A cleaning process for improving the surface cleanliness of large size polished wafers

Abstract

The present application provides a kind of cleaning process for improving the surface cleanliness of large size polished wafer, steps include: multiple times to silicon wafer execution pickling and alkaline cleaning, wherein, all alkaline cleaning is carried out between pickling, and all alkaline cleaning is continuous arrangement.The present application provides a kind of cleaning process for improving the surface cleanliness of large size polished wafer, by redesigning cleaning process, and re-adjusting pickling and alkaline cleaning component, to thoroughly clean the metal ions on the surface of silicon wafer, at the same time, mechanical damage and other particle impurities on the surface of silicon wafer are also removed, to ensure that the surface cleanliness of silicon wafer can reach 65nm particle growth≤60, so that the dirty wafer rate is reduced by 0.3%.

Description

Technical Field

[0001] The invention belongs to the technical field of silicon wafer cleaning, and in particular relates to a cleaning process for improving the surface cleanliness of large-size polishing wafers. Background Art

[0002] The cleaning performance of silicon wafers directly impacts the conversion efficiency of solar cells. Currently, alkaline and acid washes alone cannot completely remove particulate impurities and metal ions from the silicon wafer surface, and therefore cannot guarantee complete surface cleanliness. Furthermore, as silicon wafer size increases, the proportion of dirty wafers increases significantly with current cleaning methods, resulting in poor cleanliness. Summary of the Invention

[0003] The present invention provides a cleaning process for improving the surface cleanliness of large-size polishing wafers, solving the technical problem in the prior art that irrational cleaning processes lead to incomplete removal of particle impurities and metal ions on the surface of silicon wafers.

[0004] In order to solve at least one of the above technical problems, the technical solution adopted by the present invention is:

[0005] A cleaning process for improving the surface cleanliness of large-size polishing wafers comprises the following steps: performing acid washing and alkaline washing on silicon wafers multiple times, wherein all alkaline washings are performed between acid washings and all alkaline washings are performed continuously.

[0006] Furthermore, it includes two acid washes and two alkali washes, and the acid wash is performed once and then the alkali wash is performed.

[0007] Furthermore, the chemical compositions of the two pickling processes were different;

[0008] The liquid composition of the two alkaline washes is the same, and the concentration of the liquid of the first alkaline wash is greater than that of the second alkaline wash.

[0009] Furthermore, the first pickling solution is a diluted hydrofluoric acid solution, and the mass fraction of the hydrofluoric acid is 30-40%;

[0010] The secondary pickling solution is a mixed solution of hydrochloric acid and hydrogen peroxide, wherein the volume ratio of hydrochloric acid to hydrogen peroxide is 1:1;

[0011] The pickling time for both times was 230-330s; and the pickling was performed at room temperature.

[0012] Furthermore, the liquid used in both alkaline washes is a mixed solution of ammonia water and hydrogen peroxide; and the concentration of the liquid used in the first alkaline wash is 30% greater than that used in the second alkaline wash.

[0013] Furthermore, in the first alkaline washing solution, the volume ratio of ammonia water to hydrogen peroxide is 1:2; wherein the ammonia water is a solution with a mass fraction of 44-46%, and the hydrogen peroxide is a solution with a mass fraction of 30-32%.

[0014] Furthermore, the cleaning time of the two alkaline washings is the same, both are 250-350s; the alkaline washing temperature is both 50-70°C;

[0015] And ultrasonic vibration is also provided during the alkaline cleaning process.

[0016] Furthermore, pure water cleaning is performed before and after each pickling, and the cleaning time is the same, both 230-350s; the cleaning temperature is room temperature.

[0017] Furthermore, after the secondary pickling, the silicon wafer is further subjected to oxidation cleaning, and the oxidation cleaning liquid is ozone water;

[0018] The time and temperature of oxidation cleaning are the same as those of each pure water cleaning.

[0019] Furthermore, after the oxidation cleaning, the method further includes sequentially performing slow pull cleaning and thermal drying on the silicon wafer, wherein:

[0020] Slow pull cleaning is carried out in pure water at a temperature of 40-60°C, and the cleaning time is 20-40s;

[0021] After slow pulling and cleaning, slowly extract the silicon wafer from the pure water surface. The extraction time is 100-200 seconds.

[0022] Thermal drying is carried out in an oven at a temperature of 45-65°C and a drying time of 100-200s.

