NL1031734C2 - Process for purifying a semiconductor material using an oxidation-reduction reaction. - Google Patents

Description

Brief description: Process for purifying a semiconductor material using an oxidation-reduction reaction

DESCRIPTION

The present invention relates to a method for purifying a semiconductor material using an oxidation-reduction reaction.

Silicon is one of the best known and most widely used semiconductor materials and is used extensively in industry, such as for example in solar cells, diodes, transistors, integrated circuits (chips) and other areas of electronics. Such applications require a high purity of silicon, such as so-called SoG silicon (Solar Grade, purity of> 99.9999% (6N)) or EG silicon (Electronic Grade, purity of> 99.99999999% (> 9N) )). In order to obtain such a pure silicon, so-called MG silicon (Metallurgy Grade, purity of 98 - 99%) is frequently used. Such silicon materials are generally recovered from SiO 2 (e.g., sand) by reduction with carbon (C) to form carbon monoxide (carbothermal reduction).

Examples of impurities that may be present in MG silicon are carbon, boron or phosphorus compounds and metallic elements, such as for example iron, aluminum, titanium and vanadium. Such impurities cause, among other things, a deterioration of the semiconductor properties of the material, which may, for example, lead to a reduced efficiency for solar cells. In addition, such degraded semiconductor properties are also undesirable for other electronic applications.

Certain impurities, such as, for example, iron, aluminum, boron, phosphorus, titanium and vanadium, are more harmful than others and, moreover, certain impurities are more difficult to remove from silicon than others. The simplicity of removal has to do with, among other things, the separation coefficient or the difference in solubility of a certain element in liquid and solid silicon.

The standard process to purify silicon which is currently being applied consisting of a number of steps, namely a first step in which MG-Si is chlorinated with HCl gas up to 90% SiHCl 3 (trichlorosilane) and 10% SiCl 4 (silicon tetrachloride) with hydrogen gas is being formed.

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Subsequently, the silicon compounds formed are purified in the gas phase by fractional distillation until the amount of impurities is at the level of ppb (parts per billion, parts per billion) and ppm (parts per million, parts per million).

In a next step, the so-called "Siemens process", deposition of purified silicon takes place using the purified gaseous silicon compounds and hydrogen. This reaction is carried out at a temperature of, for example, 900-1100 ° C on an electrically heated graphite electrode to form polycrystalline silicon with a purity of 99.999999% (7N) and HCl. A similar method for purifying silicon is disclosed in U.S. Patent No. 4,213,937.

The disadvantage of such a method is, firstly, that corrosive gas such as HCl and explosive gas such as hydrogen are required for the reaction, which imposes strict requirements on the equipment used and safety regulations. Secondly, the process is very energy consuming because of the thermal processes that take place during the conversion of silicon compounds to crystalline silicon.

The electrochemical deposition of silicon on a silicon cathode using silicon tetrachloride (SiCl4) as a starting material is known from T. Matsuda et al., Chemistry Letters, 1996, pages 569-570.

The electrochemical synthesis of silane (SiH4) from MG silicon using hydrogen gas is known from T. Nohira et al., Electrochemistry 2005, 73 (8), pages 692-699.

JP 06/173064 discloses a method for electrolytically purifying titanium by electrolysis in molten salts.

WO 02/099166 discloses a method for making silicon by dissolving SiO 2 in an electrolyte consisting of CaCl 2 and CaO.

It is an object of the present invention to provide a method for purifying semiconductor materials that is energy efficient.

It is also an object of the present invention to provide such a method In which method no corrosive or explosive gases are necessary.

In addition, it is an object of the present invention to provide such a method which can be carried out with simple equipment without strict safety regulations.

