KR101431730B1 - Texturing method of solar cell wafer - Google Patents

Abstract

The present invention relates to a surface treatment method of a substrate for a solar cell. According to the surface treatment method of the present invention, a first wet etching process, a reactive ion etching process, and a second wet etching process are performed on a substrate for a solar cell to form a pyramid structure on the substrate surface. Such a pyramid structure contributes to improving the light absorption efficiency of the solar cell by lowering the light reflectance. According to the present invention, the efficiency of a solar cell can be improved by effectively performing surface treatment on a pseudo single crystal substrate irrespective of the kind and orientation of crystals.

Description

TECHNICAL FIELD [0001] The present invention relates to a surface treatment method for a solar cell substrate,

The present invention relates to a surface treatment method of a substrate for a solar cell. More particularly, the present invention relates to a surface treatment method for a substrate for a solar cell, which can effectively structure the surface regardless of the type and quality of the lattice structure of the substrate.

With the recent depletion of existing energy sources such as oil and coal, interest in alternative energy to replace them is increasing. Among them, solar cells are attracting attention as next-generation batteries using semiconductor devices that convert solar energy directly into electric energy. Solar cells are divided into silicon solar cell, compound semiconductor solar cell and tandem solar cell.

Silicon-based solar cells, which are currently mass-producing solar cells, use silicon as a semiconductor substrate. Silicon is an indirect interband transition semiconductor in which only light having energy higher than the band gap of silicon is used as an electron- Hole pairs can be generated, so that the light absorption rate is low. Therefore, the silicon-based solar cell reflects at least 30% of the light incident into the solar cell on the surface of the silicon wafer as the substrate, thereby lowering the efficiency of the solar cell.

In order to reduce such optical loss, a texturing method is used in a silicon solar cell. The surface treatment method is to increase the surface roughness by forming irregularities on the silicon substrate surface of the silicon solar cell in order to absorb the maximum amount of light energy into the wafer substrate. When the first light arrives and strikes a sloping pyramid wall, some are absorbed and some are reflected back, which causes the backlight to continue to strike the surrounding pyramid walls to increase the light absorption. Thus, the light absorption amount is increased due to the pyramidal structure, and as a result, the cell efficiency can be improved. Therefore, when a silicon solar cell substrate is manufactured through a surface treatment method, it is possible to realize reduction of surface reflection of solar cell, improvement of carrier collection effect, and light shielding effect due to internal reflection of the solar cell.

There are two methods of surface treatment: wet surface treatment using acidic or alkaline solution and dry surface treatment using reactive ion gas. Alkali solution or acid solution is used for wet surface treatment, alkali solution is suitable for single crystal wafer and acid solution is suitable for surface treatment of polycrystalline wafer. In addition, dry surface treatment uses reactive ion gas and is suitable for polycrystalline wafers.

Meanwhile, recently, a method of using a pseudo single crystal substrate having high cost competitiveness for manufacturing a solar cell has been developed, because the process is simple and a large amount can be formed at a time.

As for the pseudo single crystal silicon, for example, a method of growing pseudo single crystal silicon using a seed as disclosed in U.S. Publication Nos. 20100193031 and 0748884 is disclosed.

Since pseudocrystalline silicon includes both single crystal and polycrystalline regions according to the growth method, it is difficult to uniformly treat all regions. Therefore, in order to develop pseudo-crystalline silicon for solar cells, it is necessary to study the surface treatment technique to maximize the light absorption of the substrate.

In order to solve the above problems, it is an object of the present invention to provide a method for efficiently performing surface treatment on a substrate for a solar cell, particularly a pseudo single crystal substrate, irrespective of the kind and orientation of the crystal.

In order to accomplish the above object, the present invention provides a method of manufacturing a solar cell, comprising: performing a first wet etching on a substrate for a solar cell; Performing reactive ion etching; And performing a second wet etching on the surface of the substrate.

According to the present invention, by effectively treating the surface of a solar cell substrate, it is possible to manufacture a solar cell with high efficiency by lowering the reflectivity and increasing the light absorption rate.

In addition, according to the crystallinity, uniform surface treatment can be performed by only one technique without being limited by the kinds of the pseudo-crystal substrates divided into various kinds, so that the process can be simplified and the cost can be reduced.

