Back polishing method of silicon wafer and preparation method of TOPcon battery

Abstract

The invention provides a back polishing method of a silicon wafer and a preparation method of a TOPcon battery, and belongs to the technical field of TOPcon batteries. Through the back polishing method of the silicon wafer, a natural oxide layer can be prevented from growing on the back surface of the silicon wafer, and the thickness of the tunneling oxide layer and the consistency of the thicknesses of all parts of the tunneling oxide layer can be effectively controlled in a subsequent tunneling oxide layer deposition process, so that the photoelectric conversion efficiency of a TOPcon battery is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of TOPcon batteries, and in particular relates to a back-polishing method for a silicon wafer and a preparation method for a TOPcon battery. Background Art

[0002] The process route of TOPcon cells is mainly to first texturize the double sides of the silicon wafer, then prepare a PN junction on the front side of the texturized silicon wafer, then back-polish the back side of the silicon wafer after preparing the PN junction to obtain a back polished surface, and then use LPCVD or PECVD technology to prepare a tunneling oxide layer and a doped amorphous silicon layer on the back polished surface, and then after annealing, the doped amorphous silicon layer is transformed into a doped polycrystalline silicon layer. The tunneling oxide layer and the doped polycrystalline silicon layer together constitute a back contact passivation structure, and then an anti-reflection layer is prepared on the front and / or back side of the battery, and then the front electrode is screen-printed on the front side of the battery, and the back electrode is screen-printed on the back side of the battery, and finally sintered to form an ohmic contact.

[0003] Among them, the back polishing process can be carried out by acid etching, alkali etching or a combination of the two. Taking the commonly used alkali etching for back polishing as an example, the back polishing process can include the following steps: S1: Back polishing: Pre-attach a protective film to the front of the silicon wafer after preparing the PN junction, and use NaOH / KOH and polishing additives for back polishing; S2: Cleaning: First use a mixed solution of NaOH / KOH and H2O2 for cleaning, H2O2 decomposes the organic matter remaining in the polishing additive, and the formed product produces a water-soluble complex with the alkali to achieve the purpose of cleaning; then use a mixed solution of HF and HCl for cleaning to neutralize the alkali residue of the previous treatment; S3: Slow pull cleaning: Slowly pull and clean the silicon wafer at room temperature to 80°C, while preventing the silicon wafer from sticking; S4: Drying: Dry the silicon wafer at about 100°C. Summary of the invention

[0004] The present invention is based on the inventors' discovery and understanding of the following facts and problems: Since the silicon wafer is in an environment of oxidant H2O2 and high temperature and humidity during the back-polishing process, a natural oxide layer will grow on the back of the silicon wafer during the process. Since the naturally grown natural oxide layer has no regularity, it may be thicker or thinner in some places. When the tunneling oxide layer is subsequently grown on the back-polished surface of the silicon wafer using PECVD or LPCVD technology, the natural oxide layer will cause the total thickness of the oxide layer to be thicker or the overall uniformity to deteriorate, ultimately resulting in a decrease in the conversion efficiency of the TOPcon battery. Therefore, it is necessary to design a back-polishing method for silicon wafers and a method for preparing TOPcon batteries to improve the controllability of the oxide layer thickness and the consistency of the thickness of the oxide layer at various locations.

[0005] The present invention aims to solve one of the technical problems in the related art to a certain extent. To this end, the embodiments of the present invention provide a back-polishing method for a silicon wafer and a method for preparing a TOPcon battery.

[0006] In a first aspect, an embodiment of the present invention provides a method for cleaning a silicon wafer after back polishing, comprising the following steps:

[0007] S1. A protective film is attached to the front of the silicon wafer after the PN junction is prepared in advance, and a polishing agent is used to back-polish the back of the silicon wafer after the PN junction is prepared to obtain a back-polished silicon wafer; wherein the polishing agent is a mixed solution of a strong base and a polishing additive;

[0008] S2. The back-polished silicon wafer is first cleaned using a first cleaning agent to obtain a first cleaned silicon wafer; wherein the first cleaning agent is a mixed solution of a strong base and hydrogen peroxide;

[0009] S3. The silicon wafer after the first cleaning is cleaned for a second time using a second cleaning agent to obtain a silicon wafer after the second cleaning; wherein the second cleaning agent is a mixed solution of hydrofluoric acid and nitric acid;

[0010] S4. The silicon wafer after the second cleaning is cleaned for a third time with water to obtain a silicon wafer after the third cleaning;

[0011] S5. Drying the silicon wafer after the third cleaning at 90-110° C. in an inert gas atmosphere, and then removing the protective film to obtain a finished silicon wafer.

