HK1197278B - Purification of a metalloid by consumable electrode vacuum arc remelt process - Google Patents

Description

Refining metalloids using consumable electrode vacuum arc melting process

Cross Reference to Related Applications

The benefit of U.S. provisional application No. 61/527,799, filed on 26/8/2011, which is incorporated herein by reference in its entirety, is claimed.

Technical Field

The present application relates to the production of refined metalloids, such as silicon, using a consumable electrode vacuum arc smelting process.

Background

Many types of electronic components, such as silicon electric crystals, silicon integrated circuits, and silicon solar cells, require high purity silicon. Since the first silicon transistor was invented, many processes have been developed for producing silicon with the desired level of purity.

Processes that have been used to manufacture high quality metals such as steel, nickel-based superalloys, titanium, and the like are known as consumable electrode vacuum arc melting (CEVAR) processes. See, for example, U.S. patent No. 3,187,07 (Pestel); number 3, 344, 840 (Buehl et al); number 3, 480, 716 (Lynch et al); number 4, 303, 797 (Roberts); no. 4, 569, 056 (Veil, Jr.); and U.S. patent application publication No. 2008/0142188Al (Ishigami) for various state of the art CEVAR processes, all of which are incorporated herein by reference in their entirety. The CEVAR process differs from non-consumable electrode vacuum arc melting in which a non-consumable electrode, such as a graphite or tungsten electrode, is used to melt titanium or zirconium, for example, as disclosed in U.S. patent No. 3,546,348 (DeCorso). U.S. patent application publication No. 2010/0154475Al (Matheson et Al) discloses a primary silicon refining process similar to the Kroll refining process for titanium and briefly relates to a secondary silicon composition refining process for high temperature vacuum melting of a silicon composition comprising silicon doped with boron and phosphorus, wherein the silicon purity is in the range of 99.99% to 99.9999%.

Generally, the CEVAR process produces refined metals through these four steps: (1) evaporating impurities as the metal electrode melts in the CEVAR furnace and is exposed to vacuum; (2) floating out liquid (molten) metal impurities having a density lower than that of the melting metal electrode; (3) separating the molecular impurities by exposing them to a high energy plasma in an arc zone between the lower end of the electrode and the beach molten (liquid) metal above the ingot being formed; and (4) solidification segregation, which causes the impurity level of the solidified metal in the ingot to be lower for certain elements than the impurity level in the adjacent liquid metal forming the solid ingot.

In a normal CEVAR process, a room temperature metal electrode is charged into a CEVAR furnace, which is then evacuated to vacuum. A high magnitude direct current (DC current) arc is then achieved between the lower end of the electrode and the CEVAR water-cooled crucible. The arc causes the lower end of the electrode to melt, whereupon the molten metal falls into a closed bottom crucible where it solidifies and cools to form a refined ingot.

Although the CEVAR process has the ability to refine a variety of metals, it is not known to use the process for refining metalloids such as silicon. Since silicon is a semiconductor and not a metal in its relatively pure state (although further refining is required for the end use mentioned above), it has a relatively high resistivity at or near room temperature. In fact, a silicon electrode that is sufficiently pure to be a candidate for refining in the CEVAR process will have a resistance at or near room temperature that is too high to permit such high arc currents to pass at any reasonably applied voltage.

The metal of the solidified ingot formed in the conventional CEVAR process is first at its solidus temperature and then gradually cooled in a water-cooled crucible, where the edge of the ingot cools faster than the center due to its proximity to the water-cooled walls of the adjacent crucible. Stresses are generated in the ingot due to differential thermal contraction, subjecting the ingot surface to tension and the center to compression. This is not a problem for metals that are typically melted using the CEVAR process, since these metals are relatively ductile, in other words, crack resistant. However, in the case of any conventional CEVAR process for melting silicon, which is brittle over a wide temperature range, such ingots will be prone to undesirable cracking.

It is an object of the present invention to provide apparatus and methods for refining metalloids such as silicon, including CEVAR furnaces and processes.

