Abstract:
A system and method for performing diffusion on a three-dimensional substrate is provided. The system includes a furnace for providing a doped (e.g., p-type) molten semiconductor material and a dropper for converting the molten semiconductor material into a series of uniformly sized droplets. The droplets are then provided to a first tube where they solidify into a semiconductor crystals. The semiconductor crystals are then heated for a predetermined period of time until an outer layer of the semiconductor crystals is melted. The melted outer layer can then be doped (e.g., n-type) and then allowed to re-solidify. As a result, a plurality of spherical shaped p-n devices is created.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to semiconductor devices, and more particularly, to an apparatus and method for performing diffusion on a device such as a spherical-shaped semiconductor diode. 
     Conventional integrated circuits, or “chips,” are formed from a flat surface semiconductor wafer. The semiconductor wafer is first manufactured in a semiconductor material manufacturing facility and is then provided to a fabrication facility. At the latter facility, several layers are processed onto the semiconductor wafer surface. Once completed, the wafer is then cut into one or more chips and assembled into packages. Although the processed chip includes several layers fabricated thereon, the chip still remains relatively flat. 
     Manufacturing the wafers requires creating rod-form polycrystalline semiconductor material; precisely cutting ingots from the semiconductor rods; cleaning and drying the cut ingots; manufacturing a large single crystal from the ingots by melting them in a quartz crucible; grinding, etching, and cleaning the surface of the crystal; cutting, lapping and polishing wafers from the crystal; and heat processing the wafers. Moreover, the wafers produced by the above process typically have many defects. These defects can be attributed to the difficulty in making a single, highly pure crystal due to the cutting, grinding and cleaning processes as well as impurities associated with containers used in forming the crystals. These defects become more and more prevalent as the integrated circuits formed on these wafers contain smaller and smaller dimensions. 
     In U.S. Pat. No. 5,955,776, which is hereby incorporated by reference, a method and apparatus for manufacturing spherical-shaped semiconductor integrated circuit devices is disclosed. Although certain systems and methods for performing various processing operations are discussed in the above-referenced patent, it is desired to further improve on the operations. For example, in making a p-n junction diode, a first type (e.g. n-type) outer layer is diffused onto a second type (e.g., p-type) spherical shaped semiconductor substrate. It is desired that both the outer layer and the inner substrate are maintained at an appropriate shape, thickness, and diffusion concentration. 
     U.S. Pat. Ser. Nos. 09/490,650 and 09/489,782, which are hereby incorporated by reference, provide improved methods for doping material on a spherical shaped substrate in a non-contact environment. These methods can be used to make spherical p-n junction diodes for solar cell applications. It is desired, however, to make uniform sized spherical p-n diodes in a continuous operation (e.g., a single step). 
     SUMMARY OF THE INVENTION 
     The present invention, accordingly, provides a system and method for performing diffusion on a three-dimensional substrate. In one embodiment, the system includes a furnace for providing a doped (e.g., p-type) molten semiconductor material and a dropper for converting the molten semiconductor material into a series of uniformly sized-droplets. The droplets are then provided to a first tube where they solidify into semiconductor crystals. 
     The semiconductor crystals are then heated for a predetermined period of time until an outer layer of the semiconductor crystals is melted. The melted outer layer can be doped (e.g., n-type) using liquid state diffusion, and then allowed to re-solidify. As a result, a plurality of spherical shaped p-n devices is created. 
     In some embodiments, the semiconductor crystals are polished before they are melted. The polishing helps to remove deformities and better insure that the outer layer is of a desired thickness. 
     In some embodiments, the dropper utilizes a vibrating nozzle. 
     One embodiment of the method for making a p-n junction on a three-dimensional substrate includes forming a solid spherical shaped semiconductor crystal of a first dopant type. An outer layer of the spherical shaped semiconductor crystal is then melted to a predetermined thickness. A second dopant type can then be provided to the melted outer layer to be diffused into the outer layer. As a result, the doped and melted outer layer can be solidified to form the a p-n junction device. 
     Therefore, what is provided is an improved system and method for performing diffusion on a three-dimensional substrate. In the present example, the system and method can be used to make spherical shaped diodes with a uniform layer thickness in a single step operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a side, cut-away view of a processor according to one embodiment of the invention. 
     FIG. 2 is an extended view of the processor of FIG.  1 . 