[0023] The present invention adopts a cleaning process designed to improve the surface cleanliness of large-size polishing wafers. By redesigning the cleaning process and readjusting the acid and alkaline cleaning components, the metal ions on the surface of the silicon wafer are thoroughly cleaned. At the same time, mechanical damage and other particulate impurities on the surface of the silicon wafer are also removed to ensure that the surface cleanliness of the silicon wafer can reach 65nm particle growth ≤ 60 particles, reducing the dirty wafer rate by 0.3%. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The present invention is a flowchart of a cleaning process for improving the surface cleanliness of a large-sized polishing wafer according to an embodiment of the present invention. DETAILED DESCRIPTION

[0025] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0026] This embodiment proposes a cleaning process to improve the surface cleanliness of large-sized polishing wafers. Figure 1 As shown, the steps include:

[0027] The cleaning unit includes performing acid cleaning and alkaline cleaning on the silicon wafer multiple times, wherein all alkaline cleanings are performed between acid cleanings and all alkaline cleanings are set continuously.

[0028] Preferably, the present embodiment includes two acid washes and two alkaline washes, and the acid wash is performed once, followed by two consecutive alkaline washes, and then the second acid wash is performed. The acid wash is mainly used to remove metal ions on the surface of the silicon wafer. At the same time, there is a natural oxide layer on the silicon wafer.

[0029] Furthermore, the two pickling solutions have different compositions. The first pickling solution is a diluted hydrofluoric acid solution with a mass fraction of 30-40%; the second pickling solution is a mixed solution of hydrochloric acid and hydrogen peroxide, wherein the volume ratio of hydrochloric acid to hydrogen peroxide is 1:1. Both pickling times are 230-330 seconds; and the pickling is performed at room temperature. This is because a single pickling cannot completely remove the metal ions, requiring at least two pickling cycles to completely remove them. Although the pickling is performed multiple times, the compositions of the pickling solutions used in the two pickling cycles are different. The purpose is to have different amounts of metal ions at different pickling locations, resulting in different corresponding pickling intensities, and thus different pickling solution compositions.

[0030] Specifically, the first pickling is mainly to remove the natural oxide layer and metal ions on the surface of the silicon wafer. The pickling solution is a diluted hydrofluoric acid solution, wherein the mass fraction of the hydrofluoric acid is 30-40%.

[0031] The secondary pickling process is performed after the two alkaline washes. It primarily corrodes the silicon wafer, removing metal ions from the surface. It also neutralizes any remaining alkaline solution, removing mechanical damage. The secondary pickling solution is a mixture of hydrochloric acid and hydrogen peroxide, with a volume ratio of 1:1. The acid solution is prepared according to this ratio. The secondary pickling process lasts the same 250-350 seconds as the initial pickling process.

[0032] The two alkaline washes use the same solution composition, and the concentration of the first alkaline wash is higher than that of the second. Alkaline washes primarily corrode the silicon wafer surface to remove particulate matter and mechanical damage. A single alkaline wash cannot completely remove particulate matter and mechanical damage from the wafer surface. Furthermore, since the initial alkaline wash produces a high concentration of particulate matter and mechanical damage on the wafer surface, a higher concentration of alkaline solution is required. After the initial alkaline wash, the number of particulate matter and mechanical damage on the wafer surface decreases, requiring a more diluted concentration than the initial wash to completely remove the particulate matter and mechanical damage. If the second alkaline wash uses the same concentration as the initial wash, it will not only remove particulate matter and mechanical damage, but also further damage the wafer surface quality, alkalizing the wafer substrate and compromising its surface quality. Furthermore, a high and excessive concentration of alkaline solution on the wafer surface is detrimental to subsequent cleaning and the secondary acid wash.

[0033] Preferably, the liquids for both alkaline washes are mixed solutions of ammonia water and hydrogen peroxide; and the concentration of the liquid for the first alkaline wash is 30% greater than that for the second alkaline wash; in the liquid for the first alkaline wash, the volume ratio of ammonia water to hydrogen peroxide is 1:2; wherein the ammonia water is a solution with a mass fraction of 44-46%, and the hydrogen peroxide is a solution with a mass fraction of 30-32%, that is, the alkaline solution is a solution with a volume ratio of ammonia water to hydrogen peroxide of 1:2, and the alkaline solution is prepared according to this volume ratio. In other words, the liquid components for the two alkaline washes are the same, that is, the liquid ratio is the same, still a volume ratio of ammonia water to hydrogen peroxide of 1:2; wherein the ammonia water is a solution with a mass fraction of 44-46%, and the hydrogen peroxide is a solution with a mass fraction of 30-32%; but the proportion of solute water is different, and the proportion of solute water in the second alkaline wash is greater than that in the first alkaline wash, thereby making the liquid concentration of the second alkaline wash lower than that of the first alkaline wash. Furthermore, when the concentration of the secondary alkali solution is 30% lower than that of the first alkali solution, the treatment of particle impurities and mechanical damage on the surface of the silicon wafer is optimal, and not much alkali solution will remain on the surface of the silicon wafer.

