Ultra-high-purity oxygen-free copper and preparation method therefor

Abstract

The present invention relates to an ultra-high purity oxygen-free copper, and a preparation method therefor. In the preparation method, first, cathode copper is added to a graphite crucible of an electric vacuum high-temperature purification furnace, to be melted into liquid copper; next, after the liquid copper is left to stand and maintain temperature, a heating cover of the electric vacuum high-temperature purification furnace is slowly lifted upwards, and the liquid copper in the graphite crucible slowly solidifies from the bottom until the solution is completely solidified; then, the solidified ingot is removed, both ends of the ingot are cut off, and the ingot remaining after cutting repeats the previous two steps. After 3-5 cycles, an ultra-high purity oxygen-free copper ingot is obtained. The prepared ultra-high purity oxygen-free copper has a purity of 5N-6N, an oxygen content of less than 2 ppm, and a hydrogen content of less than 1 ppm. The present invention has simple and reliable operation, and can control preparation of ingots having different copper contents according to different application directions; in addition, due to the extremely low gas content and good compactness of the ingots, the oxygen content is also controlled at an extremely low level.

Description

Ultra-high purity oxygen-free copper and preparation method thereof TECHNICAL FIELD

[0001] The present application belongs to the field of non-ferrous smelting and processing, and particularly relates to an ultra-high purity oxygen-free copper and a preparation method thereof. BACKGROUND

[0002] Oxygen-free copper refers to pure copper without oxygen and any deoxidizer residue. However, it actually contains very small amounts of oxygen and some impurities. According to the standard, the oxygen content is not more than 0.003%, the total impurity content is not more than 0.05%, and the purity of copper is more than 99.95%, which belongs to oxygen-free copper.

[0003] The application of ultra-high purity oxygen-free copper is more and more widely used in the current high-tech field, and the purity requirement of the ultra-high purity oxygen-free copper is also higher and higher. With the development of large-scale integrated circuits and large-size displays, the copper interconnection leads required by chips and displays are mostly plated on the circuit by the method of magnetron sputtering, and the purity, density and oxygen content of copper are required to be higher. The purity of copper used for display lead is required to be 5N or above, and the purity of copper used for chips is required to be 6N or above; the oxygen content is required to be less than 5ppm, and there cannot be defects such as loose and pores. However, the purity of conventional cathode copper is currently between 99.90-99.9935%, and the trace impurities therein will affect the use properties of copper.

[0004] At present, the conventional copper purification methods mainly include electron beam melting, suspension melting, zone melting and other methods. These methods have problems of high production cost and great operation difficulty, which limits the development and application of large-scale integrated circuits and large-size displays. SUMMARY

[0005] The purpose of the present application is to provide an ultra-high purity oxygen-free copper and a preparation method thereof. The trace impurity elements in the solid phase and the remaining liquid phase are different when they are precipitated under the condition of approaching equilibrium solidification in a vacuum environment, and the copper liquid is repeatedly purified to achieve the purpose of preparing ultra-high purity oxygen-free copper.

[0006] In order to achieve the above purpose, the technical scheme adopted by the present application is as follows: a preparation method of ultra-high purity oxygen-free copper, comprising the following steps:

[0007] S1, cathode copper is added to a graphite crucible of an electric vacuum high-temperature purification furnace, and the cathode copper is melted into copper liquid in the vacuum and high-temperature environment of the electric vacuum high-temperature purification furnace;

[0008] S2, after the copper liquid is kept warm, the heating cover of the electric vacuum high-temperature purification furnace is slowly lifted upward, the heating cover is away from the graphite crucible, the copper liquid in the graphite crucible is slowly solidified from the bottom, and the solution is completely solidified;

[0009] S3, the solidified ingot is taken out, the two ends of the ingot are cut off, and the remaining ingot after cutting is repeated according to steps S1 and S2, and after 3-5 cycles, an ultra-high purity oxygen-free copper ingot is obtained.

[0010] In step S1, the purity of the cathode copper is 3N-5N.