1 is a photograph of a single crystal substrate and a polycrystalline substrate photographed by a scanning electron microscope at a magnification of 5,000 times.
Figure 2 is a view showing substrates of various qualities.
Fig. 3 is a photograph of a pseudodocrystal substrate obtained by enlarging a surface of a base solution and an acidic solution using a microscope.
FIG. 4 is a photograph of the surface of the substrate after performing the steps up to the reactive ion etching process in Example 1, taken at a magnification of 70,000 times using a scanning electron microscope.
FIG. 5 is a photograph of the surface of the substrate after the reactive ion etching process and the second wet etching process performed in Example 1 is enlarged 5,000 times using a scanning electron microscope.
Fig. 6 is a graph showing the reflectance of the substrate before the surface treatment and the reflectance of the substrate after the surface treatment in the embodiment 1 and the comparative example 1. Fig.

In the surface treatment method of the present invention,

Performing a first wet etching on the solar cell substrate;

Performing reactive ion etching; And

And performing a second wet etch.

In the present invention, the terms first, second, etc. are used to describe various components, and the terms are used only for the purpose of distinguishing one component from another.

Moreover, the terminology used herein is for the purpose of describing exemplary embodiments only and is not intended to be limiting of the present invention. The singular expressions include plural expressions unless the context clearly dictates otherwise. In this specification, the terms "comprising," "comprising," or "having ", and the like are intended to specify the presence of stated features, But do not preclude the presence or addition of one or more other features, integers, steps, components, or combinations thereof.

Also in the present invention, when referring to each layer or element being "on" or "on" each layer or element, it is meant that each layer or element is formed directly on each layer or element, Layer or element may be additionally formed between each layer, the object, and the substrate.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a surface treatment method of a substrate for a solar cell of the present invention will be described in detail with reference to the drawings.

Substrates required for solar cell semiconductors vary in quality and price depending on the purity and the precision of the technology used for substrate preparation. The most important conditions are high purity to minimize the concentration of impurities, high quality to minimize crystal defects, and low cost to mass-produce. These are the most important parts for determining the mass production of solar cells. Recently, recently developed mono-like multi-wafer (MLM wafer) has both monocrystalline and polycrystalline forms.

Similar monocrystalline substrates combine the advantages of single crystals and polycrystals, and are produced using polycrystalline ingot growth methods. It can be used to produce high efficiency solar cell by making monocrystalline ingot by utilizing heat exchange method (HEM? Eat Exchange Method) which is one of the methods of growing polycrystalline ingot. The polycrystalline ingot growth method has a simple process and can produce a large amount at a time, and has high price competitiveness. Thus, the polycrystalline ingot is being developed for use as a substrate for a solar cell.

On the other hand, a process of texturing the substrate surface (also called "surface structuring" or "surface treatment") is performed to increase the light absorption of the solar cell. When the solar cell substrate is manufactured through texturing, the surface reflection of the solar cell can be reduced, the carrier collection effect can be improved, and the light confinement effect can be realized by the internal reflection of the solar cell.

The monocrystalline substrate refers to a single crystal substrate having a single crystal orientation as a whole, and in the case of the monocrystalline substrate, anisotropic etching that textures the substrate surface with a basic solution such as NaOH or KOH is effective.

On the other hand, a polycrystalline substrate refers to a crystal substrate in which a plurality of randomly oriented crystals are present in the body of the substrate. In the case of the polycrystalline substrate, the isotropic etching using an acid solution is more effective than the base because the crystal orientation is not uniform.

1 is a photograph of a single crystal substrate and a polycrystalline substrate photographed by a scanning electron microscope at a magnification of 5,000 times.

In Fig. 1, the photograph on the left is a photograph of a wet-type surface treatment of a monocrystalline substrate with a basic solution, and the photograph on the right shows a wet-surface treatment of a polycrystalline substrate with an acidic solution. Referring to FIG. 1, the monocrystalline substrate and the polycrystalline substrate exhibit a uniform etching pattern in a single substrate, although their shapes vary depending on the characteristics of the crystal.

However, the pseudocrystalline substrate is divided into a 100% single crystal substrate, a substrate in which a single crystal and a polycrystalline region are mixed, and a 100% polycrystalline substrate depending on the growth method and conditions. Since the single crystal and the polycrystalline region are all included together, It is difficult to surface.

