Abstract:
A method for doping crystals is disclosed. The method includes a receiver for receiving semiconductor spheres and a dopant. The semiconductor spheres are heated to a molten state. The dopant is absorbed by the semiconductor spheres. The semiconductor spheres are cooled to produce doped semiconductor spheres. The method is performed in a non-contact environment.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to patent application Ser. No. 09/209,653, filed on Dec. 10, 1998, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates generally to semiconductor devices, and more particularly, to a method for doping spherical-shaped semiconductors. 
     The doping process involves the controlled introduction of an impurity to a substrate, which produces subtle changes in the electrical resistivity of the substrate. Such characteristics are necessary for solid-state electronic semiconductor devices, such as a transistor or integrated circuit. 
     A current method to produce doped, spherical single crystal substrates involves doping the surface of a polycrystalline silicon granule, and then melting it to create a homogeneously doped crystal. Both the doping and the melting comprise two separate steps that are carried out in a furnace in sequence. A third step involves processing (i.e. partial melting/recrystallizing) the homogeneously doped crystal to produce a doped single crystal. 
     In U.S. Pat. Nos. 5,278,097, 5,955,776, and 5,223,452, methods and apparatuses for doping spherical-shaped semiconductors are disclosed. However, an improved method of doping the spherical shaped semiconductors, which is simpler and more economical, is desired. 
     SUMMARY OF THE INVENTION 
     The present invention, accordingly, provides a method for doping spherical semiconductors. To this end, one embodiment provides a chamber for levitating semiconductor spheres. The semiconductor spheres are then heated and melted within the chamber. A dopant is introduced into the chamber to diffuse into the molten semiconductor spheres. 
     In one embodiment, the method of doping a three dimensional substrate in a non-contact environment includes: receiving a three dimensional substrate; melting the three dimensional substrate to a liquid state; doping the three dimensional substrate with a dopant to create a doped three dimensional substrate; and recrystallizing the doped three dimensional substrate to create a doped three dimensional single crystal. 
     In one embodiment, the three dimensional substrate is a spherical shaped semiconductor. 
     In one embodiment, the non-contact environment can be achieved by floating or levitating the substrate, such as through aerodynamic levitation, acoustic levitation, electromagnetic levitation or a drop tube. 
     In one embodiment, melting the spherical shaped semiconductor to a liquid state includes placing the spherical shaped semiconductor in a chamber connected to a heat source. The heat source may be an Inductively Coupled Plasma (ICP) torch, which may also serve to maintain the non-contact environment. Other heat sources include an infrared lamp, a resistance furnace, an electromagnetic radiation heater, or a laser. 
     In one embodiment, doping the spherical shaped semiconductor includes diffusing the dopant through the spherical shaped semiconductor. The dopant may be a dopant plasma produced by passing a solid dopant mixed with an inert gas through an ICP torch or by lining an ICP torch with a solid dopant. The solid dopant may be boron nitride (BN), phosphorous (P), or antimony (Sb). 
     In one embodiment, the dopant is a dopant vapor produced by bubbling an inert gas through a dopant liquid, mixing a dopant gas with an inert gas, or passing an inert gas over a liquified solid dopant. The dopant liquid may be phosphorous oxychloride (POCl 3 ) or boron tribromide (BBr 3 ). 
     In one embodiment, the liquified solid dopant may be antimony (Sb), phosphorous (P), or gallium (Ga). The dopant gas may be phosphine (PH 3 ) or diborane (B 2 H 6 ). 
     In one embodiment, the dopant is a dopant plasma comprised of passing the dopant vapor through an ICP torch. 
     In one embodiment, the dopant is a dopant vapor produced from lining the chamber with a solid dopant. 
     In one embodiment, the solid dopant is selected from the group consisting of boron nitride (BN), phosphorous (P), or antimony (Sb). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of an apparatus for use in doping spherical semiconductors according to one embodiment of the present invention. 
     FIG. 2 is a flow chart of an embodiment of a method for doping a spherical shaped semiconductor using the apparatus of FIG.  1 . 
     FIG. 3 is a cross-sectional view of an alternate embodiment of an apparatus for use with the method of FIG.  2 . 
     FIG. 4 is a cross-sectional view of an alternate embodiment of an apparatus for use with the method of FIG.  2 . 
     FIG. 5 is a cross-sectional view of an alternate embodiment of an apparatus for use with the method of FIG.  2 . 