[0011] In step S1, the vacuum degree of the electric vacuum high-temperature purification furnace is less than 5 Pa.

[0012] In step S2, the holding temperature of the copper liquid is 1160-1300℃.

[0013] In step S2, the holding time of the copper liquid is 0.5-1.5h.

[0014] In step S2, the rising speed of the heating cover is 10-50mm / h.

[0015] In step S3, the weight of the part cut off from the two ends of the ingot each time accounts for 1-25% of the weight of the ingot before cutting.

[0016] In step S3, the total length of the ingot cut off is 6-12mm.

[0017] In the process of 3-5 cycles, the lifting speed of the heating cover is gradually reduced, and the lifting speed of the heating cover in the later cycle is reduced by 7%-14% compared with the lifting speed of the heating cover in the previous cycle.

[0018] The application further provides an ultra-high purity oxygen-free copper, which is prepared by the preparation method of the ultra-high purity oxygen-free copper, has a purity of 5N-6N, an oxygen content of <2ppm, and a hydrogen content of <1ppm.

[0019] The process principle of the application is as follows: according to the solidification theory, trace impurity elements (solute) in the copper liquid will be redistributed in the process of approaching equilibrium solidification, and the solid phase and the remaining liquid phase exist in the case of unequal distribution. The solid phase that solidifies first contains higher concentration of impurities (solute) than the remaining liquid phase; at the end of solidification, the remaining liquid phase contains higher concentration of impurities (solute) than the solid phase that solidifies. The closer to equilibrium solidification, the more obvious the difference, the slower the lifting speed of the heating cover, the closer to equilibrium solidification, and the more obvious the solute distribution difference, which is more conducive to purification and obtaining high-purity oxygen-free copper. In the process of multiple cycles, the product purity is higher and further purification is more difficult, so the lifting speed of the heating cover needs to be gradually reduced to be closer to equilibrium solidification and realize further purification and impurity removal.

[0020] According to the solute distribution principle in the solidification process, if the solid phase in the initial solidification process and the part of the remaining liquid phase solidified when the solidification is close to completion are removed respectively, the impurities (solute) with different properties in the copper liquid can be removed in the initial solidification stage and the final solidification stage, and the purity of the copper can be continuously improved through multiple iterations.

[0021] In the process of purification and impurity removal close to equilibrium solidification, trace oxygen and carbon in the graphite crucible are reduced to CO, and together with the extremely small amount of hydrogen, they escape from the melt and are removed. Thus, ultra-high purity and high density low-oxygen oxygen-free copper is obtained.

[0022] The beneficial effects of the present application are: during the preparation process, when the copper liquid is close to equilibrium solidification crystallization under vacuum conditions, the metal impurity elements are enriched at the beginning of solidification and at the end of final solidification due to directional solidification, and are removed in the process of cutting off the ends of the ingot; at the same time, trace oxygen and carbon in the graphite are reduced to CO, and together with the extremely small amount of hydrogen, they escape from the melt and are removed; the process is repeated continuously, thereby achieving the purpose of preparing ultra-high purity, high density, low-oxygen oxygen-free copper, and the prepared oxygen-free copper has a purity of 99.999-99.9999% (5N-6N), an oxygen content of <2ppm, and a hydrogen content of <1ppm.

[0023] The preparation method is simple and reliable, and can control the preparation of ingots with different copper contents according to different application directions. At the same time, due to the extremely low gas content of the ingot and good density, the oxygen content is also controlled at a very low level. The entire preparation process is carried out under the condition of close to equilibrium solidification, and there will be no fatal defects such as ingot inclusions. DETAILED DESCRIPTION

[0024] The present application will be further described in detail below in conjunction with examples, but it is not as a basis for any limitation on the invention.

