Abstract:
The present invention overcomes the limitations of Siemens reactors by providing for the deposition reaction to occur inside of a sealed crucible rather than inside of the overall cavity of a water-cooled reactor. The crucible itself is positioned inside of a cartridge reactor, which can have heat shields between crucible and the reactor walls to significantly reduce radiant energy losses. Additionally, the ratio of deposition surface area to cavity volume in the crucible is much higher than that in the ratio of rod deposition surface area to overall cavity volume in Siemens reactors, which results in a much higher contact percentage of gas molecules with the deposition surfaces. This in turn results in a much higher actual conversion ratio of material in the gas to material on the deposition surfaces.

Description:
[0001]    The present patent application incorporates by reference in its entirety U.S. patent application Ser. No. 12/597,151 (the “&#39;151 patent application”), Deposition of high-purity silicon via high-surface-area gas-solid or gas-liquid interfaces and recovery via liquid phase, filed Oct. 22, 2009. This application also incorporates by reference in its entirety the co-pending application entitled: DEPOSITION CARTRIDGE FOR PRODUCTION OF MATERIALS VIA THE CHEMICAL VAPOR DEPOSITION PROCESS filed concurrently herewith (whose application Ser. No. ______ will be added once known). The present patent application also claims benefit of U.S. provisional patent application No. 61/504,148 (the “&#39;148 provisional patent application”), Deposition cartridge for production of high-purity amorphous and crystalline silicon and other materials, filed Jul. 1, 2011, and provisional patent application No. 61/504,145 (the “&#39;145 provisional patent application”), filed Jul. 1, 2011, Cartridge reactor for production of high-purity amorphous and crystalline silicon and other materials, which are both hereby incorporated herein in their entireties. In the &#39;151 patent application, the term “deposition plates” is defined as the surfaces upon which the silicon is deposited, but for the purposes of enhanced clarity when describing actual physical components in this patent application, a “deposition surface” is defined as a surface upon which materials are deposited and a “deposition plate” is defined as an actual physical flat plate (an object with significantly larger surface areas on its sides relative to its edges) upon which materials are deposited, preferably on both sides as well as one or more edges. Thus the sides and edges of a deposition plate are deposition surfaces. The term “deposition cartridge” is defined as the combination of distribution rods and a solid deposition plate or as simply a meander patterned deposition plate, either of which can incorporate an insulative layer or spacer. The term “Siemens reactor” is defined as a deposition reactor that has originally been designed to utilize seed rods. 
     
    
     BACKGROUND 
       [0002]    The &#39;151 patent application describes the limitations of Siemens reactors as including:
       1. The low average surface area of the polysilicon rods which results in a low volumetric deposition rate and hence low Siemens reactor productivity (as measured by the mass of polysilicon produced over a given period of time, typically metric tons per year)   2. The low ratio of surface area to volume of the polysilicon rods, which results in high energy consumption in order to maintain the surface temperature required to achieve deposition for the extended period of time required to achieve a meaningful deposition volume.   3. The labor-intensive and contamination-prone nature of the rod harvesting process       
 
         [0006]    The invention described in the &#39;151 patent application overcomes these limitations by providing high-surface-area electrically heated deposition plates. Silicon is deposited at a high volumetric rate onto these plates through the CVD process and then recovered by additional heating of the plates. The additional heating causes a very thin layer of the deposited polysilicon at the plate interfaces to liquefy and the solid crust of deposited polysilicon can be pulled away from the plates either mechanically or by gravity. Using large-sized plates in a Siemens reactor increases the productivity of the reactor relative to using conventional seed rods whereas using smaller-sized plates reduces the energy consumption of the reactor while maintaining the same productivity relative to using seed rods. However, further limitations of Siemens reactors remain, including but not limited to:
       1. High radiant energy loss from the rods to the reactor walls which must be cooled in order to prevent deposition of polysilicon onto the walls in addition to the rods   2. Low contact percentage of deposition gas molecules with the deposition surface area due to the low ratio of deposition surface area to reactor overall cavity volume. The low actual conversion ratio of silicon in the gas to silicon on the rods, relative to the theoretical conversion ratio, which is governed by reaction equilibria, is the result of low contact percentage.       
