Patent Publication Number: US-2007119366-A1

Title: Section forming method &amp; construction for wafer ingot growth

Description:
BACKGROUND OF THE INVENTION  
      (a) Field of the Invention  
      The present invention is related to a wafer ingot processing technology, and more particularly, to one that allows wafer growing into a shape that is comparatively closer to a polygonal section to minimize the amount of wastes resulted from the subsequent process in slicing the ingot into chips when the arc perimeter areas must be cut off; and to significantly increase output and reduce production cost especially in growing a square ingot for solar cell since the square ingot is at its best to minimize the raw materials to be cut off.  
      (b) Description of the Prior Art  
      As a member in the family of semiconductor, the solar cell is also known as a solar chip and silicon is so far a representative material for manufacturing solar cells generally available in the market. The power generation of the solar cell works on converting solar energy into electrical energy. There are may types of chip materials for the manufacturing of the solar PV cell and can be roughly grouped into mono-crystalline silicon, polycrystalline/Multi-crystalline silicon, and amorphous silicon, and other non-silicon materials, e.g., compound semiconductor materials including CdTe, InGaAs, and GaAs.  
      Furthermore, silicon is available in mono-crystalline and polycrystalline. Wherein, the constituent atoms of the mono-crystalline silicon are arranged in given rules; therefore, the product conversion efficiency is higher. In the manufacturing process of the mono-crystalline silicon, silicon metal of 99.999999999% purity is melted in a crucible  11  as illustrated in  FIG. 1  of the accompanying drawings, and a mono-crystalline silicon seed in the direction marked as &lt;100&gt; is inserted into the liquid of the melted silicon and rotates at a rate of 2˜20 circles per minute while being pulled up at a rate of 0.3˜10 mm per minute to go through neck growth, dome growth, crystalline growth, and pedestal growth to grow into a mono-crystalline silicon ingot  20  in a diameter of 4″˜8″. This growing process is referred as a Czochralski Method.  
      As illustrate din  FIG. 1 , in the system comprised of a wafer growing furnace  10  for the manufacturing of the mono-crystalline silicon ingot, a heater  12  to heat the material inside the crucible  11  is provide in the peripheral of the crucible  11 ; a thermal shield  13  is provided externally to the heater  12  and the crucible  11 ; a thermal shield  14  is disposed over the crucible to define a thermal field for the oven  10  so to reduce thermal loss and production cost of the wafer ingot. Furthermore, a gas vent  131  is disposed below the thermal insulation  13  provided externally to both of the heater  12  and the crucible  11  to produce gas flow passing through the thermal insulation of the furnace for expelling oxides that are vulnerable to form foreign matters.  
      In the prior art as described above, the section of the ingot relates to circular section as illustrated in  FIG. 3 . When an ingot  20  is sliced into square chips that can be used in the manufacturing of the solar cell, the parts of the ingot  20  in the shadowed areas as illustrated in  FIG. 3  are deemed as wastes  21  and must be cut off. The massive waste of the ingot material naturally increases the production cost of the chip.  
     SUMMARY OF THE INVENTION  
      The primary purpose of the present invention is to provide a method and construction of growing wafer ingot for minimizing the material to be cut off in the subsequent process of slicing wafer ingot into chips by controlling gas current, heat conduction and heat radiation in ingot growth to allow an isotherm of solidification temperature of growth to approach a preferred straight side line status for the ingot to grow into a form approximating the preset sectional form of a polygon.  
      To achieve the purpose, a thermal shield disposed on an opening of a crucible, an opening approximating a polygonal contour disposed on the thermal shied to control gas current, heat conduction and heat radiation in ingot growth, an isotherm of solidification temperature in ingot growth approaching a polygonal form to grow the wafer into a form approximating the preset sectional form of a polygon.  
      Alternatively, a cover is placed at the opening of the crucible, and an opening is provided on the cover in a contour approximating that of a polygon. By controlling the weight of the cover the downward pressure of the cover, the melting liquid of silicon in the crucible is extruded towards the opening of the cover to produce an ingot in a form approximating the preset sectional form of a polygon. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic view showing a construction of an oven for growing wafer ingot.  
       FIG. 2  is a schematic view showing a construction of another oven for growing wafer ingot.  
       FIG. 3  is a schematic view showing a section of a wafer ingot formed by a growing oven of the prior art.  
       FIGS. 4, 5 ,  6 , and  7  are schematic views respectively showing a shape of a cover in the preferred embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      A method and construction of growing wafer ingot of the present inventions is essentially comprised of controlling gas currents, heat conduction, and heat radiation by means of a thermal shield disposed at the opening of a crucible for the isotherm of solidification temperature of the thermal filed in growing the wafer ingot to approximate a polygonal status; thus to provide a section that is comparatively closer to a polygonal section as preset to minimize the possible wastes to be cut off from the ingot when the ingot is sliced into chips in the subsequent manufacturing process.  
      The present invention is essentially comprised of a growth furnace  10  with a crucible  11  to contain a crystal material as illustrated in  FIG. 1  for the manufacturing of a wafer ingot. The crystal material relates to silicon or non-silicon material, e.g., compound semiconductor material including CdTe, InGaAs, or GaAs. A heater  12  to heat the raw material placed in the crucible  11  is disposed to the peripheral of the furnace  10  for reducing thermal loss. A thermal insulation  13  is provided externally to the heater  12  and the crucible  11 . A gas vent is disposed below the thermal shield  13  to create gas currents to pass through a thermal field in the furnace for discharging oxides that are vulnerable to become foreign matters.  
      A thermal shield  14  is disposed at the opening over the curable  11  in the furnace  10  to define the thermal filed for the furnace  10  in conjunction with the thermal insulation  13 . The thermal shield  14  is made of a material with excellent insulation performance and is provided with an opening  141  approximating a polygonal contour to allow the ingot  20  to pass through and to control the air currents flowing through the thermal field of the furnace  10 . As illustrated in  FIG. 4 , the opening  141  for the thermal curtain  14  to control gas currents to be made in a square is preferred; or alternatively, in a shape as respectively illustrated in  FIGS. 5, 6 , and  7  with the corners of the polygonal opening  141  to be elongated or designed with a proper form for the isotherm of the solidification temperature of wafer growth to approximate a preset form, thus to allow the section of the ingot  20  to comparatively get closer to a polygonal section as present.  
      In another preferred embodiment of the present invention as illustrated in  FIG. 2 , it is also essentially comprised of the oven with the crucible to contain the crystal material. The heater  12  is provided to heat the raw material contained in the crucible  11 . The thermal insulation  13  is disposed externally to the heater and the crucible to reduce thermal loss. A cover  15  is provided by contact at the opening of the oven  10 . An opening  151  in a contour resembling that of a polygon is disposed in the cover  15 . The melted silicon in the crucible is extruded towards the opening  151  by the downward pressure from the cover  15 . Similarly, the ingot  20  grows with a section that is comparatively closer to a polygonal section as present.  
      Both preferred embodiments of the present invention are capable of producing a wafer ingot with a non-circular section to effectively minimize the amount of wastes to be cut off from the side areas of a round ingot when sliced into chips in the subsequent manufacturing process as found with the prior art.  
      The prevent invention provides a method of producing a wafer ingot with non-circular section, and the application for a patent is duly filed accordingly. However, it is to be noted that the preferred embodiments disclosed in the specification and the accompanying drawings are not limiting the present invention; and that any construction, installation, or characteristics that is same or similar to that of the present invention should fall within the scope of the purposes and claims of the present invention.