CA1193522A - Growing silicon ingots - Google Patents
Growing silicon ingotsInfo
- Publication number
- CA1193522A CA1193522A CA000386705A CA386705A CA1193522A CA 1193522 A CA1193522 A CA 1193522A CA 000386705 A CA000386705 A CA 000386705A CA 386705 A CA386705 A CA 386705A CA 1193522 A CA1193522 A CA 1193522A
- Authority
- CA
- Canada
- Prior art keywords
- silicon
- improvement
- crucible
- impurities
- melt
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 title claims abstract description 57
- 229910052710 silicon Inorganic materials 0.000 title claims abstract description 57
- 239000010703 silicon Substances 0.000 title claims abstract description 57
- 239000012535 impurity Substances 0.000 claims abstract description 44
- 230000008018 melting Effects 0.000 claims abstract description 11
- 238000002844 melting Methods 0.000 claims abstract description 11
- 238000010438 heat treatment Methods 0.000 claims abstract description 10
- 238000004519 manufacturing process Methods 0.000 claims abstract description 6
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims description 51
- 239000000377 silicon dioxide Substances 0.000 claims description 25
- 239000000155 melt Substances 0.000 claims description 19
- 238000000034 method Methods 0.000 claims description 19
- 239000007795 chemical reaction product Substances 0.000 claims description 5
- ZAMOUSCENKQFHK-UHFFFAOYSA-N Chlorine atom Chemical compound [Cl] ZAMOUSCENKQFHK-UHFFFAOYSA-N 0.000 claims description 3
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 claims description 3
- 230000015572 biosynthetic process Effects 0.000 claims description 3
- 239000000460 chlorine Substances 0.000 claims description 3
- 229910052801 chlorine Inorganic materials 0.000 claims description 3
- 239000001257 hydrogen Substances 0.000 claims description 3
- 229910052739 hydrogen Inorganic materials 0.000 claims description 3
- 239000000126 substance Substances 0.000 claims description 2
- 239000013078 crystal Substances 0.000 description 20
- 239000007788 liquid Substances 0.000 description 13
- HBMJWWWQQXIZIP-UHFFFAOYSA-N silicon carbide Chemical compound [Si+]#[C-] HBMJWWWQQXIZIP-UHFFFAOYSA-N 0.000 description 11
- 229910010271 silicon carbide Inorganic materials 0.000 description 11
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 10
- 239000007787 solid Substances 0.000 description 10
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 description 8
- 229910052750 molybdenum Inorganic materials 0.000 description 8
- 239000011733 molybdenum Substances 0.000 description 8
- LIVNPJMFVYWSIS-UHFFFAOYSA-N silicon monoxide Chemical compound [Si-]#[O+] LIVNPJMFVYWSIS-UHFFFAOYSA-N 0.000 description 8
- 238000006243 chemical reaction Methods 0.000 description 6
- 229910002804 graphite Inorganic materials 0.000 description 6
- 239000010439 graphite Substances 0.000 description 6
- 238000007711 solidification Methods 0.000 description 6
- 230000008023 solidification Effects 0.000 description 6
- 229910052681 coesite Inorganic materials 0.000 description 5
- 229910052906 cristobalite Inorganic materials 0.000 description 5
- 235000012239 silicon dioxide Nutrition 0.000 description 5
- 229910052682 stishovite Inorganic materials 0.000 description 5
- 229910052905 tridymite Inorganic materials 0.000 description 5
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 description 4
- 229910052796 boron Inorganic materials 0.000 description 4
- 229910052799 carbon Inorganic materials 0.000 description 4
- 239000002585 base Substances 0.000 description 3
- 239000007789 gas Substances 0.000 description 3
- 238000009413 insulation Methods 0.000 description 3
- 239000000463 material Substances 0.000 description 3
- 239000002245 particle Substances 0.000 description 3
- 238000007670 refining Methods 0.000 description 3
- 239000002893 slag Substances 0.000 description 3
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 2
- 229910002092 carbon dioxide Inorganic materials 0.000 description 2
- 238000005266 casting Methods 0.000 description 2
- 230000015556 catabolic process Effects 0.000 description 2
- BHEPBYXIRTUNPN-UHFFFAOYSA-N hydridophosphorus(.) (triplet) Chemical compound [PH] BHEPBYXIRTUNPN-UHFFFAOYSA-N 0.000 description 2
- 230000001965 increasing effect Effects 0.000 description 2
- 238000003756 stirring Methods 0.000 description 2
- -1 trichorlosilane) Chemical class 0.000 description 2
- UGFAIRIUMAVXCW-UHFFFAOYSA-N Carbon monoxide Chemical compound [O+]#[C-] UGFAIRIUMAVXCW-UHFFFAOYSA-N 0.000 description 1
- 238000002231 Czochralski process Methods 0.000 description 1
- 229910004298 SiO 2 Inorganic materials 0.000 description 1
- 230000001464 adherent effect Effects 0.000 description 1
- 229910052783 alkali metal Inorganic materials 0.000 description 1
