WO2008026931A1

WO2008026931A1 - Method and equipment for manufacturing multi-crystalline solar grade silicon from metallurgical silicon

Info

Publication number: WO2008026931A1
Application number: PCT/NO2007/000296
Authority: WO
Inventors: Dag ØVREBØ; William George Clark
Original assignee: Norsk Hydro Asa
Priority date: 2006-08-30
Filing date: 2007-08-22
Publication date: 2008-03-06

Abstract

A method and apparatus for manufacturing multi-crystalline solar grade silicon from metallurgical silicon. The silicon is provided in a boat (2) in a closed housing (3), and is subjected to horizontal refining in a repeatable process where one or more zones (X) of the silicon at a time is heated by a heater (1) at preferably constant speed along the total length of the boat (2), whereby a purging gas under the refining operation is passed along the boat (2) from one inlet (4) end of the housing to an outlet (5) at the other end of the housing purging of any impurities being generated under the refining operation.

Description

Method and equipment for manufacturing multi-crystalline solar grade silicon from metallurgical silicon

The present invention relates to a method and apparatus for manufacturing multi- crystalline solar grade silicon from metallurgical silicon.

Multi-crystalline silicon wafers for photovoltaic solar cells are predominantly produced by melting refined silicon into blocks which is cut into wafers. The starting material for this process is metallurgical silicon, a typically 99% pure material made from carbo-thermal reduction of silicon oxide (silica).

Metallurgical silicon has to be purified before it can be used for PV wafer production. This is now commonly done via the so-called silane route which was developed by Union Carbide and others in the 80's. In this process, the silicon is chlorinated with hydrogen and made into trichlorosilane and then further processed into silane (SiH₄). The silane is then thermally decomposed into poly silicon - a silicon powder/granule with less than 25 ppm metallic impurity. This process is both capital and energy intensive, and the yield is only 20%, the rest comes off as oxidised silicon (SiCI₂), which will have to be disposed or sold as a low value by-product.

The refined poly silicon (also called Solar Grade Silicon- SGS) is then shipped to wafer producers, who melt the metal in large blocks and who subsequently cut the large blocks into smaller blocks suitable for wafer sawing by wire. The wafer manufacturing process has a yield of 35% (Si into the plant vs. Si out as wafer). Approx 30% is further lost in wafer cutting and the remaining is lost as scrap and off cuts in melting and block preparation. Hence, by today's known method the Si yield in the total process from metallurgical silicon to wafer is only 7% which is partly the reason for the high cost. Due to the above low yield process, a number of attempts have been made to find alternative routes, the most common one being some sort of metallurgical refining, i.e. avoiding the silane gas phase. It is commonly known that trials have been made to do vertical zone refining where silicon is melted in a pot, and directionally solidified from the bottom and upwards in order to bring the impurities to the top. This works for most metallic impurities as the solubility in liquid is much higher than in a solid, and at the solid/liquid interface, the impurities will migrate to the liquid phase and become concentrated in this phase. This does not, however, work for boron and phosphorus which has got similar solubility in liquid and solid. Hence, various ways of gassing has been tried to remove remaining impurities. None of these attempts have shown to be commercially viable, mostly due to the need to re-melt several times to achieve the required low impurity.

With the present invention is provided a process, which provides the required SGS quality and which at the same time is simple and cheap compare to the known SGS grade processes.

The method according to the invention is characterized in that the silicon, being provided in a boat in a closed housing, is subjected to horizontal refining in a repeatable process where one or more zones of the silicon at a time is heated by a heater at preferably constant speed along the total length of the boat, whereby a purging gas under the refining is passed along the boat from one inlet end of the housing to an outlet end of the housing purging off any impurities generated under the refining operation, as defined in the attached independent claim 1.

The apparatus is characterised in that the equipment is of the horizontal refining type and includes a boat for the silicon to be provided in a closable housing, which housing at one end is provided with an inlet for purge gas and an outlet for purge gas together with any impurities being purged off the silicon during refining, a heater provided around the boat or around the housing for heating a zone or zones of the silicon, and that the boat or heater is movable in relation to one another in the longitudinal direction of the boat along its total length, as defined in the independent claim 6.

Preferred embodiments of the invention are defined in the subordinate claims 2 - 5 and 7 - 11. The inventive process has a number of advantages over vertical zone refining:

• The molten metal will circulate within the slice due to the temperature gradient and magnetic field from the RF coil, and thereby continuously bring impurities to the top where it comes in contact with a gas thereby enabling continuous gas purification. It is known that a small amount of water in argon or another inert gas will oxidise carbon and boron, which can then be flushed away as boron and carbon oxide gas. Phosphorous will evaporate if the gas is at slight vacuum and thereby be removed as a volatile gas. • The rate of purification can be increased by placing several induction coils, say four coils which travel together. When VA of the bar is done, they are all shifted back VA distance and coil no 2 takes over where coil no 1 left and the process is repeated.

• The molten slice will form at a slight angle, and hence there will be a zone refining effect from the top to the bottom as well as from the right to the left (zone travelling left as shown in drawing).

• Only a small amount of the metal is melted at any time (as opposed to conventional vertical zone refining where the entire pot is melted), which makes the process energy efficient.

