KR101095615B1 - Chemical mechanical polishing slurry - Google Patents

Abstract

The CMP slurry of the present invention includes a compound having a nitrogen-containing aromatic group and a polyethylene oxide group at the same time, and relates to a CMP slurry that can minimize the occurrence of dishing, erosion, etc. of the wiring layer.

Description

CPM Slurry {CHEMICAL MECHANICAL POLISHING SLURRY}

The present invention relates to a CMP slurry, and more particularly, to a CMP slurry capable of minimizing the occurrence of dishing, erosion, etc. of a wiring layer, without reducing the removal rate (hereinafter, polishing rate) of excess wiring material.

In order to achieve high integration of ULSI, the current semiconductor manufacturing process is required to have a large diameter of wafers, and the demand for a strict manufacturing environment is required such that the minimum line width required for device manufacturing is gradually reduced to 0.13 µm or less.

Accordingly, a damascene process is used to form metal wirings. This is to form a groove by forming a groove or the like in the insulating film on the process wafer, embedding wiring material such as tungsten, aluminum, copper, and the like, and then removing the excess wiring material.

At this time, as a method of removing the excess wiring material, the recent chemical mechanical polishing (CMP) method is mainly used, which presses against the surface of the polishing pad which rotates relatively to the wafer surface, and simultaneously chemically By supplying a reaction slurry, it is a technique of leveling the wafer surface by chemical mechanical action.

However, in the polishing technique described above, there is a problem in that the surface to be polished is not uniformly polished, such as dishing or erosion of the wiring layer, thereby lowering the reliability of the circuit. In order to solve this problem, a method of protecting the wiring layer using an appropriate corrosion inhibitor has been proposed, but in this case, the polishing rate is significantly lowered.

The present invention is to provide a CMP slurry that can minimize the occurrence of dishing, erosion, etc. of the wiring layer, without lowering the polishing rate. More specifically, the present invention is to provide a CMP slurry comprising a compound having a nitrogen-containing aromatic group and a polyethylene oxide group at the same time as a corrosion inhibitor.

The present invention is an abrasive; Oxidizing agents; Complex formers; Corrosion inhibitors; And water, wherein the corrosion inhibitor comprises: a CMP slurry comprising a compound having a nitrogen-containing aromatic group and a polyethylene oxide group at the same time; And a chemical mechanical polishing (CMP) method using the CMP slurry to planarize a metal film, an oxide film, an insulating film, or a metal wiring.

The present invention also provides novel compounds.

CMP slurry of the present invention can minimize the occurrence of dishing, erosion, etc. of the wiring layer, without lowering the polishing rate, it is possible to improve the economics and the reliability of the circuit.

CMP slurries for wiring formation generally include abrasives; Oxidizing agents; Complex formers; And water.

On the other hand, the polishing process to remove the excess wiring using the CMP slurry is generally mechanical polishing through physical friction between the excess wiring material and the abrasive; And chemical polishing which oxidizes the excess wiring material with an oxidant and removes the excess wiring material by forming a complex with the oxidized metal ions.

However, dishing, erosion, etc. of the wiring layer may occur during the polishing process, which may impair the reliability of the circuit. That is, the concave region among the uneven portions of the buried wiring material is polished mainly by a chemical mechanism because the polishing pad is not closed and mechanical force cannot be applied properly. However, in this case, since it is difficult to control the degree of removal of the wiring material, the wiring material for forming the wiring layer is polished in addition to the excess wiring material, and the dishing and erosion of the wiring layer, such as the recessed central portion of the surface of the wiring layer, is performed. erosion) may occur. This problem can be more pronounced when the wiring forming material is copper which is susceptible to corrosion.

Accordingly, a method of adding a corrosion inhibitor (eg, benzotriazole (BTA)) capable of physically adsorbing on the surface of the wiring material embedded in the CMP slurry to protect the wiring layer has been proposed. However, conventional corrosion inhibitors interfere with chemical and mechanical polishing of convex regions as well as concave regions of the uneven portions of the buried wiring material, thereby inevitably reducing the polishing rate.

The present invention includes a compound having a nitrogen (N) -containing aromatic group and a polyethylene oxide group at the same time as a corrosion inhibitor, a CMP slurry that can minimize the occurrence of local corrosion, dishing, erosion, etc. of the wiring layer without lowering the polishing rate It is characteristic to provide.