[0012] The advantages and technical effects of the back-polishing method of the silicon wafer according to the embodiment of the present invention are as follows:

[0013] (1) After the back polishing in step S1 and the first cleaning in step S2, the silicon wafer is cleaned for the second time using a second cleaning agent in step S3 to remove the natural oxide layer that may have grown on the back of the silicon wafer and remove the residual alkali solution.

[0014] (2) The second cleaning agent used in step S3 is a mixed solution of hydrofluoric acid and nitric acid. Hydrofluoric acid is the main substance for removing the natural oxide layer, and nitric acid is added to enhance the acidity of the second cleaning agent, which has a better cleaning effect than using only hydrofluoric acid for the second cleaning.

[0015] (3) The water washing in step S4 can remove the acid residue brought by step S3, and the drying in an inert atmosphere in step S5 can further prevent the formation of a natural oxide layer.

[0016] (4) Since the back-polishing process does not result in the production of a natural oxide layer with poor uniformity, the thickness of the subsequently deposited tunnel oxide layer can be precisely controlled, and the consistency of the thickness of the tunnel oxide layer can be greatly improved, thereby effectively improving the photoelectric conversion efficiency of the TOPcon cell.

[0017] In some embodiments, the volume fraction of hydrofluoric acid in the second cleaning agent is 1.5-2.5%.

[0018] In some embodiments, the volume fraction of hydrofluoric acid in the second cleaning agent is 1.8-2.0%.

[0019] In some embodiments, the volume fraction of nitric acid in the second cleaning agent is 0.5-1.0%.

[0020] In some embodiments, the volume fraction of nitric acid in the second cleaning agent is 0.6-1.0%.

[0021] In some embodiments, in the second cleaning agent, the volume fraction ratio of hydrofluoric acid to nitric acid is (2.5-5.0):1.

[0022] In some embodiments, in the second cleaning agent, the volume fraction ratio of hydrofluoric acid to nitric acid is (2.5-3.0):1.

[0023] In some embodiments, the second cleaning time is 20-40 seconds.

[0024] In some embodiments, after the first cleaning, the silicon wafer contacts the second cleaning agent with its back side facing downward.

[0025] In a second aspect, an embodiment of the present invention further provides a method for preparing a TOPcon battery, including the back-polishing method of the silicon wafer of the first aspect.

[0026] The advantages and technical effects brought by the preparation method of the TOPcon battery of the embodiment of the present invention are:

[0027] After the back-polishing process of the first aspect is completed, the tunnel oxide layer deposition process is entered. Since the back-polishing process does not result in the production of a natural oxide layer with poor uniformity, the thickness of the tunnel oxide layer obtained by subsequent deposition can be accurately controlled, and the consistency of the thickness of the tunnel oxide layer at various locations is greatly improved, thereby effectively improving the photoelectric conversion efficiency of the TOPcon cell. DETAILED DESCRIPTION

[0028] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and intended to be used to explain the present invention, but should not be construed as limiting the present invention.

[0029] In a first aspect, an embodiment of the present invention provides a method for back-polishing a silicon wafer, comprising the following steps:

[0030] S1. A protective film is attached to the front of the silicon wafer after the PN junction is prepared in advance, and a polishing agent is used to back-polish the back of the silicon wafer after the PN junction is prepared to obtain a back-polished silicon wafer; wherein the polishing agent is a mixed solution of a strong base and a polishing additive;

[0031] S2. The back-polished silicon wafer is first cleaned using a first cleaning agent to obtain a first cleaned silicon wafer; wherein the first cleaning agent is a mixed solution of a strong base and hydrogen peroxide;

[0032] S3. The silicon wafer after the first cleaning is cleaned for a second time using a second cleaning agent to obtain a silicon wafer after the second cleaning; wherein the second cleaning agent is a mixed solution of hydrofluoric acid and nitric acid;

[0033] S4. The silicon wafer after the second cleaning is cleaned for a third time with water to obtain a silicon wafer after the third cleaning;

[0034] S5. Drying the silicon wafer after the third cleaning at 90-110° C. in an inert gas atmosphere, and then removing the protective film to obtain a finished silicon wafer.

[0035] Working principle: After back polishing in step S1 and the first cleaning in step S2, the silicon wafer is cleaned for the second time with a second cleaning agent in step S3 to remove the natural oxide layer that may have grown on the back of the silicon wafer and remove the residual alkali solution.

[0036] The second cleaning agent used in step S3 is a mixed solution of hydrofluoric acid and nitric acid, wherein hydrofluoric acid is a weak acid that can dissolve the natural oxide layer (silicon dioxide). The specific reaction involved is that hydrofluoric acid reacts with silicon dioxide to generate volatile silicon tetrafluoride gas that escapes from the solution, thereby clearing the natural oxide layer; and adding strong acid nitric acid to the second cleaning agent is to enhance the acidity of the second cleaning agent, which is better than using a single hydrofluoric acid for the second cleaning effect, and is more conducive to completely clearing the natural oxide layer. Step S4 water washing can remove the acid residue brought by step S3, and step S5 drying under an inert atmosphere can further prevent the formation of a natural oxide layer.