 
       SUMMARY 
       [0009]    The present invention overcomes the limitations of Siemens reactors described above by providing for the deposition reaction to occur inside of a sealed crucible rather than inside of the overall cavity of a water-cooled reactor. Deposition onto the inner walls of the reactor is undesirable as it results in loss of the material to be produced, whereas deposition onto the inner walls of the crucible is actually desirable as it increases the volumetric deposition rate due to the addition of deposition surface area. The crucible itself is positioned inside of a cartridge reactor, which can have heat shields between the crucible and the reactor walls to significantly reduce radiant energy losses. Typically up to 60-70% of the energy used by Siemens reactors is lost to their unshielded water-cooled walls. 
         [0010]    Additionally, the ratio of deposition surface area to cavity volume in the crucible is much higher than that in the ratio of rod deposition surface area to overall cavity volume in Siemens reactors, which results in a much higher contact percentage of gas molecules with the deposition surfaces. This in turn results in a much higher actual conversion ratio of material in the gas to material on the deposition surfaces. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  shows an elevation section of one preferred embodiment of the main components of the cartridge reactor 
           [0012]      FIG. 2  shows plan sections of one preferred embodiment of the main components of the cartridge reactor 
           [0013]      FIG. 3  shows a perspective of one preferred embodiment of deposition cartridges for the cartridge reactor 
           [0014]      FIG. 4  shows an elevation section of one preferred embodiment of the cartridge reactor with the bottom assembly lowered and the crucible being loaded 
           [0015]      FIG. 5  shows an elevation section of one preferred embodiment of the cartridge reactor with the bottom assembly raised and the reactor pressurized with inert gas 
           [0016]      FIG. 6  shows an elevation section of one preferred embodiment of the cartridge reactor with the crucible raised and the deposition cartridges preheated 
           [0017]      FIG. 7  shows an elevation section of one preferred embodiment of the cartridge reactor during the deposition sequence 
           [0018]      FIG. 8  shows an elevation section of one preferred embodiment of the cartridge reactor during directional solidification with inert gas in the reactor 
           [0019]      FIG. 9  shows an elevation section of one preferred embodiment of the cartridge reactor during cool down and air purge 
           [0020]      FIG. 10  shows an elevation section of one preferred embodiment of the cartridge reactor with the bottom assembly lowered and the crucible being unloaded 
           [0021]      FIG. 11  shows a side elevation section of one preferred embodiment of the reactor top assembly 
           [0022]      FIG. 12  shows a front elevation section of one preferred embodiment of the reactor top assembly 
           [0023]      FIG. 13  shows a plan section (looking up) of one preferred embodiment of the reactor top assembly 
           [0024]      FIG. 14  shows a side elevation section of one preferred embodiment of the crucible during deposition showing gas flow patterns 
           [0025]      FIG. 15  shows a plan section of one preferred embodiment of the crucible after deposition showing material crusts 
       
    
    
     DESCRIPTION 
       [0026]    The main components of one preferred embodiment of the cartridge reactor  50  for the production of materials via the CVD process are shown in  FIG. 1 . In this embodiment, the reactor top assembly  1  functions to support the deposition cartridges  2  (which are described in the &#39;148 and &#39;145 patent applications and in the DEPOSITION CARTRIDGE FOR PRODUCTION OF MATERIALS VIA THE CHEMICAL VAPOR DEPOSITION PROCESS application filed concurrently herewith), distribute the deposition gas mix over the deposition surfaces of the deposition cartridges  2 , remove the vent gas, and to affect heat exchange between the vent gas and the deposition gas mix. The array of deposition cartridges  2  preferably has a square plan section if the desired final product is multicrystalline material or preferably a circular plan section if the desired final product is monocrystalline material. The reactor top assembly  1  is attached to the reactor middle assembly  3  by the reactor flanges  9  which incorporate an airtight seal. The reactor middle assembly  3  houses a crystallization heater  4 . The reactor bottom assembly  6 , which can be raised to and lowered from the reactor middle assembly  3 , houses a crucible pedestal  5  which is equipped with a bottom cooler  10  for cooling the crucible during directional solidification and which is capable of vertical travel. All assemblies of the reactor incorporate heat shields to minimize radiant energy losses. 