Disclosure of Invention

In one aspect, the invention is an apparatus and method for producing ingots of refined metalloids, such as silicon. The silicon electrode can be formed from one or more pieces of silicon. The electrode is preheated to a temperature at which it becomes sufficiently conductive to pass current without excessive pressure drop and electrode cracking in subsequent CEVAR refining process steps, and then melted in a CEVAR refining process comprising a low CEVAR bottom-opening and water-cooled crucible. Transferring the hot ingot resulting from the CEVAR process while the ingot is still hot into a heating system adjacent the open bottom of the low CEVAR bottom-opening crucible, wherein the heating system is controlled to prevent cracking of the silicon ingot as it cools.

In another aspect, the invention is a metalloid refining CEVAR furnace system comprising a low CEVAR bottom opening crucible having means for containing an electric arc in a CEVAR process. A heating system is provided adjacent the open bottom of the low CEVAR open-bottom crucible and has means to provide controlled cooling of a hot ingot formed in the low CEVAR open-bottom crucible to prevent cracking of the ingot as it cools. An ingot withdrawal drive system is provided to withdraw the ingot from the crucible at a rate equal to its vertical growth rate during steady state of the CEVAR refining process, so that the arc zone and the top of the solidified ingot remain within the CEVAR crucible. Optionally, a crucible/heater drive system may be provided to lift the low CEVAR bottom opening crucible, electrodes and a heater that provides a temperature controlled thermal environment for the hot ingot while the ingot remains stationary.

The above and other aspects of the invention are set out in the description and the appended claims.

Drawings

The drawings illustrate one or more non-limiting modes of practicing the invention, along with the description and examples. The invention is not limited to the depicted layout and content of the drawings.

FIG. 1 is a simplified cross-sectional view of one embodiment of a CEVAR furnace system of the present invention.

Detailed Description

In the present invention of making silicon ingots from silicon electrodes in a CEVAR refining process, the initial process step is to preheat the silicon electrodes used in the CEVAR process. The resistivity of silicon drops rapidly with increasing temperature, so a silicon electrode that has been preheated to a sufficiently high temperature, but held below its melting temperature so that it remains solid (a prerequisite for a CEVAR melting process), will permit sufficient arc current to pass through to allow the CEVAR process to begin. The pre-heating temperature required for the electrode in a particular CEVAR melt process may be specified by a CEVAR process resistivity determined by the process parameters for the particular CEVAR melt process. Such a preheating temperature would require at least several hundred degrees celsius. In addition, increasing the preheat temperature of the electrodes reduces the initial voltage drop in the electrodes, thus permitting the use of lower voltage, less expensive power inductors.

Preheating of the electrodes can be done either inside or outside the CEVAR furnace. For example, external heating in a resistance furnace with a vacuum or inert gas (controlled) atmosphere, as the electrode in air is transferred into the CEVAR furnace, can cause oxygen and nitrogen to be picked up at the electrode surface, which has the risk of increasing the impurity level in the subsequent CEVAR ingot. Optionally, a vacuum lock chamber may be provided between the external furnace chamber and the CEVAR furnace to establish a controlled environment without exposing the electrodes to air during transfer.

Passage of the arc current in the CEVAR process may be used to maintain the temperature of the electrode as the heated electrode melts in the CEVAR furnace, or an auxiliary electrode heating system inside the CEVAR furnace may be used to maintain the temperature of the electrode during the CEVAR process. In either case, providing thermal insulation around the electrode inside the CEVAR furnace facilitates reducing the energy consumed during the process. For example, carbon fiber thermal insulation material may be utilized to at least partially surround an electrode in a CEVAR furnace.

In the present invention, it is preferred to utilize a low CEVAR crucible (for use in CEVAR melting) having an internal height h in the range of about the diameter d of the ingot formed in the crucible; for example, the interior height of the low CEVAR crucible may be greater than 60% and less than 120% of the diameter of the formed silicon ingot. Alternatively, if the cross-sectional shape of the inner wall of the low CEVAR crucible is rectangular, the interior height of the crucible is approximately within the range of the length of the rectangular side of the ingot formed in the crucible; for example, the interior height of the rectangular low CEVAR crucible may be greater than 60% and less than 120% of the rectangular side of the formed silicon ingot. In a conventional CEVAR process with a closed bottom crucible, the interior height of the crucible will be much greater than the ingot height disclosed in, for example, U.S. patent No. 4,131,754 (Roberts).