- 150000001340 alkali metals Chemical class 0.000 description 1
- 229910052782 aluminium Inorganic materials 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- 229910052810 boron oxide Inorganic materials 0.000 description 1
- 230000005587 bubbling Effects 0.000 description 1
- 239000001569 carbon dioxide Substances 0.000 description 1
- 229910002091 carbon monoxide Inorganic materials 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 238000013016 damping Methods 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- JKWMSGQKBLHBQQ-UHFFFAOYSA-N diboron trioxide Chemical compound O=BOB=O JKWMSGQKBLHBQQ-UHFFFAOYSA-N 0.000 description 1
- 230000002708 enhancing effect Effects 0.000 description 1
- 230000001747 exhibiting effect Effects 0.000 description 1
- 239000011521 glass Substances 0.000 description 1
- 239000001307 helium Substances 0.000 description 1
- 229910052734 helium Inorganic materials 0.000 description 1
- SWQJXJOGLNCZEY-UHFFFAOYSA-N helium atom Chemical compound [He] SWQJXJOGLNCZEY-UHFFFAOYSA-N 0.000 description 1
- 230000002401 inhibitory effect Effects 0.000 description 1
- FBAFATDZDUQKNH-UHFFFAOYSA-M iron chloride Chemical compound [Cl-].[Fe] FBAFATDZDUQKNH-UHFFFAOYSA-M 0.000 description 1
- 229910021421 monocrystalline silicon Inorganic materials 0.000 description 1
- 239000012071 phase Substances 0.000 description 1
- 239000000843 powder Substances 0.000 description 1
- 238000005204 segregation Methods 0.000 description 1
- 239000004065 semiconductor Substances 0.000 description 1
- 150000003376 silicon Chemical class 0.000 description 1
- 229910052814 silicon oxide Inorganic materials 0.000 description 1
- 239000011863 silicon-based powder Substances 0.000 description 1
- 239000007858 starting material Substances 0.000 description 1
- 239000012808 vapor phase Substances 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C30—CRYSTAL GROWTH
- C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
- C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
- C30B29/02—Elements
- C30B29/06—Silicon
-
- C—CHEMISTRY; METALLURGY
- C30—CRYSTAL GROWTH
- C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
- C30B11/00—Single-crystal growth by normal freezing or freezing under temperature gradient, e.g. Bridgman-Stockbarger method
- C30B11/003—Heating or cooling of the melt or the crystallised material
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F28—HEAT EXCHANGE IN GENERAL
- F28D—HEAT-EXCHANGE APPARATUS, NOT PROVIDED FOR IN ANOTHER SUBCLASS, IN WHICH THE HEAT-EXCHANGE MEDIA DO NOT COME INTO DIRECT CONTACT
- F28D7/00—Heat-exchange apparatus having stationary tubular conduit assemblies for both heat-exchange media, the media being in contact with different sides of a conduit wall
- F28D7/10—Heat-exchange apparatus having stationary tubular conduit assemblies for both heat-exchange media, the media being in contact with different sides of a conduit wall the conduits being arranged one within the other, e.g. concentrically
- F28D7/12—Heat-exchange apparatus having stationary tubular conduit assemblies for both heat-exchange media, the media being in contact with different sides of a conduit wall the conduits being arranged one within the other, e.g. concentrically the surrounding tube being closed at one end, e.g. return type
Abstract
ABSTRACT OF THE DISCLOSURE
The production of silicon ingots of substantially single crystallinity from metallurgical grade silicon by heating it in a crucible to above its melt-ing point to melt it and then extracting heat from the bottom of the crucible with a heat exchanger in heat conducting relationship with the bottom, said metallurgical grade silicon having an impurity level greater than 100 ppm, this level being greater than silicon melt stocks previously used.
The production of silicon ingots of substantially single crystallinity from metallurgical grade silicon by heating it in a crucible to above its melt-ing point to melt it and then extracting heat from the bottom of the crucible with a heat exchanger in heat conducting relationship with the bottom, said metallurgical grade silicon having an impurity level greater than 100 ppm, this level being greater than silicon melt stocks previously used.
Description
~;33~2 This invention relates to the manufacture of silicon crystals suitable for use in photovoltaic cells from low purity silicon melt stock.
The best solar cells have been fabricated from high-purity, single-crystal silicon, the making of which by conventional processes involves many steps. Metallurgical grade silicon (98-99~ pure, an impurity level inhibiting single crystal growth and exhibiting conductivity too high for solar cells, owing primarily to the presence of boron and phosphorous) is typically produced in large quantities in arc furnaces by the carbothermic reduction of silica.
