A new type of electronic grade polysilicon cleaning technology for integrated circuits

Abstract

The present invention relates to a novel electronic-grade polysilicon cleaning process technology for integrated circuits, and belongs to the technical field of electronic-grade polysilicon core cleaning. The No. 1 tank body is a high-purity water soaking tank, the No. 2 tank body is a degreasing and degreasing agent soaking tank, the No. 3 tank is an ultrasonic + high-purity water rinsing tank, the No. 4 tank is a high-purity water rinse tank, the No. 5 tank is an ultrapure water rinse tank, the No. 6 tank is a hydrofluoric acid + nitric acid mixed acid etching tank, the No. 7 tank is a hydrofluoric acid + nitric acid mixed acid pickling correction tank, the No. 8 tank is a high-purity water rinse tank, the No. 9 tank is a high-purity water rinse tank, the No. 10 tank is a high-purity water rinse tank, the No. 11 tank is a high-purity water soaking tank, and the No. 12 tank is nitrogen hot air drying. It mainly includes one degreasing and degreasing cleaning, two pickling and nitrogen drying, and auxiliary ultrasonic cleaning. The silicon material is placed in a rectangular material basket made of PVDF material, and the basket is grabbed by a manipulator and passes through each working tank one by one to complete the silicon material cleaning operation.

Description

Technical Field

[0001] The invention belongs to the technical field of electronic-grade polysilicon core cleaning, and relates to a novel electronic-grade polysilicon cleaning process technology for integrated circuits. Background Art

[0002] Electronic-grade polysilicon materials are the upstream raw materials of semiconductors that are gradually developing and maturing worldwide. They are widely used in many fields of silicon-based semiconductor chips and devices. At present, the demand and application in the fields of mobile phones, computers, photovoltaics, 5G communications and electronic products are increasing. It is believed that in the near future, silicon-based semiconductor materials will be used in a wider range of fields.

[0003] At present, the most commonly used method for producing electronic-grade polysilicon in the industry is the Modified Siemens Process (MSP), which uses chlorine and hydrogen to synthesize hydrogen chloride (or purchase hydrogen chloride), hydrogen chloride and industrial silicon powder to synthesize trichlorosilane at a certain temperature, and then separates and rectifies the trichlorosilane. The purified trichlorosilane undergoes a CVD reaction in a reduction furnace. The CVD reaction process is to reduce high-purity trichlorosilane with high-purity hydrogen on a high-purity silicon core at about 1100°C, generate polysilicon atomic particles and deposit them on the silicon core, and finally generate silicon rods. The surface of the produced silicon rods is easily contaminated due to factors such as contact with air or human factors, and does not meet the needs of downstream products. Therefore, the back-end cleaning process determines the recognition of electronic-grade polysilicon materials in the downstream industry to a certain extent. Surface metal is an important quality indicator of electronic-grade polysilicon. The surface metal impurity content of electronic-grade polysilicon products of national standard I is required to be less than 0.1ppbw. The surface metal impurity content of high-quality silicon materials on the current electronic-grade polysilicon market can meet national standard I, and some products are of better quality. At present, the main cleaning method in the silicon material cleaning industry is to etch the impurities on the surface of the silicon block with acid or mixed acid of different concentrations and different components after crushing and screening the silicon material, and then dry and clean it and package it into products after etching. This cleaning method has the following main disadvantages: First, the broken silicon blocks have different shapes, and the acid concentration ratio required for etching is extremely high, making it difficult to immerse and clean them all; Second, there are tiny gaps and cracks on the surface of the broken silicon blocks, and various pollutant particles in the air are easy to adhere to the vicinity. Some pollutants will diffuse into the gaps and cracks of the silicon material over time, and the cleaning time is difficult to control; Third, some fine-grained silicon materials are easily directly "dissolved" (reacted by acid) during the etching process; In summary, how to effectively improve the above problems is the top priority for improving product quality and increasing production capacity and benefits.

