Abstract:
A pull rod for use in producing a single crystal from a molten alloy is provided that includes an elongated rod having a first end and a second end, a first cavity defined at the first end and a second cavity defined at the first end and in communication with the first cavity. The first cavity receives the molten alloy and the second cavity vents a gas from the molten alloy to thereby template a single crystal when the pull rod is dipped into and extracted from the molten alloy.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent application Ser. No. 61/945,900 entitled “CAVITY PULL ROD: DEVICE TO PROMOTE SINGLE CRYSTAL GROWTH FROM THE MELT” filed on Feb. 28, 2014. The entirety of the above-noted application is incorporated herein by reference. 
    
    
     ORIGIN OF THE INVENTION 
     The invention described herein was made by an employee (or employees, as appropriate) of the United States Government and may be manufactured and used by or for the Government for Government purposes without the payment of any royalties thereon or therefore. 
    
    
     BACKGROUND 
     The unique problem of growing single crystal from a melt involves the selective growth of a single or dominate growth direction from a number of possible paths. The current state of the art for the growth of a single crystal for metals includes the floating zone process, the Czochralski process, and the Bridgman method. These methods require a seed or template crystal with the same composition as the desired sample or the use of a crucible to contain the melted sample, which may introduce contamination or reaction with the melt. 
     Other methods disclose producing a single crystal without the use of a seed crystal. For example, U.S. Pat. No. 7,175,705 to Fujimura et al. discloses a process to produce a semiconductor single crystal without the use of a seed crystal. Fujimura, however, requires that solid raw material be left in the raw material disposed in the crucible to promote nucleation and therefore, prevent the raw material melt from being supercooled in order to produce a single crystal. The disadvantage to Fujimura is that the use of a solid material left in the raw material leads to waste and, thus, is more costly. 
     Another method, such as the one disclosed in U.S. Pat. No. 6,153,007 to Nakata discloses a method of producing a single crystal without the use of a seed crystal. Nakata, however, requires that the method be performed under micro-gravitational conditions, which are achieved in space stations, space shuttles, rockets and aircraft. Thus, Nakata requires simulations of micro-gravitational conditions, which requires additional equipment and, thus, an increase in cost, in order to produce a single crystal without the use of a seed crystal. 
     SUMMARY 
     The following presents a simplified summary in order to provide a basic understanding of some aspects of the innovation. This summary is not an extensive overview of the innovation. It is not intended to identify key/critical elements or to delineate the scope of the innovation. Its sole purpose is to present some concepts of the innovation in a simplified form as a prelude to the more detailed description that is presented later. 
     Thus, what is required is a device, system and method to produce a single crystal without the use of a seed crystal that overcomes the aforementioned disadvantages. 
     In one aspect of the innovation, the innovation overcomes the above mentioned disadvantages by providing an innovative cavity pull rod that produces a single to a small number of directionally grown polycrystalline metal alloys without the use of a seed or template crystal. More specifically, the innovation allows for the extraction of a single crystal from a melted alloy. 
     In another aspect of the innovation, the innovation overcomes the above mentioned disadvantages by providing an innovative a cavity pull rod having an innovative double cavity is provided that is used with the Czochralski process to produce a single or large poly crystalline metal at high temperatures without the use of a seed crystal. 
     In yet another aspect of the innovation, a pull rod for use in producing a single crystal from a molten alloy is disclosed that includes an elongated rod having a first end and a second end, a first cavity defined at the first end, a second cavity defined at the first end and in communication with the first cavity, wherein the first cavity is adapted to receive the molten alloy and the second cavity is adapted to vent a gas from the molten alloy to thereby template a single crystal when the pull rod is dipped into and extracted from the molten alloy. 
     In yet another aspect of the innovation, a system for forming a single crystal from a molten alloy is disclosed and includes a crystal growth apparatus that includes a crucible to hold the molten alloy and a heating element that melts the molten alloy. A pull rod is included that has an elongated rod having a first end and a second end, a first cavity defined at the first end, a second cavity defined at the first end and in communication with the first cavity, wherein the first cavity is adapted to receive the molten alloy and the second cavity is adapted to vent a gas from the molten alloy to thereby template a single crystal when the pull rod is dipped into and extracted from the molten alloy. 
     In still yet another aspect of the innovation, a method of forming a single crystal from a molten alloy is disclosed that includes melting an alloy in a crystal growth apparatus thereby creating a melt, introducing a dopant into the melt, dipping a cavity pull rod into the melt such that the melt enters a first part of a first cavity defined in a first end of the cavity pull rod, venting a gas from the melt through a second cavity defined in the first end of the cavity pull rod that is in communication with the first cavity, forming a single crystal in situ in the first part of the first cavity, extracting the cavity pull rod from the melt, controlling a thermal gradient of the crystal growth apparatus, a rate of extraction, and a rate of rotation of the cavity pull rod, and growing a single crystal ingot. 
     To accomplish the foregoing and related ends, certain illustrative aspects of the innovation are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the innovation can be employed and the subject innovation is intended to include all such aspects and their equivalents. Other advantages and novel features of the innovation will become apparent from the following detailed description of the innovation when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conventional seed crystal puller rod. 