The functional mechanism of the compound having the nitrogen-containing aromatic group and the polyethylene oxide group at the same time can be estimated as follows, but is not limited thereto.

In the corrosion inhibitor of the present invention, the nitrogen portion of the aromatic group maintains an equilibrium level of adsorption / desorption with the metal as the wiring material, and the polyethylene oxide group forms a weak van der Waals bond with the metal, thereby forming a metal wiring layer. I can protect it.

In particular, the corrosion inhibitor of the present invention comprises a nitrogen-containing aromatic group and a polyethylene oxide group at the same time, the relatively high molecular weight. Thus, during the polishing process, the flow of the slurry and the contact pressure with the pads may place more in the concave area than in the convex area (see FIG. 1). Thus, the wiring material in the concave area can be well protected from chemical polishing due to the corrosion inhibitor. On the other hand, in the convex region, the protective action of the corrosion inhibitor on the wiring material is relatively small, and the corrosion inhibitor is easily desorbed by the polishing pad or the abrasive, so that the excess wiring material can be smoothly polished. As a result, in the present invention, dishing of the wiring layer can be reduced without lowering the polishing rate.

On the other hand, the corrosion inhibitor of the present invention contains a large amount of oxygen (O), so that the solubility in water is high, it is less likely to impair the dispersion stability of the slurry.

The corrosion inhibitor of the present invention is not particularly limited as long as it is a compound having a nitrogen (N) -containing aromatic group and a polyethylene oxide group, and may be a compound represented by the following Chemical Formula 1. In this case, the nitrogen-containing aromatic group is not particularly limited as long as it is an atomic group containing nitrogen in the aromatic ring, and may have a monocyclic or heterocyclic structure. Examples thereof include pyrrolinyl group, imidazole group, pyrazole group, pyridinyl group, pyrimidinyl group, indolinyl group, quinolinyl group and benzotriazole group.

In addition, in order to increase the solubility of the corrosion inhibitor in water, the end group of the polyethylene oxide group may be substituted with OH, SO ₃ H, CO ₂ H, or PO ₃ H.

[Formula 1]

In Formula 1, X is a nitrogen (N) containing aromatic group; Y is a C ₁ to C ₆ alkyl group, a C ₂ to C ₆ alkenyl group, OH, CO ₂ H, SO ₃ H, or PO ₃ H; n is an integer of 5 to 500.

On the other hand, the weight average molecular weight of the corrosion inhibitor is preferably from 200 to 1,000,000, more preferably from 800 to 1,000,000. When the molecular weight is too small, the dishing suppression effect may be insignificant. When the molecular weight is too large, dispersion stability or polishing rate of the slurry may be lowered.

In addition, the content of the corrosion inhibitor in the slurry is 0.001 to 2% by weight, preferably 0.01 to 1% by weight. When the content of the corrosion inhibitor is less than 0.001% by weight, the corrosion inhibiting activity may not be sufficient, so that dishing and erosion of the wiring layer may occur. When the content exceeds 2% by weight, the polishing rate is greatly reduced, and the slurry is dispersed. Stability may be degraded.

CMP slurries of the present invention include conventional CMP slurry constituents known in the art, such as abrasives, except that they include compounds having a nitrogen (N) containing aromatic group and a polyethylene oxide group as corrosion inhibitors; Oxidizing agents; Complex formers; And water.

The present invention can be used alone or mixed with a metal oxide, organic particles, or organic-inorganic composite particles as the abrasive.

The metal oxides include silica (SiO ₂ ), alumina (Al ₂ O ₃ ), ceria (CeO ₂ ), zirconia (ZrO ₂ ), titania (titanium oxide), zeolite, and the like. Do. In addition, these may be used alone or in combination, and the abrasive may be used by any one of the fume method and the sol-gel method.

Examples of the organic particles include polymers such as (i) polystyrene, styrene copolymers, poly (meth) acrylates, (meth) acrylate copolymers, polyvinyl chloride, polyamides, polycarbonates, and polyimides; Or (ii) particles of a core / shell structure in which the polymer (s) comprise a core, a shell, or both, among which polystyrene is preferably used. In addition, these may be used alone or in combination, and the organic particles may be prepared by emulsion polymerization, suspension polymerization, or the like.

On the other hand, when metal oxides are used, the primary particle size of these particles is 10 to 200 nm, preferably 10 to 100 nm, and when organic particles are used, the primary particle size of these particles is 10 to 500 nm. And preferably 50 to 300 nm. If the particle size of the abrasive is too small, the polishing rate is lowered, and if the particle is too large, there is a disadvantage that the dispersion stability is poor.

