Abstract:
Systems and methods for continuous sapphire growth are disclosed. One embodiment may take the form of a method including feeding a base material into a crucible located within a growth chamber, heating the crucible to melt the base material and initiating crystalline growth in the melted base material to create a crystal structure. Additionally, the method includes pulling the crystal structure away from crucible and feeding the crystal structure out of the growth chamber.

Description:
TECHNICAL FIELD 
     The present application is directed to sapphire growth and, more particularly, to systems and methods for continuous sapphire growth. 
     BACKGROUND 
     Corundum is a crystalline form of aluminum oxide and is found in various different colors, all of which are generally commonly referred to as sapphire except for red corundum which is commonly known as ruby and pinkish-orange corundum which is known as padparadscha. Transparent forms of corundum are considered precious stones or gems. Generally, corundum is extraordinarily hard with pure corundum defined to have 9.0 Mohs and, as such, is capable of scratching nearly all other minerals. 
     As may be appreciated, due to certain characteristics of corundum, including its hardness and transparent characteristics, among others, it may be useful in a variety of different applications. However, the same characteristics that are beneficial for particular applications commonly increase both the cost and difficulty in processing and preparing the sapphire for those applications. As such, beyond costs associated with it being a precious stone, the costs of preparing the corundum for particular uses is often prohibitive. For example, the sapphire&#39;s hardness makes cutting and polishing the material both difficult and time consuming when conventional processing techniques are implemented. Further, conventional processing tools such as cutters experience relatively rapid wear when used on corundum. 
     SUMMARY 
     Systems and methods for continuous sapphire growth are disclosed. One embodiment may take the form of a method including feeding a base material into a crucible located within a growth chamber, heating the crucible to melt the base material and initiating crystalline growth in the melted base material to create a crystal structure. Additionally, the method includes pulling the crystal structure away from crucible and feeding the crystal structure out of the growth chamber. 
     Another embodiment may take the form of a system for continuous sapphire growth including a vertical growth chamber and a crucible positioned within the growth chamber. The crucible includes a die set and is configured to hold molten alumina. The system also includes a heater configured to heat the crucible and a feeding system for continuously feeding alumina into the crucible. A pulling system is provided and configured to contact a seed crystal with molten alumina at the top of the die set and pull a crystal ribbon upwardly and out of the growth chamber. 
     While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following Detailed Description. As will be realized, the embodiments are capable of modifications in various aspects, all without departing from the spirit and scope of the embodiments. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive. 
    
    
     
       BRIEF SUMMARY OF THE DRAWINGS 
         FIG. 1  illustrates an example continuous crystal growth system in accordance with an example embodiment. 
         FIG. 2  is a top-down view of a crucible of the continuous growth system of  FIG. 1  showing inlets for continuous feeding of alumina. 
         FIG. 3  illustrates zoomed in view of the continuous growth system with crystal ribbons exiting a growth chamber. 
         FIG. 4  is a plot illustrating force versus length of a ribbon. 
         FIG. 5  illustrates an integrated pulling system in accordance with an alternative example embodiment. 
         FIG. 6  illustrates a cutting system and a testing system for the continuous crystal growth system. 
         FIG. 7  illustrates a portion of a crystal ribbon having defects. 
         FIG. 8  is an example flowchart illustrating a method for continuous sapphire growth. 
     
    
    
     DETAILED DESCRIPTION 
     Generally, all sapphire growth methods are batch in nature. At the start of each growth cycle, at a minimum, a precisely oriented seed crystal is positioned within the growth chamber and contacted with molten alumina to propagate crystal growth. Often the growth furnace is cooled and the crucible refilled with alumina prior to each new growth cycle. 
     The present continuous growth techniques helps to avoid down-time associated with the batch methods by eliminating all but the initial seeding operations, and eliminating the need to cool and re-heat the crucible. Equipment up time could be greatly increased as down time would only occur to replace worn components (something that generally does not occur until after many growth cycles). Further energy costs may be reduced by eliminating and/or reducing the cooling and heating cycles of the conventional batch processing techniques. 
     The present continuous growth method is based upon the EFG (edge-defined film fed growth) process. In this process, ‘ribbons’ of sapphire are grown by pulling them through a die set that defines the shape of the solidifying crystal. This process is typically a batch process because of either the emptying of the crucible through growth of crystals or by the length of grown ribbon reaching a maximum as allowed by the growth chamber. 
     To convert the process into a continuous process, alumina may be continuously fed into the crucible and the growth chamber may be configured to open up to allow the sapphire ribbon to pass through. This introduces several new problems. In particular, the growth chamber is typically filled with an inert gas during growth. The open top would allow that gas to escape. Additionally, heat would tend to escape from the top of the chamber, thereby reducing efficiency. Further, the sapphire ribbons are generally pulled via an actuator holding the seed crystal, and this would not be able to continue pulling upwards indefinitely. 