     FIGS. 3 a - 3   d  illustrate different stages of a semiconductor device being processed by the processor of FIGS.  1  and  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present disclosure relates to semiconductor processing. It is understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components, sizes, and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to limit the invention from that described in the claims. 
     Referring to FIG. 1, the reference numeral  10  designates, in general, one embodiment of a processor for forming spherical shaped semiconductor substrates. Formation of the substrates may be facilitated in different manners by varying parameters described herein, including repetitive processing through portions of the processor  10 . 
     The processor  10  can be separated into three sections: an input section  12 , a main furnace section  14 , and a drop section  16 . The input section  12  includes a receiver tube  18  for receiving processing materials, such as granules, gases and the like. The receiver tube  18  is about  2  centimeters in diameter and registers with the main furnace section  14 . 
     An enclosure  20  surrounds the main furnace section  14  and supports a general environment for processing. The enclosure is filled with an insulative material  22  to contain the relatively high temperatures produced in the main furnace section. The enclosure  20  and insulative material  22  provide an inert atmosphere, which prevents burnout of the insulation material  22  and other components stored therein. Disposed within the insulative material is a crucible  24 . The crucible serves to hold molten semiconductor material, yet not react with the material. 
     A lid  26  of the crucible  24  connects to the receiver tube  18 . In the present embodiment, the lid is threadably engaged to the receiver tube to facilitate removal and separation of the various components. The lid  26  further maintains the inert atmosphere inside the enclosure. Alternative embodiments may have other types of lids that either temporarily or permanently secure the receiver tube  18  to the crucible  24 . The receiver tube  18  can either batch feed or continuously feed raw semiconductor material from the crucible  24 . For each type of feeding, a different lid  26  may be required. 
     Immediately surrounding the outside of the crucible  24  is a furnace  30 . In the present embodiment, the furnace is a fluid-heat type furnace, although other sources of heat may be used. The furnace  30  includes a fluid nozzle  32  through which the fluid may pass. The fluid nozzle  32  further maintains the inert atmosphere inside the enclosure  20 . Although not shown, another device may be used to heat the fluid before it passes through the fluid nozzle  32 . Also, a heat measurement device  34 , such as a thermocouple, is attached to the furnace  30  for monitoring the temperature of the furnace  30  and of the crucible  24 . 
     The enclosure  20 , along with the crucible  24 , rests-on a support platform  36 . The platform has several apertures to facilitate the various devices and processes herein disclosed. The platform  36  is also able to withstand some of the severe heat that radiates from the furnace  30  while maintaining the inert atmosphere inside the enclosure  20 . 
     Referring also to FIG. 2, attached to a bottom portion of the crucible  24 , as seen in FIG. 1, is a dropper  40 . The dropper  40  may include a nozzle  50  that injects precise sized droplets of molten semiconductor material from the crucible  24  and into the drop section  16 . In one embodiment, the nozzle  50  is further connected to a vibrating plate  52  connected to a piezo-electric (PZT) vibrator  54 . The vibrating plate  52  can be positioned in several different locations, such as a position  52   a  illustrated in FIG. 2 in phantom. The PZT vibrator  54  can be controlled to produce a precise movement, which in turn creates a precise size droplet. Alternatively, or in combination with the nozzle, inert gas may also be applied to facilitate the precise amounts of molten semiconductor material being injected into the drop section  16 . 
     The drop section  16  may be further divided into a first drop section  16   a;  and a second drop section  16   b.  The first drop section  16   a  includes a long drop tube  62 . For example, the drop tube  62  may be stainless steel with an electro-polished inside finish, about five to ten centimeters in diameter and about ten meters in length. The drop tube  62  may include apertures through which a cooling gas  64  may flow. The cooling gas may also include impurities for doping the semiconductor material to a desired level. In some embodiments, a first heater  66  is placed adjacent to the drop tube  62 . The first heater  66  maintains a temperature below the melting point of the semiconductor material. However, the temperature is high enough to slow the cooling process of the semiconductor material to thereby reduce the number of different crystalline growth directions formed during solidification. 
     Connected to the first drop section  16   a  is the second drop section  16   b,  which includes a second heater  70 . The second drop section  16   b  may also be stainless steel with an electro-polished inside finish, about five to ten centimeters in diameter and about ten meters in length. The second heater  70  maintains a temperature above the melting point of the semiconductor material. It is understood, however, that in some embodiments, the first heater  66  does not exist. Therefore, in these embodiments, the second heater  70  is the only heater in the drop section  16 . 