The carbothermic process causes the presence of significant amounts oE carbon, primarily in the form of silicon carbide, and sînce the silicon is poured in air, the surface of the silicon is oxidized to silica. This grade of silicon is then chemically converted by another process to an intermediate compound (e.g., trichorlosilane), which is in turn converted by still another process (e.g., Siemens process) to semiconductor grade silicon (having impurities in the ppb range), which in turn is used to grow a single crystal suitable for use in a solar cell. ~ method that has proved in growing crystals from such high purity silicon (e.g., impurities less than 10 ppb) is the Heat Exchanger Method, which involves heating material in a crucible to above its melting point in vacuum to melt the material therein and thereafter extracting heat Erom the bottom oE the crucible by providing a heat exchanger in heat conducting relatlonship with the bottom. The Heat Exchanger Method is described in United States Patent~s Nos. 3,653,432, 3,898,05L and 4,256,530.
By using selected sllica and carbon in an arc furnace, Dow Corning Corporatlon has demonstrated the production of metallurgical silicon that is 3~
about 99.8% pure and has low concentrations of boron and phosphorous, impurities which have high segregation coefficients and are tharefore difficult to segregate during directiona1 solidification. This silicon was poured in air, resulting in a silica layer, which was etched away prior to growing an ingot by the Czoch-ralski process ~ a directional solidification process). Loss of single crystall-inity still resulted; but growth of a second crystal, using the best portions from the :Eirst growth as starting material, enabled the production of a single crystal material suitable for solar cell production.
It has now been unexpectedly discovered that silicon with impurity levels greater than 100 ppm ~e.g., metallurgical grade silicon that is less than 99% pure) can be grown into a single crystal ingot using the Heat ExchangerMethod ~HE~ in a single step.
Thus, in accordance with the present invention; there is provided in the process of producing silicon ingots of substant~i~lly single crystallinity ;S~ incl~ r,c~ rl ~ S
by heating a melt stock of silicon ~ p~t~ in a crucible to above its melting point and solidifying melted silicon by extracting heat from the bottom of said crucible with a heat exchanger in heat-conductillg relationship with a portion of the bottom oE said crucible, the improvement wherein said melt stock used in said heating step is silicon having an impurity level greater than 100 ppm, whereby impurities in said silicon are caused to migrate to exterior surfaces.
Preferably said silicon used is less than 99% pure, and the silicon used may be metallurgical grade silicon prod~ced by the carbothermic reduction of silica.
It has also been discovered that refining reactions can be employed in the Heat Eixchanger Method prior to or during crystal growth. In preferred embodiments the silica covering of metallurgical grade silicon is not etched pr:ior to melting in order to promote silicon carbide removal by slagging of ~2-3~ii2~
the silicon oxide, and in a most preferred embodiment pure powdered silica (in its amorphous phase, i.e., glass) is added to the melt prior to crystal growth. In other embodiments the melt is stirred, moist hydrogen is passed through the melt prior to crystal growth to remove boron impurities, chlorine is passed through ~he melt resulting in volatile reaction products and also causing the removal of impurities, and the melt is heated to high temperatures, prior to crystal growing at a lower temperature, to remove impurities.
In all of the above methods zone refining is promoted by the expand-ing solid/liquid interface (as opposed to constant interface area in directiona]
solidification or shrinking interface area when the exterior solidifies before the interior), which limits increase in impurity concentration at the interface, the increase often causing interface breakdown and loss of single crystallinity.
Also, impurities are transported to exterior surfaces where they can be easily cropped off, and the temperature gradient with the hottest melt at the top promotes stable impurity gradients and liquid motion. The silica slag layer floats on the surface of the melt and does not interfere with the solid/liquid interface. In the gas bubbling and stirring of the melt embodiments, the incr-eased turbulence promotes removal of impurities from the interface and their transport to the upper surface.
The vacuum operation of llEM with a high impurity content silicon (such as metallurgical silicon) allows further refinement by vapori~ation of high vapor pressure species. These species are impurities (such as alkali metals, mcmganese, etc.) that have a tendency to go into vapor phase in preference to staying in the silicon melt. Under vacuum operation (e.g., below 30 torr and preferably near 0.1 torr), the impurity vapor is continuously removed from the site of the reaction in preference to building up near the melt surface, thereby 35i2Z
enhancing removal of these impurities from the melt.
Objects, features and advantages of tlle invention will appear from the following detailed description of the structure and use of a preferred prac~e~ thereof, taken together with the attached drawing in which:
Figure 1 is a schematic view, partially in section, of a crucible, molybdenum retainer, conducting graphite plug, and insulation within the heating chamber of a casting furnace.