     FIG. 6 is a cross-sectional view of an alternate embodiment of an apparatus for use with the method of FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a method for doping substrates. The following description provides many different embodiments, or examples, for implementing different features of the invention. Certain techniques and components described in these different embodiments may be combined to form more embodiments. Also, specific examples of components, chemicals, and processes are described to help clarify the invention. These are, of course, merely examples and are not intended to limit the invention from that described in the claims. 
     Referring to FIG. 1, an embodiment of an apparatus for melting spherical semiconductors includes an inductively coupled plasma (“ICP”) torch  10  positioned above a drop tube  26 . The ICP torch  10  includes a quartz tube  12  made of a high temperature material, for example, ceramic. Surrounding the quartz tube  12  is a cooling system  14 . The cooling system  14  may include water or any other type of coolant to help prevent the quartz tube  12  from melting. 
     Also, surrounding the quartz tube  12  is a conductive coil  16 . In the present embodiment, the conductive coil  16  is a hollow copper coil attached to a radio frequency (“RF”) energy generator  18 . Because the conductive coil  16  is hollow, air or other fluid can flow there through to help lower the temperature of the coil. 
     An entry tube  20  is attached to the quartz tube  12  for receiving a plurality of spherical semiconductors  104  and directing them towards a central chamber  22  of the quartz tube. Although not shown, a shield may be provided to prevent radiation losses from plasma in the entry tube  20 . 
     In operation, a plasma gas  108  flows at atmospheric pressure inside the quartz tube  12 . The RF generator  18  operates at a desired frequency to heat the plasma gas  108 , thereby creating a plasma flame  24 . The plasma flame  24  is preferably at a temperature between 8000°K to 10,000°K, which depends on the gas flow rate and torch dimensions. A portion of the flame  24   a  extends into the entry tube  20  and serves as a preheater. 
     When the spherical semiconductors  104  enter the entry tube  20 , they are quickly preheated by the plasma flame  24   a . The spherical semiconductors  104  then enter the central chamber  22  and are melted by the plasma flame  24 . Because the temperature of the flame  24  is so high, impurities in the spherical semiconductors  104  will vaporize. As the spherical semiconductors  104  exit the central chamber  22 , they solidify to form crystals  104   a . The crystals  104   a  advance through the drop tube  26  where they cool. 
     The drop tube  26  will allow the crystals  104   a  to cool and form a single crystal structure. The drop tube  26  includes an aperture  28  so that a cooling gas  30  may flow therein. In some embodiments, a heater  32  is placed adjacent to the drop tube  26 . The heater  32  helps to reduce the number of different crystalline growth directions by slowing the cooling process. The cooling gases  30  may be, for example, helium, hydrogen, argon, or nitrogen to facilitate the cooling of the crystals  104   a . The cooling gases  30  may also be used to control the rate of descent of the crystals  104   a . The drop tube  26  may be stainless steel with an electro-polished inside finish, about five to ten centimeters in diameter and about ten meters in length. 
     Referring to FIG. 2, a method  100  may be used in conjunction with the apparatus  10 . At step  102 , a plurality of spherical semiconductors  104  are received in the entry tube  20  of the ICP torch  10 . The spherical semiconductors  104  may be, for example, any commercially available spherical semiconductor material. In a preferred embodiment, the spherical semiconductors  104  are silicon. 
     At step  106 , a plasma gas  108  flows at atmospheric pressure inside the quartz tube  12 . The RF generator  18  operates at a desired frequency to heat the plasma gas  108 , thereby creating the plasma flame  24 . The spherical semiconductors  104  enter the central chamber  22  where the plasma flame  24  melts them. The plasma gas  108  may be, for example, a mixture of a gas phase dopant and an inert gas. In a preferred embodiment, the plasma gas  108  is a mixture of phosphine (PH 3 ) and argon (Ar). In an alternate embodiment, the plasma gas  108  is a mixture of diborane (B 2 H 6 ) and argon (Ar). The amount of gas phase dopant in the plasma gas  108  is chosen to maximize the amount of the gas phase dopant in the plasma gas  108 . The plasma flame  24  includes ions of the plasma gas  108 . 
     At step  110 , the spherical semiconductors  104 , now in the liquid state from the plasma flame  24 , absorb the ions of the plasma gas  108  in the plasma flame  24 . As the spherical semiconductors  104  exit the central chamber  12 , they solidify to form crystals  104   a . The crystals  104   a  include the dopant ions in the lattice of the crystals  104   a . The crystals  104   a  are cooled in the drop tube  26 . 
