci 56 and 58. In this connection it is to be noted that the growth interface may be tapered as shown or may be substantially flatter. The growth interface tends to be less tapered as the radial dimension of the capillary decreases (due apparently to a more isothermal condition in the growth pool of melt) and also as the pulling speed is increased.

Although the forming members of FIGS. 1-3 are for growing tubular bodies, it is to be understood that forming members for growing other shapes, e.g. rods, filaments, ribbons, etc., according to the EFG and SFT processes, also may be used in the practice of this invention since, regardless of the shape of the growing body, the growth pool of melt is still characterized by at least one meniscus. In the case of rods, filaments and ribbons, there is only one meniscus (at the outside of the growth pool of melt) since the growing body is as a solid. Growth of tubular bodies other than round tubes, e.g. hollow bodies having a rectangular, square or triangular cross-section, also is characterized by both inner and outer menisci.

As is obvious from my aforesaid U.S. Patents, the EFG and SFT processes improved according to this invention may be used to grow crystalline bodies of a wide variety of materials, including but not limited to alumina (sapphire) ruby, barium titanate, beryllium oxide, titanium dioxide, chromium oxide (Cr.sub.2 O.sub.3), lithium niobate, lithium fluoride (LiF), calcium fluoride (CaF.sub.2) and sodium chloride. The products produced are monocrystalline or may consist of two to four crystals growing together.

The following example provides further details of how to practice the invention.

EXAMPLE

An EFG molybdenum die arrangement substantially as shown in FIGS. 4 and 5 is assembled with a molybdenum crucible and the crucible is filled with a supply of solid particles of high purity (99.sup.+%) alumina. The crucible and die assembly are mounted in a crystal growing furnace of the type shown in my U.S. Pat. Nos. 3,591,348 and 3,471,266. With reference to FIG. 4, the crucible 2 is mounted on short tungsten rods 80 within a cylindrical carbon heat susceptor 82 which in turn is attached to and supported by a tungsten rod 84 that is mounted in the bed plate (not shown) of the furnace, substantially in the manner illustrated in the aforementioned U. S. Patents. A cylindrical radiation shield 85 made of carbon cloth is wrapped around the carbon susceptor. The R.F. heating coil 86 of the furnace is disposed so that it surrounds the carbon susceptor 82 as shown.

The three die assemblies 4a-c are identical. Each is made of molybdenum, as is the plate 16 and filter tube 59. Delivery tube 61 is made of alumina. With reference to FIGS. 1 and 4, the annular upper end surface 10 of each capillary rod 6 has an outside diameter of 0.378 inch and an inside diameter (i.e. the diameter of hole 8) of 0.315 inch while each of the capillaries 12 has a diameter of 0.012 inch. The crucible has an internal depth of about 11/2 inch and an internal diameter of about 11/2 inch. Each rod 6 has an overall length of about 13/8 inch and its bottom end is spaced about one-eighth inch from the bottom of the crucible. The upper ends of the rods 6 project about one-sixteenth inch above the level of plate 16. Filler tube 59 has an o.d. of about 0.378 inch and an inner diameter of about 0.325 inch, and its length is such that its bottom end is about one-eighth inch from the bottom of the crucible and its top end projects about one-sixteenth inch above plate 16.

Three idential seeds 72 are attached to holder 70. The seeds are substantially monocrystalline alumina tubes previously grown by means of the same crucible-capillary arrangement. Cooling water is introduced in the water jacket defined by quartz tubes 60 and 62 and the furnace enclosure 88 is evacuated and then filled with argon to a pressure of about 1 atmosphere. The RF coil is energized by a 500 KHz power supply which is operated so that the alumina charge in crucible 2 is melted and the upper surface of the die is at an average temperature of about 10.degree.-20.degree.C above the melting point of alumina. In this molten condition, the alumina rises in and fully fills each of the capillaries 12. Then the furnace's pulling mechanism is actuated and operated so that the three seeds are lowered into contact with the upper end surfaces 10 of the three die units 4a-c.