     To address these issues, several mechanisms may be implemented. First, sets of precise rollers could be introduced near the top of the furnace to take over pulling the continuous sapphire ribbons after an initial pull by an actuator. Second, beyond the rollers, a seal mechanism could close the gaps around and in between the ribbons to minimize heat and gas loss. Additionally, by making the furnace tall, the effect of heat loss could be significantly separated from the hot zone and crystallization points to minimize effect. This would also allow the ribbons sufficient time within the chamber to slowly cool without developing significant thermal induced stresses. Third, the slow leak of gas from a seal could be compensated for by a continuous input of inert gas. Also, by positioning the seals above the rollers, any friction effects from the seal would be separated by the rollers from the growth point, minimizing their effects. 
     Turning to  FIG. 1 , a continuous sapphire growth system  100  is illustrated. The growth system  100  may generally be constructed to grow sapphire in accordance with the EFG process. However, it should be appreciated that aspects of this example embodiment may be implemented in other sapphire growth processes to achieve continuous or nearly continuous sapphire growth. 
     The continuous sapphire growth system  100  includes a growth chamber  102  that houses the component parts that facilitate sapphire growth. Specifically, the chamber  102  houses a crucible  104 , a die set  106 , heaters  108 , and insulation  110 . The crucible  104  configured to hold molten alumina. The crucible  104  may be heated by the heater  108  to temperatures above the melting point of alumina so that the alumina remains in a molten state. The heater  108  may generally take any suitable form, and in one embodiment may take the form of one or more electric heaters. In some embodiments, a preheating system (not shown) may be implemented to help efficiently bring the temperature of the crucible  104  and/or the growth chamber  102  to a first temperature level before the heater  108  raises the temperature to above the melting point of alumina. 
     Generally, the die set  106  may include multiple sets of parallel plates  112 . Each set of parallel plates  112  forms a slit  114  which draws the molten alumina upward through capillary action. Each set of parallel plates  112  includes a die tip  116  located at the top of the plates  112 . The die tip  116  determines the shape of a crystal formed from the molten alumina drawn up the slits  114  of the parallel plates  112 . 
     A pulling system  120  is provided which is configured to pull the crystal from the die tip  116 . In some embodiments, the pulling system  120  may be configured to move within the growth chamber  102 , whereas in other embodiments the pulling system is fixed at or near the top of the growth chamber. In still other embodiments, the pulling system may be positioned at some point above the crucible  104  within the growth chamber. In further embodiments, the pulling system or parts of the pulling system may be positioned outside of the growth chamber  102 . 
     In  FIG. 1 , the pulling system  120  may be configured to move vertically within the growth chamber  120 . A support member  122  may be coupled to the pulling system  122  and configured to move the pulling system vertically. The pulling system  120  may include a plurality of pulling mechanisms, each corresponding to a die tip of the die set. Each pulling mechanism may initially support a seed crystal. The pulling system  120  may be lowered so that the seed crystal is in contact with the molten alumina at the die tip so that crystal growth is propagated. Once crystal growth is initiated, the pulling system  120  is pulled away from the crucible and crystal ribbons  124  are pulled upwardly from the die set. 
     As the crystal ribbons  124  are pulled upwardly, the molten alumina in the crucible is used up. The molten alumina is replenished by feeding alumina into the crucible  104  at a rate corresponding to the rate at which crystal is grown. Arrows  130  illustrate the feeding of the alumina into the crucible  104 . The alumina may generally be fed into the crucible in a solid form. As may be appreciated, the feeding of the solid alumina into the molten alumina will have a slight cooling effect on the molten alumina. In one embodiment, the alumina is fed into the crucible  104  at multiple locations equidistantly located about the perimeter of the crucible. Thus, the cooling effects are evenly distributed.  FIG. 2  is a top-down view of the crucible  104  showing the inlet points  132  for the alumina being evenly distributed about the perimeter of the crucible. In other embodiments, more or fewer inlets  132  may be provided. Additionally, in some embodiments, the inlets  132  may be located at positions other than about the periphery of the crucible  104 . For example, one or more inlets may be located more inwardly from the periphery and/or near the center of the crucible  104 . 