[0025] Example 1: A preparation method of ultra-high purity oxygen-free copper, comprising the following steps:

[0026] S1, place 500 kg of dry cathode copper with oil-free and water-free surface, cut off the edge part, and with a purity of 99.995% (4N5) into a graphite crucible with a diameter of 400 mm, a height of 800 mm, and a wall thickness of 20 mm in an electric vacuum high-temperature purification furnace, and cover it with a heating cover to completely cover the graphite crucible; use a vacuum pump to pump the vacuum degree in the graphite crucible to below 3 Pa, and power the resistance wire of the heating cover to gradually increase the temperature in the furnace to 1160-1200℃, so that the cathode copper is gradually melted into liquid state in the vacuum and high-temperature environment;

[0027] S2: keep the temperature of the copper liquid at 1160-1200℃, and keep still for 0.5 h; after the end of the heat preservation process, slowly lift the heating cover at a moving speed of 20 mm / h; due to the disappearance of the heat source, the copper liquid in the graphite crucible gradually and slowly solidifies from the bottom until the copper liquid in the crucible completely solidifies, and a copper ingot is obtained;

[0028] S3: after the copper ingot is taken out, 4 mm is cut off from each of the head and tail to remove the part with higher impurity element content; the remaining copper ingot is put back into the electric vacuum high-temperature purification furnace, and the above-mentioned S1 and S2 operations are repeated to continuously improve the purity of the oxygen-free copper; after 3 cycles, the final ultra-high-purity oxygen-free copper with a purity of 99.9999% (6N), an oxygen content of 0.6 ppm, and a hydrogen content of 0.5 ppm is obtained.

[0029] It should be noted that the lifting speed of the heating cover is gradually reduced in the 3-cycle purification process, and in order to operate, the lifting speed of the heating cover in each cycle adopts an integer value; specifically, the lifting speed of the heating cover in the second cycle of purification is 18 mm / h, which is reduced by 10% compared with the lifting speed of the heating cover in the first cycle of purification, i.e. 20 mm / h; in the third cycle of purification, the lifting speed of the heating cover is 16 mm / h, which is reduced by 11% compared with the second cycle. In each cycle, except for the lifting speed of the heating cover, other process parameters remain unchanged.

[0030] Embodiment 2: A preparation method of ultra-high-purity oxygen-free copper, comprising the following steps:

[0031] S1, put the 450 kg dry cathode copper with a purity of 99.998% (4N8) and without oil and water on the surface and with the edge cut off into the graphite crucible with a diameter of 400 mm, a height of 800 mm and a wall thickness of 20 mm in the electric vacuum high-temperature purification furnace, and cover the heating cover to completely cover the graphite crucible; use the vacuum pump to pump the vacuum degree in the graphite crucible to below 3 Pa, power on the resistance wire of the heating cover to heat, and gradually increase the temperature in the furnace to 1200-1240℃ to gradually melt the cathode copper into a liquid state in the vacuum and high-temperature environment;

[0032] S2: keep the temperature of the copper liquid at 1200-1240℃, and keep still for 1 h; after the end of the heat preservation process, slowly lift the heating cover at a moving speed of 10 mm / h; due to the disappearance of the heat source, the copper liquid in the graphite crucible gradually and slowly solidifies from the bottom until the copper liquid in the crucible completely solidifies, and a copper ingot is obtained;

[0033] S3: After the copper ingot is taken out, 5mm is cut off from each of the head and tail to remove the part with higher impurity element content; the remaining copper ingot is put back into the electric vacuum high-temperature purification furnace, and the above-mentioned S1 and S2 operations are repeated to continuously improve the purity of the oxygen-free copper; after 5 cycles, the ultra-high-purity oxygen-free copper with a purity of 99.99992% (6N2) and oxygen content <0.5ppm and hydrogen content <0.5ppm is finally obtained.

[0034] It should be noted that the lifting speed of the heating cover is gradually reduced in the 5-cycle purification process, and for operation, the lifting speed of the heating cover in each cycle adopts an integer value; specifically, the lifting speed of the heating cover in the second cycle of purification is 9mm / h, which is reduced by 10% compared with the lifting speed of the heating cover in the first cycle of purification, which is 10mm / h; in the third cycle of purification, the lifting speed of the heating cover is 8mm / h, which is reduced by 11% compared with the second cycle; in the fourth cycle of purification, the lifting speed of the heating cover is 7mm / h, which is reduced by 13% compared with the third cycle; in the fifth cycle of purification, the lifting speed of the heating cover is 6mm / h, which is reduced by 14% compared with the fourth cycle. In each cycle, except for the lifting speed of the heating cover, other process parameters remain unchanged.