         FIG. 2  is a perspective view of an innovative cavity pull rod in accordance with an aspect of the innovation. 
         FIG. 3  is a cross section view of the cavity pull rod of  FIG. 2  taken along lines  3 - 3  in accordance with an aspect of the innovation. 
         FIG. 4  is an example crystal growth apparatus used to heat a molten alloy for the formation of a single crystal in accordance with an aspect of the innovation. 
         FIG. 5  is a flow chart illustrating the extraction of a single crystal from a melted alloy using the innovative cavity pull rod in accordance with an aspect of the innovation. 
     
    
    
     DETAILED DESCRIPTION 
     The innovation is now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the subject innovation. It may be evident, however, that the innovation can be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing the innovation. 
     While specific characteristics are described herein (e.g., thickness), it is to be understood that the features, functions and benefits of the innovation can employ characteristics that vary from those described herein. These alternatives are to be included within the scope of the innovation and claims appended hereto. 
     While, for purposes of simplicity of explanation, the one or more methodologies shown herein, e.g., in the form of a flow chart, are shown and described as a series of acts, it is to be understood and appreciated that the subject innovation is not limited by the order of acts, as some acts may, in accordance with the innovation, occur in a different order and/or concurrently with other acts from that shown and described herein. For example, those skilled in the art will understand and appreciate that a methodology could alternatively be represented as a series of interrelated states or events, such as in a state diagram. Moreover, not all illustrated acts may be required to implement a methodology in accordance with the innovation. 
     Referring to  FIG. 1 ,  FIG. 1  illustrates a conventional puller rod  10  used in conjunction with methods, such as but not limited to the Czochralski process to produce a single crystal that acts a seed crystal to extract a single crystal from a molten alloy, such as silicon. The puller rod  10  is made from the same material (e.g., silicon) as the molten alloy. The puller rod  10  includes a shiny tip  12  that, as mentioned above, acts as a seed crystal when extracting the single crystal from the molten alloy. The puller rod  10  further includes a notch  14  for engagement with a chuck to hold the puller rod  10  in place during the process. The disadvantage to the puller rod  10  is that this puller rod  10  must be made from the same material as the molten alloy ion order to act as a seed crystal. Whereas, as will be seen below, the innovative cavity pull rod disclosed herein can be made from any material since the cavity pull rod is constructed such that a seed crystal is not required. 
     Referring now to  FIGS. 2 and 3 , disclosed herein is an innovative device, system and method to produce a single crystal without the use of a seed crystal in accordance with one aspect of the innovation. The device includes a cavity pull rod  100  that is an elongated, cylindrical, solid rod and includes an innovative double cavity configured to extract a single crystal from a melted alloy (hereinafter “melt”) without the use of a seed crystal. The double cavity configuration allows the melt to enter the one cavity of the cavity pull rod while allowing the inert gas to vent from the another cavity (exit port) of the cavity pull rod. This in turn reduces the amount of possible nucleation sites, which, reduces polycrystalline development throughout the crystal. 
     The cavity pull rod  100  may be used with conventional methods, such as but not limited to the Czochralski process, the Bridgman method, the floating zone process, etc. to produce a single or large poly crystalline metal at high temperatures without the use of a seed crystal. The cavity pull rod  100  may be used to extract a single crystals of semiconductors from a melt, such as but not limited to silicon, germanium, gallium arsenide, etc.). For simplicity, the disclosed innovation will be described using the Czochralski process using silicon as the melt. Thus, the example embodiment disclosed herein is for illustrative purposes only and is not intended to limit the scope of the innovation. 
     The cavity pull rod  100  has a first end  200  and a second end  300 . The first end  200  includes a radial channel (groove)  202  that circumferences the first end  200 , whereby the radial channel  202  is longitudinally spaced a distance d from an edge  204  of the first end  200 . The first end  200  further includes a double cavity configuration defined therein comprised of a first cavity  206  and a second cavity  208 . The first cavity  206  follows the cylindrical shape of the cavity pull rod  100  and is, thus, circular in shape. The first cavity  200  includes a first part  210 , a second part  212 , and a third part  214 . 
     The first part  210  is a tapered cavity that includes a tapered wall  216  that extends from the edge  204  of the first end  200  inward toward a radial center of the cavity pull rod  100  and, hence toward the second part  212  at an angle of approximately 20-50 degrees with respect to a longitudinal axis A. The first part  210  has an isosceles trapezoidal cross-section, as illustrated in  FIG. 3 . The first part  210  has a depth of d−x and as a result, a depth of the first part  210  is less than the distance d that the radial channel  202  is from the edge  204  of the first part  210 . The significance of this will be explained further below. 
     The second part  212  intersects the first part  210  at a first intersection defined by points P that define a circle and extends cylindrically from the first intersection toward the second end  300  a distance d 1 . Thus, the second part  212  has a cylindrical cross section and is substantially parallel with the longitudinal axis A, as illustrated in  FIG. 3 . 