Figure 2 is a view showing substrates of various grades.

Referring to FIG. 2, a mono wafer of 100% single crystal, a pseudo single crystal substrate (MLM wafer grade A, B) mixed with a single crystal and a polycrystalline region, and a multi wafer of 100% Can be found.

In general, a pseudo-crystalline substrate is classified into three grades with a ratio of monocrystalline to monocrystalline in the ratio of monocrystalline to monocrystal of 90% or more, grade B of 70% or more, and grade C of 25% or more can do.

Generally, the grade A pseudocrystalline substrate is almost the same as a single crystal wafer and can be etched using a basic solution. However, since the low-grade substrate, that is, the grade B or C substrate, contains a large number of single crystal and polycrystalline regions, it is difficult to realize an optimal texture in the treatment of a basic or acidic solution.

FIG. 3 is a photograph of a pseudodocrystal substrate obtained by surface-treating a basic solution and an acidic solution using a microscope. 3, the left photograph is a photograph of the surface of a pseudo single crystal substrate surface treated with a basic solution, and the right photograph shows a surface of a pseudo single crystal substrate surface treated with an acidic solution.

When the basic solution is subjected to surface texture for a pseudo single crystal substrate, the single crystal region is etched by a fine pyramid structure having a low reflectance because the crystal directions are the same. However, since the polycrystalline region has various crystal orientations, . Due to the uneven surface texture of the single crystals and the polycrystals, a maple-shape phenomenon such as a leaf of a maple tree is generated as shown in the left photograph of FIG. This phenomenon is not suitable because it causes the value of the product to deteriorate.

On the other hand, when surface-structuring is performed with an acid solution, a random pyramid is formed at the same etching rate irrespective of the crystal orientation, so that the maple-shape does not appear as shown in the right side of FIG. However, a substrate surface-structured by an acidic solution exhibits a higher reflectivity than a substrate surface-structured by a basic solution. Therefore, surface organization by acidic solution is also not suitable.

According to the surface treatment method of the present invention, it is possible to perform surface texture regardless of the lattice structure such as the single crystal region or the polycrystalline region on the surface of the substrate. Therefore, it is possible to solve the problem of maple-shape by realizing uniform surface texture and different surface texture according to the lattice structure, and thereby preventing the product value from falling due to the occurrence of maple-shape . In addition, there is an advantage that the process time is short and the cost is reduced because the process procedure is simple.

In the surface treatment method of the present invention, the substrate for the solar cell to be subjected to the surface treatment may be a pseudo single crystal substrate. As described above, the pseudo single crystal substrate may be a single crystal substrate, a substrate in which a single crystal and a polycrystalline region are mixed, or a polycrystalline substrate, and may be a substrate of various grades. According to the surface treatment method of the present invention, the process can be applied regardless of the type and grade of the substrate. Therefore, even if a low-grade substrate having a relatively low price is used, a surface treatment similar to the case of using a high-grade substrate can be obtained, which can significantly reduce the production cost.

According to one embodiment of the present invention, the solar cell substrate may be a substrate made of, for example, p-type conductivity type silicon. When the solar cell substrate has a p-type conductivity type, it contains an impurity of a trivalent element such as boron (B), gallium, indium and the like.

Alternatively, the solar cell substrate may be a substrate made of silicon of n-type conductivity type. When the positive electrode for a solar cell has an n-type conductivity type, impurities such as phosphorus (P), arsenic (As), antimony (Sb) and the like may be contained.

First, a first wet etching is performed on the substrate for a solar cell.

The first wet etching process can simultaneously achieve sawing damage removal and first texturing on the substrate surface. The cutting damage is a step of removing defects due to cutting of the substrate and removing an oxide film formed on the surface.

According to an embodiment of the present invention, the first wet etching is performed using an acid solution.

More specifically, the acid solution used in the first wet etching process comprises HF, HNO ₃ , and H ₂ O, HF: HNO _3: it may comprise a volume ratio of 2: H ₂ O of about 1: 2 to 4: 1 to 3, preferably about 1: 3.

The first wet etching may also be performed by immersing the substrate in the solution at a temperature of about 5 to about 10 DEG C for about 1 minute to about 5 minutes, preferably about 1 to about 2 minutes.

