Abstract:
Method and apparatus for producing purified bulk silicon from highly impure metallurgical-grade silicon source material at atmospheric pressure. Method involves: (1) initially reacting iodine and metallurgical-grade silicon to create silicon tetraiodide and impurity iodide byproducts in a cold-wall reactor chamber; (2) isolating silicon tetraiodide from the impurity iodide byproducts and purifying it by distillation in a distillation chamber; and (3) transferring the purified silicon tetraiodide back to the cold-wall reactor chamber, reacting it with additional iodine and metallurgical-grade silicon to produce silicon diiodide and depositing the silicon diiodide onto a substrate within the cold-wall reactor chamber. The two chambers are at atmospheric pressure and the system is open to allow the introduction of additional source material and to remove and replace finished substrates.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
       [0001]    This patent application is a divisional application of Ser. No. 09/941,490, filed Aug. 28, 2001, which is a continuation-in-part of U.S. patent application Ser. No. 09/334,166, filed on Jun. 15, 1999, now U.S. Pat. No. 6,281,098. 
     
    
       [0002] The United States Government has rights in this invention pursuant to Contract No. DE-AC36-99GO10337 between the United States Department of Energy and the National Renewable Energy Laboratory, a division of Midwest Research Institute. 
     
    
     
       FIELD OF INVENTION  
         [0003]    The present invention pertains generally to producing silicon feedstock for the semiconductor industry, and more specifically, to purifying metallurgical-grade silicon by means of iodine chemical vapor transport to produce pure silicon feedstock for use in fabricating photovoltaic and other semiconductor devices.  
         BACKGROUND OF INVENTION  
         [0004]    About 85% of the photovoltaic modules sold annually are made from silicon. Manufacturers have repeatedly expressed concern about the future supply of low-cost feedstock as this market continues to grow at a rate exceeding 30% each year. Recent reports project that demand for silicon from the electronics industry will exceed the current supply levels by a factor of 2 to 4 within the next decade. This projection does not represent a fundamental material shortage problem because the technology, quartzite, and coke needed to make feedstock are in abundant supply. Rather, the issue is how best to supply the required feedstock with the requisite purity (˜99.999%) to manufacturers at an acceptable cost. Several methods exist for the manufacture of silicon feedstock that meet at least a portion of the manufacturing sector&#39;s requirements, including the widely used silicon chlorosilane method. However, in general, the existing methods are complicated, generate a significant amount of hazardous by-products, require a vacuum system, and are, therefore, quite expensive.  
           [0005]    A number of new methods are under consideration for the purification of metallurgical-grade silicon (MG-Si), including: (1) repetitive porous MG-Si etching, gettering and surface-removal of impurities; (2) MG-Si gaseous melt-treatment; and (3) MG-Si purification by recrystallization of Si from MG-Si/metal solutions. Many of these potential methods improve upon the deficiencies of the existing techniques, yet most of the above-referenced techniques still contain some of the above-listed drawbacks, including specifically, the level of complexity of the processes used to generate consistent and predictable results, and which also increase the already high costs associated with producing pure feedstock products. Specifically, the porous silicon etch/gettering removal of impurities, although effective in the near surface region, appears impractical for bulk purification because of the large number of process cycles that would be required and that would thus drive up the time and cost needed to produce purified feedstock in sufficient quantities. The gaseous melt treatment using moist argon appears promising for reducing boron levels from the MG-Si source material, but requires much longer treatment times and more efficient exposure to the liquid silicon in order to be cost-efficient at the level required for this specific problem. Finally, the recrystallization of silicon from MG-Si/metal solutions remains essentially theoretical at this time and is not the short-term solution needed to address current commercial concerns.  
         SUMMARY OF INVENTION  
         [0006]    Accordingly, an object of the present invention is to is to provide a high deposition rate process for producing pure silicon feedstock from metallurgical-grade silicon.  
           [0007]    Another object of the present invention is to provide a viable, economical and high through-put method of depositing pure silicon feedstock for solar cells and other applications.  
           [0008]    Yet another object of the present invention is to provide an apparatus by which to produce pure silicon feedstock according to the method of the present invention.  