[0034] Specifically, the two alkaline washes primarily corrode the silicon wafer surface, removing impurities and mechanical damage. Both washes take the same 250-350 seconds, and the wash temperature is 50-70°C. This equals the cleaning time of a single alkaline wash, shortening the wash cycle and improving overall wash efficiency while reducing alkali solution waste.

[0035] To enhance cleaning effectiveness during the two alkaline washes, both alkaline wash tanks are equipped with ultrasonic vibrations, with an output power of 1500-2500W. Ultrasonic generators are installed in the alkaline wash tanks, enabling ultrasonic cleaning of silicon wafers. Furthermore, both tanks are equipped with heating functions to heat the alkaline solution within them, ensuring that the solution is heated to 50-70°C during wafer cleaning, rapidly improving cleaning effectiveness.

[0036] Furthermore, pure water cleaning is performed before and after each pickling, and the cleaning time is the same. In order to ensure the consistency of the overall cleaning rhythm, the cleaning time is 230-350s; the cleaning temperature is room temperature.

[0037] Among them, the cleaning before the first pickling is mainly to remove large water-washable particle molecules on the surface of the silicon wafer.

[0038] In the secondary cleaning after the first pickling, the cleaning tank is filled with pure water cleaning liquid, and the silicon wafers that have undergone the first pickling are cleaned with pure water. Since there is residual hydrofluoric acid diluted solution on the surface of the silicon wafer, the hydrofluoric acid solution on the surface of the silicon wafer is cleaned through this pure water cleaning. The pure water tanks for the first and second cleanings have overflow functions.

[0039] The third cleaning after the second alkaline cleaning, that is, the cleaning before the second acid cleaning, is also done with pure water. It is mainly used to clean the silicon wafers after the second alkaline cleaning, and to clean the alkaline solution on the surface of the silicon wafers. The pure water cleaning tank also has an overflow function.

[0040] The four subsequent rinses after the secondary pickling process primarily involve pure water cleaning of the wafers, removing the hydrochloric acid and hydrogen peroxide mixture from the wafer surfaces. This pure water rinse utilizes a rapid drainage method, with multiple water supply and drain cycles for rapid cleaning and drainage. This rapid drainage method facilitates the complete removal of the acid solution from the wafer surfaces.

[0041] Furthermore, after the second pickling and after the fourth cleaning, the silicon wafer is also subjected to oxidation cleaning. This is the last step of the cleaning unit. The oxidation cleaning liquid is ozone water. It mainly uses ozone (O3) aqueous solution to oxidize the surface of the silicon wafer to make the surface of the silicon wafer reach a certain cleanliness standard and reduce the particle contamination of the silicon wafer surface by particles in the environment. The oxidation cleaning time is the same as the pure water cleaning time, both are 250-350s; and the oxidation cleaning temperature is also the same as the temperature of each pure water cleaning, both are room temperature.

[0042] At this point, the cleaning unit ends.

[0043] A drying unit is provided after the cleaning unit, which performs slow pull cleaning and thermal drying on the silicon wafer in sequence.

[0044] Slow pull cleaning is performed in pure water at a temperature of 40-60°C, bringing the silicon wafer surface to a certain cleanliness standard and reducing contamination from particles in the environment. The cleaning time is 20-40 seconds. After the slow pull cleaning, the silicon wafer is slowly extracted from the pure water surface for 100-200 seconds.

[0045] Thermal drying is to place the silicon wafer in an oven at a temperature of 45-65°C to completely remove the water molecules on the surface of the silicon wafer so that the surface cleanliness of the silicon wafer meets the specified requirements. The drying time is 100-200s.

[0046] After drying is completed, the silicon wafer is unloaded from the cleaning machine.