[0035] Embodiment 3: A preparation method of ultra-high-purity oxygen-free copper, comprising the following steps:

[0036] S1, 580kg of dry cathode copper with a purity of 99.95% (3N5) and without oil and water on the surface is placed in a graphite crucible with a diameter of 400mm, a height of 800mm and a wall thickness of 20mm in the electric vacuum high-temperature purification furnace, and a heating cover is covered to completely cover the graphite crucible; the vacuum degree in the graphite crucible is pumped to below 5Pa by a vacuum pump, the resistance wire of the heating cover is powered to heat, and the temperature in the furnace is gradually increased to 1250-1300℃, so that the cathode copper is gradually melted into a liquid state in a vacuum and high-temperature environment;

[0037] S2: The temperature of the copper liquid is kept at 1250-1300℃, and the heating cover is slowly lifted at a moving speed of 30mm / h; due to the disappearance of the heat source, the copper liquid in the graphite crucible gradually and slowly solidifies from the bottom until the copper liquid in the crucible is completely solidified, and a copper ingot is obtained;

[0038] S3: After the copper ingot is taken out, 5mm is cut off from each of the head and tail to remove the part with higher impurity element content; the remaining copper ingot is put back into the electric vacuum high-temperature purification furnace, and the above-mentioned S1 and S2 operations are repeated to continuously improve the purity of the oxygen-free copper; after 5 cycles, the ultra-high-purity oxygen-free copper with a purity of 99.99992% (6N2) and oxygen content <0.5ppm and hydrogen content <0.5ppm is finally obtained.

[0039] It should be noted that in the 4-cycle purification process, the lifting speed of the heating cover is gradually reduced, and in order to operate, the lifting speed of the heating cover in each cycle adopts an integer value; specifically, the lifting speed of the heating cover in the second cycle purification is 27 mm / h, which is reduced by 10% compared with the lifting speed of the heating cover in the first cycle purification, which is 30 mm / h; in the third cycle purification, the lifting speed of the heating cover is 25 mm / h, which is reduced by 7% compared with the second; in the fourth cycle purification, the lifting speed of the heating cover is 23 mm / h, which is reduced by 8% compared with the third. In each cycle, in addition to the lifting speed of the heating cover, other process parameters remain unchanged.

[0040] Embodiment 4: A preparation method of ultra-high purity oxygen-free copper, comprising the following steps:

[0041] S1, 500 kg of dry cathode copper with oil-free and water-free surface, cut-off edge, purity of 99.95% (3N5) is placed in a graphite crucible with a diameter of 400 mm, a height of 800 mm and a wall thickness of 20 mm in an electric vacuum high-temperature purification furnace, and a heating cover is covered to completely cover the graphite crucible; the vacuum degree in the graphite crucible is pumped to below 4 Pa by a vacuum pump, the resistance wire of the heating cover is powered to heat, and the temperature in the furnace is gradually increased to 1180-1220℃, so that the cathode copper is gradually melted into a liquid state in a vacuum and high-temperature environment;

[0042] S2: keep the temperature of the copper liquid at 1180-1220℃, and stand for 0.6h; after the heat preservation process is completed, the heating cover is slowly lifted at a moving speed of 50 mm / h; due to the disappearance of the heat source, the copper liquid in the graphite crucible gradually and slowly solidifies from the bottom until the copper liquid in the crucible is completely solidified, and a copper ingot is obtained;

[0043] S3: after the copper ingot is taken out, 5mm is cut off from the head and tail of the copper ingot respectively to remove the part with high impurity element content; the remaining copper ingot is re-placed in the electric vacuum high-temperature purification furnace, and the above-mentioned S1 and S2 operations are repeated to continuously improve the purity of the oxygen-free copper; after 3 cycles, ultra-high purity oxygen-free copper with a purity of 99.999% (5N) and an oxygen content of 1.9 ppm and a hydrogen content of 0.9 ppm is finally obtained.