The CEVAR refining process utilized in the present invention is generally similar to that set forth in the background art, such as disclosed above, except for the preheating of the silicon electrode used in the CEVAR process and the use of a low CEVAR bottom-opening, water-cooled metal crucible disclosed herein. Generally for the present invention, during the CEVAR refining process, a preheated silicon electrode is placed in a low-height CEVAR open-bottom crucible within the CEVAR furnace made gas tight and brought to a vacuum or other controlled environment. During the process, a direct current (DC current) flows through the electrode, and the melt formed below the electrode establishes an arc between the lower end of the electrode and the top of the melt, with an arc zone maintained within the height of the low CEVAR bottom-opening crucible, such that the hot (at a temperature above room temperature) solidified ingot exits the bottom of the low CEVAR bottom-opening crucible. As further described below, further controlled cooling of the hot solidified ingot exiting from the low CEVAR bottom-opening crucible is performed to substantially avoid cracking of the solidified ingot.

The ingot withdrawn from the low CEVAR crucible is placed into a heating system that provides controlled cooling in a temperature range where the ingot is likely to crack. The ingot withdrawal rate substantially matches the growth rate of the ingot during steady state operation such that the arc zone and the top of the ingot are maintained within the crucible. In an alternative configuration of the invention, the ingot is held stationary and the crucible, electrode and crucible exit heater are raised together to substantially match the growth rate of the ingot.

In the practice of one embodiment of the invention, the following process steps are performed: (1) forming an electrode from one or more pieces of silicon; (2) preheating the electrode to a temperature (by way of example and not limitation, between 800 and 1200 degrees celsius) at which it becomes sufficiently conductive (with a CEVAR process resistivity) to pass current without excessive voltage drop and prevent electrode cracking in subsequent CEVAR process steps; (3) melting the electrode by a CEVAR process; (4) when the ingot is at a sufficiently high temperature, causing the resulting hot ingot to pass to a heating system adjacent to the open-bottom CEVAR crucible to prevent cracking; and (5) controlling the heating system to prevent cracking of the silicon ingot as it cools.

In an alternative embodiment of the present invention, the preheating process step (2) described above may be performed within or outside the CEVAR furnace, as described above.

In an alternative embodiment of the invention, the above-mentioned melting of the electrode may comprise a process step of thermally insulating the electrode when the melting step is performed.

FIG. 1 depicts one embodiment of a CEVAR furnace system 10 of the present invention. A DC circuit is formed between the electrode 90 and the low CEVAR bottom opening to the crucible 12, with conductors 92 and 94 diagrammatically showing connections to an external DC power source. Electrode 94 (typically a positive potential electrode) is electrically connected to base 32 (or alternatively drives actuator 34).

Fig. 1 shows a CEVAR furnace system 10 in an intermediate (steady state) CEVAR melting process, wherein a hot solidified ingot 96 is partially formed within the interior height of the crucible. The pool of molten (liquid) metal 98 at the top of the ingot forms as a molten metal droplet that falls from the preheated electrode 90 through the arc zone AZ. Providing a heating system adjacent to the open bottom of the low CEVAR open-bottom crucible, wherein the heating system provides controlled cooling of an ingot formed in the crucible to prevent cracking of the ingot. The heating system includes an ingot heater 22 surrounding the hot ingot exiting the open-bottom crucible, and an ingot heater controller 24 that provides a temperature controlled thermal environment as the ingot passes through the vacuum heater. The temperature controlled environment is provided to allow controlled conductive heating (sometimes referred to as thermal "soak") into the interior of the ingot as it cools and controlled thermal radiation from the exterior surface of the ingot to avoid cracking.

Fig. 1 diagrammatically depicts, in dashed lines, a hermetic CEVAR furnace chamber 11 including a hermetic seal for driving an actuator 34, which is described further below.