[0047] Table 1 shows the changes in silicon wafer cleanliness and dirty wafer rate achieved using the present cleaning process compared to the prior art. The table shows that the prior art cleaning process achieves a silicon wafer cleanliness level of ≤100 65nm particle growth. However, after two acid washes and two alkaline washes, the silicon wafer surface cleanliness level reaches ≤60 65nm particle growth, a significant improvement in cleanliness. The dirty wafer rate with the prior art cleaning process was 0.5%, but this was reduced to 0.2% with the present cleaning process, significantly reducing the dirty wafer rate.

[0048] Table 1 Changes in cleanliness and dirty wafer rate obtained by the cleaning process of this application compared with the existing technology

[0049]

[0050] The present invention adopts a cleaning process designed to improve the surface cleanliness of large-size polishing wafers. By redesigning the cleaning process and readjusting the acid and alkaline cleaning components, the metal ions on the surface of the silicon wafer are thoroughly cleaned. At the same time, mechanical damage and other particulate impurities on the surface of the silicon wafer are also removed to ensure that the surface cleanliness of the silicon wafer can reach 65nm particle growth ≤ 60 particles, reducing the dirty wafer rate by 0.3%.

[0051] The above embodiments of the present invention are described in detail. The contents described are only preferred embodiments of the present invention and should not be considered to limit the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the patent coverage of the present invention.

Claims

1. A cleaning process for improving the surface cleanliness of large-size polishing wafers, characterized in that the steps include: The silicon wafer is acid-washed and alkaline-washed multiple times, wherein all alkaline washes are performed between acid washes and all alkaline washes are set continuously; the cleaning steps are: primary cleaning, primary acid wash, secondary cleaning, primary alkaline wash, secondary alkaline wash, tertiary cleaning, secondary acid wash, fourth cleaning, and oxidation cleaning; The liquid used for both alkaline washes is a mixture of ammonia and hydrogen peroxide; and the concentration of the liquid used for the first wash is 30% higher than that used for the second wash. In the first alkaline washing solution, the volume ratio of ammonia water to hydrogen peroxide is 1:2; wherein, ammonia water is a 44-46% by mass solution, and hydrogen peroxide is a 30-32% by mass solution; The cleaning time of the two alkaline washes is the same, both are 250-350s; the alkaline wash temperature is 50-70℃; The cleaning liquid for the first, second, third and fourth cleanings is pure water, the liquid for oxidation cleaning is ozone water, and the oxidation cleaning time and pure water cleaning time are both 250-350s.

2. A cleaning process for improving the surface cleanliness of a large-size polishing sheet according to claim 1, characterized in that: It includes two acid washes and two alkaline washes, and the acid wash is performed once and then the alkaline wash is performed.

3. A cleaning process for improving the surface cleanliness of a large-size polishing sheet according to claim 1 or 2, characterized in that: The chemical composition of the two pickling solutions is different; The liquid composition of the two alkaline washes is the same.

4. A cleaning process for improving the surface cleanliness of a large-size polishing sheet according to claim 3, characterized in that: The first pickling solution is a dilute solution of hydrofluoric acid, and the mass fraction of hydrofluoric acid is 30-40%; The secondary pickling solution is a mixed solution of hydrochloric acid and hydrogen peroxide, wherein the volume ratio of hydrochloric acid to hydrogen peroxide is 1:1; The pickling time for both times was 230-330s; and the pickling was performed at room temperature.

5. A cleaning process for improving the surface cleanliness of a large-size polishing wafer according to claim 4, characterized in that: Ultrasonic vibration is also provided during the alkaline cleaning process.

6. A cleaning process for improving the surface cleanliness of a large-sized polishing wafer according to any one of claims 1-2, 4-5, characterized in that: A pure water rinse is performed before and after each pickling; the cleaning temperature is room temperature.

7. A cleaning process for improving the surface cleanliness of a large-size polishing wafer according to claim 6, characterized in that: After the secondary pickling, the silicon wafer is also subjected to oxidation cleaning; The oxidation cleaning temperature is the same as the pure water cleaning temperature each time.

8. A cleaning process for improving the surface cleanliness of a large-size polishing wafer according to claim 7, characterized in that: After the oxidation cleaning, the process also includes performing slow pull cleaning and thermal drying on the silicon wafer in sequence, wherein: Slow pull cleaning is carried out in pure water at a temperature of 40-60°C, and the cleaning time is 20-40s; After slow pulling and cleaning, slowly extract the silicon wafer from the pure water surface. The extraction time is 100-200 seconds. Thermal drying is carried out in an oven at a temperature of 45-65°C and a drying time of 100-200s.

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