           [0009]    Additional objects, advantages and novel features of the invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the invention. The objects and the advantages of the invention may be realized and attained by means of the instrumentalities and in combinations particularly pointed out in the appended claims.  
           [0010]    To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the method of this invention may comprise producing pure silicon feedstock by first placing solid metallurgical-grade silicon and solid iodine in the bottom portion of a cold-wall reactor, heating the bottom portion of the cold-wall reactor so as to create a thermal gradient while vaporizing the MG-Si and the iodine, which react chemically to produce SiI 2  precursor, drive a portion of the SiI 2  to a lower temperature and to thereby deposit the silicon upon a substrate within the cold-wall reactor chamber, and, by taking advantage of a variance in the partial pressures of the metal iodides vapors formed, separate the desirable iodides from the undesirable byproduct iodides by condensation of the desirable iodides on surfaces in the reaction chamber, capturing the iodide condensate in the reaction chamber, and transferring the condensate to a distillation chamber. In the distillation chamber, the condensate of desirable iodides is vaporized, and, once again taking advantage of a variance in the partial pressures of the metal iodide vapors formed to further separate residual undesirable iodide condensates from the desirable SiI 4  condensate, collect the SiI 4  condensate, and return it to the cold-wall reaction chamber for further cyclical processing until a desired quantity of pure silicon is deposited on the substrate within the cold-wall reactor chamber that it can be removed and replaced with a new substrate.  
           [0011]    To produce feedstock using the method described herein, the apparatus of this invention may comprise a plurality of interconnected chambers that are at about atmospheric pressure. A first chamber may have a bottom portion, a mid-portion and a top portion, along with a plurality of inlets and a plurality of outlets. A second chamber may also have a bottom portion, a mid-portion and a top portion, as well as an inlet and a plurality of outlets. A third chamber may have an inlet and an outlet. The metallurgical-grade silicon may be deposited in the first chamber along with an amount of iodine source material. The bottom portion of the first chamber may be heated, thus producing a temperature gradient within the first chamber and also vaporizing a portion of the MG-Si and the I. Some of the vaporized material will form SiI 2  which may be deposited upon a substrate in the mid-portion of the first chamber. Additionally, many byproduct metal iodide vapors will be formed, some of which will be separated and removed from the first chamber permanently and some of which will be separated and removed from the first chamber and transferred to the second chamber as liquid condensate.  
           [0012]    The second chamber may also be heated, thus producing another temperature gradient and also vaporizing a portion of the liquid condensate. Some of the vaporized condensate will form a SiI 4  vapor which will be separated from other metal iodide vapors formed, collected, and transferred to a third chamber to be subsequently returned to the first chamber for re-use. The remaining undesirable metal iodide vapors formed will be separated and removed from the second chamber permanently. Other embodiments and variations based upon the above-described process and apparatus, as well as that which will be disclosed in more detail below, will be recognized by those persons skilled in the art. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The accompanying drawings, which are incorporated herein and form a part of the specification illustrate preferred embodiments of the present invention, and together with the description, serve to explain the principles of the invention.  
         [0014]    [0014]FIG. 1 is a cut-away schematic of the apparatus used to practice the process of the present invention.  
         [0015]    [0015]FIG. 2 is a graph showing relative Iodide vapor pressures.  
         [0016]    [0016]FIG. 3 is a graph showing impurity levels in the MG-Si source material and in an epitaxial silicon layer grown by the process and apparatus of the present invention.  
         [0017]    [0017]FIG. 4 is a graph showing diagnostic solar cell parameters for a wafer from a crystal grown using ICVT-purified MG-Si produced by the process and apparatus of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    A pure silicon feedstock can be produced at high deposition rates and at low costs with the process and apparatus disclosed herein. Referring generally to FIG. 1, a silicon purification system  10  according to this invention has three interconnected chambers  20 ,  50 ,  80 . In the first chamber  20 , also called a cold-wall reactor chamber, solid pieces of metallurgical-grade silicon (MG-Si) II and iodine  12  are heated and evaporated in a bottom portion  21  of chamber  20  to create a vapor mixture of silicon and iodine, which react chemically to form SiI 2  gas. The SiI 2  gas rises to a slightly cooler region  22  in the chamber  20 , where it goes through a disproportionation reaction to produce Si and SiI 4 . The Si deposits on a substrate  40 , while the SiI 4  condenses on the interior wall  28  of the chamber  20 , as indicated by droplets  13 . The SiI 4  condensate  13  is collected by an annular catch ring  40  at the bottom portion of region  22  of chamber  20 , which ring  40  routes the condensate  13  to a conduit  29  that is connected to a second chamber  50 , which is also called a distillation chamber.  
