Abstract:
An apparatus for growing ingots by the Czochralski method includes a growth chamber defining an enclosure configured to circulate a purge gas about the growing ingot and a crucible provided in the growth chamber configured to hold the molten silicon. A weir is supported in the crucible and is configured to separate the molten silicon into an inner growth region surrounding the melt/crystal interface from an outer region configured to receive the crystalline feedstock. The weir comprises at least one sidewall extending vertically and a cap extending substantially perpendicularly to the sidewall.

Description:
FIELD 
       [0001]    The field of the disclosure relates generally to growing single crystal semiconductor material by the Czochralski process. In particular, the field of the disclosure relates to a continuous Czochralski process employing a cap weir. The cap weir is configured for modifying the flow and pressure of a gas that lowers the rate of erosion of the weir. 
       BACKGROUND 
       [0002]    In a continuous Czochralski (CZ) crystal growth process, the melt is supplemented or recharged as the crystal is growing. This is in contrast with batch recharge wherein the melt is recharged after the melt is depleted by a completion of a crystal growing process. In either case the melt can be supplemented either with solid feedstock or molten feedstock. 
         [0003]    In contrast to batch recharge, there are advantages of a continuous Czochralski process for growing crystal silicon ingots. The melt height remains substantially constant and therefore the growth conditions at the melt-crystal interface can be maintained more uniformly for optimal crystal growth. The cycle time may also be reduced because the melt conditions are not suddenly changed by the addition of a large quantity of feedstock. 
         [0004]    A conventional weir arrangement in a conventional continuous crystal growth crucible is shown in  FIG. 1 . In the conventional Czochralski system, a crucible  100  holds a quantity of molten silicon  102  in which a single crystal ingot  104  is grown and pulled in a vertical direction indicated by arrow  105  from a crystal/melt interface  106 . A weir  108 , typically shaped as a cylinder is positioned on the floor of the crucible  100  and extends vertically above the melt as shown. The weir  108  defines an inner growth region  110  and an outer melt supplementing region  112 . Subsurface passageways  114  connect the first or melt supplementing region  112  with the inner growth region  110 . 
         [0005]    A heat shield  116  is conical in shape and extends downwardly at an angle to create an annular opening disposed about the growing crystal or ingot  104  to permit the growing ingot to radiate its latent heat of solidification and thermal flux from the melt. The top of the heat shield  116  has a first diameter much wider than the diameter forming the annular opening around the ingot  104 . The top of the heat shield  116  is supportably held by an insulating lid or insulation pack. The insulating lid is omitted from the drawing for the sake of simplicity. A flow of an inert gas, such as Argon, is typically provided along the length of the growing crystal as indicated at  117 . 
         [0006]    A feed supply  118  provides a quantity of silicon feedstock to the melt supplement region  112  of the crucible  100 . The silicon feedstock may be in the form of solid chunks of silicon feedstock provided directly to melt region  112 . In either case, addition of feedstock to the melt region is often accompanied by particles of dust transported by aerostatic forces over the top of weir  108 . The dust or unmelted silicon particles contaminate the growth region  110  and can become attached to the growing ingot  104 , thereby causing it to lose its single silicon structure. 
         [0007]    Although the conventional weir  108  arrangement of  FIG. 1  may help to limit transmission of dust and un-melted particles of silicon by means of melt flowing from the melt supplementing region to the crystal growth region, it fails to address the problem of high erosion rates of the weir. As shown in  FIG. 5 , conventional quartz weir  108  is subjected to silicon monoxide and Argon gas. Argon gas is pumped over the liquid silicon to remove silicon monoxide gases and to reduce the concentration of oxygen incorporated in the grown crystal. The Argon gas and silicon monoxide gas are indicated by streamlines  500 . A side effect of the gas flow is that the weir undergoes rapid erosion at the melt-gas contact line  119  due to surface kinetics and eventually is cut-through to the extent that it no longer functions as a sufficient barrier to the solid polysilicon. Such erosion poses a problem because it requires that the weirs be frequently replaced, causing downtime and increased cost for replacement. 
         [0008]    While this conventional weir  108  arrangement may be adequate for preventing unmelted silicon from the melt supplementing region to reach the crystal growth region, the arrangement fails to address the problem of rapid erosion of the weir. This rapid erosion may increase the cost of the process and may decrease throughput by causing downtime to replace the weir. 
         [0009]    This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
       BRIEF DESCRIPTION 
       [0010]    In one aspect, an apparatus for growing an ingot by the Czochralski method is disclosed. The ingot is grown from a melt/crystal interface in a quantity of molten silicon replenished by crystalline feedstock. The apparatus includes a growth chamber defining an enclosure configured to circulate a purge gas about the growing ingot and a crucible provided in the growth chamber configured to hold the molten silicon. A weir is mounted in the crucible and configured to separate the molten silicon into an inner growth region surrounding the melt/crystal interface from an outer region configured to receive the crystalline feedstock. The weir includes at least one sidewall extending vertically and a cap extending substantially perpendicularly to the sidewall. 
         [0011]    In another aspect, a method for continuous Czochralski crystal growing is disclosed. For such method, one or more crystal ingots are pulled in a growth chamber from a melt/crystal interface defined in a crucible containing molten crystalline material that is replenished by feedstock. The method includes separating the molten crystalline material into an inner growth region surrounding the melt/crystal interface and an outer region for receiving the feedstock using a weir. An inert gas is flowed in contact with the weir and the melt such that a partial pressure of a silicon monoxide gas released from the melt is increased at a maximum weir erosion point. 
         [0012]    In yet another aspect, a system for growing ingots by the Czochralski method is disclosed. The ingots are drawn from a melt/crystal interface in a quantity of molten silicon replenished by crystalline feedstock. The apparatus includes a growth chamber defining an enclosure configured to circulate a purge gas about the growing ingot and a crucible provided in the growth chamber configured to hold the molten silicon. A feed supply is provided for supplying the crystalline feedstock. A heater heats the crystalline feedstock and the molten silicon. A weir is supported in the crucible and configured to separate the molten silicon into an inner growth region surrounding the melt/crystal interface from an outer region configured to receive the crystalline feedstock. The weir includes at least one sidewall extending vertically and a cap extending substantially perpendicularly to the sidewall. 
     