The seed crystals are allowed to rest in contact with the die units for about 5-10 seconds, during which time the ends of the seeds melt and form films as shown at 20 in FIG. 1 which overlie and substantially fully cover the end surfaces 10. In each case the film 20 connects with the columns of melt in the capillaries 12. Then the pulling mechanism is operated so as to withdraw the three tubular seeds at a rate of about one-fourth to one-fifth inch/minute. The initial withdrawal of the seeds is accompanied by solidification of melt therein from the films and as the seeds continue to be withdrawn continuous crystal growth occurs on the end of each seed. The crystal growth on the seeds tends to deplete the films 20, but the films are replenished by additional melt fed by the capillaries.

As crystal growth occurs on each of the three seeds, the meniscus 24 of the film 20 overlying the upper end surface of the capillary unit 4c is optically monitored by means of an American Optical Model 562B-L1 stereo-microscope disposed as shown in FIG. 4 and modified with eyepieces and an eyepiece disc as previously described. It is desired to maintain the height of the meniscus 24 to approximately halfway between the limits of 0.004 and 0.001 inch. Accordingly, as the crystal growth occurs, the power input to the RF heating coil 86 is modified to raise or lower the temperature of the melt films 20 so as to increase or decrease the height of the meniscus 24 of the unit 4c as required. The pulling speed is maintained constant at the aforementioned rate as crystal growth processes. Additional powdered alumina is periodically delivered to the crucible via tubes 59 and 61 to replenish the supply of melt. The crystal growth is continued for approximately 4 hours, at which time the pulling rate is increased to approximately 1.0 inch per hour so as to cause the growing crystal to be pulled free from the melt films 20. The power supply to the heater is then turned off and the furnace allowed to cool. Then the seeds and grown tubes are removed from the seed holder.

Crystalline bodies grown according to the procedure of this Example are tubular and substantially monocrystalline. Furthermore, at substantially all points along their lengths each of the bodies has a diameter which is within the limits of 0.375 inch .+-. 0.003 inch.

The advantages of the invention can be readily determined by modifying the procedure of the foregoing example in either of two ways. The first modification consists of the same procedure except that (a) the height of the film meniscus is not measured and (b) the rate of power input to the RF coil is maintained constant during crystal growth at the level which was found adequate to initially raise the temperature of the upper end surface of the die to about 10.degree.-20.degree.C above the melting point of alumina. The second modification consists of the same procedure as in the Example except that (a) the height of the film meniscus is not measured and (b) the temperature of the edge of the film is constantly measured with an optical pyrometer and the rate of power input to the RF coil is adjusted as required so that the apparent temperature of the edge of the film is kept within 10-20.degree.C above the melting poing of alumina.

With the first modification, stress and grain boundaries tend to appear in the later grown portions of the crystal bodies and frequently crystal growth terminates prematurely due to solidification of the melt on the end surface of the die assembly. Further, the outside diameters of the crystal bodies tend to vary substantially. The difficulties are due to the fact that the temperature of the melt film can vary as much as 20.degree.C in either direction due to unavoidable instabilities in the system and also because the temperature of the melt film tends to drop as the crystal grows longer and as the supply of melt in the crucible is depleted.

With the second modification the problems of stresses and grain boundaries and premature termination of crystal growth due to the melt freezing to the die are minimized. However, the tubes have varying outside diameters and the differences in o.d. at different points along one tube typically will exceed 0.003 inch and not all of the three tubes will meet the o.d. requirement of 0.375 inch .+-. 0.003 inch. This is due, as previously noted, to (1) the fact that changes in emissivity will case the pyrometer to give erroneous readings and (2) the fact that initially the operator has no way of knowing whether the tube being pulled from the film which is monitored by the pyrometer has an o.d. that is exactly 0.375 inch or is nearer 0.375 .+-. 0.003 or 0.375 -- 0.003 inch. Thus it is apparent that the invention, as herein described, offers the advantages of increased product yield, particularly where several products are required to be grown at the same time to the same close tolerances as in the foregoing Example.

It is believed to be obvious that, if desired, the invention may be practiced by maintaining the temperature at the solidliquid interface substantially constant, and varying the pulling speed so as to maintain the height of the meniscus 24 (or the meniscus 56 in the case of the SFT process) within limits that will assure production of bodies of substantially constant outside dimensions that meet predetermined tolerances.