According to an embodiment of the present invention, the first wet etching process may be performed, followed by a first rinse with DI water, a neutralization process with a basic solution such as KOH, a second rinse with DI water, a HF cleaning And a third rinsing step with DI water.

By performing the first wet etching as described above, the damage due to the cutting of the substrate is removed, and the surface of the substrate is primarily etched to form a concavo-convex structure having a height of about 2 to about 10 mu m, preferably about 3 to about 5 mu m Can be formed.

Next, a reactive ion etching (RIE) process is performed on the substrate subjected to the first wet etching process.

The reactive ion etching process may be performed by converting an etching gas into a plasma and activating the etching gas in a reactive state to collide with the surface of the substrate. As the etching gas, F ₂ , SF ₆ , CF ₄ A chlorine-based gas such as Cl ₂ , O ₂ , and the like, or a mixture of two or more thereof.

According to one embodiment of the present invention, in the reactive ion etching process, the etching gas may include O ₂ in order to control the etching pattern. By including O ₂ in the etching gas, an oxide film serving as a mask is formed on the side where ion bombardment does not occur, and a film is not formed on the lower side where ion bombardment is performed. .

According to an embodiment of the present invention, by using Cl ₂ , SF ₆ , and O ₂ as the etching gas An etching process can be performed.

More specifically, after the substrate is placed in the chamber, an etching gas is injected into the chamber. Next, electric power of a constant magnitude is applied to two electrodes provided between the substrates to generate a plasma based on the etching gas in a space between the two electrodes. The generated plasma has reactive radicals and ions. The plasma particles thus generated are accelerated to collide with the surface of the substrate. Accordingly, a plurality of uneven structures can be formed on the surface of the substrate by the combination of physical impact and chemical reaction.

In the reactive ion etching process, a higher energy ionic charge than the normal plasma etching is applied to the substrate because a higher negative potential is formed on the electrode compared to the grounded electrode.

By performing the second texturing on the substrate surface by the reactive ion etching process as described above, a pyramid structure is formed on the surface of the substrate.

The pyramid structure contributes to improve the light absorption efficiency of the solar cell by lowering the light reflectance of the substrate surface. The pyramid structure may be formed in various shapes and sizes by adjusting process conditions of a reactive ion etching process, that is, etching gas, pressure, temperature, electrode height, and power.

According to an embodiment of the present invention, the height / width of the pyramid structure may be about 0.75 to about 1.1. Further, in the range of maintaining the above ratio, it may have a height and a width of several nanometers to several hundred nanometers. For example, the width of the pyramid may be from about 100 to about 300 nm and the height from about 150 to about 350 nm. According to the present invention, by performing the first wet etching and the reactive ion etching process stepwise as described above, a uniform pyramid structure can be formed on the surface regardless of the crystal structure of the substrate. Accordingly, the amount of light reflected from the surface is reduced, so that a low reflectivity can be realized.

However, surface damage caused by the plasma impact of the reactive ion etching process may occur. If the surface damage is not removed, the current value is decreased due to the increase of the surface recombination speed, and it is difficult to expect an increase in the cell conversion efficiency. That is, although the reflectance is lowered by the reactive ion etching process, the light absorption can be increased. However, due to the surface damage caused by the reactive ion etching process, the disappearance of the electron-hole pairs is also accelerated so that the increased light absorption effect can not be obtained. This causes low Voc and FF.

Also, if the pyramid structure produced by the reactive ion etching process is held too sharp, a high leakage current may be generated in the upper layer portion of the pyramid.

Therefore, after the reactive ion etching process, a damage removal etching (DRE) process is performed to remove electrical surface damage while maintaining a low reflectance.

The surface damage removal process is to remove the surface damage caused by the reactive ion etching process. However, if the surface damage removal process is performed excessively, the surface is excessively etched and the pyramid formed by the reactive ion etching process is removed, Can not be obtained. Therefore, it is necessary to perform the surface damage removing process in an optimal condition that can maintain the pyramid structure formed by the reactive ion etching process, remove the oxide film generated by O ₂ , and effectively remove surface damage.