The invention will be further described in the following by way of example and with reference to the drawings where:

Fig. 1 shows a schematic view of a horizontal zone refining apparatus according to the present invention,

Fig. 2 shows in larger scale part of the apparatus according to the invention shown in Fig. 1

The method according to the present invention is, as stated above, based on horizontal zone refining. Referring to Fig. 1 and 2, a travelling heater unit 1 , positioned at one end of a boat (metal basin) 2, melts a narrow 'X' section of the charge. The boat is provided in a closable housing 3 to obtain a controlled atmosphere in a space 7 surrounding the boat. Purge gas is provided to be supplied through an inlet 4 at one end of the housing 3 and is further provided to be purged together with impurities from an outlet 5 at the other end of the housing. The housing may preferably be of cylindrical shape and made of a suitable material such as quartz. The heater 1 , in the form of an electromagnetic coil, is provided on the outside of the housing. The conducting threads of the electromagnetic coil 1 form an angle to the boat and direction of travel (arrow 6 in Fig. 2), thereby providing a vertical (transversal relative to the boat) zone refining effect in addition to the horizontal effect. Further, the magnetic field of the coil induce a rotation in the molten zone obtained by the coil and thereby makes gas purging more effective as impurities continually are being are being brought to the gas/liquid interface by the rotation.

When performing the method according to the invention metallurgical silicon is provided in the boat 2, which in turn is provided in the housing 4 being closed off from the surroundings. The heater is switched on and a purge gas in the form of Ar, O₂ H₂O or other suitable gasses, or combination of gasses, is admitted to the space 7 between the housing and boat.

After the molten zone is established the heater unit 1 is traversed along the length of the boat at a constant speed. The first 'pass' is usually quicker, with high heat, to consolidate the material. Subsequent passes are slower to accomplish purification. Two or three passes may be required to achieve desired purity. Purification is carried out by the act of crystallization: as the narrow molten zone travels along the bar, impurities are pushed forward and carried with the melt. The molten metal will circulate within the slice due to the temperature gradient and magnetic field from the coil, and thereby continuously bring impurities to the top where it comes in contact with the flush gas thereby enabling combined gas purification. It is known that a small amount of water in argon will oxidise carbon and boron, which then can be flushed away as boron and carbon oxide gas. Phosphorous will evaporate if the chamber formed by the housing 13 is at slight vacuum and thereby be removed as a volatile gas. The molten slice will form at a slight angle, and hence there will be a zone refining effect from the top to the bottom as well as from the right to the left (zone travelling left as shown in drawing). At the conclusion of the process the bar is assessed and the end where all of the impurities are agglomerated is cut off. This may represent 5 - 10% of the initial, total batch material. The rate of purification can be increased by placing, in stead of one, several induction coils, say four coils which travel together. When VΛ of the bar is done, they are all shifted back 14 distance and coil no 2 takes over where coil no 1 left and the process is repeated.

As a further improvement of the above method a silicon bar with rectangular cross section may be made, say with a height and width of 15 x 15 cm directly by zone refining. This zone-refined bar, combined with gas purification would have the required purity for wafer production. Hence the bar can be cut into wafers directly. In this way one will save a number of process steps like crushing, re-melting and cutting of silicon blocks into bars suitable for wafer cutting. The overall process will also be more efficient and increase the overall Si yield from 7% to 60%.

The invention defined in the attached claims is not limited to the figures and examples as described above. Thus, the coil may instead of being provided on the outside of the housing, be provided inside the housing. Further, the electromechanical coil may be stationary and the boat moving, instead of vice versa.

Claims

1. A method for manufacturing multi-crystalline solar grade silicon from metallurgical silicon, characterised in that the silicon, being provided in a boat (2) in a closed housing (3), is subjected to horizontal zone refining in a repeatable process where one or more zones (X) of the silicon at a time is heated by a heater or heating elements (1 ) at preferably constant speed along the total length of the boat (2), whereby a purging gas under the refining operation is passed along the boat (2) from one inlet (4) end of the housing to an outlet end (5) of the housing purging off any impurities generated under the refining operation.

2. A method in accordance with claim 1 , characterised in that the heater in the form of one or more electromagnetic coils (1) is used.

3. A method in accordance with claim 1 and 2, characterised in that the heater (1) is movable along the outside of the housing from inlet (4) to the outlet end (5).

4. A method in accordance with claim 1 and 2, characterised in that the heater is stationary, while the boat moves inside the housing.

5. A method in accordance with claim 1 -3, characterised in that the purge gas is Ar, O₂ H₂O or a combination of these gasses.

6. Equipment for manufacturing multi-crystalline solar grade silicon from metallurgical silicon in the form of a bar, characterised in that the equipment is of the horizontal refining type and includes a boat (2) for the silicon to be provided in a closable housing (3), which housing (3) at one end is provided with an inlet (4) for purge gas and an outlet (5) for purge gas together with any impurities being purged off the silicon during refining, a heater (1) provided around the boat or around the housing for heating a zone (X) or zones of the silicon, and that the boat or heater (1) is movable in relation to one another in the longitudinal direction of the boat along its total length.

7. Equipment according to claim 6, characterised in that the space within the boat (2) corresponding to the shape of the silicon bar is of square cross section.

8. Equipment according to claims 6 and 7, characterised in that the heater (1) includes two or more heating elements spaced apart, one after the other and being designed to heat two or more zones (X) at a time.

9. Equipment according to claim 7 - 8, characterised in that the heater or heating elements (1) is in the form of an electromagnetic coil or coils.

10. Equipment according to claim 7, characterised in that the electrical windings (8) of the electromagnetic coil (1) are provided with an angle relative to the direction of movement between the heater and boat (2).

1. Equipment according to claim 7 - 9, characterised in that the boat (2) is stationary provided within the housing (3), while the heater (1) is provided to be moved along the boat (2) or vice versa.