     In an alternate embodiment, the plasma gas  108  is argon gas. A solid liner sleeve of solid dopant material is inserted into the central chamber  22  such that the plasma flame  24  passes through the solid dopant material ,and the argon gas carries the solid dopant material throughout the central chamber  22 . The solid dopant material is ionized by the plasma flame  24  so that the plasma flame  24  contains a certain concentration of the dopant material. The solid dopant material may be, for example, boron nitride (BN), phosphorus (P), or antimony (Sb). 
     The plasma gas may be produced in many different ways. For one, the plasma gas  108  may be produced by mixing a very fine powder of the solid dopant material with argon gas. The solid dopant material would be vaporized and ionized in the plasma flame  24 . Alternatively, the plasma gas  108  may be produced by passing argon (Ar) through liquid phosphorus oxychloride (POCl 3 ). The plasma gas  108  may be produced by passing argon (Ar) through liquid boron tribromide (BBr 3 ). The plasma gas  108  may be produced by heating solid antimony (Sb) to a liquid state in a separate chamber and passing argon over the liquid. The plasma gas  108  may be produced by heating solid phosphorus (P) to a liquid state in a separate chamber and passing argon over the liquid. The plasma gas  108  may be produced by heating solid gallium (Ga) to a liquid state in a separate chamber and passing argon over the liquid. 
     Referring to FIG. 3, the reference numeral  40  designates, in general, an embodiment of an apparatus for melting spherical semiconductors. The apparatus includes a drop tube  42 . The drop tube  42  includes an aperture  44  so that a cooling gas  112  may flow therein. In some embodiments, a heater  46  is placed adjacent to the drop tube  42 . 
     In operation, the heater  46  melts the spherical semiconductors  104 . The heater  46  may be, for example, an infrared lamp, a resistance furnace, an electromagnetic radiation heater or a laser. As the spherical semiconductors  104  cool, they solidify to form crystals  104 a. The cooling gases  112  may be, for example, helium, hydrogen, argon, or nitrogen to facilitate the cooling of the crystals  104   a . The cooling gases  112  may also be used to control the rate of descent of the spherical semiconductors  104 . The drop tube  42  may be stainless steel with an electro-polished inside finish, about five to ten centimeters in diameter and about ten meters in length. 
     Referring also to FIG. 2, the apparatus  40  may be used with the method  100 . At step  102 , the spherical semiconductors  104  are placed in the drop tube  42 . At step  106 , the heater  46  melts the spherical semiconductors  104  to the molten state. At step  110 , the cooling gas  112  flows at atmospheric pressure through the aperture  44  and into the drop tube  42 . 
     In a preferred embodiment, the cooling gas  112  may be a mixture of a gas phase dopant and an inert gas. The cooling gas  112  may be, for example, a mixture of phosphine (PH 3 ) and argon (Ar). The amount of gas phase dopant in the cooling gas  112  is chosen to maximize the amount of the gas phase dopant in the cooling gas  112 . The molten spherical semiconductor  104  absorbs the dopant. As the spherical semiconductors  104  free fall through the drop tube  42 , they solidify to form crystals  104   a . The crystals  104   a  include the dopant in the lattice of the crystals  104   a.    
     In some embodiments, the apparatus  40  controls the rate at which the crystals  104   a  cool. This may occur by many different methods. For example, the cooling gases  112  may be heated. 
     There are various embodiments of the cooling gas. The cooling gas  112  may be a mixture of diborane (B 2 H 6 ) and argon (Ar). The cooling gas  112  may be produced by passing argon (Ar) through liquid phosphorus oxychloride (POCl 3 ). The cooling gas  112  may be produced by passing argon (Ar) through liquid boron tribromide (BBr 3 ). The cooling gas  112  may be produced by heating solid antimony (Sb) to a liquid state in a separate chamber and passing argon over the liquid. The cooling gas  112  may be produced by heating solid phosphorus (P) to a liquid state in a separate chamber and passing argon over the liquid. The cooling gas  112  may be produced by heating solid gallium (Ga) to a liquid state in a separate chamber and passing argon over the liquid. 
     Referring to FIG. 4, the reference numeral  50  designates, in general, an embodiment of an aerodynamic chamber for melting spherical semiconductors. The apparatus includes a cone  52  and a drop tube  54 . The drop tube  54  includes an aperture  56  so that a levitating gas  114  may flow therein. The levitating gas  114  levitates the spherical semiconductors  104 . A heater  58  is connected to the cone  52 . 