     The second drop section  16   b  also includes an inlet  72  for providing a dopant gas. The dopant gas includes impurities for doping semiconductor material in the drop section  16   b.    
     In some embodiments, the first and second drop sections  16   a,    16   b  are connected to form one single drop tube  16 . In other embodiments, the drop sections  16   a,    16   b  are separated, and material must be transported from the first drop section to the second. 
     In some embodiments, a polishing system  80  is provided between the first and second drop sections  16   a,    16   b.  For example, the polishing system  80  may be a simple “barreling” type of polisher that roughly polishes the outer surface of a spherical substrate. One such polishing system is described in U.S. Pat. No. 5,955,776, which is hereby incorporated by reference. 
     In operation, material  104  is placed into the receiver tube  18 . For the sake of example, the material includes silicon, it being understood that different types of semiconductor material may also be used. The material  104  may also include an inert carrier gas, such as argon, and one or more dopant materials. The material  104  passes through the lid  26  and into the main furnace section  14 . The furnace  30  can produce temperatures of about 1600° C., which far exceed the melting point of silicon (about 1410° C.). This high temperature causes the material  104  to become a molten mass  108 . 
     Referring also to FIG. 3 a,  the nozzle  40  allows droplets  112  of the molten mass to leave the crucible  24  and enter the drop section  16 . The droplets  112  fall down the drop tube  62 . The drop tube will allow the droplets to cool and form a polycrystalline structure. The cooling gases  64  may be, for example, helium, hydrogen, argon, or nitrogen to facilitate the cooling of the droplets. The cooling gases  64  may also be used to control the rate of descent of the droplets  112 . 
     In some embodiments, the processor  10  controls the rate at which the droplets  112  cool. This may occur by many different methods. For example, the cooling gases  64  may be heated. Also, the drop tube  62  may be heated by the heaters  66 . As a result, the droplets  112  will cool very slowly, thereby forming crystals. 
     Referring to also to FIG. 3 b,  as the droplets  112  approach the bottom portion of the first drop section  16   a,  as seen in FIG. 2, the cooled droplets form a solid, spherical shaped crystal substrate  114 . In the present example, the spherical shaped crystal substrate  114  has a diameter of about 1 millimeter. In some embodiments, the substrate  114  is a single crystal, while in other embodiments, the substrate is a polycrystal. The substrate  114  has been doped by one or more of the materials provided in the receiver tube  18 , and/or the cooling gas  64 . For the sake of example, the substrate  114  will be deemed to be doped with P-type material. 
     It is noted that sometimes, a plurality of deformities, such as spikes  116 , are formed during the cooling process. These spikes  116  can be removed by the polishing system  80 . By removing the spikes, later processing of the device can be more precisely controlled. However, as will become more evident in the following discussion, the spikes  116  may eventually be removed from the substrate  114  by later processing. The substrate  114  (polished or not) is then provided into the second drop section  16   b.    
     Referring now to FIG. 3 c,  the relatively high temperature created by the second heater  70  exceeds the melting point of the semiconductor material. As a result, the substrate, now designated with the reference numeral  118 , has a solid core portion  120  and a melted outer layer  122 . The thickness of the melted outer layer  122  can be precisely controlled by the amount of time the substrate  118  is in the second drop section  16   b  and the temperature inside the second drop section. For example, the dopant gas from inlet  72  may serve to float (to control or sustain the descent of) the substrate  118  inside the second drop section  16   b  for a predetermined period of time. In another example, the overall length of the second drop section  16   b  can be precisely determined to accommodate the desired thickness of the melted outer layer  122 . 
     Once the melted outer layer  122  is formed, it is susceptible to receive impurities. These impurities can be provided by the dopant gas from the inlet  72 . In continuance of the previous example, the impurities may be ntype, as contrasted with the p-type core portion  120 . 
     Referring now to FIG. 3 d,  once the impurities have been deposited into the melted outer layer  122 , the substrate, now designated with the reference numeral  124 , can be cooled. Once cooled, a solidified outer layer  126  is formed, having a precise and uniform thickness. In the present example, a P-N diode of precise dimension has thereby been formed. 
     It is understood that several variations may be made in the foregoing. For example, different heating steps may be used in different parts of the processor. Further still, a catcher (not shown) may be included to receive the material and facilitate the heating or cooling process. The catcher may also be used to return the material to a furnace section for additional processing. 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.