    
     
       SUMMARY 
         [0013]      FIG. 1  is a partially schematic view showing a conventional crucible with peripheral heaters in a Czochralski crystal growing system. 
           [0014]      FIG. 2  is a partially schematic view showing a cap weir in a Czochralski crystal growing system according to an embodiment of this disclosure. 
           [0015]      FIG. 3  is an enlarged view of the cap weir showing gas streamlines around the cap weir. 
           [0016]      FIG. 4  is a schematic diagram of another cap weir in a Czochralski crystal growing system according to another embodiment. 
           [0017]      FIG. 5  is an enlarged view of the conventional crucible of  FIG. 1  showing gas streamlines. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Referring to  FIG. 2 , in accordance with an embodiment of the disclosure, a cap weir  208  is provided in a crucible  200 . The cap weir  208  has generally a cylindrical shape with vertically extending sidewalls  222  mounted at the bottom of the crucible  200  that define a growth region in silicon melt  202 . The growth region  210  in silicon melt  202  is defined as the region encompassed by sidewalls  222  (i.e., radially inward of the sidewalls). An outer feed supplement region  212  is defined as a region outside of the sidewalls  222  (i.e., radially outward of the sidewalls). As such, the cap weir  208  separates the growth region  210  from a first region or melt supplement region  212  to isolate and prevent thermal and mechanical disturbances from affecting the growing crystal in the growth region  210 . 
         [0019]    In some embodiments a passageway  214  is defined in sidewalls  222  for controlling melt flow between the melt supplement region  212  and growth region  210 . A feed supply  221  supplies a source of solid silicon feedstock to melt supplement region  212 . A heat shield  216  may be provided to shield the melt/crystal interface  206  and the ingot  204  from thermal perturbations. In the exemplary embodiment, the heat shield is conical in shape, and angles or tapers radially inwardly in the downward direction, such that the top of the heat shield is much wider than the bottom of the heat shield. The sidewalls of the conical heat shield  216  depend downwardly from the base and at an angle such that a smaller diameter distal end of the heat shield defines a central annular opening  205 , large enough to receive the growing ingot, as the single crystal ingot  204  is pulled vertically as shown. In some embodiments, one or more bottom heaters  218  and side heaters  219  are in thermal communication with crucible  200  to supply heat to the melt  202 . The bottom heaters may be independently controlled annular heaters  218  disposed in a radial pattern beneath the base of the crucible  200  in addition to side heaters  219 , which may provide a more controlled temperature distribution through melt  212 . 
         [0020]    The cap weir  208  includes a cap  207  extending from an upper portion of the sidewalls  222 . In the exemplary embodiment, the cap extends substantially perpendicularly to sidewalls  222 , in a radially inward direction and a radially outward direction. In other embodiments, cap  207  may extend in only a radially outward direction from sidewalls  222 , or only in a radially inward direction from sidewalls  222 . 
         [0021]    The cap weir  208  comprises a generally cylindrical shaped body mounted on the base of the crucible  200 . The sidewalls  222  of the cap weir  208  extend vertically upward to form and define a modified flow region  215  with the melt  202 . It will be appreciated that the modified flow region  215  between the underside of the cap  207  and the melt  202  is optimized to reduce the erosion of the weir by creating gas flow patterns ( FIG. 3 ) in such a way that the partial pressure of gas, such as silicon monoxide gas, is locally increased at the maximum weir erosion point  230 . 
         [0022]      FIG. 3  shows exemplary modified gas streamlines  300  in modified flow region  215 , having an interface height  304 , in accordance with an embodiment. The dimensions of the cap  207  are chosen such that the extended sides of the cap weir  208  in combination with the adjacent melt  206  provide a modified flow path, represented in  FIG. 3  as streamlines  300 , for the outflow of the argon purge gas from the ingot. In one embodiment, the outflow of Argon purge gas and/or silicon monoxide gas is supplemented by modifying the pressure within the chamber, for example by using vacuum pump  220  ( FIG. 2 ). That is, the modified flow region  215  is dimensioned, by way of sizing of cap portion  207 , to provide a narrowed, focused flow path for the outflow of the purge gas which has the effect of increasing the pressure of the outflow purge gas with respect to flow region  302 , which is outside of the modified flow region  215 . This local increase in gas pressure advantageously reduces the erosion of the weir at sidewalls  222 , thus increasing the usable life of the weir. The diameter of cap weir  208  is selected to as to provide sufficient melt volume in the melt region  212 , such that the latent heat of fusion and thermal energy necessary to maintain the silicon at or above its melting temperature is maintained. 
         [0023]    Table 1 below shows exemplary performance results of the cap weir in comparison to a Comparative weir, such as the weir of  FIG. 1 . 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Weir Performance Results 
               