Referring now to Figure l, there is illustrated a silica crucible lO
within the cylindrical heating chamber defined by the resistance heater 12 of a casting furnace of the type disclosed in United States Patent No. 3,898,051.
The crucible 10 rests on a molybdenum disc ll which itself is supported by graph-ite rods 14 mounted on a graphite support plate 16 on the bottom 18 of the heat-ing chamber, and is surrounded by a cylindrical molybdenum retainer 9. A helium cooled molybdenum heat exchanger 20, of the type disclosed in United States Patent No. 3,653,432, extends through openings in the center of the plate 16 and bottom 18.
Crucible 10 is about 6 in. (15 cm.) in height and diameter and its cylindrical wall 22 and base 24 are 0-15 in. (3.7 mm.) thick. Molybdenum disc 11 is about 0.040 in. (1 mm.) thick, and molybdenum retainer 9, comprising a sheet of the same thickness rolled into cylindrical form, engages the exterior of cylinder wall 22. A silicon ingot 26, partially solidified according to the process described in aforementioned patents, is shown within the crucible, the solid-liquid interface 28 having advanced from the seed (shown in dashed lines at 30).
A stepped cylindrical graphite plug 50 (upper portion diameter 1.9 in., and lower portion diameter 2.5 in.) extends from bottom 18 upwardly through r ~
coaxial holes 52, 54, 56 in, respectively) plate 16, molybdenum disc 11 and crucible base 24. The top 58 of plug 50 is flush with the inside bottom surface of crucible base 24. The seed 30 is placed over the plug 50 and the adjacent portion of crucible bot~om 2~ so as to cover opening 56. The exterior of the plug upper portion fits loosely in openings 54, 56 to allow for thermal expansion; and the step 60 between the plug's upper smaller diameter and lower larger diameter portions engages the underside of plate 11. A small quantity of silicon powder is placed in the area of opening 56 where seed 30, crucible 10 and graphite plug 50 are in proximity. Heat exchanger 20 fits within a coaxial recess 62 in the bottom of plug 50, with the top of the heat exchanger about 1/8 in. below the top 58 of the plug. A graphite felt insulation and/or molybdenum heat shield sleeve 6~ closely surrounds the larger diameter portion of plug 50, extending axially of the plug the full distance between bottom 18 and plate 11. As shown~ the exterior surface of insulation sleeve 64 engages the interior of opening 52.
In an embodiment described below, movable sil:ica tube 66 is suspended (by means not shown) so that one end extends into crucible 10 and the other end is connected to a gas supply (not shown~.
The apparatus described above and the operating conditions and methods disclosed in the above-mentioned patents and patent applications were used in growing single crystals from metallurgical grade silicon. First, etched metal-lurgical grade silicon was upwardly and outwardly solidified in 6 inch crucible 10 using the ~leat Exchanger Method (~IEM). The melt stock was heated under vacuum con~ition (0.1 torr pressure) furnace temperature was kept to less than 3C above melting point, the heat exchanger temperature was kept 113C below the melting point, the heat exchanger temperature was decreased during growth ~L~3~
at a rate of ~20C/hr., ~he furnace temperature was kept constant, and crystal growth lasted about 7.75 hrs. A single cystal ingot with impurities segregated to the outside of the ingot resulted. Even impurities present in the form of solid particles that did not float or sink but remained suspended did not prevent single crystallinity owing to the very stable solid/liquid interface, temperature and impurity gradients and to the damping of mechanical vibrations of~ and temperature variations in, the heating element by the liquid buffer region between the solid/liquid interface 28 and the crucible wall 22. An important feature of HEM growth that is useful in removing impurities from metallurgical grade silicon is that the crystal grows outwardly from the bottom center so that the last regions to solidify are at the upper surface and at the crucible walls, As solidification proceeds, impurities are segregated in front of the solid/
liquid interface, causing an increase in impurity concentration in the remaining liquid. Although the increase in impurities concentration in front of the inter-face causes interface breakdown and loss of single crystallinity in unidirectioaal solidification processes, because the HEM interface expands, this impurity build-up is distributed over a larger interface area; hence, concentration buildup is not as rapid as for unidirectional solidification. Therefore, by using the HEM
process, higher impurities were tolerated without loss of structure. The impurities are transported to exterior surfaces where they can be easily cropped off. Concentration of the impurities at the solid/liquid interface 28 was also minimized by stirring the melt.
The higll carbon content of this grade of silicon (up to 0.5%) resulted in formation of sil:icon carbide particles both at the surface of the ingot, where they ~ould be easily removed, and within the crystal, where they lowered the purity of the crystal but did not prevent single crystallinity.
