Pickling method of gradient acid for electronic-grade polycrystalline silicon

Abstract

The invention discloses a gradient acid pickling method for electronic-grade polycrystalline silicon, which comprises the following steps of: performing three-stage acid pickling on polycrystalline silicon blocks, and taking a pickling solution used in each stage as a discharge solution of the next-stage acid pickling; performing high-purity water cleaning on the polycrystalline silicon block material before each time of pickling, and performing high-purity water spraying after the first time of high-purity water cleaning and the last time of high-purity water cleaning; and finally, the polycrystalline silicon block material is subjected to nitrogen drying after being sprayed and cleaned. The method has the advantages that the utilization rate of the pickling solution is effectively improved and the operation cost is reduced through an ordered and multi-stage pickling utilization mode. Meanwhile, the acid concentration of the pickling solution is reduced, the ion concentration is high, the surface tension of the acid solution is effectively reduced, the wettability of the pickling solution is higher, the pickling solution can deepen into gaps of the silicon block, and the cleaning quality is improved. Compared with other electronic-grade polycrystalline silicon processes, the method has the advantages that the process is simple, multiple acid preparation links are reduced, acid liquor discharge is reduced, the treatment cost is saved, the method is more environment-friendly, and the personnel safety problem is greatly reduced.

Description

Technical Field

[0001] The invention relates to the technical field of polysilicon pickling, and in particular to a method for pickling electronic-grade polysilicon with gradient acid. Background Art

[0002] Electronic grade polysilicon is a kind of polysilicon with a very high purity level. With the continuous advancement of semiconductor process technology, the purity and quality requirements for electronic grade polysilicon are getting higher and higher. Electronic grade polysilicon produced by vapor deposition method is a large rod-shaped material, which needs to be broken into blocks before it can be used downstream. The current silicon rod crushing process, whether it is manual crushing or mechanical crushing, or in the subsequent screening process, will inevitably introduce various impurities. If the impurities on the surface of the silicon block are not removed, the impurities will enter the silicon rod body along with the crystal pulling, reducing the purity.

[0003] In the current production process of polysilicon, the cleaning process is mainly achieved through the following steps: multiple high-purity water spray washing, then nitric acid and hydrofluoric acid mixed acid etching, and then high-purity water spray washing again. After multiple times of the above cleaning process, it is finally dried.

[0004] When cleaning polysilicon, the following two problems are still not effectively solved: 1. As the number of cleaning batches increases, the etched silicon block surface consumes the acid concentration of the mixed acid, resulting in a continuous decrease in the acid concentration. The reduction in acid concentration reduces the cleaning quality of polysilicon, so that the polysilicon cleaning cannot reach the required etching thickness. The existing solution is to continuously replenish and replace the newly prepared nitric acid / hydrofluoric acid mixed acid to maintain the reaction rate to meet the etching thickness of the silicon block within the time limit of the process production cycle.

[0005] However, frequent acid changes increase cleaning costs and reduce cleaning efficiency.

[0006] 2. There are tiny gaps and cracks on the surface of the broken polysilicon block. Metal particles in the air or particles on the utensils in contact with the silicon block, in addition to being adsorbed on the surface of the silicon block, will gradually diffuse into the gaps and cracks of the silicon block over time. However, due to its surface tension, the mixed acid solution cannot enter the gaps and is difficult to remove. If you want to clean the impurities diffused into the gaps and cracks of the silicon block, you need to use a mixed acid of nitric acid and hydrofluoric acid, increase the concentration of the mixed acid in the solution, and then increase the reaction etching time to increase the etching thickness until the outer layer of the silicon block containing gaps and cracks is completely etched to achieve the purpose of eliminating impurities in the gaps.

[0007] However, this method will increase the reaction time, slow down the yield, and waste high-purity nitric acid and hydrofluoric acid, resulting in cost waste. Summary of the invention

[0008] The object of the present invention is to provide a method for pickling electronic grade polysilicon with gradient acid.

[0009] The present invention is implemented by the following technical solutions: Initial cleaning: The polysilicon block is subjected to three-stage pickling, and the pickling liquid used in each stage is a newly configured pickling liquid, and the acidity of the pickling liquid used in each stage is lower than that of the pickling liquid in the previous stage; the polysilicon block is cleaned with high-purity water before each pickling, and high-purity water is sprayed after the first high-purity water cleaning and the last high-purity water cleaning; finally, the polysilicon block is dried with nitrogen after spray cleaning; Normal cleaning: The initial cleaning process is still used for cleaning. After completing the initial cleaning process, the pickling liquid after the first and second pickling is discharged. The first-pole pickling uses the discharge liquid of the second-level pickling, and the second-level pickling uses the discharge liquid of the third-level pickling, and then the normal cleaning process is carried out.

