GB2084978A - Growing silicon ingots - Google Patents

Abstract

In the process of producing silicon ingots of substantially single cystallinity by heating silicon in a crucible to above its melting point and solidifying the melted silicon by extracting heat from the bottom of said crucible with a heat exchanger in heat-conducting relationship with a portion of the bottom of said crucible the improvement wherein said silicon used in said heating step has an impurity level greater than 100 ppm, whereby impurities in said silicon are caused to migrate to exterior surfaces.

Description

SPECIFICATION Growing silicon ingots 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 of carbon, primarily in the form of silicon carbide, and since 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. trichlorosilane), 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. A method that has proved useful 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 from the bottom of the crucible by providing a heat exchanger in heat conducting relationship with the bottom. The Heat Exchanger Method is described in U.S. Patents Nos.

3,653,432 and 3,898,051 and Applications Serial Nos. 4,465 filed January 18, 1979 and 967,114 filed December 7, 1978, all of which are hereby incorporated by reference.

By using selected silica and carbon in an arc furnace, Dow Corning Corporation has demonstrated the production of metallurgical silicon that is about 99.8% pure and has low concentrations of boron and phosphorous, impurities which have high segregation coefficients and are therefore difficult to segregate during directional solidification. This silicon was poured in air, resulting in a silica layer, which was etched away prior to growing an ingot by the Czochralski process (a directional solidification process). Loss of single crystallinity still resulted; but growth of a second crystal, using the best portions from the first growth as starting material, enabled the production of a single crystal material suitable for solar cell production.

We have 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 Exchanger Method (HEM) in a single step.

We have also discovered that refining reactions can be employed in the Heat Exchanger Method prior to or during crystal growth. In preferred embodiments the silica covering of metallurgical grade silicon is not etched prior to melting in order to promote silicon carbide removal by slagging of 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 the 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 expanding solid/liquid interface (as opposed to constant interface area in directional 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 increased turbulence promotes removal of impurities from the interface and their transport to the upper surface.

The vacuum operation of HEM with a high impurity content silicon (such as metallurgical silicon) allows further refinement by vaporization of high vapor pressure species. These species are impurities (such as alkali metals, manganese, 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 enhancing removal of these impurities from the melt.

A preferred embodiment of the invention will now be described with reference to the accompanying drawing, which 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.

In detail, the drawing illustrates a silica crucible 10 within the cylindrical heating chamber defined by the resistance heater 1 2 of a casting furnace of the type disclosed in U.S. Patent No. 3,898,051.

The crucible 10 rests on a molybdenum disc 11 which itself is supported by graphite rods 14 mounted on a graphite support plate 1 6 on the bottom 1 8 of the heating chamber, and is surrounded by a cylindrical molybdenum retainer 9. A helium cooled molybdenum heat exchanger 20, of the type disclosed in U.S. 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 iines at 30).

A stepped cylindrical graphite plug 50 (upper portion diameter 1.9 in., and lower portion diameter 2.5 in.) extends from bottom 1 8 upwardly through 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 bottom 24 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 si in. below the top 58 of the plug. A graphite felt insulation and/or molybdenum heat shield sleeve 64 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 silica tube 66 is suspended (by means not shown) so that one end extends into crucible 1 0 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 metallurgical grade silicon was upwardly and outwardly solidified in 6 inch crucible 10 using the Heat Exchanger Method (HEM). The melt stock was heated under vacuum condition (0.1 torr pressure) furnace temperature was kept to less than 30C above melting point, the heat exchanger temperature was kept 11 30C below the melting point, the heat exchanger temperature was decreased during growth at a rate of 4200C/hr., the furnace temperature was kept constant, and crystal growth lasted about 7.75 hrs. A single crystal 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 iast regions to solidify are at the upper surface and at the crucible wails. 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 interface causes interface breakdown and loss of single crystallinity in unidirectional solidification processes, because the HEM interface expands, this impurity buildup 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 high carbon content of this grade of silicon (up to 0.5%) resulted in formation of silicon carbide particles both at the surface of the ingot, where they could 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 was 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 + CO SiC + SiO, -, SiO + CO + Si 2 SiC + Six, 3 Si + 2 CO SiC + SiO2- < 2 Si + CO2 SiC + SiO2 < C + 2 SiO 2 Si + CO~SiC + SiO SiO2 + 3C < SiC +2 2 CO SiO2 + C -t 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 monoxide created by these reactions form bubbles, 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 interfere with the solid/liquid interface, and the impurities are, therefore, not incorporated in the crystal.

High purity silica powder has also been added to the melt stock prior to crystal growth by the HEM to further reduce silicon carbide content. In the 6" crucible 10,150 grams of silica (99% pure and in powdered form with 100,um particles) is added to 3 kilograms of metallurgical grade silicon.

In both the unetched 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 photovoltaic cells.

Solar cells fabricated from such silicon have shown up to 12.33% conversion efficiency.

In addition 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 from the melt by passing via tube 66 gasses that react with the impurities to form reaction products that are volatile or will otherwise remove themselves from the melt. Specifically, 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 1 000C above the silicon melting point to improve volatization of impurities.

After sufficient removal of impurities, the temperature is then lowered to 30C above melting point to allow crystal growth.

Claims (14)

1. A process for the production of a silicon ingot of substantially single crystallinity which process comprises heating silicon having an impurity level greater than 100 ppm, in a crucible to above its melting point and solidifying the melted silicon by extracting heat from the bottom of said crucible with a heat exchanger in heat-conducting relationship with a portion of the bottom of said crucible, whereby impurities in said silicon are caused to migrate to exterior surfaces.

2. A process as claimed in claim 1, wherein the silicon used is less than 99% pure.

3. A process as claimed in claim 2 wherein the silicon used is metallurgical grade silicon produced by the carbothermic reduction of silica.

4. A process as claimed in claim 3 wherein the metallurgical grade silicon has not been etched to remove silica that has formed on it during its manufacture.

5. A process as claimed in any one of the preceding claims further comprising adding silica to the silicon.

6. A process as claimed in any one of the preceding claims wherein the pressure in the chamber is less than 30 torr to promote removal of volatile impurities.

7. A process as claimed in claim 6 wherein the pressure is about 0.1 torr.

8. A process as claimed in any one of the preceding claims wherein the melt stock is heated to from 50 to 1 000C above the melting point of silicon to promote removal of volatile impurities.

9. A process as claimed in any one of the preceding claims 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. A process as claimed in claim 9 wherein the said reacting step comprises passing moist hydrogen through the melted silicon.

11. A process as claimed in claim 9 wherein the said reacting step comprises passing chlorine through the melted silicon.

1 2. A process for the production of a silicon ingot of substantially single crystallining substantially as hereinbefore described with reference to the accompanying drawing.

13. A silicon ingot prepared by a process as claimed in any one of the preceding claims.

14. A photovoltaic cell produced from an ingot as claimed in claim 13.