     Returning to  FIG. 1 , the system  100  may include a controller  156  that is configured to control the various operations of the system  100 . For example, the controller  156  may control the speed of the pulling system  120 , the rate at which alumina is fed into the crucible  104 , as well as the temperature of the crucible and/or the growth chamber  102 . Additionally, the controller  156  may control the supply of inert gas that fills the growth chamber  102 . Generally, the growth chamber is filled with an inert gas such as argon. As some inert gas will be lost through the present process, a supply of the inert gas may be provided to maintain proper conditions within the growth chamber  102 . Generally, the controller  156  may include a processor coupled to memory with inputs and outputs that enable the controller to control one or more operations of the system  100  autonomously in accordance with a set of desired operation parameters. In some embodiments, the controller  156  may take the form of a desktop computer, a laptop computer or other such computing device. In such embodiments, the controller  156  may be configured to allow user input to help modify operating parameters. Further, the controller may record operations of the system  100  and provide reports to a user. In other embodiments, the controller  156  may take the form of an application specific integrated circuit or a system on a chip device. 
     Referring to  FIG. 3 , the pulling system  120  is illustrated as having reached the top of the growth chamber  102 . In the illustrated embodiment, the growth chamber  102  may have a convertible top  140  which is configured to open to allow the crystal ribbons  124  to pass through. As illustrated, the convertible top  140  may take the form of hinged doors  142  that open to allow the crystal ribbons  124  pass through. In other embodiments, the convertible top  140  may take other forms. For example, the convertible top  140  may include one or more sliding members that open the top  140 . Generally, any suitable mechanism for opening the top  140  to allow the crystal ribbons  124  to pass through may be implemented. 
     One or more insulative features  144  or seals may be provided to help provide a thermal and/or gas seal for between the pulling system  120  and the top  140  of the growth chamber  102 . The insulative features  144  may take the form of a gasket that may be affixed to either the pulling system  120  of to an internal side of the top  140  of the growth chamber  102 . The insulative features  144  may take other suitable forms, however. Additionally insulative features or seals may be provided adjacent to the crystal ribbons  124  to help prevent gas and/or heat from escaping from the growth chamber  102  while the crystal ribbon exits. 
     In some embodiments, the pulling system  120  may include one or more sets of rollers  150  for each crystal ribbon  124  to move the ribbons after the pulling system has reached the top  140  of the growth chamber  102 . The rollers  150  may be configured to engage a lead  152  which may support the seed crystal initially. The rollers  150  may thus be configured to help lower the seed crystal to contact the molten alumina. Additionally, the rollers  150  may operate once the pulling system  120  reaches the top  140  of the growth chamber  102  to pull the crystal ribbons  124  from the crucible  104  and push them out of the growth chamber. Alternatively, the rollers  150  may be configured to engage the sapphire ribbons  124  after the pulling system  120  reaches the top  140  of the growth chamber  102 . That is, the rollers  150  may not be in contact with the lead  152  and/or the ribbons  124  until the pulling system  120  has reached the top  140  of the growth chamber  102 . As such, the rollers  150  may be articulated from a rest position to an engaged position. The articulation may be achieved through any suitable technique. Further, as the lead  152  and the ribbons  124  may have different dimensions, the rollers  150  may be configured to move and/or otherwise adjust to the different sizes. For example, the rollers  150  may be spring loaded. 
     The rollers  150  may be driven by tunable motors (not shown). Additionally, the rollers  150  may include torque sensors  154  that are configured to sense the torque on the rollers and/or the amount of force being applied to the ribbon  124 . Generally, it is desirable to maintain a relatively constant amount of force pulling the ribbons  124  to help avoid breakage. Additionally, the constant force helps maintain a desired thickness and/or growth rate of the ribbon to help reduce or eliminate the creation of defects in the ribbon. As such, each set of rollers  150  may include one or more sensors  154  configured to sense forces and/or perturbations that may result in defective growth. The sensors  154  may be in communication with the controller  156  of  FIG. 1 . The controller  156  may, in turn, control the operation (e.g., the speed) of the rollers  150 . 
     The rollers  150  may be tuned to try to maintain a consistent growth rate or uniform cross-section of the ribbons  124 .  FIG. 4  illustrates a force versus length curve  160 , with force on the vertical axis and length on the horizontal axis. The curve  160  is merely illustrative and as such, the length and force values are arbitrary. As shown, the curve  160  generally follows a straight line. The curve  160  does show minor deviations from a straight line which may indicate either excessive force or too little force was being exerted on the ribbon  124  as being affected by the solidification rate which is driven by temperature. As such, as the force curve  160  indicates a deviation from a straight line, a speed of the rollers  150  may either increase or decrease so that a desired amount of force is applied. Generally, the force required to pull the ribbon  124  may increase as the length of the ribbon increases. However, once the length of the ribbon  124  reaches a threshold level (for example, once the ribbon has reached a length that it extends out of the growth chamber  102  and is being supported by other mechanisms and/or is being cut, the curve may flatten out, as shown). 