[0010] Preferably, the equipment used for the first-level pickling is a first-level pickling tank, the equipment used for the second-level pickling is a second-level pickling tank, and the equipment used for the third-level pickling is a third-level pickling tank.

[0011] Preferably, the pickling time of the three-stage pickling is 150~300s.

[0012] Preferably, the pickling solutions in the primary pickling tank, the secondary pickling tank and the tertiary pickling tank are all mixed acid solutions of nitric acid and hydrofluoric acid.

[0013] Preferably, the equipment used for high-purity water cleaning is a high-purity water cleaning tank.

[0014] Preferably, the cleaning time of the high-purity water cleaning is 150~300s.

[0015] Preferably, the equipment used for the high-purity water spraying is a high-purity water spray tank and a self-rotating cleaning head.

[0016] Preferably, the water pressure of the high-purity water spray is 0.2 MPa, and the flushing time is 150-300 s.

[0017] Preferably, the nitrogen drying is carried out by blowing nitrogen hot air in a nitrogen drying tank, the hot air temperature is 60-120°C, the air volume is 1000m³ / h, and the drying time is 150-300s.

[0018] The advantages of the present invention are as follows: through an orderly, multi-stage pickling utilization method, the utilization rate of the pickling solution is effectively improved and the operating cost is reduced. At the same time, the pickling solution utilized in multiple stages has a lower acid concentration and a higher ion concentration, which effectively reduces the surface tension of the acid solution. The pickling solution has higher wettability and can penetrate deeper into the gaps of the silicon blocks, thereby improving the cleaning quality. Compared with other electronic-grade polysilicon processes, the process is simple, multiple acid preparation links are reduced, and acid discharge is reduced, which not only saves processing costs, is more environmentally friendly, but also greatly reduces personnel safety issues. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0020] Figure 1 It is a schematic diagram of the overall structure of the present invention.

[0021] In the figure: primary pickling tank 1, secondary pickling tank 2, tertiary pickling tank 3, high-purity water cleaning tank 4, high-purity water spray tank 5, nitrogen drying tank 6. DETAILED DESCRIPTION

[0022] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0023] like Figure 1 As shown, an acid pickling method for electronic grade polysilicon gradient is used for initial cleaning: The polysilicon block is subjected to three-stage pickling, and the pickling liquid used in each stage is a newly configured pickling liquid, and the acidity of the pickling liquid used in each stage is lower than that of the pickling liquid in the previous stage; the polysilicon block is cleaned with high-purity water before each pickling, and high-purity water is sprayed after the first high-purity water cleaning and the last high-purity water cleaning; finally, the polysilicon block is dried with nitrogen after spray cleaning; Normal cleaning: The initial cleaning process is still used for cleaning. After completing the initial cleaning process, the pickling liquid after the first and second pickling is discharged. The first-pole pickling uses the discharge liquid of the second-level pickling, and the second-level pickling uses the discharge liquid of the third-level pickling, and then the normal cleaning process is carried out.

[0024] The equipment used for the first-level pickling is the first-level pickling tank 1, the equipment used for the second-level pickling is the second-level pickling tank 2, and the equipment used for the third-level pickling is the third-level pickling tank 3.

[0025] The pickling time for the three levels of pickling is 150~300s.

[0026] The pickling solutions in the primary pickling tank 1, the secondary pickling tank 2 and the tertiary pickling tank 3 are all mixed acid solutions of nitric acid and hydrofluoric acid.

[0027] The equipment used for high-purity water cleaning is a high-purity water cleaning tank 4.

[0028] The cleaning time for high-purity water cleaning is 150~300s.

[0029] The equipment used for high-purity water spraying is a high-purity water spray tank 5 and a self-rotating cleaning head.

[0030] The water pressure of high-purity water spray is 0.2Mpa, and the flushing time is 150~300s.

[0031] Nitrogen drying is carried out by blowing nitrogen hot air in the nitrogen drying tank 6, the hot air temperature is 60~120℃, the air volume is 1000m³ / h, and the drying time is 150~300s.