Next, unetched silica with its adherent silica layer ~as used to reduce silicon carbide content of the end product. Silica reacts with silicon carbide according to the following reactions:
SiC ~ 2 SiO2 > 3 SiO + C0 sic + sio2 - > sio + co + s
The best solar cells have been fabricated from high-purity, single-crystal silicon, the making of which by conventional processes involves many steps. Metallurgical grade silicon (98-99~ pure, an impurity level inhibiting single crystal growth and exhibiting conductivity too high for solar cells, owing primarily to the presence of boron and phosphorous) is typically produced in large quantities in arc furnaces by the carbothermic reduction of silica.
The carbothermic process causes the presence of significant amounts oE carbon, primarily in the form of silicon carbide, and sînce the silicon is poured in air, the surface of the silicon is oxidized to silica. This grade of silicon is then chemically converted by another process to an intermediate compound (e.g., trichorlosilane), which is in turn converted by still another process (e.g., Siemens process) to semiconductor grade silicon (having impurities in the ppb range), which in turn is used to grow a single crystal suitable for use in a solar cell. ~ method that has proved in growing crystals from such high purity silicon (e.g., impurities less than 10 ppb) is the Heat Exchanger Method, which involves heating material in a crucible to above its melting point in vacuum to melt the material therein and thereafter extracting heat Erom the bottom oE the crucible by providing a heat exchanger in heat conducting relatlonship with the bottom. The Heat Exchanger Method is described in United States Patent~s Nos. 3,653,432, 3,898,05L and 4,256,530.
By using selected sllica and carbon in an arc furnace, Dow Corning Corporatlon has demonstrated the production of metallurgical silicon that is 3~
about 99.8% pure and has low concentrations of boron and phosphorous, impurities which have high segregation coefficients and are tharefore difficult to segregate during directiona1 solidification. This silicon was poured in air, resulting in a silica layer, which was etched away prior to growing an ingot by the Czoch-ralski process ~ a directional solidification process). Loss of single crystall-inity still resulted; but growth of a second crystal, using the best portions from the :Eirst growth as starting material, enabled the production of a single crystal material suitable for solar cell production.
It has now been unexpectedly discovered that silicon with impurity levels greater than 100 ppm ~e.g., metallurgical grade silicon that is less than 99% pure) can be grown into a single crystal ingot using the Heat ExchangerMethod ~HE~ in a single step.
Thus, in accordance with the present invention; there is provided in the process of producing silicon ingots of substant~i~lly single crystallinity ;S~ incl~ r,c~ rl ~ S
by heating a melt stock of silicon ~ p~t~ in a crucible to above its melting point and solidifying melted silicon by extracting heat from the bottom of said crucible with a heat exchanger in heat-conductillg relationship with a portion of the bottom oE said crucible, the improvement wherein said melt stock used in said heating step is silicon having an impurity level greater than 100 ppm, whereby impurities in said silicon are caused to migrate to exterior surfaces.
Preferably said silicon used is less than 99% pure, and the silicon used may be metallurgical grade silicon prod~ced by the carbothermic reduction of silica.
It has also been discovered that refining reactions can be employed in the Heat Eixchanger Method prior to or during crystal growth. In preferred embodiments the silica covering of metallurgical grade silicon is not etched pr:ior to melting in order to promote silicon carbide removal by slagging of ~2-3~ii2~
the silicon oxide, and in a most preferred embodiment pure powdered silica (in its amorphous phase, i.e., glass) is added to the melt prior to crystal growth. In other embodiments the melt is stirred, moist hydrogen is passed through the melt prior to crystal growth to remove boron impurities, chlorine is passed through ~he melt resulting in volatile reaction products and also causing the removal of impurities, and the melt is heated to high temperatures, prior to crystal growing at a lower temperature, to remove impurities.
In all of the above methods zone refining is promoted by the expand-ing solid/liquid interface (as opposed to constant interface area in directiona]
solidification or shrinking interface area when the exterior solidifies before the interior), which limits increase in impurity concentration at the interface, the increase often causing interface breakdown and loss of single crystallinity.
Also, impurities are transported to exterior surfaces where they can be easily cropped off, and the temperature gradient with the hottest melt at the top promotes stable impurity gradients and liquid motion. The silica slag layer floats on the surface of the melt and does not interfere with the solid/liquid interface. In the gas bubbling and stirring of the melt embodiments, the incr-eased turbulence promotes removal of impurities from the interface and their transport to the upper surface.
The vacuum operation of llEM with a high impurity content silicon (such as metallurgical silicon) allows further refinement by vapori~ation of high vapor pressure species. These species are impurities (such as alkali metals, mcmganese, etc.) that have a tendency to go into vapor phase in preference to staying in the silicon melt. Under vacuum operation (e.g., below 30 torr and preferably near 0.1 torr), the impurity vapor is continuously removed from the site of the reaction in preference to building up near the melt surface, thereby 35i2Z
enhancing removal of these impurities from the melt.