One or more of the above objectives are achieved by the method according to the preamble, characterized in that the method comprises the steps of: a) oxidizing an anode placed in an electrolyte of a semiconductor material containing one or more impurities, in which electrolyte a or more ionic compounds are present. which ionic compounds enter into reactions with the semiconductor material and the one or more impurities to form one or more compounds of the formula I: AX 2 H (y.2), (I) wherein A = semiconductor ion X = halogen ion H = hydrogen

15 y = valence of A

z = integer greater than or equal to z and one or more associated reaction products.

b) adding the one or more compounds of the formula I obtained in step a) to a cathode placed in an electrolyte and performing the reduction there to form purified semiconductor material on the cathode, during which reduction also a or more ionic compounds are formed; wherein the one or more ionic compounds formed in step b) are returned to step a) and wherein the anode and cathode are interconnected for electron transfer.

The present invention will be further elucidated with reference to silicon as an example of a semiconductor material, however, the present invention also applies to other semiconductor materials and conductor materials, such as, for example, germanium, titanium and aluminum.

In the present process, two semi-reactions are coupled, namely an anodic reaction in which crude silicon is converted into halogenated silicon compounds and a cathodic reaction in which purified halogenated silicon compounds are converted into purified silicon.

The present invention provides an energy efficient method 4 by performing the first reaction (step a according to the present method) at an anode and carrying out the second reaction (step b according to the present invention) at a cathode. The electrons that are formed at the anode are used at the cathode, so that in principle no additional energy is required since a reversible (equilibrium) reaction is used. Once the reaction has been started, it is able to sustain itself. However, the supply of additional energy by increasing the electrical potential difference can considerably speed up the reaction.

A further advantage of the present invention is that HCl gas and hydrogen gas are no longer required since ionic compounds, such as, for example, chlorine ions or other ions, are used instead of gaseous compounds. These ionic compounds react with the semiconductor material of the anode to form the compounds of formula I, such as, for example, halogenated silicon compounds if a silicon anode is used. This achieves one or more of the above objectives.

The impurities present in the semiconductor material to be purified also enter into one or more reactions with the one or more ionic compounds to form corresponding reaction products. Such associated reaction products. if MG-Si is used as an anode, for example, FeCl3, PCI3, BCI3, TiCl4, AICI3 and VCI4.

The electrolyte serves as a transport medium for the ionic reaction products from the cathode to the anode. The recycling of these ionic reaction products is therefore preferably carried out by the electrolyte.

The addition of the one or more compounds of formula I to the cathode can for instance take place by bubbling such compounds along the cathode as a gas. However, other methods can also be applied.

It is preferred that the compounds of formula I formed according to the present invention be purified. This can be done by a preferred embodiment of the present method, comprising an additional step c) which is performed after step a) and before step b) wherein step c) comprises two sub-steps, c1) and c2), namely

c1) withdrawing the one or more compound of formula I

5 and the one or more associated reaction products on the electrolyte; c2) separating the one or more compounds of formula I from the one or more associated products.

In the case of MG-Si as an anode, in this embodiment both the halogenated silicon compounds formed and the associated reaction products, preferably as gas, are withdrawn from the electrolyte in which the anode is placed and, after purification thereof, are added to the cathode where the reduction can take place to form silicon and ionic compounds (chlorine ions). The ionic compounds preferably move through the electrolyte to the anode where they react with MG-Si to form additional halogenated silicon compounds.

Such an embodiment of the present invention ensures excellent purification and prevents contamination of the electrolyte. Any impurities present in the starting material such as, for example, boron and phosphorus are removed before the purified halogenated silicon compounds lead to pure silicon on the cathode by deposition. In particular, the removal of boron and phosphorus are of great importance since these elements are used as dopants in further processing of pure silicon. The amount of such dopants must be determined very accurately in order to obtain the desired properties and it is therefore of great importance that these elements are present as little as possible in the purified silicon.

Step c1) can be carried out, for example, by phase separation. Halogenated silicon compounds are volatile at the temperatures used and can therefore be captured as gas from the liquid electrolyte. However, other extraction methods can also be applied.

Step c2) can be carried out, for example, by fractional distillation. The halogenated silicon compounds and the halogenated impurities have different boiling points and can therefore be separated by distillation. However, other separation methods can also be used.