[0032] The advantages of the present invention are as follows: through an orderly, multi-stage pickling utilization method, the utilization rate of the pickling solution is effectively improved and the operating cost is reduced. At the same time, the pickling solution utilized in multiple stages has a lower acid concentration and a higher ion concentration, which effectively reduces the surface tension of the acid solution. The pickling solution has higher wettability and can penetrate deeper into the gaps of the silicon blocks, thereby improving the cleaning quality. Compared with other electronic-grade polysilicon processes, the process is simple, multiple acid preparation links are reduced, and acid discharge is reduced, which not only saves processing costs, is more environmentally friendly, but also greatly reduces personnel safety issues.

[0033] Working principle: When the present invention is used: Pickling process direction: The silicon block is placed in a rectangular material basket made of PVDF material, grabbed by a robot, and successively passes through the first high-purity water cleaning tank 4, the first high-purity water spray tank 5, the first pickling tank 1, the second high-purity water cleaning tank 4, the second pickling tank 2, the third high-purity water cleaning tank 4, the third pickling tank 3, the fourth high-purity water cleaning tank, the second high-purity water spray tank 5, and the nitrogen drying tank 6. The impurities are removed. The other water washing, soaking, and spraying are all for removing the impurities or solution residues in the previous process, and they are not recycled.

[0034] (1) The polysilicon block is immersed in the first high-purity water cleaning tank 4 to precipitate surface impurities, particles, etc. The conductivity of high-purity water is required to reach 15-18MΩ / CM; (2) The polysilicon blocks are sprayed and rinsed in the first high-purity water spray tank 5 to remove the granular silicon on the surface of the silicon blocks and other particulate impurities that fall off after being soaked in high-purity water; (3) In the initial startup phase of the primary pickling tank 1, a pickling solution with a lower acidity than the secondary pickling tank 2 can be used. After the production is stable, the discharge liquid from the secondary pickling tank 2 can be used. The low surface tension can clean impurities in the gaps, while saving acid and reducing costs. (4) Clean in the second high-purity water cleaning tank 4, and use high-purity water to clean the residual liquid in the first pickling tank 1 to avoid transferring impurities in the first pickling tank 1 into the second pickling tank 2 to prevent cross contamination.

[0035] (5) For pickling in the secondary pickling tank 2, in the initial startup stage, a pickling solution with a lower acidity than the tertiary pickling tank 3 can be used. After the production is stable, the effluent from the tertiary pickling tank 3 is used, which still has a high acid concentration. A light etching step can be performed first to increase the etching thickness, while saving acid and reducing costs; (6) The third high-purity water cleaning tank 4 is cleaned with high-purity water. The residual liquid in the secondary pickling tank 2 is cleaned with high-purity water to avoid transferring impurities in the secondary pickling tank 2 into the tertiary pickling tank 3 to prevent cross contamination.

[0036] (7) Acid pickling in the tertiary pickling tank 3, which is the main working tank, contains a mixed acid pickling solution composed of nitric acid and hydrofluoric acid, which is highly acidic. The silicon block first reacts with nitric acid to generate silicon dioxide, and the generated silicon dioxide then reacts with hydrofluoric acid to generate hexafluoride, while releasing a certain amount of nitrogen oxides. The molar mass ratio of hydrofluoric acid to nitric acid is 1:5~1:10, the mass concentration of hydrofluoric acid is 15~25wt%, the mass concentration of nitric acid is 45~75wt%, and the rest is water; (8) The fourth high-purity water cleaning tank is used to soak and clean the residual acid; (9) The second high-purity water spray tank 5 evenly rinses and sprays all surfaces of the basket; (10) The polysilicon blocks are dried by blowing with hot nitrogen air in the nitrogen drying tank 6 .