[0044] It should be noted that in the 3-cycle purification process, the lifting speed of the heating cover is gradually reduced, and in order to operate, the lifting speed of the heating cover in each cycle adopts an integer value; specifically, the lifting speed of the heating cover in the second cycle purification is 45 mm / h, which is reduced by 10% compared with the lifting speed of the heating cover in the first cycle purification, which is 50 mm / h; in the third cycle purification, the lifting speed of the heating cover is 40 mm / h, which is reduced by 11% compared with the second. In each cycle, in addition to the lifting speed of the heating cover, other process parameters remain unchanged.

[0045] Example 5: A preparation method of ultra-high purity oxygen-free copper, comprising the following steps:

[0046] S1, 600 kg of dry cathode copper with surface free of oil and water, cut-off edge, purity of 99.996% (4N6) is placed in a graphite crucible with a diameter of 400 mm, a height of 800 mm and a wall thickness of 20 mm in an electric vacuum high-temperature purification furnace, and a heating cover is covered to completely cover the graphite crucible; the vacuum degree in the graphite crucible is pumped to below 3 Pa by a vacuum pump, the resistance wire of the heating cover is powered to heat, and the temperature in the furnace is gradually increased to 1220-1250℃, so that the cathode copper is gradually melted into a liquid state in a vacuum and high-temperature environment;

[0047] S2: the temperature of the copper liquid is kept at 1220-1250℃, and the heating cover is slowly lifted at a moving speed of 30 mm / h; due to the disappearance of the heat source, the copper liquid in the graphite crucible gradually and slowly solidifies from the bottom until the copper liquid in the crucible is completely solidified, and a copper ingot is obtained;

[0048] S3: after the copper ingot is taken out, 3 mm is cut off from the head and tail of the copper ingot respectively to remove the part with high impurity element content; the remaining copper ingot is placed into the electric vacuum high-temperature purification furnace again, and the above-mentioned S1 and S2 operations are repeated to continuously improve the purity of the oxygen-free copper; after 3 cycles, the final ultra-high purity oxygen-free copper with a purity of 99.9996% (5N6), an oxygen content of 1.8 ppm and a hydrogen content of 0.7 ppm is obtained.

[0049] It should be noted that the lifting speed of the heating cover is gradually reduced during the 3 cycles of purification, and in order to operate, the lifting speed of the heating cover in each cycle adopts an integer value; specifically, the lifting speed of the heating cover in the second cycle of purification is 27 mm / h, which is reduced by 10% compared with the lifting speed of the heating cover in the first cycle of purification, i.e. 30 mm / h; the lifting speed of the heating cover in the third cycle of purification is 25 mm / h, which is reduced by 7% compared with the second cycle. In each cycle, except for the lifting speed of the heating cover, other process parameters remain unchanged.

[0050] The following design of the comparative example is compared with the above-mentioned example to illustrate the beneficial effects of the technical scheme of the present application.

[0051] Comparative Example 1: Based on Example 1, the lifting speed of the heating cover remains unchanged in each cycle of purification, which is 20 mm / h, and other process parameters remain unchanged. The purity of the finally prepared ultra-high purity oxygen-free copper is 99.998%, i.e. 4N8, the oxygen content is 0.8 ppm, and the hydrogen content is 1.7 ppm.

[0052] Compared with Example 1, the purity of the oxygen-free copper is reduced and is lower than 5N, and the oxygen content and the hydrogen content are increased, and the hydrogen content is increased by a larger margin and is higher than 1ppm required by the present application. This is because in the process of step-by-step purification, the migration speed of trace solutes in the melt will be slower and slower, and a slower solidification process is required for purification. If the same migration speed is adopted, the trace impurity solutes and the oxygen and hydrogen contents will be difficult to completely remove from the melt, and a longer time is required to achieve a better removal effect.