It is preferable that the compounds of formula 1 are separated in step c2) until they have a purity of greater than or equal to 99.99% (4N), preferably 99.999% (5N), in particular 99.9999% ( 6N) and 6 in particular 99.99999% (7N). The desired purity will depend on the desired application, as already explained above.

In particular, silicon is used as the oat conductor material in step a), and in particular MG silicon, because of the good availability of the latter.

The material of the cathode is preferably selected from the group consisting of silicon, carbon, silver, molybdenum, platinum and tungsten and one or more combinations thereof. However, other conductive materials are also applicable. In particular, silicon and in particular silicon with at least the desired purity, that is, a purity of greater than or equal to 99.99% (4N) and preferably 99.9999% 6N) are preferred as cathode material.

Such materials give sufficiently good electron conduction properties and they are inert. These materials have a low resistance to electron conduction at the operating temperature, give minimal fouling of the silicon to be purified, minimum fouling of the electrolyte and contain spots on the surface with a low activation energy for easy crystallization of silicon.

The advantage of using silicon and in particular silicon with at least the desired purity as material for the cathode is that it is easier to remove the silicon formed from the cathode, for example by scraping it off, without any impurity being formed in the silicon formed due to the scraping of material from the cathode.

The material for the anode preferably consists of the semiconductor material to be purified, it being possible for the semiconductor material to be purified to form the complete anode, that is to say that the anode consists of, for example, a wire, rod or chunks of MG silicon . It is also possible for the anode to consist of a core of an inert material that serves as a support material, which inert material is preferably not a conductor for electrons, which inert material is surrounded by a sheath of the semiconductor material. Such a jacket can for instance be applied to the core by means of deposition, sputtering or coating.

It is particularly preferred if the semiconductor material to be purified is porous or powdered. As a result, the material to be purified has a large surface area, which increases the reaction rate.

7

In the method according to the present invention, two electrolytes are used, namely an electrolyte in which the anode is placed and an electrolyte in which the cathode is placed. It is preferred that these electrolytes are the same. This makes the reactor design simpler since the anode and the cathode can be present in a single vessel. This also simplifies the return of the ionic connections from the cathode to the anode.

The electrolyte in which the cathode is placed preferably meets the following requirements: high ion conduction (low resistance) to minimize energy loss due to resistance and to effect mass transfer of the formed ionic compounds; - higher standard reduction potential (Eo) than the compounds of formula I to prevent electrolysis of the electrolyte; a high standard reduction potential is preferable because this allows the application of larger potential differences to the electrodes (see also below) whereby the reaction rate increases; low solubility for the semiconductor material formed on the cathode to prevent loss thereof in the electrolyte; Low solubility and diffusion rate in the semiconductor material formed on the cathode to prevent contamination thereof; - high solubility in water to enable easy removal of the electrolyte during washing of the semiconductor material formed on the cathode.

The electrolyte in which the anode is placed preferably meets the following requirements: high ion conduction (low resistance) to minimize energy loss due to resistance and to effect mass transfer of the formed ionic compounds; - high standard reduction potential (Eo) to prevent electrolysis of the electrolyte; low solubility for the anode material to prevent contamination of the electrolyte, since such contamination of the electrolyte can lead to contamination of the semiconductor material formed on the cathode.

- low solubility of compounds of formula I and associated reaction products to simplify their extraction - low solubility of associated reaction products to prevent contamination of the electrolyte.

Melted salts may be mentioned as electrolyte. It is preferred that the electrolyte is a salt of one or more halogens and one or more (earth) alkali metals or one or more combinations thereof, in particular calcium chloride (CaCl 2). However, also KCl, LiCl 2 and BaCl 2, but also nitrates, carbonates and sulfates and combinations thereof can be mentioned as electrolyte. Such electrolytes can be used both as an electrolyte in which the anode is placed and as an electrolyte in which the cathode is placed.