         [0019]    The MG-Si feedstock  11  also contains other elements, such as boron (B) and phosphorus (P), which also react with iodine to produce undesirable iodides, such as BI 3  and PI 3 . However, because they have different vapor pressures than the silicon iodides, these undesirable iodides do not enter into the SiI 2  disproportionation process described above, so the B and P do not deposit on the substrate  40 . Instead, they are substantially condensed and drawn off in a still cooler, top portion  23  of the reaction chamber  20 , by an annular catch ring  41 , as indicated by droplet  14  in conduit outlet  27 . However, this separation process is not perfect, and some amount of BI 3  and PI 3  condenses below the outlet  27  in the mid-portion  22  of the reaction chamber  20 , thus mixing with the SiI 4  condensate  13  that runs via conduit  29  into the distillation chamber  50 . Other impurities, such as carbon tetraiodide (CI 4 ) may also be produced in the reaction chamber  20  and also mix with the SiI 4  condensate  13  and run via conduit  29  into the distillation chamber  50 .  
         [0020]    The purpose of the distillation chamber  50 , therefore, is to further purify the SiI 4  by separating it from the undesirable iodides, e.g., BI 3 , PI 3 , CI 4 , and others. To do so, SiI 4 , BI 3 , PI 3 , CI 4 , and others mixed together in the pool  15  in the bottom portion  51  of the distillation chamber  50  are vaporized. The SiI 4 , BI 3 , PI 3 , and CI 4  vapors rise in the distillation chamber  50 , where temperature decreases as distance from the bottom increases. The SiI 4  condenses at a higher temperature than BI 3  and PI 3 , so the SiI 4  condenses, as illustrated by droplets  16 , in the mid-portion  52  and is drawn out of the distillation chamber  50  by an annular catch ring  42  and conduit  56 . Meanwhile, the undesirable BI 3  and PI 3  condense at a cooler temperature in the top portion  53  of distillation chamber  50 , as indicated by droplets  17 , and is captured and drawn out of the distillation chamber  50  by an annular catch ring  43  and conduit  55 . The CI 3  is condensed at a higher temperature in a lower portion of the distillation chamber  50 , as indicated by droplets  18 , and is captured and drawn out of chamber  50  by annular catch ring  44  and conduit  57 .  
         [0021]    Only the middle conduit  56 , carrying the SiI 4  condensate  16  is connected to the third chamber or reservoir  80 . Therefore, the reservoir  80  collects and holds the purified S 114  condensate  16 . A feed tube  81  connects the reservoir  80  back to the bottom portion  21  of the reaction chamber  20 , and a metering valve  82  meters the purified SiI 4  into the reaction chamber  20 , where it is joined in the reaction cycle by combining with additional Si from the vaporied MG-Si feedstock  11  to produce more SiI 2 , which cools and disproportionates in mid-portion  22  to deposit more Si on the substrate  40  and produce more SiI 4  to be repurified in the distillation chamber  50 , as described above. When a desired amount of pure Si is deposited on the substrate  40 , the substrate  40  can be removed and replaced with new substrates  40 .  
         [0022]    With this overview of the silicon purification system  10  of this invention in mind, a more detailed explanation of the components and process steps is provided below. The reaction chamber  20 , as mentioned above, has a bottom portion  21 , a mid-portion  22  and a top portion  23 . Cold-wall reactor chamber  20  further has a first inlet  24  through which metallurgical-grade silicon (MG-Si) source material II and iodine source material  12  are introduced into the chamber  20  and placed into the bottom portion  21 . A purge line  25  may be connected to the system  10  through the first inlet  24  and is used initially to introduce gas (i.e. hydrogen) with which to drive all foreign vapors from the system  10  prior to operation. During operation, the purge line  25  is used to introduce a blanketing gas (i.e. hydrogen or any other blanketing gas that is less dense than, and non-reactive with, iodine vapor) with which to keep air out of the otherwise open system  10 .  