Claims

What is claimed is:

1. In a method of growing a crystalline body of a selected material so that said body has a selected cross-sectional shape for an indefinite distance along its length, said method comprising growing and pulling said crystalline body from a growth pool of melt which is replenished via a capillary member by action of capillary rise from a reservoir supply of melt, said growth pool of melt also being characterized by a vertical meniscus which extends between the interface thereof with said crystalline body and an edge of said capillary member, the improvement comprising:

optically monitoring and measuring the height of said meniscus; and

adjusting as required the speed at which said body is being pulled or the temperature of said growth pool of melt so as to maintain the height of said meniscus within predetermined limits, whereby to cause said body to grow with a cross-section of substantially constant size.

2. A method according to claim 1 wherein said growth pool of melt has an edge configuration that corresponds to said selected cross-sectional shape.

3. A method according to claim 2 wherein said capillary member has substantially horizontally extending top end surface with an edge configuration corresponding to said selected cross-sectional shape, and further wherein said growth pool of melt is a film that overlies said top end surface and said meniscus constitutes an edge of said film.

4. A method according to claim 1 wherein said growth pool of melt is a continuation of a column of melt in said capillary member.

5. A method according to claim 4 wherein said capillary member has a substantially horizontal top end surface and said growth pool of melt is a film overlying said top end surface.

6. A method according to claim 4 wherein said capillary member comprises a single capillary filled with said column of melt and said growth pool of melt has substantially the same cross-sectional shape as said capillary.

7. A method of growing a crystalline body of a selected material comprising establishing a growth pool of melt from a reservoir supply of melt by means of a capillary member having at least one capillary whose bottom end is positioned so that melt will fill said capillary by action of capillary rise from said reservoir supply, growing a substantially monocrystalline body from said growth pool of melt and pulling said body from said growth pool of melt as growth occurs thereon, said growth pool of melt being an extension of a column of melt in said at least one capillary and being bounded by a meniscus that extends from the interface thereof with said growing body down to an edge of said capillary member, optically monitoring and measuring the height of said meniscus, holding substantially constant the rate at which said growing body is pulled from said growth pool of melt, and adjusting the temperature of said growth pool of melt as required to maintain the height of said meniscus substantially constant within predetermined limits.

8. In the process of growing a crystalline body of a selected material from the upper end of a column of a melt of said material in a heated capillary member having a bottom end that is connected to a reservoir pool of said melt, said column being formed by action of capillary rise from said reservoir pool and the interface of said body and said column of melt being characterized by a vertical meniscus formed by said melt above said capillary member, the improvement comprising:

optically monitoring said meniscus to determine changes in the height thereof; and

adjusting the rate at which heat is supplied to said capillary member so as to maintain the height of said meniscus substantially constant within predetermined limits.

9. A method of growing a substantially monocrystalline body of a selected material so that said body has a controlled cross-sectional size comprising:

establishing a growth pool of a melt of said material at the upper end of a vertically-extending capillary formed in a member that is disposed so that the bottom end of said capillary is in direct communication with a reservoir pool of melt of said material;

growing and pulling a substantially monocrystalline body of said material from said growth pool of melt with the surface tension of the melt of said growth pool causing it to adhere to said body and to form a vertical meniscus that extends between an upper end edge of said member and said growing body;

replenishing the melt in said growth pool by inflow from said reservoir pool via said capillary as said body is grown and pulled;

optically monitoring and measuring the height of said meniscus; and

adjusting the temperature of said growth pool of melt so as to maintain the height of said meniscus substantially constant between predetermined limits, whereby said growing body has a controlled substantially constant cross-sectional size.

10. In a method of simultaneously growing two or more crystalline bodies of a selected material so that each of said bodies has a selected cross-sectional shape and size for an indefinite distance along its length with the cross-sectional size of each body being within prescribed maximum and minimum limits, said method comprising growing and pulling said crystalline bodies from individual growth pools of melt which are replenished via capillary members from a reservoir supply of melt, said growth pools of melt each having a vertical meniscus which extends between the interface thereof with one of said bodies and an edge of one of said capillary members, the improvement comprising:

pulling all of said bodies at the same pulling rate and maintaining said pulling rate substantially constant;

optically monitoring and measuring the height of a single meniscus; and

adjusting as required the rate of heating of said growth pools of melt so as to maintain the height of the monitored meniscus substantially constant at a level at which the body associated with the monitored meniscus will grow to a cross-sectional size substantially half-way between said maximum and minimum limits.