[0004] The cleaning process routes commonly used in the current industry are: cleaning, high-purity water washing, mild etching pickling, pure water washing, pure water rinsing, mixed acid etching pickling, pure water washing, desalted water washing, rinsing, drying and discharging; this process route is difficult to control the cleaning of fine silicon particles and the impurity pollution in the gaps of some silicon blocks during the cleaning process. Due to the limitations of the cleaning process, the surface grease of some silicon materials is also difficult to clean, which makes it difficult to control the carbon and oxygen concentrations on the surface of the silicon materials. Patent CN 114653658 discloses a cleaning process, multiple high-purity water spray washing → nitric acid / hydrofluoric acid mixed acid etching → high-purity water spray washing again → drying. The carbon content of the product produced by this cleaning process is high because it is difficult to wash off the grease attached to the surface of the silicon material with multiple high-purity water; Patent CN202011290207.9 discloses that before using mixed acid cleaning, a small amount of alkali solution is used for cleaning to eliminate carbon-containing impurities such as organic matter and grease, but this cleaning process still fails to completely deal with the impurity pollution in the gaps of the silicon material. Summary of the invention

[0005] The purpose of the present invention is to provide a novel electronic-grade polysilicon cleaning process for integrated circuits, which has the characteristics of high cleaning efficiency and good cleaning effect.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] A novel electronic grade polysilicon cleaning process technology for integrated circuits, the specific steps of the process technology are as follows:

[0008] S1: First, soak the silicon material in high-purity water in tank No. 1. The water temperature of the high-purity water is 50-60°C and the soaking time is 150-300s;

[0009] S2: Transfer the silicon material to tank No. 2 and soak it in a degreasing agent at a temperature of 50 to 60°C for 100 to 300 seconds.

[0010] S3: Transfer the silicon material to tank No. 3 for overflow rinsing with high-purity water. The high-purity water temperature is room temperature. Rinsing is performed under ultrasonic conditions. The rinsing time is 150 to 300 seconds. The ultrasonic power is 28 to 40 kHz.

[0011] S4: Transfer the silicon material to tank No. 4 for a single rinse with high-purity water. The water temperature of the high-purity water is room temperature, the water pressure of the high-purity water is 0.1-0.2Mpa, and the rinsing time is 150-300s.

[0012] S5: Transfer the silicon material to tank No. 5 for secondary rinsing with ultrapure water. The ultrapure water temperature is room temperature, the ultrapure water pressure is 0.1-0.2Mpa, and the rinsing time is 150-300s.

[0013] S6: Transfer the silicon material to tank No. 6 for pickling treatment, use a mixed solution of nitric acid and hydrofluoric acid for pickling, the mass ratio of hydrofluoric acid to nitric acid is 1: (5-10), and the pickling time is 100-200s;

[0014] S7: Transfer the silicon material to tank No. 7 for correction pickling treatment, use a mixed solution of nitric acid and hydrofluoric acid for correction pickling, the mass ratio of hydrofluoric acid to nitric acid is 1: (10-12), while the correction pickling, the silicon material tank is slightly shaken horizontally in the acid solution, and the cleaning time is 150-300s;

[0015] S8: Transfer the silicon material to tank No. 8 for overflow rinsing with high-purity water. The water temperature of the high-purity water is room temperature and the cleaning time is 150 to 300 seconds.

[0016] S9: Transfer the silicon material to tank No. 9 for high-purity water washing. The water temperature of the high-purity water is room temperature, the water pressure of the high-purity water is 0.1-0.2Mpa, and the cleaning time is 150-300s.

[0017] S10: Transfer the silicon material to tank No. 10 for overflow rinsing with high-purity water. The temperature of the high-purity water is 55°C and the cleaning time is 150 to 300 seconds.

[0018] S11: Transfer the silicon material to tank No. 11 for immersion in high-purity water. The water temperature of the high-purity water is 90°C and the immersion time is 150 to 300 seconds.

[0019] S12: Transfer the silicon material to tank No. 12 and dry it with nitrogen purge at a temperature of 80-120°C and an air volume of 800-1200m 3 / h, the drying time is 200-600s, and the cleaning process is completed.

[0020] Furthermore, the silicon material has a particle size range of 1 to 150 mm.

[0021] Furthermore, the degreasing agent in S2 is CCl 4 Or hydrogen peroxide.

[0022] Furthermore, the CCl 4 The concentration is 10 to 30 wt%.