[0053] Comparative Example 2: Based on Example 2, the number of purification cycles is 6, and the purity of the obtained oxygen-free copper is 99.99993%, i.e. 6N3, the oxygen content is <0.5ppm, and the hydrogen content is <0.5ppm.

[0054] Compared with Example 2, the number of purification cycles in the present comparative example is increased and is more than 3-5 required by the present application, and the purity of the final copper will be increased, and the lower the purity of the copper, the greater the increase in purity after purification, and the higher the purity, the less the increase in purity after purification. When the purity of the copper approaches or reaches 6N, the increase in the purity of the copper after purification is very small. Therefore, although the increase in the number of cycles can improve the purity of the product, too many cycles will increase the preparation cost of the product, and as the number of purification cycles increases, the improvement in the purity of the product in the later stage will become more and more difficult, and therefore the number of cycles is not the more the better.

[0055] Comparative Example 3: Based on Example 2, the number of purification cycles is 2, and the purity of the obtained oxygen-free copper is 99.991%, i.e. 4N1, the oxygen content is 2.3ppm, and the hydrogen content is 1.9ppm.

[0056] Compared with Example 2, after the purification cycles in the present comparative example, the purity of the oxygen-free copper is only 4N1, which is lower than 5N-6N required by the present application, and the oxygen content and the hydrogen content both exceed the requirements of the present application. This is because as the number of purification cycles increases, the impurities in the oxygen-free copper will be gradually removed, the purity will gradually increase, and the oxygen content and the hydrogen content will also decrease, and therefore a certain number of cycles is required.

[0057] Through the comparison of Comparative Examples 2 and 3, it is illustrated that in the preparation of oxygen-free copper, the number of purification cycles is important, too low a number of cycles will affect the purity of the product, and too high a number of cycles will increase the overall preparation cost, and therefore the number of cycles of 3-5 proposed by the present application can take into account the quality and the preparation cost of the final product.

[0058] Comparative Example 4: Based on Example 3, the holding temperature of the copper liquid is 1320-1350℃, and the other process parameters are the same as those of Example 3, and the purity of the obtained oxygen-free copper is 99.999%, i.e. 5N, the oxygen content is 2.0ppm, and the hydrogen content is 1.1ppm.

[0059] Compared with Example 3, in the comparative example, the temperature of the holding is higher than the range of 1160-1300℃ required by the present application, so the oxygen content and the hydrogen content of the oxygen-free copper prepared are slightly higher than the range required by the present application (oxygen content <2ppm, hydrogen content <1ppm). The analysis is as follows:

[0060] The change rule of the hydrogen and oxygen contents and other impurities is related to the state of the initial copper. However, under normal circumstances, with the increase of the number of purification cycles, the hydrogen and oxygen contents of the melt are basically removed and are lower than the solubility of hydrogen and oxygen in the solid copper when the number of purification cycles is more than 3, so the use requirement is met. However, when the temperature of the melt is increased, the solubility of hydrogen and oxygen in the melt is correspondingly increased, the removal difficulty is increased, and the residual amount in the copper is correspondingly increased.

[0061] In addition, if a lower holding temperature, such as 1130℃, is used on the basis of Example 3, the flowability of the melt is poor, and the migration speed of the impurities in the melt is slow, so the purity of the oxygen-free copper obtained is lower and the hydrogen and oxygen contents are relatively higher than those of Example 3 under the same number of purification cycles. Therefore, the holding temperature of the copper liquid is set to 1160-1300℃ in the present application.

[0062] Comparative Example 5: On the basis of Example 4, the vacuum degree of the electro-vacuum high-temperature purification furnace is 6-8Pa, and other process parameters are the same as those of Example 4. The purity of the oxygen-free copper obtained is 99.998%, i.e. 4N8, the oxygen content is 2.5ppm, and the hydrogen content is 1.8ppm.