According to the surface treatment method of the present invention, the second wet etching is performed in the surface damage removing step. More specifically, the second wet-etch process is performed using an acid solution comprising HF, HNO ₃ , and H ₂ O, and a ratio of HF: HNO ₃ : H ₂ O of about 1:13 to 17:15 to 19 , Preferably in a volume ratio of about 1: 15: 17. By performing the surface damage removing process under the above conditions, the structure of the pyramid formed by the reactive ion etching process can be maintained without excessive etching of the surface, and the height ratio of the height of the pyramid structure to the width of the pyramid structure can be maintained, So as to improve the reflectivity and prevent the leakage current from being generated. In addition, it is possible to deposit an effective anti-reflection film in the subsequent process of forming an anti-reflection film, and the synergy effect of the anti-reflection film can further improve the reflectivity. According to an embodiment of the present invention, by implementing the surface damage removal process, the reflectance can be improved by about 1 to about 3% more than before the surface damage removal process is performed.

The process temperature of the second wet etching process may be room temperature, for example, from about 20 to about 30 캜, and preferably from about 10 seconds to about 60 seconds, preferably from about 20 seconds to about 50 seconds Followed by immersion.

According to an embodiment of the present invention, the etching amount by the second wet etching process is about 0.009 to about 0.035%, relative to the weight of the substrate before the second wet etching process The second wet etching process may be performed.

The substrate having been subjected to the surface treatment using the surface treatment method of the present invention may exhibit a surface reflectance of about 9 to about 12%.

A high efficiency solar cell can be obtained by forming an emitter layer, an antireflection film, a front electrode, and a rear electrode on a substrate subjected to the surface treatment process as described above according to a general manufacturing method of a solar cell.

More specifically, an emitter layer is formed on the surface-processed solar cell substrate. The P-N junction (P-N junction) can be formed by doping the emitter layer with an impurity opposite to the substrate. According to an embodiment of the present invention, the emitter layer may be formed in a shallow emitter layer having a thickness of about 100 nm to about 500 nm to be applied to a high efficiency solar cell.

Also, according to an embodiment of the present invention, the emitter layer may have a high sheet resistance with high photoelectric conversion efficiency, for example, a sheet resistance of about 85 to about 100? / Sq.

Next, an antireflection film is formed on the upper portion of the emitter layer.

The antireflection film may be formed by vacuum deposition, chemical vapor deposition, spin coating, screen printing or spray coating, but is not limited thereto. The antireflection film may be formed of any one single film selected from the group consisting of, for example, a silicon nitride film, a silicon nitride film including hydrogen, a silicon oxide film, a silicon oxynitride film, MgF ₂ , ZnS, TiO ₂ and CeO ₂ , But it is not limited thereto. According to one embodiment of the present invention, the anti-reflection film is a silicon nitride film having a refractive index of about 2.0 to about 2.2, and may have a thickness of about 75 to about 85 nm.

Next, a silver paste is screen printed and then heat treated to form a front electrode, and an aluminum paste is printed on the back surface of the substrate, followed by heat treatment to form a rear electrode, thereby manufacturing a solar cell. During the heat treatment of the aluminum (Al) paste, aluminum may diffuse through the backside of the substrate, thereby forming a back surface field layer at the interface between the backside electrode and the substrate. If the rear layer is formed, the carrier can be prevented from moving to the rear surface of the substrate and recombined. If the recombination of the carriers is prevented, the open voltage increases and the efficiency of the solar cell can be improved.

Hereinafter, the present invention will be described in more detail with reference to examples according to the present invention. It is to be understood, however, that these embodiments are merely illustrative of the invention and are not intended to limit the scope of the invention.

< Example >

Example  One

Doped Group III element impurity A p-type similar single crystal substrate (MLM wafer, Grade B) was prepared. To the substrate HF: HNO _3: a cut damage removal of the first texturing at the same time by performing the first wet etching by immersing in a temperature 7 ℃ to a solution comprising 2 volume ratio for 1.5 minutes: the H ₂ O 1: 3 . Etching was performed at a depth of 3 to 5 탆 from the surface by the first texturing.

Next, a reactive ion etching process was performed using Cl ₂ / SF ₆ / O ₂ as an etching gas. The pyramid structures produced by the reactive ion etching process exhibited a ratio of height to width H / W of 0.75 to 1.1.