[0023] Furthermore, the concentration of the hydrogen peroxide is 15-25wt%.

[0024] Furthermore, the mass concentration of hydrofluoric acid in S6 is 15-25wt%, and the mass concentration of nitric acid is 45-75wt%.

[0025] Furthermore, the mass concentration of hydrofluoric acid in S7 is 15-25wt%, and the mass concentration of nitric acid is 45-75wt%.

[0026] Furthermore, the nitrogen in S12 is blown in using a centrifugal fan with a power of 5 to 6 kW.

[0027] Tank No. 1 is a high-purity water immersion tank, which is used to remove particulate impurities on the surface of the silicon material and reduce the surface tension of the silicon material; Tank No. 2 is a degreasing agent immersion tank, which is used to remove grease pollutants on the surface of the silicon material. The degreasing agent is CCl 4 or hydrogen peroxide; Tank 3 is an ultrasonic + high-purity water rinse tank; Tank 4 is a high-purity water rinse tank, the purpose of which is to preliminarily clean the degreasing agent and other contaminants on the surface of the silicon material; Tank 5 is an ultrapure water rinse tank, the purpose of which is to completely clean the degreasing agent and other contaminants on the surface of the silicon material; Tank 6 is a hydrofluoric acid + nitric acid mixed acid etching tank, the silicon material 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; Tank 7 is a hydrofluoric acid + nitric acid mixed acid pickling calibration tank, the cleaning reaction principle Consistent with the previous slot, the supporting robot controls the silicon material slot to shake slightly horizontally in the acid solution to ensure that the etching degree of the silicon material surface meets the corresponding process requirements. The hydrofluoric acid used in the present invention is UPS grade; slot No. 8 is a high-purity water rinsing slot, the purpose of which is to preliminarily clean the residual acid on the surface of the silicon material; slot No. 9 is a high-purity water flushing slot, the purpose of which is to secondary clean the residual acid on the surface of the silicon material; slot No. 10 is a high-purity water rinsing slot, the purpose of which is to further clean the residual acid on the surface of the silicon material; slot No. 11 is a high-purity water immersion slot, the purpose of which is to completely clean the residual acid on the surface of the silicon material; slot No. 12 is nitrogen dried.

[0028] The present invention mainly includes one degreasing and degreasing cleaning, two acid washings and nitrogen drying, and auxiliary ultrasonic cleaning. In the present invention, the silicon material is placed in a rectangular material basket made of PVDF material, and the basket is grabbed by a manipulator and passes through each working tank one by one to complete the silicon material cleaning operation. Each time the material basket is rinsed, the six surfaces are evenly rinsed and sprayed.

[0029] The cleaning process of the present invention etches the surface through mixed acid, and through ultrasonic and manipulator-assisted cleaning, the pollutants in the gaps and cracks of the silicon material can be cleaned, multiple high-purity water rinses can effectively clean the particle pollutants attached to the surface of the silicon material, and one deoiling and degreasing soaking can effectively clean the surface organic matter. The first pickling can dissolve silicon oxide on the surface of the silicon material at a low cost without consuming nitric acid. The consumption of UP-grade nitric acid in the entire pickling process can be reduced by 5% to 10%. Correction pickling can effectively etch the surface to remove impurities and effectively modify the surface. Assisted by horizontal shaking of the manipulator, the mixed acid solution can be fully infiltrated into the cracks and gaps, effectively removing pollutants and impurities, improving cleaning efficiency, and reducing the cleaning time by 15% to 20%, effectively increasing the yield. Through this method, the surface metal can reach less than 1ppbw. DETAILED DESCRIPTION

[0030] In order to further illustrate the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the specific implementation methods, structures, features and effects of the present invention are described in detail below in combination with the embodiments.

[0031] Example 1

[0032] S1: First, soak the silicon material with a particle size of 1 to 8 mm in high-purity water in tank 1. The water temperature of the high-purity water is 55°C and the soaking time is 300 seconds.

[0033] S2: Transfer the silicon material to tank 2 and use CCl with a concentration of 30wt%. 4 Soaking, the soaking temperature is 55℃, and the soaking cleaning time is 300s;

[0034] S3: The silicon material is transferred to tank No. 3 for overflow rinsing with high-purity water. The high-purity water temperature is room temperature. The rinsing is performed under ultrasonic conditions. The rinsing time is 300 seconds and the ultrasonic power is 40KHz.