         [0023]    Heater  26  at least partially surrounds the bottom portion  21  of the chamber  20 . Once MG-Si source material  11  and iodine source material  12  are introduced into the bottom portion  21  of the chamber  20 , heat from heater  26  will be applied to the bottom portion  21  of chamber  20  to vaporize a portion of both the MG-Si source material and the iodine source material. The heat applied by heater  26  will also create a temperature gradient to form within chamber  20 , such that the temperature To at the bottom portion  21  of the chamber  20  is warmer than the temperature T 1  at the mid-portion  22  of the chamber  20 , which in turn is warmer than the temperature T 2  at the top portion  23  of the chamber. During the first stage of operation, the heater  26  initially applies enough heat to bring the temperature (T 0 ) at the bottom portion  21  of the chamber 20 to between 500° and 1000° C. At this range of temperatures, some silicon (Si) present in the MG-Si source material II will vaporize and react with a vapor of the iodine (I) source material  12  to form silicon tetraiodide (SiI 4 ) vapor and iodine (I) vapor. Several other impurities present in the MG-Si source material  11  (i.e., those impurities, e.g., boron (B), phosphorus (P), iron (Fe), and aluminum (Al), with free energies of formation with iodine greater in absolute value than that of silicon and iodine) will likewise react with the iodine source material  12  to form several impurity iodides (those impurities present in the MG-Si source material with free energies of formation with iodine less in absolute value than that of silicon and iodine will be retarded). The silicon tetraiodide (SiI 4 ) vapor and the several impurity iodide vapors (e.g., BI 3  and PI 3 ) are then driven upwardly through the mid-portion  22  of the chamber  20  and into the top portion  23  of the chamber  20 , where the lower temperature (T 2  is about 120° C.) causes some of the impurity iodide vapors (e.g., BI 3  and PI 3 ) to condense, as indicated by droplets  14 . The condensed impurity iodides  14  are then mostly collected at a first outlet  27  by annular catch ring  41  and drawn out of the reaction chamber  20  by way of a cold trap or such other mechanism that will be instantly recognized by and familiar to those persons skilled in the art, the details of which need not be further discussed herein.  
         [0024]    The purer vapors, which include silicon tetraiodide (SiI 4 ) vapor, will condense on the interior wall  28  of the chamber  20 , as indicated by droplets  13 , at some point positioned lower than the first outlet  27 . It is important to keep the temperature (T 1 ) of the interior wall  28  “cold”, i.e., the temperature T 1  of the interior wall  28  should be maintained between 120° and 700° C., in order to prevent any silicon deposition along the wall  28 . The condensate  13  of the purer vapors will run by gravity down the interior wall  28  of chamber  20  and will subsequently be collected by an annular catch ring  40  and transported out of chamber  20  through second outlet  29  and into second chamber  50 . Second chamber  50  is a distillation tower, which, similar to the cold-wall reactor chamber  20 , has a bottom portion  51 , a mid-portion  52  and a top portion  53 , and a first outlet  55 , a second outlet  56  and a third outlet  57 . The bottom portion  51  is at least partially surrounded by heater  54 , which will heat the bottom portion  51  of chamber  50  to a temperature (T 3 ) of about 310° C. Similar to the temperature gradient formed by heater  26  in chamber  20 , a temperature gradient is formed in chamber  50  such that the temperature (T 4 ) at the first outlet  55  is lower than the temperature (T 5 ) at the second outlet, which is lower than the temperature (T 6 ) at the third outlet, which is lower than T 3 .  