[0037] Since the back-polishing process does not result in a natural oxide layer with poor uniformity, the thickness of the subsequently deposited tunnel oxide layer can be precisely controlled, and the consistency of the thickness of the tunnel oxide layer can be greatly improved, thereby effectively improving the photoelectric conversion efficiency of the TOPcon cell.

[0038] In some embodiments, the volume fraction of hydrofluoric acid in the second cleaning agent is 1.5-2.5%, such as 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, etc. When the volume fraction of hydrofluoric acid is too small, it is not conducive to fully removing the natural oxide layer that may be produced. When the volume fraction of hydrofluoric acid is too large, too much acid remains on the surface of the silicon wafer after the second cleaning, and too much water resources need to be wasted in the subsequent step S4 water washing. Preferably, in some embodiments, the volume fraction of hydrofluoric acid in the second cleaning agent is 1.8-2.0%.

[0039] In some embodiments, the volume fraction of nitric acid in the second cleaning agent is 0.5-1.0%, such as 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, etc. When the volume fraction of nitric acid is too small, it is not conducive to enhancing the acidity of the second cleaning agent and is not conducive to fully removing the natural oxide layer that may be produced. When the volume fraction of nitric acid is too much, there is too much acid remaining on the surface of the silicon wafer after the second cleaning, and too much water resources need to be wasted during the subsequent step S4 water washing. Preferably, in some embodiments, the volume fraction of nitric acid in the second cleaning agent is 0.6-1.0%.

[0040] In some embodiments, the volume fraction ratio of hydrofluoric acid to nitric acid in the second cleaning agent is (2.5-5.0):1, such as 2.5:1, 3.0:1, 3.5:1, 4.0:1, 4.5:1, 5.0:1, etc. When the volume fraction ratio is too small or too large, it is not conducive to fully removing the natural oxide layer that may be generated. Preferably, in some embodiments, the volume fraction ratio of hydrofluoric acid to nitric acid in the second cleaning agent is (2.5-3.0):1.

[0041] In some embodiments, the second cleaning time is 20-40s, such as 20s, 25s, 30s, 35s, 40s, etc. The thickness of the natural oxide layer is generally thin, so the second cleaning time within the above range can ensure that it is fully removed. When the second cleaning time is too short, it is not conducive to fully removing the generated natural oxide layer. When the second cleaning time is too long, it is not conducive to improving production efficiency.

[0042] In some embodiments, after the first cleaning, the back side of the silicon wafer faces downward and contacts the second cleaning agent, which is more conducive to protecting the PN junction on the front side of the silicon wafer from being damaged by the second cleaning agent.

[0043] The purpose of the drying process is to remove the moisture remaining on the silicon wafer from the washing process as quickly as possible, so as to facilitate the subsequent tunnel oxide layer preparation process as soon as possible.

[0044] In a second aspect, an embodiment of the present invention provides a method for preparing a TOPcon battery, including the back-polishing method of the silicon wafer of the first aspect.

[0045] Working principle: After the back-polishing process in the first aspect is completed, the tunnel oxide layer deposition process is entered. Since the back-polishing process does not result in the production of a natural oxide layer with poor uniformity, the thickness of the tunnel oxide layer deposited subsequently can be precisely controlled, and the consistency of the thickness of the tunnel oxide layer at various locations is greatly improved, thereby effectively improving the photoelectric conversion efficiency of the TOPcon cell.

[0046] The present invention is described in detail below with reference to embodiments.

[0047] Example 1

[0048] A method for preparing an N-type TOPcon battery comprises the following steps:

[0049] (1) Double-sided texturing is performed on a silicon wafer to obtain a double-sided texturing silicon wafer.

[0050] (2) A PN junction is prepared on the front side of the silicon wafer after double-sided texturing to obtain a silicon wafer after the PN junction is prepared.

[0051] (3) A protective film is attached to the front side of the silicon wafer after the PN junction is prepared, and a polishing agent is used to back-polish the back side of the silicon wafer after the PN junction is prepared to obtain a back-polished silicon wafer; wherein the polishing agent is a mixed solution of a strong base and a polishing additive.

[0052] (4) Using a first cleaning agent to perform a first cleaning on the back-polished silicon wafer to obtain a first-cleaned silicon wafer; wherein the first cleaning agent is a mixed solution of a strong alkali and hydrogen peroxide.

[0053] (5) Using a second cleaning agent to clean the silicon wafer after the first cleaning for a second time to obtain a silicon wafer after the second cleaning; wherein the second cleaning agent is a mixed solution of hydrofluoric acid and nitric acid.