         [0027]    As shown in  FIG. 2 , the reactor walls  35  of the reactor top assembly  1 , reactor middle assembly  3 , and reactor bottom assembly  6  are preferably circular in plan section, and they are also preferably water cooled. The plan sections of the heat shields  13 , array of deposition cartridges  2 , crystallization heater  4 , and bottom cooler  10  are preferably square if multicrystalline material is desired and preferably circular if monocrystalline material is desired. 
         [0028]      FIG. 3  shows a perspective of one preferred embodiment of the array of deposition cartridges  2  that are fitted to the reactor top assembly. The deposition cartridges  2  are connected, by their electrode tabs  53 , to the distribution bar  32  by electrode brackets  57 . There are  16  deposition cartridges  2  which are spaced approximately 5 cm apart and which have a height of approximately 42 cm and a length of approximately 75 cm. Assuming a deposition crust thickness of approximately 2 cm on the deposition cartridges  2  and on the inner walls of the crucible, the array of deposition cartridges  2  in this preferred embodiment is designed to fit inside of an 85 cm by 85 cm crucible typically used for the crystallization of deposition materials, including but not limited to polysilicon. 
         [0029]    This preferred embodiment of the cartridge reactor  50  is operated in the following preferred seven steps:
       1. A crucible-loading step is shown in  FIG. 4 . Preferably, the reactor bottom assembly  6  is lowered and the crucible  11 , which is preferably quartz, is precisely positioned onto the crucible pedestal  5 .   2. An inert gas purge step is shown in  FIG. 5 . Preferably, the reactor bottom assembly  6  is raised and the airtight reactor flanges  6  of the reactor bottom assembly and of the reactor middle assembly  3  are sealed. The reactor cavity is purged with an inert gas, preferably nitrogen, using the reactor gas inlets  18  and the reactor top assembly  1  gas inlets and outlets. Preferably, the cartridge reactor  50  is also brought up to operating pressure, (preferably in the range of 6 bar).   3. A preheating step is shown in  FIG. 6 . Preferably, the crucible pedestal  5  is raised so that the top edges of the crucible  11  press against the gas seal  19  and form an airtight seal. Preferably, the deposition cartridges  2  are then electrically preheated to the optimal deposition temperature, which is preferably in the range of 850° C. to 1,150° C. when the deposition material is polysilicon. Heat shields  13  in the cartridge reactor  50  minimize radiant energy losses and minimize the cooling duty of the water-cooled reactor walls  35 .   4. A deposition sequence step is shown in  FIG. 7 . Preferably, the deposition gas mix, which is preferably trichlorosilane and hydrogen or monosilane when the deposition material is polysilicon, is pumped into the crucible  11  from gas inlets in the reactor top assembly  1  while inert gas, which is preferably nitrogen, is maintained in the rest of the reactor cavity outside of the crucible. Preferably, for safety, the inert gas is at a slightly higher pressure than the deposition gas so that in the unlikely event of a leak in the gas seal  19 , inert gas will leak into the crucible  11  rather than flammable deposition gas mix leaking outside of the crucible  11 . Alternatively in this preferred embodiment, if there is a leak in the reactor flanges  9 , inert gas will leak outside of the cartridge reactor  50  rather than flammable deposition gas mix leaking outside of the cartridge reactor  50 , which is an additional safety improvement over Siemens reactors. The gas seal  19  is preferably chosen to withstand relatively high temperatures, for which there are preferred seal materials, such as carbon-based materials, but the gas seal preferably experiences a relatively small pressure differential. Preferably, the deposition gas mix that is pumped into the crucible  11  comes into contact with the heated deposition surfaces of the deposition cartridges  2 , undergoes the deposition reaction, converts into the vent gas and is removed through gas outlets in the reactor top assembly  1 . In this preferred embodiment, this process continues until a material crust  14  has accumulated on the deposition surfaces such that most of the void volume inside the crucible  11  is filled. At this point, both the inside and outside of the crucible  11  are purged with a suitable inert gas, preferably argon, and preferably a vacuum is drawn both inside and outside of the crucible  11 . Then, the deposition surfaces are further heated to or above the melting point of the material, causing a thin layer of the material at the deposition surfaces of the deposition cartridges  2  to liquefy and the material crust to detach from the deposition cartridges  2 .   