         [0025]    Referring to FIG. 2, it is shown that most of the potential impurity iodides (i.e., FeI 2 , AlI 3 , etc.) have vapor pressures lower than the vapor pressure of silicon tetraioidide, thus those potential impurity iodides will remain in the liquid mix  15  in the bottom portion  51  of the chamber, while boron triiodide, phosphorus triiodide and carbon tetraiodide, that have higher partial pressures than silicon tetraiodide, will vaporize along with silicon tetraiodide at temperature T 3  and at about atmospheric pressure. As can be seen in FIG. 2, at one atmosphere, boron triiodide and phosphorus triiodide boil at about 63° C. lower than silicon tetraiodide, and carbon tetraiodide boils at about 19° C. higher than silicon tetraiodide.  
         [0026]    Thus, the three outlets  55 ,  56 ,  57  of chamber  50  of the preferred embodiment are positioned such that first outlet  55  is positioned in the top portion  53  of chamber  50  at temperature T 4 , the second outlet  56  is positioned below first outlet  55  at a temperature T 5 , and third outlet  57  is positioned below second outlet  56  but above the bottom portion  51  of chamber  50  and is at temperature T 6 . Upon the application of heat by heater  54  to the bottom portion  51  of chamber  50 , the silicon tetraiodide and the other impurity iodides are vaporized and driven upwardly through the chamber  50 . At the first outlet  55 , temperature T 4  is about 120° C. and the vaporized impurity iodides that have lower boiling points than silicon tetraiodide vapor (i.e. BI 3  and PI 3 ) condense, as is indicated by droplets  17 , and are collected by annular catch ring  43  and removed from chamber  50  at a cold trap similar to the way that the impurity iodides are trapped in, and removed from, the cold-wall reaction chamber  20 , as disclosed above. Similarly, at the second outlet  56 , temperature T 5  is about 180° C., the temperature at which silicon tetraiodide condenses as shown by droplets  16 . Condensed silicon tetraiodide  16  is collected by annular catch ring  42  and removed from chamber  50  via second outlet  56 , which is connected to chamber  80 , which in the preferred embodiment, is a reservoir used to temporarily store liquid silicon tetraiodide  16 . Lastly, third outlet  57  is maintained at temperature T 6  at about 205° C., the temperature at which carbon tetraiodide is condensed, as illustrated by droplets  18 , collected by annular catch ring  44 , and removed at a cold trap in the same manner as has been previously discussed.  
         [0027]    At this stage of the operating cycle, the purified silicon tetraiodide  16  that has been collected in chamber  80 , is returned to chamber  20  via outlet  81  through the opening of valve  82 . When the purified liquid silicon tetraiodide  16  collects in the bottom portion  21  of chamber  20 , the heat applied to bottom portion  21  is increased to a temperature (T f ) in excess of 1000° C., and preferably in the range of 1000° to 1400° C. At temperature T f , the purified silicon tetraiodide (SiI 4 )  16  further reacts with the still-present silicon from the MG-Si source material  11  to form silicon diiodide (SiI 2 ) vapor. The silicon diiodide vapor is very unstable and, as the vapor is driven upwardly into the mid-portion  22  of chamber  20 , the chemical reaction, Si 2 →Si+Si 4 , drives the disproportionation of SiI 2  and the deposition of silicon (Si) onto substrate(s)  40 . Other metal iodides (i.e., AlI 3 , etc.) have very large and negative values of free energy of formation, and while they will form vapors readily, within the deposition zone, they exhibit a very small tendency to be reduced and thus the silicon deposition on the substrate(s)  40  is quite pure. Substrate(s)  40  are preferably high purity silicon slim rods or tubes or carbon tubes or rods, and are heated by heater  30  to a temperature (T s ) of about 750° C. The temperature T f  is continuously applied to bottom portion  21  of chamber  20  for the remainder of the cycle. The silicon tetraiodide vapor and the other metal impurity iodide vapors generated by the secondary reaction at temperature T f  go through the distillation process as described above for a second (or third, fourth, etc) time before again returning as purified silicon tetraiodide  16  from outlet  81  of reservoir  80  to the bottom portion  21  of chamber  20 .  