     An example force perturbation that may result in a breakage is illustrated at  162 . Such perturbations may generally be avoided through individual fine control of the rollers  150  to maintain a constant or linear curve. It is anticipated that the crucible  104  may have some variation in temperature, for example due to feeding in alumina, some ribbons  124  may grow faster or slower than others. As such, the individual control of sets of rollers  150  provides flexibility in the growth rates of the individual ribbons to help prevent breakage events and/or defective crystal growth. 
       FIG. 5  illustrates a pulling system  170  in accordance with an alternative embodiment. The pulling system  170  is integrated with the growth chamber  102 . The pulling system  170  is configured to lower the leads  152  with rollers  150  to contact the molten alumina and propagate crystal growth. The rollers  150  then begin to pull the crystal ribbons  124  upwardly away from the crucible  104  and out of the growth chamber  102 . As with the prior described embodiment, sensors  154  may be provided to sense the forces applied to the ribbons  124  and the rollers  150  may be controlled to apply a desired level of force to help avoid breakage of the ribbons and/or defective growth patterns. 
     The integrated pulling system  170  may advantageously reduce the amount of heat and gas loss associated with the convertible top embodiment discussed above. Additionally, it may provide for more exact control over the pulling rate, as each set of rollers  150  may be individually controlled throughout the entire growth process. As such, fewer defects and/or breakage events may occur in comparison with the more common pulling systems that have a single force feedback and control. 
     Multiple sets of rollers  150  may be provided to help stabilize and position the ribbons as they are pulled. In some embodiments, one set of rollers  150  may be included as part of the pulling system and a second (or more) set may be provided outside of the growth chamber  102  and pulling system. For example, a set of rollers  180  may be provided outside of the growth chamber  102  as illustrated in  FIG. 6 . The rollers  180  may have their own motors, but may be driven at speeds generally the same as those of the rollers  150 . The rollers  180  may include their own sensors, in some embodiments, and may be controlled in accordance with the same force curve as rollers  150 . In other embodiments, the rollers  180  may not be driven, but merely rotate as the ribbon  124  rolls in between the rollers. 
     The rollers  180  may help to decouple the growth of the crystal ribbons  124  from a cutting process. A cutting system may generally take the form of laser cutter  184  configured to cut the ribbons  124  as they are moving. As such, the cutter  184  may be configured to move with crystal ribbon in order to achieve a straight cut or alternatively may simply move laterally relative to the movement of the ribbon and achieve a diagonal cut of the ribbon. In other embodiments the cutter  184  may take the from of a mechanical device that may operate in accordance with a scribe and break technique, where a surface of the ribbon is scratched and then pressure or a thermal gradient is applied to cause fracture of the ribbon along the scratch line. In the cutting process, the ribbons  124  may be cut into discrete parts that may at least approximate the size and shape of a sapphire part for use for example in electronic devices. 
     In some embodiments, a vision test system  182  may be provided to scan the ribbons  124  for defects prior to cutting the ribbons. The vision test system  182  may generally take the form of a light based system that directs light or electromagnetic energy within a certain range of wavelengths towards the ribbons  124  and detects defects based on either reflected light patterns or light patterns that pass through the ribbons. In some embodiments, the vision test system may take the form of an infrared or ultraviolet sensor. In still other embodiments, a sensor other than a light based sensor may be implemented. 
       FIG. 7  illustrates a ribbon  124  with discrete part patterns  190  on the ribbon. Generally, the centermost part of the ribbon  124  is suitable for use. However, occasionally, gas bubbles  192  or other defects may be found in this region. With the vision test system  182 , such defects  192  may be found and the ribbon  124  may be cut so that such defects do not become integrated into a part. Specifically, cuts may be made along the dashed lines to avoid integrating the defects into a part. Thus, the vision test system  182  helps to achieve an efficient use of the grown sapphire and reduce waste of the grown crystal. 
       FIG. 8  is a flowchart illustrating an example method  200  for continuous sapphire growth in accordance with an example embodiment. Generally, the method may start with alumina being loaded into the crucible (Block  202 ) and heating the crucible (Block  204 ). The crucible is heated to beyond the melting point of alumina. A seed crystal is lowered on to touch on the die tips to begin crystallization of molten alumina (Block  206 ). The seed crystal is withdrawn vertically (Block  208 ) once crystal propagation starts. The crucible is continuously replenished through continuous feeding of alumina into the crucible (Block  210 ). Additionally, an inert gas such as argon may be fed into the chamber to replenish any gas that is escaping the chamber throughout the process. A cutting process then trims the ribbons into finite lengths above the top of the growth chamber (Block  218 ). 
     Although the foregoing discussion has presented specific embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the embodiments. Accordingly, the specific embodiments described herein should be understood as examples and not limiting the scope thereof.