Objects, features and advantages of tlle invention will appear from the following detailed description of the structure and use of a preferred prac~e~ thereof, taken together with the attached drawing in which:
Figure 1 is a schematic view, partially in section, of a crucible, molybdenum retainer, conducting graphite plug, and insulation within the heating chamber of a casting furnace.
Referring now to Figure l, there is illustrated a silica crucible lO
within the cylindrical heating chamber defined by the resistance heater 12 of a casting furnace of the type disclosed in United States Patent No. 3,898,051.
The crucible 10 rests on a molybdenum disc ll which itself is supported by graph-ite rods 14 mounted on a graphite support plate 16 on the bottom 18 of the heat-ing chamber, and is surrounded by a cylindrical molybdenum retainer 9. A helium cooled molybdenum heat exchanger 20, of the type disclosed in United States Patent No. 3,653,432, extends through openings in the center of the plate 16 and bottom 18.
Crucible 10 is about 6 in. (15 cm.) in height and diameter and its cylindrical wall 22 and base 24 are 0-15 in. (3.7 mm.) thick. Molybdenum disc 11 is about 0.040 in. (1 mm.) thick, and molybdenum retainer 9, comprising a sheet of the same thickness rolled into cylindrical form, engages the exterior of cylinder wall 22. A silicon ingot 26, partially solidified according to the process described in aforementioned patents, is shown within the crucible, the solid-liquid interface 28 having advanced from the seed (shown in dashed lines at 30).
A stepped cylindrical graphite plug 50 (upper portion diameter 1.9 in., and lower portion diameter 2.5 in.) extends from bottom 18 upwardly through r ~
coaxial holes 52, 54, 56 in, respectively) plate 16, molybdenum disc 11 and crucible base 24. The top 58 of plug 50 is flush with the inside bottom surface of crucible base 24. The seed 30 is placed over the plug 50 and the adjacent portion of crucible bot~om 2~ so as to cover opening 56. The exterior of the plug upper portion fits loosely in openings 54, 56 to allow for thermal expansion; and the step 60 between the plug's upper smaller diameter and lower larger diameter portions engages the underside of plate 11. A small quantity of silicon powder is placed in the area of opening 56 where seed 30, crucible 10 and graphite plug 50 are in proximity. Heat exchanger 20 fits within a coaxial recess 62 in the bottom of plug 50, with the top of the heat exchanger about 1/8 in. below the top 58 of the plug. A graphite felt insulation and/or molybdenum heat shield sleeve 6~ closely surrounds the larger diameter portion of plug 50, extending axially of the plug the full distance between bottom 18 and plate 11. As shown~ the exterior surface of insulation sleeve 64 engages the interior of opening 52.
In an embodiment described below, movable sil:ica tube 66 is suspended (by means not shown) so that one end extends into crucible 10 and the other end is connected to a gas supply (not shown~.
The apparatus described above and the operating conditions and methods disclosed in the above-mentioned patents and patent applications were used in growing single crystals from metallurgical grade silicon. First, etched metal-lurgical grade silicon was upwardly and outwardly solidified in 6 inch crucible 10 using the ~leat Exchanger Method (~IEM). The melt stock was heated under vacuum con~ition (0.1 torr pressure) furnace temperature was kept to less than 3C above melting point, the heat exchanger temperature was kept 113C below the melting point, the heat exchanger temperature was decreased during growth ~L~3~
at a rate of ~20C/hr., ~he furnace temperature was kept constant, and crystal growth lasted about 7.75 hrs. A single cystal ingot with impurities segregated to the outside of the ingot resulted. Even impurities present in the form of solid particles that did not float or sink but remained suspended did not prevent single crystallinity owing to the very stable solid/liquid interface, temperature and impurity gradients and to the damping of mechanical vibrations of~ and temperature variations in, the heating element by the liquid buffer region between the solid/liquid interface 28 and the crucible wall 22. An important feature of HEM growth that is useful in removing impurities from metallurgical grade silicon is that the crystal grows outwardly from the bottom center so that the last regions to solidify are at the upper surface and at the crucible walls, As solidification proceeds, impurities are segregated in front of the solid/
liquid interface, causing an increase in impurity concentration in the remaining liquid. Although the increase in impurities concentration in front of the inter-face causes interface breakdown and loss of single crystallinity in unidirectioaal solidification processes, because the HEM interface expands, this impurity build-up is distributed over a larger interface area; hence, concentration buildup is not as rapid as for unidirectional solidification. Therefore, by using the HEM
process, higher impurities were tolerated without loss of structure. The impurities are transported to exterior surfaces where they can be easily cropped off. Concentration of the impurities at the solid/liquid interface 28 was also minimized by stirring the melt.
The higll carbon content of this grade of silicon (up to 0.5%) resulted in formation of sil:icon carbide particles both at the surface of the ingot, where they ~ould be easily removed, and within the crystal, where they lowered the purity of the crystal but did not prevent single crystallinity.