Calcium chloride has a low resistance (2 ohm'1.cm'1 at 782 eC), a high reduction potential (-3.887 V at 25 eC and -3.296 V at 850 ° C), a low diffusion speed in silicon (6.10'14 cm2) / s at 1100 ° C), high solubility in water (56% by weight at 50 ° C), low solubility for silicon and halogenated silicon compounds, high surface tension - high internal gas pressure (0.1423 N / m, at 927 ° C) and low toxicity, but also has a high melting point (782 ° C).

In an embodiment of the present invention, a membrane is used that is permeable to the one or more ionic compounds formed in step b). The membrane is present between the electrolyte in which the anode is placed and the electrolyte in which the cathode is placed.

Such a membrane should be selectively permeable to the passage of ionic reduction compounds from cathode to anode and not to oxidation products from anode to cathode. A membrane can prevent contamination of the electrolyte in which the cathode is placed, which contamination can consist of positively charged ions (metal and others) that can deposit on the cathode and thus contaminate the purified semiconductor material.

The membrane is preferably made from a ceramic material, such as, for example, La-M-O-CI (M is alkaline earth metal, such as, for example, Ca / Sr). Such a ceramic material is an example of a so-called "Solid Electrolyte" which is used in fuel cells.

9

It is also possible to use a capture electrode which is connected to an external voltage source and which has an equal or slightly higher potential as the cathode, the capture electrode being placed in the electrolyte between the anode and the cathode.

The capture electrode is not exposed to the compounds of formula I that is added to the cathode and the sole purpose of this capture electrode is to provide a site for depositing and securing impurities (oxidation products) that are released to the anode.

In an embodiment of the present invention, one or more compounds of formula I is additionally added in step 10 b) to compensate for any loss observed in the purification according to step c2). Such compounds should be of the desired purity, i.e. a purity of at least 99.99% (4N) and in particular at least 99.9999% (6N).

The conditions for carrying out the present reaction, such as pressure, temperature and reduction potentials, are determined by the reaction rate of the anodic and cathodic reactions.

After this, the reaction conditions are written if silicon is used.

Cathodic reaction conditions It is preferable to obtain a crystalline, purified silicon. The minimum temperature for depositing silicon around the electrode is approximately 400 ° C due to the conductivity of silicon that increases with increasing temperature, as is usual with semiconductors. The transition from amorphous to crystalline silicon occurs at a temperature of 470 ° C and therefore the temperature at the cathode is preferably at least 400 ° C, in particular at least 470 ° C. In principle, it is also possible for amorphous silicon to be formed at the cathode which is subsequently converted to crystalline silicon at a specific temperature of the cathode, for example a temperature higher than 470 ° C. Amorphous silicon may also be used for certain applications.

Crystalline silicon can also be formed at a lower temperature (<400 ° C) by using the high resistance of silicon at low temperature, which leads to local heating of the cathode which leads to formation of crystalline silicon. If the resistance becomes higher, the temperature will also become higher, which leads to a better crystalline form of, for example, silicon.

10

However, a higher resistance also leads to a higher energy consumption, which is disadvantageous. It is therefore important to find a good compromise.

In order to obtain a high silicon deposition rate and thus a high reaction rate, the concentration of the compounds of formula I at the cathode should be as high as possible. In the case that halogenated silicon compounds are used, they are used as gas at the application temperature of at least 400 ° C because of their boiling point and therefore the reaction will be carried out in three phases, namely solid (cathode), liquid (electrolyte) and gas (halogenated silicon compound).

The pressure should preferably be high to allow a high concentration of the compounds of formula I. If the halogenated silicon compounds are added to the cathode as a gas (bubbling), the electrolyte should have the highest possible surface tension and the bubbles should be as small as possible in order to obtain the highest possible internal pressure. This is because the internal pressure in a gas bubble is determined by the external pressure of the liquid and the surface tension divided by the radius of the bubble.

If a membrane and / or a capture electrode is used, the conditions such as temperature and pressure must be adjusted accordingly.