             
          
           
               
                   
                 Comparative Weir 
                 Exemplary Cap Weir 
               
               
                   
                   
               
             
          
           
               
                 Oxygen (ppma) 
                 11.8 
                 11.9 
               
               
                 G (K/cm) 
                 49.5 
                 49.5 
               
               
                 Heater power (kW) 
                 67 
                 67 
               
               
                 Interface height (cm) 
                 10.1 
                 10.1 
               
               
                 Feed zone minimum T 
                 1699 
                 1699 
               
               
                 (K) 
               
               
                 Inner erosion rate (mum/ 
                 21.9 
                 18.8 
               
               
                 hr) 
               
               
                   
               
             
          
         
       
     
         [0024]    As shown, in a CZ process with equivalent parameters, the exemplary cap weir provided a reduced inner erosion rate in comparison to a typical non-cap weir. As used herein, the value of G is a measure of the axial temperature gradient in a crystal at the melt-crystal interface. As is known to one of skill in the art, G is a measure of how fast heat may be removed through the crystal and/or how quickly the crystal is cooled. For example, for a given crystal cooling configuration, a lower value of G may indicate that there is additional room for increasing the pull rate of the crystal. For a given configuration, an interface height is a measure of the vertical distance between the melt line and the topmost part of the melt-crystal interface, and may be used as a direct measure of how hot the crystal is. In some instances, a deeper interface may indicate that there is less room for increasing the crystal pull rate, due to a higher crystal temperature. 
         [0025]    As shown in  FIG. 4 , the crucible  200  containing the cap weir  208  may also include a second weir  408  located radially outward from cap weir  208 . Although, second weir  408  is shown radially outward from cap weir  208 , in other embodiments, second weir  408  may be located radially inward from the cap weir. The second weir defines an interconnecting region  411  between the outer feed supplement region  212  and the growth region  210  of the melt  202 . 
         [0026]    Feedstock, whether in solid or liquid form, added in at  221  to the feed supplement region  212  of the crucible should be fully melted before it arrives in the central growth region  210 , otherwise small particles in the central growth region  210 , particularly oxides of unmelted silicon feedstock, can attach themselves to the growing ingot and cause dislocations. Thus, additional time for feedstock to be melted is provided by the feedstock passing through the feed supplement region  212 , through passageway  414 , and the interconnecting region  411 . In addition, the melt in the growth region  210  is devoid of large local temperature fluctuations that can cause dislocations in the growing crystal  204 . In this embodiment, the second weir  408  is chosen to be a height that does not substantially interfere with the modified flow path  215 . The height of second weir  408  may be the same height, taller or shorter than the cap weir  208 , and in some embodiments, the second weir  408  includes a cap portion, similar to cap portion  207 . Similarly, the cap weir may be sized with a diameter, such that the second weir does not substantially interfere with the modified flow path  215 . 
         [0027]    Exemplary embodiments of the apparatus, systems and methods for improved crystal growth in a continuous Czochralski process are described above in detail. The apparatus, systems and methods are not limited to the specific embodiments described herein, but rather, components of the systems and apparatus, and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the methods may also be used in combination with other crystal forming systems, methods, and apparatuses, and are not limited to practice with only the systems, methods, and apparatus as described herein. Rather, the exemplary embodiment can be implemented and utilized in connection with many other applications. 
         [0028]    When introducing elements of the present invention or the embodiment(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
         [0029]    As various changes could be made in the above without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.