Next, unetched silica with its adherent silica layer ~as used to reduce silicon carbide content of the end product. Silica reacts with silicon carbide according to the following reactions:
SiC ~ 2 SiO2 > 3 SiO + C0 sic + sio2 - > sio + co + s
2 SiC + SiO2 > 3 Si + 2 C0 SiC + SiO2 ~ 2 Si + C02 SiC + SiO2 - > C ~ 2 SiO
2 Si + C0 > SiC + SiO
SiO2 + 3C - ~ SiC ~ 2 C0 sio2 -, c ~ sio ~ co These reactions all have negative free energy at the melting point of silicon and approximately 0.1 torr pressure, and therefore tend to proceed to the right. Because the carbon monoxide, carbon dioxide, and silicon mono-xide created by these reactions form bubblesJ which rise to the surface, a net removal of the carbon from the melt is caused. The presence of silica also causes the removal of carbide and other impurities (e.g., aluminum) by the slagging phenomenon. The slag layer rises to the melt surface where it does not lnterfere with the solid/liquid interface, and the impurities are, there-fore, not incorporated in the crystal.
High purity silica powder has also been added to the melt stockprior to crystal growth by the ~IEM to further reduce silicon carbide content.
In the 6" crucible 10, 150 grams of silica (99% pure and in powdered form with 100 ~m particles) is added to 3 kilograms of metallurgical grade silicon.
2 Si + C0 > SiC + SiO
SiO2 + 3C - ~ SiC ~ 2 C0 sio2 -, c ~ sio ~ co These reactions all have negative free energy at the melting point of silicon and approximately 0.1 torr pressure, and therefore tend to proceed to the right. Because the carbon monoxide, carbon dioxide, and silicon mono-xide created by these reactions form bubblesJ which rise to the surface, a net removal of the carbon from the melt is caused. The presence of silica also causes the removal of carbide and other impurities (e.g., aluminum) by the slagging phenomenon. The slag layer rises to the melt surface where it does not lnterfere with the solid/liquid interface, and the impurities are, there-fore, not incorporated in the crystal.
High purity silica powder has also been added to the melt stockprior to crystal growth by the ~IEM to further reduce silicon carbide content.
In the 6" crucible 10, 150 grams of silica (99% pure and in powdered form with 100 ~m particles) is added to 3 kilograms of metallurgical grade silicon.
3~
In both the une~ched and the unetched plus added silica embodiments, the slag is removed after crystal growth by cropping, In the added silica embodiment, the ingot was found to have low enough conductivity to allow use in photovolatic cells.
Solar cells fabricated from such silicon have shown up to 12.33%
conversion efficiency.
In a~dition to leaving the metallurgical grade silicon unetched and adding silica to the melt, the use of other refining processes involving reacting a substance with the impurities in the silicon to form either a solid, immiscible liquid, or gas is made possible by the stability and expanding nature of the solid/liquid interface.
For example, impurities could be stripped Erom the melt by passing via tLIbe 66 gasses that react with the impurities to form reaction products that are volatile or will otherwise remove themselves from the melt. Specific-ally, moist hydrogen will cause the removal of boron by the formation of boron oxide.
Also, chlorine will react with metallic impurities to form volatile reaction products such as iron chloride.
Finally, the melt stock temperature has been increased to 50 to 100C
above the silicon melting point to improve volatization of impurities. After sufficient removal of impurities, the temperature is then lowered to 3C above melting point to allow crystal growth.
In both the une~ched and the unetched plus added silica embodiments, the slag is removed after crystal growth by cropping, In the added silica embodiment, the ingot was found to have low enough conductivity to allow use in photovolatic cells.
Solar cells fabricated from such silicon have shown up to 12.33%
conversion efficiency.
In a~dition to leaving the metallurgical grade silicon unetched and adding silica to the melt, the use of other refining processes involving reacting a substance with the impurities in the silicon to form either a solid, immiscible liquid, or gas is made possible by the stability and expanding nature of the solid/liquid interface.
For example, impurities could be stripped Erom the melt by passing via tLIbe 66 gasses that react with the impurities to form reaction products that are volatile or will otherwise remove themselves from the melt. Specific-ally, moist hydrogen will cause the removal of boron by the formation of boron oxide.
Also, chlorine will react with metallic impurities to form volatile reaction products such as iron chloride.
Finally, the melt stock temperature has been increased to 50 to 100C
above the silicon melting point to improve volatization of impurities. After sufficient removal of impurities, the temperature is then lowered to 3C above melting point to allow crystal growth.