An ingot withdrawal drive system may be provided to withdraw the solidified ingot at a rate substantially equal to the vertical growth rate during steady state CEVAR process operation such that the arc zone and the top of the solidified ingot remain within the crucible, or alternatively, a drive system may be provided to lift the crucible, electrodes and ingot heater while the solidified ingot remains stationary. At the beginning and end of the CEVAR refining process, the ingot withdrawal rate varies due to instantaneous start-up and end process parameters. As shown in fig. 1, the ingot withdrawal drive system may include a base 32 on which the bottom of the solidified ingot rests, and a drive actuator 34 that controls the rate at which the ingot is withdrawn (dropped) from the crucible in a downward direction. The substrate 32 may be configured with a profile that enhances interlocking contact with the bottom of the solidified ingot. For example, as shown in FIG. 1, the substrate 32 may be configured with a dovetail interface with the bottom of the solidified ingot 96. It is advantageous to form a resistive contact between the solidified ingot sidewall and the interior sidewall of the low CEVAR open-bottomed crucible, since the drive actuator 34 can pull the substrate down with the interlocked solidified ingot against the sidewall resistance.

As in conventional CEVAR furnaces, an electrode drive system (not shown in the figures) is provided to lower the preheated silicon electrode as its lower end melts and drips from the electrode during the CEVAR refining process.

By way of example and not limitation, in the CEVA refining process of the present invention, melting a silicon electrode at 7,000 amps DC for a silicon electrode 200cm in length and 30cm in diameter, it may be desirable to limit the initial voltage drop in the electrode to 5 volts DC, since this is a suitable value compared to the typical CEVAR process arc voltage (pressure within the CEVAR furnace) in the range of 20 volts to 40 volts DC. In this example, conventional calculations indicate that the silicon electrode will need to be preheated to a temperature that results in a silicon resistivity of 2524 microohm-cm of the electrode (CEVAR process resistivity). The temperature required to achieve the CEVAR process resistivity, which increases as the silicon purity of the silicon electrode increases, will depend on the type and level of impurities in the silicon electrode used in the particular application of the present invention.

The shape of the formed silicon ingot, and thus the interior shape of the low CEVAR open-bottomed crucible, can be of various cross-sectional configurations including cylindrical or rectangular, with or without an upwardly tapering interior diameter or perimeter to facilitate downward movement of the ingot as the hot ingot forms.

In some embodiments of the invention, continuous charging is performed on a CEVAR furnace having a pre-heated electrode such that the resulting continuous ingot is formed from a series of pre-heated electrodes. In these embodiments, an ingot truncation device may be provided to truncate the continuous ingot produced as the CEVAR refining process continues for removal of ingot fragments.

In the above embodiments of the invention, the expression "vacuum" in CEVAR means melting at any pressure level below one atmosphere.

In other embodiments of the invention, melting is advantageously carried out in an inert gas atmosphere at atmospheric pressure or even above atmospheric pressure, and this "pressure arc melting" of silicon is within the scope of the invention.

The invention has been described in terms of preferred examples and embodiments. Equivalents, alternatives and modifications, aside from those expressly stated, are possible within the scope of this invention. Modifications thereof can be made by one skilled in the art having the benefit of the teachings of this specification without departing from the scope of the invention.

Claims (14)

1. A method of manufacturing a silicon ingot from a silicon electrode in a consumable electrode vacuum arc melting, CEVAR, refining process, the CEVAR refining process being performed in a CEVAR bottom opening crucible placed in a CEVAR furnace, the method comprising:

heating the silicon electrode to a heated temperature below the melting point of the silicon electrode prior to initiating the CEVAR refining process to form a preheated silicon electrode having a CEVAR process resistivity;

melting the preheated silicon electrode using the CEVAR refining process for forming a silicon ingot at an elevated temperature at the open bottom of the CEVAR bottom-opening crucible;

passing the silicon ingot at an elevated temperature through a heating system adjacent to the CEVAR open-bottom crucible; and

adjusting a heating system to provide a temperature controlled thermal environment for the silicon ingot at an elevated temperature to cool the silicon ingot without cracking as the silicon ingot exits the CEVAR open-bottom crucible.