5. A crystallization step is shown in  FIG. 8 . Preferably, the crucible pedestal  5  carrying the crucible  11  and the material crust  14  is lowered into the reactor middle assembly  3 , and the material crust  14  is further heated by the crystallization heater  4  until it becomes liquid material  15 . Preferably, the heat shields  13  can incorporate a reflective layer to minimize radiant energy losses and an insulating layer outside of the reflective layer to minimize convective and conductive energy losses. In this preferred embodiment, directional solidification is achieved through one or more means including activation of the bottom cooler  10 , control of the crystallization heater  4 , and/or movement of the crucible pedestal  5  away from the crystallization heater  4 . During this crystallization step, the rotating heat shield  12  is closed to provide insulation over the top of the crucible  11  in order to minimize energy losses. The solidification front  16  moves upward through the liquid material  15 , forming a crystalline material ingot  17  behind it. In another preferred embodiment of the above crystallization step, the material crust  14  can be fully melted by the deposition cartridges  2  while the crucible  11  is still in the fully raised position. The crucible  11  can then be lowered in a controlled manner while the deposition cartridges  2  continue to heat the liquid silicon and the bottom cooler  10  is activated to initiate directional solidification. This preferred embodiment has the potential to accelerate the crystallization process as well as produce higher quality crystalline silicon by keeping the solidification front  16  more planar. Both preferred embodiments described above result in the production of multicrystalline material and a square plan section geometry for the array of deposition cartridges  2 , the crucible  11 , and the bottom cooler  10  is preferred. However, in another preferred embodiment, if this plan section geometry is circular and a rotating puller rod is introduced from the reactor top assembly  1  into the liquid material  15 , a monocrystalline ingot can also be produced, by the Czochralski crystallization process. Finally, in another preferred embodiment, this entire crystallization step can be omitted and the cartridge reactor  50  can be used to produce just amorphous material in crucibles for further processing elsewhere.   6. A cool down and air purge step is shown in  FIG. 9 , where the vacuum is replaced with circulating inert gas, preferably argon, for convective cooling. After sufficient cooling of the crucible to facilitate subsequent handling, the inert gas is purged with air in preparation for unsealing and lowering the reactor bottom assembly  6 . In the preferred embodiment where the crystallization step is omitted, cooling of the crucible  11  and material crust  14  can also be omitted so that energy consumption in subsequent processing steps can be minimized as applicable.   7. A crucible-unloading step is shown in  FIG. 10 . Preferably, the reactor bottom assembly is unsealed and lowered and the crucible  11  with the crystalline material ingot  17  is unloaded.       
 
         [0037]    A feature of the preferred embodiment of the cartridge reactor  50  is the effective distribution and preheating of the deposition gas mix that is achieved in the reactor top assembly  1 . In  FIG. 11 , which is a side elevation section of the reactor top assembly  1 , the deposition gas mix enters into the deposition gas mix inlet manifold  29  through the deposition gas mix inlet  20 . In this preferred embodiment, the deposition gas mix is routed into a multiplicity of deposition gas mix inlet nozzles  24  which extend downward and open at the bottom surface of the reactor top assembly  1  directly above the deposition cartridges  2 . The deposition gas mix shoots out through each deposition gas mix nozzle  24 , travels downward between the deposition cartridges  2 , and strikes the bottom of the crucible  11 . The blocking effect of adjacent streams of deposition gas mix striking the bottom of the crucible  11  minimizes the lateral spread of the deposition gas mix and forces it to flow predominantly back up, preferably in a swirling or turbulent motion, between the deposition gas mix exiting the deposition gas mix nozzle  24 , preferably in a downward stream, and the deposition cartridges  2  (see also  FIGS. 12 ,  13 , and particularly  14 ). This turbulent flow preferably results in more complete contacting of the deposition gas mix with the deposition cartridges  2  and hence more complete conversion of the material in the deposition gas mix to material on the deposition surfaces. 
         [0038]    In this preferred embodiment, the vent gas continues to travel upward where it is removed through a vent gas outlet annulus  25  which surrounds the deposition gas mix inlet nozzle  24  and which is the only escape route. This heated vent gas traveling upward through the vent gas outlet annulus  25  heats the deposition gas mix traveling downward through the deposition gas mix nozzle  24  within. It also heats the cooling water traveling outside of the vent gas outlet annulus in the vent gas aftercooler  26 . Other preferred embodiments of the deposition gas mix distribution pattern include individual alternating inlet and outlet nozzles or rows of alternating inlet and outlet nozzles. 