[0037] Flow direction of pickling liquid: When starting up for the first time or after a major overhaul, the pickling liquid added to the secondary pickling tank 2 is 50% of that to the tertiary pickling tank 3, and the pickling liquid added to the primary pickling tank 1 is 25% of that to the tertiary pickling tank 3. Among them, the tertiary pickling tank 3 is the main working tank, and the mixed acid solution therein is a newly prepared solution. After nitric acid and hydrofluoric acid are evenly mixed in the liquid preparation tank, they are transported to the tertiary pickling tank 3 by the acid pump; the solution in the secondary pickling tank 2 is the discharge liquid of the tertiary pickling tank 3. When the acid in the tertiary pickling tank 3 needs to be completely discharged and replaced, its discharge liquid is transported to the secondary pickling tank 2 by the delivery pump for reuse of the secondary pickling tank 2; the original acid in the secondary pickling tank 2 is transported to the primary pickling tank 1 by the delivery pump for reuse of the primary pickling tank 1; the original acid in the primary pickling tank 1 is discharged and discarded. From the third-level pickling tank 3 to the first-level pickling tank 1, the acid concentration decreases, the ion concentration increases, the surface tension decreases, the wettability increases, the etching ability of the silicon block decreases, the etching thickness of the silicon block decreases, but the ability to enter the tiny gaps is enhanced, which is more effective for cleaning impurities in the gaps of the silicon material.

[0038] Replacement of pickling solution: The solution in the tertiary pickling tank 3 is highly corrosive and has a high acid concentration. Direct measurement is expensive. The capacity of the acid can be indirectly measured through the weight change of the standard sample block. That is, a 2*2*2cm standard silicon block is put into the pickling process at the same time as the silicon block that needs to be cleaned. After the pickling time specified in the process, the weight difference G before and after is obtained through an analytical balance to indicate the etching capacity of the acid. For each fixed pickling process, the weight difference G of the standard sample block is basically consistent. When the actual weight difference g≤Gx%, it means that the quality of the pickling solution does not meet the requirements of the third cleaning tank 3 and needs to be replaced, where G is the standard deviation value required by the process, and x is between 85-99, which is the mass percentage of the process requirement that the actual weight must not be less than the standard weight difference. For example, the standard weight difference is G. X =0.05g, the process requires that the actual weight shall not be less than 90% of the standard weight difference. When the actual weight difference g≤0.045, change the acid, x=90.

[0039] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for pickling electronic grade polysilicon with gradient acid, characterized in that: Initial cleaning: The polysilicon block is subjected to three-stage pickling, and the pickling liquid used in each stage is a newly configured pickling liquid, and the acidity of the pickling liquid used in each stage is lower than that of the pickling liquid in the previous stage; the polysilicon block is cleaned with high-purity water before each pickling, and high-purity water is sprayed after the first high-purity water cleaning and the last high-purity water cleaning; finally, the polysilicon block is dried with nitrogen after spray cleaning; Normal cleaning: The initial cleaning process is still used for cleaning. After completing the initial cleaning process, the pickling liquid after the first and second pickling is discharged. The first-pole pickling uses the discharge liquid of the second-level pickling, and the second-level pickling uses the discharge liquid of the third-level pickling, and then the normal cleaning process is carried out.

2. The method for pickling electronic grade polysilicon with gradient acid according to claim 1, characterized in that: The equipment used for the first-level pickling is the first-level pickling tank, the equipment used for the second-level pickling is the second-level pickling tank, and the equipment used for the third-level pickling is the third-level pickling tank.

3. The method for pickling electronic grade polysilicon with gradient acid according to claim 1, characterized in that: The pickling time for the three levels of pickling is 150~300s.

4. The method for pickling electronic grade polysilicon with gradient acid according to claim 2, characterized in that: The pickling solutions in the primary pickling tank, the secondary pickling tank and the tertiary pickling tank are all mixed acid solutions of nitric acid and hydrofluoric acid.

5. The method for pickling electronic grade polysilicon with gradient acid according to claim 1, characterized in that: The equipment used for the high-purity water cleaning is a high-purity water cleaning tank.

6. The method for pickling electronic grade polysilicon with gradient acid according to claim 1, characterized in that: The cleaning time of the high-purity water cleaning is 150~300s.

7. The method for pickling electronic grade polysilicon with gradient acid according to claim 1, characterized in that: The equipment used for high-purity water spraying is a high-purity water spray tank and a self-rotating cleaning head.

8. The method for pickling electronic grade polysilicon with gradient acid according to claim 1, characterized in that: The water pressure of the high-purity water spray is 0.2Mpa, and the flushing time is 150~300s.

9. The method for pickling electronic grade polysilicon with gradient acid according to claim 1, characterized in that: The nitrogen drying is carried out by blowing nitrogen hot air in a nitrogen drying tank, the hot air temperature is 60-120°C, the air volume is 1000m³ / h, and the drying time is 150-300s.

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