         [0028]    The replenishment of MG-Si and iodine source materials  11 ,  12  is accomplished through inlet  24  of chamber  20  along with a purging gas flow at the opening  25 , and the volatile gases are kept in the system  10  by a blanketing cloud layer due to the condensation of iodine vapor plus the gravity effect as described in detail in U.S. patent application Ser. No. 09/334,166, filed on Jun. 15, 1999 by the applicants and assigned to the same entity and incorporated herein by reference. The ability to replenish the source materials, recycle byproducts, and continuously load and unload substrate(s)  40  gives the above-described process and apparatus significant cost advantages over existing and previously disclosed silicon purification systems. Further, the fact that the system  10  is an open system that operates at about atmospheric pressure eliminates the need to incorporate expensive vacuum equipment into the manufacturing cycle, further reducing total system cost. Finally, the disclosed system  10  can be modified by increasing the number of chambers to potentially include separate chambers for doping silicon feedstock and can likewise by modified by changing the number of outlets and the relative temperatures to specifically target particular impurities with defined partial pressures, vaporization points and condensation points.  
         [0029]    The above-described purification technique and apparatus provided example results as follows. With a source silicon temperature (T f )&gt;1200° C. and a substrate temperature (T s ) of about 1000° C., a purified silicon deposition rate &gt;5 μm/min was achieved. The resultant single-crystal substrates were approximately 100 μm-thick epitaxial silicon layers. Impurity levels in the layers were analyzed by secondary ion mass spectrometry (SIMS) and glow discharge mass spectroscopy (GDMS) and the results are shown in FIG. 3. Specifically, the graph shows the measured initial impurity level of the metallurgical-grade silicon which is denoted by a circular dot. The allowable solar-grade silicon (SoG-Si) content range is denoted by two connected squares, with each square representing the minimum and maximum level of each impurity necessary to meet the minimum purity requirements for SoG-Si. The triangular marks denote the impurity level results generated by the ICVT technique described above as measured by GDMS, and the diamond-shaped marks denote the same results as measured by the SIMS. With the exception of boron (and phosphorus which is not shown), all results produced by the method and apparatus disclosed by the present invention, show reductions by several orders of magnitude and are within the target ranges. And with the addition of the distillation chamber  50  to the system  10 , the levels of boron (and phosphorus) would be reduced to within the allowable SoG-Si range as well.  
         [0030]    A second experiment was conducted using multiple large area substrates for ICVT growth of thick layers that were harvested and melted as feedstock for Czochralski (CZ) crystal growth and analysis. All major impurities were greatly reduced as shown by Table 1 below:  
                                                     Element   CZ-Si from ICVT (ppma)   MG-Si source (ppma)                                B   4.157   14.548       C   14.264   107.565       O   17.554   66.706       Mg   &lt;0.001   8.204       Al   &lt;0.005   520.458       Si   matrix   matrix       P   6.801   21.762       S   &lt;0.044   0.096       K   &lt;0.007   &lt;0.036       Ca   &lt;0.007   44.849       Ti   &lt;0.001   47.526       V   &lt;0.001   143.345       Cr   &lt;0.001   19.985       Mn   &lt;0.001   19.938       Fe   &lt;0.005   553.211       Co   &lt;0.002   0.763       Ni   &lt;0.002   22.012       Cu   &lt;0.001   1.724       Zn   &lt;0.002   0.077       As   &lt;0.002   0.007       Sr   &lt;0.0003   0.353       Zr   &lt;0.0003   2.063       Mo   &lt;0.001   0.790       I   &lt;0.0002   &lt;0.001       Ba   &lt;0.0002   0.266       W   &lt;0.0003   0.024                  
 
         [0031]    All the metal impurities, as illustrated above, were below the detection limits of the GDMS technique. The resultant crystal created diagnostic solar cells with efficiencies of 9.5% and voltage v. current density characteristics as illustrated by the graph in FIG. 4. Thus, the ICVT technique disclosed and illustrated herein, along with the apparatus described above and claimed below, generated purified silicon feedstock consistently and predictably, at fast deposition rates and at low operating costs.  
         [0032]    The foregoing is considered as illustrative only of the principal of the invention. Further, since numerous modifications and changes will occur to those persons skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly all suitable modifications and equivalents may be resorted to falling within the scope of the invention as defined by the claims which follow.