Anodic reaction conditions It is desirable that all products formed at the anode are gaseous to minimize contamination of the electrolyte. Whether the products formed at the anode are gaseous depends on the temperature and pressure used in combination with the boiling points of such formed compounds.

In order to obtain gaseous products, the boiling point of the compound formed must be lower than the temperature at which the reaction is carried out. The boiling point of, for example, the desired SiCl 4 is 57.6 ° C which will be gaseous at the reaction temperature. Examples of frequently present contaminants in MG-Si that are gaseous at a reaction temperature of 400 ° C are the following: BCI3 (12.5 ° C), PCI3 (75.95 ° C), TiCl4 (136.4 ° C) ), PCI5 (160 ° C), AICI3 (182.7 ° C) and FeCl3 (315 ° C). Such by-products will be separated as a gas at a reaction temperature of 400 ° C and therefore these by-products do not cause any problems with the purification. However, FeCl 2, which also frequently occurs during the reaction, has a boiling point of 1026 ° C and will therefore not be able to be captured as a gas at a temperature of 400 ° C and must therefore be removed in another way. This requires, for example, a membrane or capture electrode.

It is preferable that an external voltage source which is not limited to a particular type is used to apply a potential difference between the anode and the cathode, which potential difference is chosen between 0.01 V and the standard reduction potential of the electrolyte.

Only very little energy is required for the actual performance of the present reaction because a reversible reaction is applied between the semiconductor and the compounds of formula I. In theory this would correspond to a potential difference of 0 V. However, in practice this would infinitely long because there is no driving force for the reaction. In order to be sure that the reaction speed is high enough to be practically applicable, a potential difference of more than 0 V must be applied, such as, for example, more of 0.01 V. The potential difference should not be higher than the standard reduction potential. of the electrolytes used to prevent electrolysis thereof. It is preferable that the potential difference is as high as possible with the electrolyte used. Therefore, it is preferable to use an electrolyte with a high standard reduction potential to allow a high reaction rate.

The reduction potential (E ') of SiCl 4 at 25 ° C is -1.451 V and at 850 ° C is -1.389 V. The reduction potential of some of the aforementioned electrolytes is as follows: MgCl 2 (-3.066 V at 25 ° C and -2.42 V at 850 ° C), CaCl 2 (-3.887 V at 25 ° C and -3.296 V at 850 ° C), NaCl (-3.982 V at 25 ° C) and KCl 25 (-4.235 V at 25 ° C)

The applicability of electrolytes depends on the mobility of the ionic compounds in the electrolyte. For example, if CaCl 2 is used as an electrolyte at a temperature of 850 ° C, a potential difference of 1.907 V (-3.296 V minus - 1.389 V) can lead to undesired electrolysis of the electrolyte.

in an embodiment of the present invention, the electrolyte is forced to flow from the cathode to the anode and vice versa. In such an embodiment, the electrolyte is fed at a certain speed into a flow of the cathode and the anode, and in particular the flow of the electrolyte from the anode to the cathode is physically separated from the flow of the electrolyte from the cathode to the anode. This is for instance possible through a closed loop through which the electrolyte is forced to flow and wherein the anode and cathode are placed at separate places in the closed loop. In particular, it is advantageous if a membrane or in particular a capture electrode is placed between the anode and the cathode.

The advantage of such a construction is that, if the flow rate is sufficient, the purification of the electrolyte is simplified. Cations present in the electrolyte want to go to the cathode and are made at an anode. Due to the flow, these can only be done via the route where the membrane / capture electrode is present. The cations cannot go against the flow at a certain flow rate and will therefore always be forced to pass the membrane or the capture electrode. A capture electrode is preferred over a membrane because a capture electrode neutralizes cations and a membrane only captures cations.

Such a circulation flow of the electrolyte can be achieved, for example, by using a pump or by applying a density difference. A density difference can for instance be brought about by a temperature difference but also by the presence of gas bubbles (bubbles) at an electrode, such as the cathode. However, other methods for establishing a circulation flow can also be used.