     In operation, the heater  58  melts the spherical semiconductors  104 . The heater  58  may be, for example, an infrared lamp, a resistance furnace, an electromagnetic radiation heater, or a laser. As the spherical semiconductors  104  cool, they solidify to form crystals  104   a . The levitating gas  114  may be, for example, helium, hydrogen, argon, or nitrogen to facilitate the cooling of the crystals  104   a . The levitating gas  114  may also be used to control the rate of descent of the spherical semiconductors  104 . The drop tube  54  may be stainless steel with an electro-polished inside finish, about five to ten centimeters in diameter and about ten meters in length. The cone  52  may be stainless steel with an electro-polished inside finish, about five to ten centimeters in diameter. 
     Referring also to FIG. 2, the apparatus  50  may be used with the method  100 . At step  102 , the spherical semiconductors  104  are placed in the cone  54 . The levitating gas  114  flows through the aperture  56  to levitate the spherical semiconductors  104 . At step  106 , the heater  58  melts the spherical semiconductors  104  to the molten state. At step  110 , the levitating gas  114  flows at atmospheric pressure through the aperture  56  and into the drop tube  54 . 
     In a preferred embodiment, the levitating gas  114  may be a mixture of a gas phase dopant and an inert gas. The levitating gas  114  may be, for example, a mixture of phosphine (PH 3 ) and argon (Ar). The amount of gas phase dopant in the levitating gas  114  may be chosen to maximize the amount of the gas phase dopant in the levitating gas  114 . The molten spherical semiconductor  104  absorbs the dopant. As the spherical semiconductors  104  free fall through the drop tube  54 , they solidify to form crystals  104   a . The crystals  104   a  include the dopant in the lattice of the crystals  104   a.    
     In an alternate embodiment, the levitating gas  114  may be a mixture of diborane (B 2 H 6 ) and argon (Ar). Alternatively, the levitating gas  114  may be produced by passing argon (Ar) through liquid phosphorus oxychloride (POCl 3 ). The levitating gas  114  may be produced by passing argon (Ar) through liquid boron tribromide (BBr 3 ). The levitating gas  114  may be produced by heating solid antimony (Sb) to a liquid state in a separate chamber and passing argon over the liquid. The levitating gas  114  may be produced by heating solid phosphorus (P) to a liquid state in a separate chamber and passing argon over the liquid. The levitating gas  114  may be produced by heating solid gallium (Ga) to a liquid state in a separate chamber and passing argon over the liquid. 
     In an alternate embodiment, the levitating gas  114  may be argon gas and the cone  52  may be lined with a solid dopant. As the cone  52  is heated by the heater  58 , the vapor pressure of the solid dopant increases above the cone  52 . The solid dopant vapor may be brought into contact with the semiconductor spheres  104  by the levitating gas  114 . The solid dopant may be, for example, boron nitride (BN), phosphorous (P), or antimony (Sb). In a preferred embodiment, the solid dopant may be boron nitride (BN). 
     Referring to FIG. 5, the reference numeral  60  designates, in general, an embodiment of an acoustic chamber for melting spherical semiconductors. The apparatus includes a chamber  62  and a sound wave producing apparatus  64 . The chamber  62  includes one or more apertures  66  so that a dopant gas  116  may flow therein. A heater  68  may be connected to the chamber  62 . 
     In operation, the sound wave producing apparatus  64  levitates the spherical semiconductors  104  by sound waves. The sound wave producing apparatus  64  may be any commercially available apparatus that produces sound waves. The heater  68  melts the spherical semiconductors  104 . The heater  68  may be, for example, an infrared lamp, a resistance furnace, an electromagnetic radiation heater, or a laser. As the spherical semiconductors  104  cool, they solidify to form crystals  104   a . The dopant gas  116  may be, for example, helium, hydrogen, argon, or nitrogen to facilitate the cooling of the crystals  104   a.    
     Referring also to FIG. 2, the apparatus  60  may be used with the method  100 . At step  102 , the spherical semiconductors  104  are placed in the chamber  62 . The dopant gas  116  flows through the apertures  66 . The sound wave producing apparatus  64  produces sound waves that levitate the spherical semiconductors  104 . At step  106 , the heater  68  melts the spherical semiconductors  104  to the molten state. At step  110 , the dopant gas  116  flows at atmospheric pressure through the aperture  66  and into chamber  62 . 