Claims (11)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In the process of producing silicon ingots of substantially single crystallinity by heating a melt stock of silicon including impurities in a crucible to above its melting point and solidifying melted silicon by extracting heat from the bottom of said crucible with a heat exchanger in heat-conducting relation-ship with a portion of the bottom of said crucible, the improvement wherein said melt stock used in said heating step is silicon having an impurity level greater than 100 ppm, whereby impurities in said silicon are caused to migrate to exterior surfaces.
2. The improvement of claim 1 in which said silicon used is less than 99% pure.
3. The improvement of claim 2 in which said silicon used is metallur-gical grade silicon produced by the carbothermic reduction of silica.
4. The improvement of claim 1 wherein said metallurgical grade silicon has not been etched to remove silica that has formed on it during its manufact-ure.
5. The improvement of claims 1 or 2 further comprising adding powdered silica to the silicon in the crucible prior to solidifying the melted silicon.
6. The improvement of claim 1 wherein the pressure in said crucible is less than 30 torr to promote removal of volatile impurities.
7. The improvement of claim 6 wherein the pressure is about 0.1 torr.
8. The improvement of claim 6 wherein the silicon in the crucible is heated to between 50 and 100°C above the melting point of silicon to promote removal of volatile impurities.
9. The improvement of claim 1 further comprising reacting impurities in the melted silicon with substances that will cause the formation of volatile reaction products that will separate from the melted silicon.
10. The improvement of claim 9 wherein said reacting step comprises passing moist hydrogen through said melted silicon.
11. The improvement of claim 9 wherein said reacting step comprises passing chlorine through said melted silicon.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US19126080A | 1980-09-26 | 1980-09-26 | |
US191,260 | 1980-09-26 |
Publications (1)
Publication Number | Publication Date |
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CA1193522A true CA1193522A (en) | 1985-09-17 |
Family
ID=22704771
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
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CA000386705A Expired CA1193522A (en) | 1980-09-26 | 1981-09-25 | Growing silicon ingots |
Country Status (9)
Country | Link |
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JP (1) | JPS5785667A (en) |
BE (1) | BE890508A (en) |
CA (1) | CA1193522A (en) |
CH (1) | CH653714A5 (en) |
DE (1) | DE3138227A1 (en) |
FR (1) | FR2491095B1 (en) |
GB (1) | GB2084978B (en) |
IT (1) | IT1144865B (en) |
NL (1) | NL8104333A (en) |
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FR2908125B1 (en) | 2006-11-02 | 2009-11-20 | Commissariat Energie Atomique | PROCESS FOR PURIFYING METALLURGICAL SILICON BY DIRECTED SOLIDIFICATION |
TW201012988A (en) * | 2008-08-27 | 2010-04-01 | Bp Corp North America Inc | Gas recirculation heat exchanger for casting silicon |
US20110180229A1 (en) * | 2010-01-28 | 2011-07-28 | Memc Singapore Pte. Ltd. (Uen200614794D) | Crucible For Use In A Directional Solidification Furnace |
US20120248286A1 (en) | 2011-03-31 | 2012-10-04 | Memc Singapore Pte. Ltd. (Uen200614794D) | Systems For Insulating Directional Solidification Furnaces |
Family Cites Families (1)
Publication number | Priority date | Publication date | Assignee | Title |
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DE2933164A1 (en) * | 1979-08-16 | 1981-02-26 | Consortium Elektrochem Ind | METHOD FOR CLEANING RAW SILICON |
-
1981
- 1981-08-26 GB GB8126055A patent/GB2084978B/en not_active Expired
- 1981-09-07 CH CH5763/81A patent/CH653714A5/en not_active IP Right Cessation
- 1981-09-16 IT IT68211/81A patent/IT1144865B/en active
- 1981-09-21 NL NL8104333A patent/NL8104333A/en not_active Application Discontinuation
- 1981-09-25 CA CA000386705A patent/CA1193522A/en not_active Expired
- 1981-09-25 JP JP56151930A patent/JPS5785667A/en active Pending
- 1981-09-25 FR FR8118149A patent/FR2491095B1/en not_active Expired
- 1981-09-25 DE DE19813138227 patent/DE3138227A1/en not_active Withdrawn
- 1981-09-25 BE BE0/206076A patent/BE890508A/en not_active IP Right Cessation
Also Published As
Publication number | Publication date |
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CH653714A5 (en) | 1986-01-15 |
NL8104333A (en) | 1982-04-16 |
BE890508A (en) | 1982-01-18 |
GB2084978A (en) | 1982-04-21 |
FR2491095A1 (en) | 1982-04-02 |
GB2084978B (en) | 1984-07-04 |
IT8168211A0 (en) | 1981-09-16 |
IT1144865B (en) | 1986-10-29 |
FR2491095B1 (en) | 1986-08-22 |
DE3138227A1 (en) | 1982-07-22 |
JPS5785667A (en) | 1982-05-28 |
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