An additional example of the use of circulation is that in the case that chlorine ions are present, they are not transported to the anode by diffusion alone and that a better transfer of dust to the electrode takes place as a result of the flow, whereby a higher conversion rate is obtained.

The present method can be carried out using any suitable equipment or combinations thereof, such as, for example, a device for processing MG-silicon (grinding, alkalis), an electrolytic cell (anode and cathode and electrolytes), a separation device for purifying of halogenated silicon compounds (distillation, absorption, absorption, membrane separation) and a device for processing pure silicon (washing, grinding).

1031734

Claims (17)

A method for purifying a semiconductor material using an oxidation-reduction reaction, characterized in that the method comprises the steps of: a) oxidizing an anode placed in an electrolyte of a semiconductor material containing one or more impurities, in which electrolyte one or more ionic compounds are present, which ionic compounds enter into reactions with the semiconductor material and the one or more impurities to form one or more compounds according to formula I: AX 2 H (y 2), (I) wherein A = ion of semiconductor X = halogen ion Method according to claim 1, wherein the method comprises an additional step c) which is carried out after step a) and before step b) wherein step c) comprises two sub-steps, c1) and c2), namely 30 c1) withdrawing the one or more compound of formula I and the one or more associated reaction products on the electrolyte; c2) separating the one or more compounds of formula I from the one or more associated products. The method according to claim 2, wherein in step c2) the one or more 1031734 compounds of formula I are separated to a purity of greater than or equal to 99.99% (4N), preferably 99.9999% (6N). The method of any one of the preceding claims, wherein silicon is used as the semiconductor material in step a). The method of claim 4, wherein MG silicon (metallurgical purity) is used as the semiconductor material in step a). 6. Method according to one or more of the preceding claims, wherein the material of the cathode is selected from the group consisting of silicon, carbon, silver, molybdenum, platinum, tungsten and one or more combinations thereof. The method of claim 6, wherein silicon with a purity of greater than or equal to 99.99% (4N), preferably 99.9999% (6N) is used as the cathode. 8. Method as claimed in one or more of the foregoing claims, wherein an anode of a core of inert material surrounded by a sheath of the semiconductor material is used. The method of any one of claims 1 to 12, wherein the electrolyte in which the anode is placed is the same as the electrolyte in which the cathode is placed. The method of claim 9, wherein the electrolyte is CaCl 2. A method according to any one of the preceding claims, wherein a membrane is used that is permeable to the one or more ionic compounds formed in step b), which membrane is present between the electrolyte in which the anode is placed and the electrolyte in which the cathode is placed. The method according to claim 11, wherein a ceramic membrane is used as the membrane. 13. Method as claimed in one or more of the claims 9-11, wherein a catching electrode is applied which is connected to an external voltage source and which catching electrode has an equal or higher potential as the cathode, the catching electrode being placed in the electrolyte between the anode and the cathode. Process according to one or more of claims 2-13, wherein in step b) additionally one or more compounds of formula I are added. Method according to one or more of the preceding claims, wherein the reduction is carried out at a temperature of at least 400 ° C, preferably at least 470 ° C. H = hydrogen y = valence of A z = integer greater than or equal to z and one or more associated reaction products. b) adding the one or more compounds of the formula I obtained in step a) to a cathode placed in an electrolyte and carrying out the reduction there to form purified semiconductor material on the cathode, during which reduction also a or more ionic compounds are formed; wherein the one or more ionic compounds formed in step b) are returned to step a) and wherein the anode and cathode are interconnected for electron transfer. 16. Method according to one or more of claims 1-15, wherein an external voltage source is used to apply a potential difference between the anode and the cathode, which potential difference is chosen between 0.01 V and the standard reduction potential of the electrolyte . The method of any one of claims 9 to 16, wherein the electrolyte is forced to flow from the cathode to the anode and vice versa. 1031734