         [0039]    The vent gas is collected into a single stream from the multiplicity of vent gas outlet annuli  25  in the vent gas outlet manifold  27  and exits the reactor top assembly through the vent gas outlet  22 . Meanwhile, cooling water that has been heated in the vent gas aftercooler  26  flows on to the deposition gas mix preheater  28  where it provides initial heating to the deposition gas mix that has just entered the deposition gas mix inlet nozzles  24 . This cooling water then exits the reactor top assembly  1  through the cooling water outlet  21 . 
         [0040]      FIGS. 11 and 13  show one preferred embodiment of the reactor top assembly  1  with the positioning of the deposition gas mix inlet nozzles  24  directly above the gap between the deposition cartridges  2  which are attached to the distribution bars  32 .  FIGS. 11 and 13  also show the deposition cartridges  2  electrically connected in parallel via the distribution bars  32 , which themselves are is connected to an electrical power supply via the distribution bar electrode  31  which forms an electrically insulated airtight seal against the side wall of the vent gas aftercooler  26 . In another preferred configuration, the electrode tabs  53  or electrode brackets  57  can be extended up and out through the top of the reactor top assembly  1  through insulated steel tubes and can be connected to the power supply at a point on top of the reactor top assembly  1 . 
         [0041]    A preferred embodiment of the crucible  11  after deposition and separation from the deposition cartridges  2  is shown in  FIG. 15 . Material which has deposited onto the inside walls of the crucible  11  and the deposition surfaces of the deposition cartridges  2  fills most of the volume of the crucible and narrow deposition cartridge voids  36  remain in place of the deposition cartridges  2 . 
         [0042]    The preferred benefits of the cartridge reactor over Siemens reactors are:
       1. Faster volumetric deposition rate due to higher surface area for deposition   2. Higher actual conversion rate of material in the deposition gas mix to material on the deposition surfaces resulting from higher ratio of deposition surface area to deposition gas mix containment volume and more complete contacting of deposition gas mix with deposition surfaces made possible by the combination of the deposition cartridge geometry and the gas inlet nozzle geometry   3. Energy savings due to minimized radiant heat loss arising from the deposition cartridge geometry. The majority of radiant heat emitted from heated deposition surfaces is absorbed by adjacent deposition surfaces.   4. Energy savings due to minimized radiant, conductive, and convective heat loss to water-cooled reactor walls. Since deposition occurs inside a sealed crucible, the reactor walls outside the crucible can be blocked by heat shields.   5. Energy savings due to melting, for crystallization, of material from deposition temperature rather than from ambient temperature. Whether crystallization occurs in the cartridge reactor or in separate crystallization equipment, the material is already in the crucible and does not need to be handled directly and therefore does not need to be cooled to ambient temperature.   6. Elimination of contamination of material from handling and elimination of operations for reduction of contamination from handling, such as acid etching   7. Elimination of operations to crush material into manageable-sized chunks for loading into crucibles   8. Faster and higher quality crystallization due to controlled withdrawal of the deposition cartridges from the melted silicon.   9. Plant savings due to more complete conversion of deposition gas mix into vent gas and therefore less vent gas to process downstream of the cartridge reactor   10. Simplified electrical system composed of a single electrode pair for connecting the deposition cartridges in parallel or in series, as compared to an individual electrode pair for each rod pair in a Siemens reactor   11. Increased safety due to flammable deposition gas mix being sealed inside the additional walls of the crucible and inert gas in the reactor cavity being maintained at slightly higher pressure than the deposition gas mix in the crucible   12. Easily scalable design. Simply increasing the plan cross-section of the cartridge reactor to include a higher number of deposition gas mix inlet nozzles and a higher number of longer deposition cartridges, and also increasing the height of the deposition cartridges can significantly increase the production capacity of the cartridge reactor without major reengineering of the rest of the cartridge reactor. Easily scalable directional solidification, which would be a challenge with external heating of the melted material can be achieved through heating and controlled withdrawal of the deposition cartridges from the melted silicon.