[0035] S4: Transfer the silicon material to tank No. 4 for a single rinse with high-purity water. The water temperature of the high-purity water is room temperature, the water pressure of the high-purity water is 0.2Mpa, and the rinsing time is 300s.

[0036] S5: Transfer the silicon material to tank No. 5 for secondary rinsing with ultrapure water. The ultrapure water temperature is room temperature, the ultrapure water pressure is 0.2Mpa, and the rinsing time is 300s.

[0037] S6: transferring the silicon material to tank No. 6 for pickling treatment, using a mixed solution of nitric acid and hydrofluoric acid for pickling, the mass concentration of hydrofluoric acid is 25wt%, the mass concentration of nitric acid is 75wt%, the mass ratio of hydrofluoric acid to nitric acid is 1:5, and the pickling time is 200s;

[0038] S7: Transfer the silicon material to tank No. 7 for correction pickling treatment, use a mixed solution of nitric acid and hydrofluoric acid for correction pickling, the mass concentration of hydrofluoric acid is 25wt%, the mass concentration of nitric acid is 75wt%, and the mass ratio of hydrofluoric acid to nitric acid is 1:10. While correcting the pickling, the silicon material tank is slightly shaken horizontally in the acid solution, and the cleaning time is 300s;

[0039] S8: Transfer the silicon material to tank No. 8 for overflow rinsing with high-purity water. The water temperature of the high-purity water is room temperature and the cleaning time is 300 seconds.

[0040] S9: Transfer the silicon material to tank No. 9 for high-purity water washing. The water temperature of the high-purity water is room temperature, the water pressure of the high-purity water is 0.2Mpa, and the cleaning time is 300s;

[0041] S10: Transfer the silicon material to tank No. 10 for overflow rinsing with high-purity water. The temperature of the high-purity water is 55°C and the cleaning time is 150 to 300 seconds.

[0042] S11: Transfer the silicon material to tank No. 11 for immersion in high-purity water. The water temperature of the high-purity water is 90°C and the immersion time is 300s.

[0043] S12: Transfer the silicon material to tank No. 12 and dry it with nitrogen purge. Nitrogen is blown in by a centrifugal fan with a power of 6kW, a temperature of 120°C, and an air volume of 1200m 3 / h, the drying time is 600s, and the cleaning process is completed.

[0044] Example 2

[0045] S1: First, soak the silicon material with a particle size of 8 to 50 mm in high-purity water in tank 1. The water temperature of the high-purity water is 50°C and the soaking time is 210 seconds;

[0046] S2: Transfer the silicon material to tank No. 2 and soak it in 20wt% hydrogen peroxide at a temperature of 50°C for 200s.

[0047] S3: The silicon material is transferred to tank No. 3 for overflow rinsing with high-purity water. The high-purity water temperature is room temperature. The rinsing is performed under ultrasonic conditions. The rinsing time is 210 seconds and the ultrasonic power is 35KHz.

[0048] S4: Transfer the silicon material to tank No. 4 for a high-purity water rinse. The high-purity water temperature is room temperature, the high-purity water pressure is 0.1Mpa, and the rinsing time is 210s.

[0049] S5: Transfer the silicon material to tank No. 5 for secondary rinsing with ultrapure water. The ultrapure water temperature is room temperature, the ultrapure water pressure is 0.1Mpa, and the rinsing time is 210s.

[0050] S6: transferring the silicon material to tank No. 6 for pickling treatment, using a mixed solution of nitric acid and hydrofluoric acid for pickling, the mass concentration of hydrofluoric acid is 20wt%, the mass concentration of nitric acid is 60wt%, the mass ratio of hydrofluoric acid to nitric acid is 1:10, and the pickling time is 150s;

[0051] S7: Transfer the silicon material to tank No. 7 for correction pickling treatment, use a mixed solution of nitric acid and hydrofluoric acid for correction pickling, the mass concentration of hydrofluoric acid is 20wt%, the mass concentration of nitric acid is 60wt%, and the mass ratio of hydrofluoric acid to nitric acid is 1:12. While correcting the pickling, the silicon material tank is slightly shaken horizontally in the acid solution, and the cleaning time is 210s;

[0052] S8: Transfer the silicon material to tank No. 8 for overflow rinsing with high-purity water. The high-purity water temperature is room temperature and the cleaning time is 210 seconds.