     The third part  214  intersects the second part  212  at a second intersection defined by points P 1  that define a circle and extends from the second intersection toward the second  300  a distance d 2 . The third part  214  is a tapered cavity that includes a tapered wall  218  that extends from the second intersection toward the center of the cavity pull rod  100  and, thus, forms a point  220  thereby closing the first cavity  206 . Thus, the third part  214  has a triangular cross-section. 
     Still referring to  FIG. 3 , the second cavity  208  intersects the second part  212  of the first cavity  206  and extends essentially in a perpendicular manner from the second part  212  toward an outer surface  222  of the cavity pull rod  100  thereby forming an opening  224  defined in the outer surface  222 . Thus, the second cavity  208  forms a port from the second part  212  of the first cavity  206  to the outer surface  222  of the cavity pull rod  100 . The second cavity  208  has a cylindrical cross section and allows gas to vent from the first cavity  206  during extraction of the single crystal from the melt, which will be explained further below. 
     Still referring to  FIGS. 2 and 3 , the second end  300  has at least one recess (or indentation, notch, etc.)  302  that is configured to receive a fastening device, such as but not limited to a set screw to hold the cavity pull rod in place during extraction of the single crystal form the melt. The recess(es)  302  may have any shape, such as but not limited to circular, oval, square, etc. In the example embodiment illustrated in the figures, the recess(es)  302  are on the same longitudinal axis as the opening  224  formed in the outer surface  222  of the cavity pull rod  100 . It is to be understood, however, that the recess(es) may be located at any location around the cavity pull rod  100  and are not required to be on the same longitudinal axis as the opening  224 . 
     The cavity pull rod  100  is made from a material, such as but not limited to tungsten, that can withstand high temperatures and that can transfer heat from the melt to the cavity pull rod  100  to facilitate solidification of the pulled crystal as the cavity pull rod  100  is extracted from the melt. More specifically, the configuration of the cavity pull rod  100  allows heat to transfer along a solid longitudinal axis of the cavity pull rod  100 , while the tapered configuration of the first cavity  206  allow the melt to remain liquid and in thermal equilibrium with the surrounding melt. 
     As mentioned above, the double cavity configuration of the innovative cavity pull rod  100  facilitates the formation of a single crystal from the melt without the need of a seed crystal. The cavity pull rod  100 , however, can be used with conventional methods, such as the Czochralski process. Thus, referring to  FIGS. 4 and 5 , a method of forming a single crystal from the melt will now be described.  FIG. 4  shows one example embodiment of a crystal growth apparatus  400  used to form the single crystal. The growth apparatus  400  includes a crucible  402 , usually made of quartz, that hold the melt (e.g., silicon)  404  and a heating element  406  to heat the melt  404 . The heating element  406  creates a thermal gradient from a bottom to a top of the crystal growth apparatus  400 . As will be described below, a single crystal ingot  408  is formed without the use of a seed crystal using the cavity pull rod  100 . 
     Still referring to  FIGS. 4 and 5 , at  502 , an alloy is heated by the heating element, thus, forming a melt  404  in the crucible  402 . At  504 , an impurity (dopant), usually boron or phosphorous, can be introduced in the melt to thereby dope the melt  404 . In the example of silicon, the dopant will change the silicon into an n-type or p-type silicon. At  506 , the cavity pull rod  100  is dipped into the melt  404  via a controlled mechanism. The cavity pull rod  100  is secured to the controlled mechanism via a fastening device, such as but not limited to set screws that engage the recess(es)  302  on the second end  300  of the cavity pull rod  100 . The cavity pull rod  100  is dipped into the melt  404  such that the melt  404  does not go past the radial channel  202  on the cavity pull rod  100 . In other words, the cavity pull rod  100  is dipped into the melt  404  a distance that is less than or equal to the distance d 1  that the radial channel is from the edge  204  of the first end  200  of the cavity pull rod  100 . At  508 , gas from the melt  404  is vented through the second cavity  208 , which as mentioned above, reduces the amount of nucleation sites thereby reducing polycrystalline development through the crystal. Thus, at  510 , the double cavity configuration of the cavity pull rod  100  results in the formation of a single crystal in situ, in part due to the solidified material of the cavity. At  512 , the cavity pull rod  100  is extracted from the melt  404  in a direction indicated by the arrow A 1  and is simultaneously rotated in a counter-clockwise direction indicated by the arrow A 2 . At  514 , as the cavity pull rod  100  is being extracted the thermal gradient, rate of extraction, and rate of rotation are precisely controlled, thus, at  516 , resulting in the growth of a single crystal ingot  408  in situ until the melt  404  is exhausted. 
     As described above, a single crystal is formed without the use of a seed crystal due to the double cavity configuration of the cavity pull rod  100 . Thus, the single crystal is formed in situ. 
     What has been described above includes examples of the innovation. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the subject innovation, but one of ordinary skill in the art may recognize that many further combinations and permutations of the innovation are possible. Accordingly, the innovation is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising” as “comprising” is interpreted when employed as a transitional word in a claim.