2. The method of claim 1, wherein heating the silicon electrode to form the preheated silicon electrode is performed in an external heating chamber prior to placing the silicon electrode in the CEVAR furnace, and transferring the preheated silicon electrode from the external heating chamber into the CEVAR furnace is accomplished in a controlled environment.

3. The method of claim 1, wherein heating the silicon electrode is performed after placing the silicon electrode within the CEVAR furnace.

4. The method of any of claims 1 to 3, wherein heating the silicon electrode further comprises: heating the silicon electrode to the heated temperature in a range of between 800 to 1200 degrees Celsius to form the preheated silicon electrode.

5. The method of any of claims 1 to 3, further comprising: thermally insulating the preheated silicon electrode while melting the preheated silicon electrode using the CEVAR refining process.

6. The method of any of claims 1 to 3, further comprising: placing a supplemental heater within the CEVAR furnace to heat the preheated silicon electrode during the CEVAR refining process.

7. A consumable electrode vacuum arc melting (CEVAR) furnace system for producing refined silicon ingots from silicon electrodes, the CEVAR furnace system comprising:

a silicon electrode heating apparatus for preheating the silicon electrode to form a preheated silicon electrode;

a gas tight CEVAR furnace chamber;

a CEVAR open-bottom crucible for containing an arc zone from a CEVAR refining process melting the preheated silicon electrode, the CEVAR open-bottom furnace chamber being placed in the gas-tight CEVAR furnace chamber;

a preheated silicon electrode drive system for lowering the preheated silicon electrode within the CEVAR open-bottomed crucible as a lower end of the preheated silicon electrode melts in the CEVAR refining process;

an ingot heating device disposed adjacent to the open bottom of the CEVAR open-bottom crucible through which the silicon ingot formed in the CEVAR refining process passes;

an ingot heating controller for controlling the ingot heating apparatus to provide a temperature controlled thermal environment for the silicon ingot passing through the ingot heating apparatus; and

an ingot withdrawal drive system for withdrawing the silicon ingot from the CEVAR open-bottom crucible, optionally at a vertical growth rate of the silicon ingot during a steady state of the CEVAR refining process, or raising the CEVAR open-bottom crucible, the preheated silicon electrode and the ingot heating apparatus at a vertical growth rate of the silicon ingot during the steady state of the CEVAR refining process.

8. A CEVAR furnace system according to claim 7 wherein the internal height of the CEVAR open-bottomed crucible is at least 60% and less than 120% of the diameter of the silicon ingot formed in the CEVAR refining process.

9. A CEVAR furnace system according to claim 7 or 8, further comprising: a vacuum lock chamber connected between the silicon electrode heating device and the CEVAR open-bottomed crucible in the airtight CEVAR furnace chamber to prevent exposure of the preheated silicon electrode to air during transfer from the silicon electrode heating device to the airtight CEVAR furnace chamber.

10. A CEVAR furnace system according to claim 7 or 8, further comprising: an auxiliary electrode heater positioned within the gas-tight CEVAR furnace chamber to heat the preheated silicon electrode during the CEVAR refining process.

11. A CEVAR furnace system according to claim 7 or 8, the ingot retrieval drive system further comprising: a base on which a bottom of the silicon ingot rests and an actuator connected to the base to control a rate of withdrawal of the silicon ingot from the CEVAR open-bottom crucible.

12. A CEVAR furnace system according to claim 11, the substrate having a profile for interlocking contact with the bottom of the silicon ingot.

13. A CEVAR furnace system according to claim 12, further comprising: a DC power supply having a first output and a second output connected between the preheated silicon electrode and the substrate or the drive actuator.

14. A CEVAR furnace system according to claim 7 or 8, the CEVAR open-bottom crucible having an inner wall with a rectangular cross-section, the inner height of the CEVAR open-bottom crucible being at least 60% and less than 120% of the length of the rectangular side of the silicon ingot formed in the CEVAR refining process.