[0053] S9: Transfer the silicon material to tank No. 9 for high-purity water washing. The water temperature of the high-purity water is room temperature, the water pressure of the high-purity water is 0.1Mpa, and the cleaning time is 210s;

[0054] S10: Transfer the silicon material to tank No. 10 for overflow rinsing with high-purity water. The temperature of the high-purity water is 55°C and the cleaning time is 210s.

[0055] S11: Transfer the silicon material to tank No. 11 for immersion in high-purity water. The water temperature of the high-purity water is 90°C and the immersion time is 210s.

[0056] S12: Transfer the silicon material to tank No. 12 and dry it with nitrogen purge. Nitrogen is blown in by a centrifugal blower with a power of 5 kW, a temperature of 100°C, and an air volume of 1000m 3 / h, the drying time is 400s, and the cleaning process is completed.

[0057] Example 3

[0058] S1: First, soak the silicon material with a particle size of 50-150 mm in high-purity water in tank 1. The water temperature of the high-purity water is 60°C and the soaking time is 150 seconds.

[0059] S2: Transfer the silicon material to tank 2 and use CCl with a concentration of 10wt%. 4 Soaking, the soaking temperature is 60℃, and the soaking cleaning time is 100s;

[0060] S3: The silicon material is transferred to tank No. 3 for overflow rinsing with high-purity water. The high-purity water temperature is room temperature. The rinsing is performed under ultrasonic conditions. The rinsing time is 150 seconds and the ultrasonic power is 28KHz.

[0061] S4: Transfer the silicon material to tank No. 4 for a single rinse with high-purity water. The water temperature of the high-purity water is room temperature, the water pressure of the high-purity water is 0.2Mpa, and the rinsing time is 150s.

[0062] S5: Transfer the silicon material to tank No. 5 for secondary rinsing with ultrapure water. The ultrapure water temperature is room temperature, the ultrapure water pressure is 0.2Mpa, and the rinsing time is 150s.

[0063] S6: transferring the silicon material to tank No. 6 for pickling treatment, using a mixed solution of nitric acid and hydrofluoric acid for pickling, the mass concentration of hydrofluoric acid is 15wt%, the mass concentration of nitric acid is 45wt%, the mass ratio of hydrofluoric acid to nitric acid is 1:5, and the pickling time is 100s;

[0064] S7: Transfer the silicon material to tank No. 7 for correction pickling treatment, use a mixed solution of nitric acid and hydrofluoric acid for correction pickling, the mass concentration of hydrofluoric acid is 15wt%, the mass concentration of nitric acid is 45wt%, and the mass ratio of hydrofluoric acid to nitric acid is 1:10. While correcting the pickling, the silicon material tank is slightly shaken horizontally in the acid solution, and the cleaning time is 150s;

[0065] S8: Transfer the silicon material to tank No. 8 for overflow rinsing with high-purity water. The high-purity water temperature is room temperature and the cleaning time is 150 seconds.

[0066] S9: Transfer the silicon material to tank No. 9 for high-purity water washing. The water temperature of the high-purity water is room temperature, the water pressure of the high-purity water is 0.2Mpa, and the cleaning time is 150s.

[0067] S10: Transfer the silicon material to tank No. 10 for overflow rinsing with high-purity water. The temperature of the high-purity water is 55°C and the cleaning time is 150 seconds.

[0068] S11: Transfer the silicon material to tank No. 11 for immersion in high-purity water. The water temperature of the high-purity water is 90°C and the immersion time is 150s.

[0069] S12: Transfer the silicon material to tank No. 12 and dry it with nitrogen purge. Nitrogen is blown in by a centrifugal blower with a power of 5kW, a temperature of 80°C, and an air volume of 800m 3 / h, the drying time is 200s, and the cleaning process is completed.