[0054] Specifically, the silicon wafer after the first cleaning is sent to the chain pickling machine, with the back side facing downward to contact the second cleaning agent, in which the volume fraction of hydrofluoric acid is 1.8%, and the volume fraction of nitric acid is 0.6%. The second cleaning time is 30 seconds.

[0055] (6) The silicon wafer after the second cleaning is cleaned for a third time with water to obtain a silicon wafer after the third cleaning.

[0056] (7) After the third cleaning, the silicon wafer is dried at 100° C. in a nitrogen atmosphere for 20 seconds, and then the protective film is removed to obtain a finished silicon wafer.

[0057] Example 2

[0058] The preparation method of the N-type TOPcon battery in this embodiment is the same as that in Embodiment 1, except that in step (5), the volume fraction of hydrofluoric acid in the second cleaning agent is 2.0%, and the volume fraction of nitric acid is 1.0%.

[0059] Example 3

[0060] The preparation method of the N-type TOPcon battery in this embodiment is the same as that in Embodiment 1, except that in step (5), the volume fraction of hydrofluoric acid in the second cleaning agent is 2.5%, and the volume fraction of nitric acid is 0.5%.

[0061] Example 4

[0062] The preparation method of the N-type TOPcon battery in this embodiment is the same as that in Embodiment 1, except that in step (5), the volume fraction of hydrofluoric acid in the second cleaning agent is 1.5%, and the volume fraction of nitric acid is 0.5%.

[0063] Comparative Example 1

[0064] The preparation method of the N-type TOPcon battery in this comparative example is the same as that in Example 1, except that step (5) is omitted.

[0065] Comparative Example 2

[0066] The preparation method of the N-type TOPcon battery in this comparative example is the same as that in Example 1, except that in step (5), the second cleaning agent is a hydrofluoric acid solution, and the volume fraction of hydrofluoric acid in the second cleaning agent is 2.4%.

[0067] Test method: XRD test and SEM test were performed on the back of the finished silicon wafers prepared in each embodiment and comparative example to detect whether there was silicon oxide on the back. The test results are shown in Table 1.

[0068] Table 1.

[0069]

[0070]

[0071] In the present invention, the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" etc. mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the described specific features, structures, materials or characteristics may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0072] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A back-polishing method for a silicon wafer, characterized in that: The following steps are involved: S1. A protective film is attached to the front of the silicon wafer after the PN junction is prepared in advance, and a polishing agent is used to back-polish the back of the silicon wafer after the PN junction is prepared to obtain a back-polished silicon wafer; wherein the polishing agent is a mixed solution of a strong base and a polishing additive; S2. The back-polished silicon wafer is first cleaned using a first cleaning agent to obtain a first cleaned silicon wafer; wherein the first cleaning agent is a mixed solution of a strong base and hydrogen peroxide; S3. The silicon wafer after the first cleaning is cleaned for a second time using a second cleaning agent to obtain a silicon wafer after the second cleaning; wherein the second cleaning agent is a mixed solution of hydrofluoric acid and nitric acid; S4. The silicon wafer after the second cleaning is cleaned for a third time with water to obtain a silicon wafer after the third cleaning; S5. Drying the silicon wafer after the third cleaning at 90-110° C. in an inert gas atmosphere, and then removing the protective film to obtain a finished silicon wafer.

2. The back-polishing method of silicon wafer according to claim 1, characterized in that: In the second cleaning agent, the volume fraction of hydrofluoric acid is 1.5-2.5%.

3. The back-polishing method of silicon wafer according to claim 2, characterized in that: In the second cleaning agent, the volume fraction of hydrofluoric acid is 1.8-2.0%.

4. The back-polishing method of silicon wafer according to claim 1, characterized in that: In the second cleaning agent, the volume fraction of nitric acid is 0.5-1.0%.

5. The back-polishing method of silicon wafer according to claim 4, characterized in that: In the second cleaning agent, the volume fraction of nitric acid is 0.6-1.0%.

6. The back-polishing method of silicon wafer according to claim 1, characterized in that: In the second cleaning agent, the volume fraction ratio of hydrofluoric acid to nitric acid is (2.5-5.0):

1.

7. The back-polishing method of silicon wafer according to claim 6, characterized in that: In the second cleaning agent, the volume fraction ratio of hydrofluoric acid to nitric acid is (2.5-3.0):

1.

8. The back-polishing method of silicon wafer according to claim 1, characterized in that: The second cleaning time is 20-40s.

9. The back-polishing method of silicon wafer according to claim 1, characterized in that: After the first cleaning, the back side of the silicon wafer faces downward and contacts the second cleaning agent.

10. A method for preparing a TOPcon battery, characterized in that: A back-polishing method for a silicon wafer comprising the steps of any one of claims 1 to 9.

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