     In a preferred embodiment, the dopant gas  116  may be a mixture of a gas phase dopant and an inert gas. The dopant gas  116  may be, for example, a mixture of phosphine (PH 3 ) and argon (Ar). The amount of gas phase dopant in the dopant gas  116  may be chosen to maximize the amount of the gas phase dopant in the dopant gas  116  so that the molten spherical semiconductor  104  absorbs the dopant. As the spherical semiconductors  104  cool in the chamber  62 , they solidify to form crystals  104   a . The crystals  104   a  include the dopant in the lattice of the crystals  104   a.    
     In an alternate embodiment, the dopant gas  116  may be a mixture of diborane (B 2 H 6 ) and argon (Ar). The dopant gas  116  may be produced by passing argon (Ar) through liquid phosphorus oxychloride (POCl 3 ). Alternatively, the dopant gas  116  may be produced by passing argon (Ar) through liquid boron tribromide (BBr 3 ). The dopant gas  116  may be produced by heating solid antimony (Sb) to a liquid state in a separate chamber and passing argon over the liquid. The dopant gas  116  may be produced by heating solid phosphorus (P) to a liquid state in a separate chamber and passing argon over the liquid. The dopant gas  116  may be produced by heating solid gallium (Ga) to a liquid state in a separate chamber and passing argon over the liquid. 
     Referring to FIG. 6, the reference numeral  70  designates, in general, an embodiment of a chamber for the doping of spherical semiconductors. The apparatus includes a chamber  72  and an electromagnetic radiation apparatus  74 . The chamber  72  includes one or more apertures  76  so that a dopant gas  118  may flow therein. 
     In operation, the electromagnetic radiation apparatus  74  levitates the spherical semiconductors  104  by electromagnetic radiation. The electromagnetic radiation apparatus  74  may be any commercially available apparatus that produces electromagnetic radiation. The electromagnetic radiation apparatus  74  also melts the spherical semiconductors  104 . As the spherical semiconductors  104  cool, they solidify to form crystals  104   a . The dopant gas  118  may be, for example, helium, hydrogen, argon, or nitrogen to facilitate the cooling of the crystals  104   a.    
     Referring also to FIG. 2, the apparatus  70  may be used with the method  100 . At step  102 , the spherical semiconductors  104  are placed in the chamber  72 . The dopant gas  118  flows through the apertures  76 . The electromagnetic radiation apparatus  74  produces electromagnetic radiation that levitates the spherical semiconductors  104 . At step  106 , the electromagnetic radiation apparatus  74  melts the spherical semiconductors  104  to the molten state. At step  110 , the dopant gas  118  flows at atmospheric pressure through the aperture  76  and into chamber  72 . 
     In a preferred embodiment, the dopant gas  118  may be a mixture of a gas phase dopant and an inert gas. The dopant gas  118  may be, for example, a mixture of phosphine (PH 3 ) and argon (Ar). The amount of gas phase dopant in the dopant gas  118  may be chosen to maximize the amount of the gas phase dopant in the dopant gas  118 . The molten spherical semiconductor  104  absorbs the dopant. As the spherical semiconductors  104  cool in the chamber  72 , they solidify to form crystals  104   a . The crystals  104   a  include the dopant the lattice of the crystals  104   a . The dopant gas  118  may be a mixture of diborane (B 2 H 6 ) and argon (Ar). The dopant gas  118  may be produced by passing argon (Ar) through liquid phosphorus oxychloride (POCl 3 ). The dopant gas  118  may be produced by passing argon (Ar) through liquid boron tribromide (BBr 3 ). The dopant gas  118  may be produced by heating solid antimony (Sb) to a liquid state in a separate chamber and passing argon over the liquid. The dopant gas  118  may be produced by heating solid phosphorus (P) to a liquid state in a separate chamber and passing argon over the liquid. The dopant gas  118  may be produced by heating solid gallium (Ga) to a liquid state in a separate chamber and passing argon over the liquid. 
     Several advantages result from the above-described embodiments. For one, the material seldom, if ever, comes in physical contact with any other device or any part of the apparatuses  10 ,  40 ,  50 ,  60 , or  70 . By melting the material without physical contact, less contaminants are introduced. Also, as the material drops or is levitated, it becomes spherical due to surface tension. Furthermore, single crystals can be formed either through controlled melting and cooling of the material, or by injecting a single crystal seed into the material. Further still, many crystals can be processed in rapid succession. 
     It is understood that several variations may be made in the foregoing. Other modifications, changes and substitutions are also intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.