[0070] Surface tests were performed on Examples 1 to 3, and the experimental results are summarized in the following table:

[0071]

[0072] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technical personnel in this field can make some changes or modify the technical contents disclosed above into equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any brief modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A novel electronic grade polysilicon cleaning process for integrated circuits, characterized in that: The specific steps of the process technology are as follows: S1: First, soak the silicon material in high-purity water in tank No.

1. The water temperature of the high-purity water is 50-60°C and the soaking time is 150-300s; S2: Transfer the silicon material to tank No. 2 and soak it in a degreasing agent at a temperature of 50 to 60°C for 100 to 300 seconds. S3: Transfer the silicon material to tank No. 3 for overflow rinsing with high-purity water. The high-purity water temperature is room temperature. Rinsing is performed under ultrasonic conditions. The rinsing time is 150 to 300 seconds. The ultrasonic power is 28 to 40 kHz. S4: Transfer the silicon material to tank No. 4 for a single rinse with high-purity water. The water temperature of the high-purity water is room temperature, the water pressure of the high-purity water is 0.1-0.2Mpa, and the rinsing time is 150-300s. S5: Transfer the silicon material to tank No. 5 for secondary rinsing with ultrapure water. The ultrapure water temperature is room temperature, the ultrapure water pressure is 0.1-0.2Mpa, and the rinsing time is 150-300s. S6: Transfer the silicon material to tank No. 6 for pickling treatment, use a mixed solution of nitric acid and hydrofluoric acid for pickling, the mass ratio of hydrofluoric acid to nitric acid is 1: (5-10), and the pickling time is 100-200s; S7: Transfer the silicon material to tank No. 7 for correction pickling treatment, use a mixed solution of nitric acid and hydrofluoric acid for correction pickling, the mass ratio of hydrofluoric acid to nitric acid is 1: (10-12), while the correction pickling, the silicon material tank is slightly shaken horizontally in the acid solution, and the cleaning time is 150-300s; S8: Transfer the silicon material to tank No. 8 for overflow rinsing with high-purity water. The water temperature of the high-purity water is room temperature and the cleaning time is 150 to 300 seconds. S9: Transfer the silicon material to tank No. 9 for high-purity water washing. The water temperature of the high-purity water is room temperature, the water pressure of the high-purity water is 0.1-0.2Mpa, and the cleaning time is 150-300s. S10: Transfer the silicon material to tank No. 10 for overflow rinsing with high-purity water. The temperature of the high-purity water is 55°C and the cleaning time is 150 to 300 seconds. S11: Transfer the silicon material to tank No. 11 for immersion in high-purity water. The water temperature of the high-purity water is 90°C and the immersion time is 150 to 300 seconds. S12: Transfer the silicon material to tank No. 12 and dry it with nitrogen purge at a temperature of 80-120°C and an air volume of 800-1200m 3 / h, the drying time is 200-600s, and the cleaning process is completed.

2. The novel electronic grade polysilicon cleaning process for integrated circuits according to claim 1 is characterized in that: The silicon material particle size ranges from 1 to 150 mm.

3. The novel electronic grade polysilicon cleaning process for integrated circuits according to claim 1 is characterized in that: The degreasing and degreasing agent in S2 is CCl4 or hydrogen peroxide.

4. The novel electronic grade polysilicon cleaning process for integrated circuits according to claim 3 is characterized in that: The CCl4 concentration is 10-30wt%.

5. The novel electronic grade polysilicon cleaning process for integrated circuits according to claim 3 is characterized in that: The concentration of the hydrogen peroxide is 15-25 wt %.

6. The novel electronic grade polysilicon cleaning process for integrated circuits according to claim 1 is characterized in that: The mass concentration of hydrofluoric acid in the S6 is 15-25wt%, and the mass concentration of nitric acid is 45-75wt%.

7. The novel electronic grade polysilicon cleaning process for integrated circuits according to claim 1 is characterized in that: The mass concentration of hydrofluoric acid in S7 is 15-25wt%, and the mass concentration of nitric acid is 45-75wt%.

8. The novel electronic grade polysilicon cleaning process for integrated circuits according to claim 1 is characterized in that: The nitrogen in S12 is blown in by a centrifugal fan with a power of 5-6 kW.

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