[0063] Compared with Example 4, the purity of the oxygen-free copper is lower and is lower than the range of 5N-6N required by the present application, and the oxygen content and the hydrogen content both exceed the requirements of the present application (oxygen content <2ppm, hydrogen content <1ppm). The analysis is as follows:

[0064] Compared with Example 4, the gas content in the melt, especially hydrogen, is removed by negative pressure under the vacuum condition. When the vacuum degree of the furnace is reduced and the sealing condition of the furnace is not enough, the melt can be in contact with the atmosphere to a certain extent, at this time, the hydrogen and oxygen in the melt and the air have a certain reaction, so there is a certain balance degree between the absorption and removal of the gas in the melt, and when the balance point is reached, the gas in the melt is difficult to remove. Therefore, when the vacuum degree is lower than the range of less than 5Pa required by the present application, it is not only the removal of the gas in the melt, but also the hydrogen and oxygen contents in the oxygen-free copper product are higher than those of Example 4. In addition, the increase of the hydrogen and oxygen contents also causes the increase of the viscosity of the copper, so that the purification effect is affected.

[0065] It should be noted that when the hydrogen and oxygen contents in the oxygen-free copper are lower than 0.5ppm, the detection instrument reports <0.5ppm.

[0066] The above examples are only used to illustrate the technical solutions of the present application but not to limit it, and those skilled in the art should understand that the specific embodiments of the present application can be modified or replaced equivalently with reference to the above examples, and any modification or equivalent replacement which does not depart from the spirit and scope of the present application is within the protection scope of the claims of the application to be granted.

Claims

1. A method for producing ultrahigh purity oxygen-free copper, characterized by, The method comprises the following steps: S1, cathode copper is added into a graphite crucible of an electric vacuum high-temperature purification furnace, and the cathode copper is melted into copper liquid in a vacuum and high-temperature environment of the electric vacuum high-temperature purification furnace; S2, after the copper liquid is kept warm, a heating cover of the electric vacuum high-temperature purification furnace is slowly lifted upwards, the heating cover is separated from the graphite crucible, the copper liquid in the graphite crucible is slowly solidified from the bottom, and the solution is completely solidified; S3, the solidified ingot is taken out, the two ends of the ingot are cut off, and the remaining ingot after cutting is repeatedly processed according to steps S1 and S2, and the ultra-high-purity oxygen-free copper ingot is obtained after 3-5 cycles.

2. The method of claim 1, wherein the ultra-high purity oxygen-free copper is prepared by the steps of: In step S1, the purity of the cathode copper is 3N-5N. ​ 3. The method of claim 1, wherein the ultra-high purity oxygen-free copper is prepared by the steps of: In step S1, the vacuum degree of the electric vacuum high-temperature purification furnace is less than 5 Pa. ​ 4. The method of claim 1, wherein the ultra-high purity oxygen-free copper is prepared by the steps of: In step S2, the temperature of the copper liquid is kept warm at 1160-1300 ℃. ​ 5. The method of claim 1, wherein the ultra-high purity oxygen-free copper is prepared by the steps of: In step S2, the copper liquid is kept warm for 0.5-1.5 h. ​ 6. The method of claim 1, wherein the ultra-high purity oxygen-free copper is prepared by the steps of: In step S2, the lifting speed of the heating cover is 10-50 mm / h. ​ 7. The method of claim 1, wherein the ultra-high purity oxygen-free copper is prepared by the steps of: In step S3, the weight of the part cut off from the two ends of the ingot each time accounts for 1-25% of the weight of the ingot before cutting. ​ 8. The method of claim 1, wherein the ultra-high purity oxygen-free copper is prepared by the steps of: In step S3, the total length of the ingot cut off is 6-12 mm. ​ 9. The method of claim 1, wherein the ultra-high purity oxygen-free copper is prepared by the steps of: In the process of 3-5 cycles, the lifting speed of the heating cover is gradually reduced, and the lifting speed of the heating cover in the last cycle is reduced by 7%-14% compared with the lifting speed of the heating cover in the previous cycle. ​ 10. An ultra-high purity oxygen-free copper, characterized by, The ultra-high-purity oxygen-free copper prepared by the method of any one of claims 1-9 has a purity of 5N-6N, an oxygen content of <2 ppm, and a hydrogen content of <1 ppm.