By etching such that the immersion of 0.025% by weight lower than those prior to performing the etching at a temperature 25 ℃ the solution containing H ₂ O in a volume ratio of 1:15:17 for 30 seconds: the second wet etch in HF: HNO ₃ The surface treatment was completed.

Phosphorus (P) was doped through a diffusion process using POCL ₃ to form an emitter layer having a resistance of 85 Ω / sq. A silicon nitride film having a refractive index in the range of 2.0 to 2.2 was formed on the emitter layer using PECVD equipment to have a total thickness of 85 nm.

Screen printing is performed on the back surface using Al paste. Dry process is performed at a temperature of less than 200 ° C. Ag paste is used to form a front electrode with a width of 70 μm. Dry process is performed at a temperature of less than 200 ° C. After roughing, the front electrode and the rear electrode were formed by sintering in a belt firing at 940 ° C.

Comparative Example  One

A solar cell was fabricated in the same manner as in Example 1 except that the reactive ion etching process was not performed.

< Experimental Example >

Evaluation of surface treatment results

FIG. 4 is a photograph of the surface of the substrate after performing the reactive ion etching process in Example 1, taken at a magnification of 70,000 times using a scanning electron microscope.

FIG. 5 is a photograph of the surface of the substrate after the reactive ion etching process and the second wet etching process performed in Example 1 is enlarged 5,000 times using a scanning electron microscope.

6 is a graph showing the reflectance of the substrate before the surface treatment is performed and the reflectance of the substrate after the surface treatment process is performed in the first and the first comparative examples.

Evaluation of electrical performance of solar cell

The electrical performances of the solar cells prepared in Example 1 and Comparative Example 1 were measured according to ASTM G-173-03 using solar tester, Halm cetis PV-products of Hanwha Solarone limited (HSOL) The results are shown in Table 1 below. In Table 1, Isc means the maximum current that is transmitted through the solar cell corresponding to the short circuit condition when the impedance is low. Also, the current flowing when the voltage across the solar cell is zero is Jsc, The voltage appearing across the solar cell when the current is zero means the maximum voltage that can be obtained from the solar cell. Parallel resistors refer to resistors connected in parallel with certain circuits, while low parallel resistors cause leakage currents to reduce the current voltage. A series resistance (RS) is a series of resistors between the top and bottom electrodes of the solar cell, which flows through the emitter and base of the solar cell. That is, the vertical resistance component is defined as RS, and the parameter that is greatly affected is FF. FF [%] is the most important measure for solar cell quality, and Fill Factor is calculated by comparing the maximum power with the theoretical power output from open-circuit voltage and short-circuit current. Eta [%] Is defined as the ratio of the output energy to the energy incident from the sun.

Eta (%) Voc (mV) Isc
(A) Jsc (mA / cm ² ) Rs (Ω) Rsh (Ω) FF (%) Comparative Example 1 17.13 629 8.59 35.29 0.0017 390 77.18 Example 1 18.09 634 8.87 36.44 0.0013 557 78.28

As shown in Table 1, in the case of the solar cell manufactured using the substrate having the surface treatment method of the present invention, the electrical performance is improved as compared with the substrate subjected to the surface treatment only by wet etching.

Claims (9)

Performing a first wet etching on a substrate for a solar cell which is a pseudo-crystalline substrate;
Performing reactive ion etching; And
HF: HNO _3: H a ₂ O 1: 13 to 17: 15 by using a solution containing in a volume ratio of 19 to surface treatment method of a solar cell substrate including the step of performing a second wet etching process.
The method according to claim 1, wherein saw damage removal and first texturing are simultaneously performed by the first wet etching.
The method of claim 1, wherein the first wet etching is performed using a solution containing HF: HNO ₃ : H ₂ O in a volume ratio of 1: 2 to 4: 1 to 3.
The method according to claim 1, wherein a second texturing is performed by the reactive ion etching to form a pyramid structure on the surface of the substrate.
The method of claim 1, wherein the reactive ion etching is performed using a gas including Cl ₂ , SF ₆ , and O ₂ .
5. The method of claim 4, wherein the pyramid structure has a height to width ratio of 0.75 to 1.1.
delete The method according to claim 1, wherein the second wet etching process is performed at a temperature of 20 to 30 占 폚 for 10 seconds to 60 seconds.
delete

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