The content in the slurry of the abrasive is 0.1 to 30% by weight, preferably 0.5 to 10% by weight of the total weight of the slurry. When the content of the abrasive particles in the slurry is less than 0.1% by weight, the polishing effect is insignificant, and when it exceeds 30% by weight, the dispersion stability of the slurry may be lowered.

The oxidant of the present invention is a material capable of oxidizing excess wiring material to form an oxide film, and conventional oxidants known in the art may be used. Non-limiting examples of the oxidizing agent include hydrogen peroxide, organic peroxide, ammonium persulfate (APS), potassium persulfate (KPS), hypochlorous acid (HOCl), potassium dichromate, potassium permanganate, iron nitrate, potassium ferricyanide, periodic acid Potassium, sodium hypochlorite (NaOCl), vanadium trioxide, potassium bromate (KBrO ₃ ), and the like. Non-limiting examples of the organic peroxide include peracetic acid, perbenzoic acid, tert-butylhyperperoxide, and the like. These oxidizing agents can be used individually or in mixture of 2 or more types, It is preferable to use hydrogen peroxide.

In addition, the content of the oxidant in the slurry is 0.01 to 10% by weight, preferably 0.02 to 5% by weight. When the content of the oxidant in the slurry exceeds 10% by weight, excessive surface corrosion may occur or local corrosion may be caused. On the other hand, if the content is less than 0.01% by weight, the polishing rate can be greatly reduced.

In the present invention, a complex former is used to easily remove oxidized metal ions and at the same time increase the polishing rate. The complex former may share an electron pair of a ligand with metal ions, thereby forming a complex having a chemically stable structure, thereby making it difficult to redeposit the metal to the metal surface.

Examples of the complexing agent include amino acid compounds, amine compounds, carboxylic acid compounds, and the like, and non-limiting examples thereof include alanine, glycine, cystine, histidine, asparagine, guanidine, tryptophan, hydrazine, ethylenediamine, 1,2- Diaminocyclohexane, diaminopropionic acid, 1,2-diaminopropane, 1,3-diaminopropane, diaminopropanol, maleic acid, malic acid, tartaric acid, citric acid, malonic acid, phthalic acid, acetic acid, lactic acid, oxalic acid, 2,3-pyridinedicarboxylic acid, ascorbic acid, aspartic acid, 3,5-pyrazoledicarboxylic acid, quinalic acid, pyridinecarboxylic acid or salts thereof. These can be used individually or in mixture of 2 or more types, Preferably glycine can be used.

The content in the slurry of the complex former is from 0.05 to 10% by weight, preferably from 0.1 to 3% by weight of the total weight of the CMP slurry. If used in excess of 10% by weight, surface corrosion is severe, and with Wafer Non-Uniformity (WIWNU) is greatly deteriorated. On the other hand, when used at less than 0.05% by weight, the effect of its use is insignificant.

On the other hand, the CMP slurry of the present invention may be added a pH adjuster for adjusting to an effective pH. Non-limiting examples of the pH adjuster is a basic regulator of potassium hydroxide, sodium hydroxide, ammonia water, rubidium hydroxide, cesium hydroxide, sodium hydrogen carbonate, sodium carbonate, acidic control of hydrofluoric acid, hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, formic acid, acetic acid, etc. I am. In the case of a strong acid or a strong base, it is preferable to dilute with water since addition may cause aggregation of the slurry due to a local pH change.

At this time, the content of the pH adjusting agent is preferably added so that the pH of the CMP slurry according to the present invention is 3 to 11. The pH is less than 3 or 11 If exceeded, there is a problem that may adversely affect the polishing rate and the polishing selectivity.

In addition, in the present invention, in order to enhance the solubility of the corrosion inhibitor in water, small amounts of organic solvents such as methanol, ethanol, isopropyl alcohol, or the like, or dodecylbenzenesulfonic acid (DBSA), dodecyl sulfate (DSA), Surfactants, such as lauryl sulfate (SDS) or these salts, can be added.

On the other hand, in the present invention, water may be included in an amount that meets 100% by weight of the total slurry.

The present invention also provides a chemical mechanical polishing (CMP) method for planarizing a metal film, an oxide film, an insulating film, or a metal wiring using the CMP slurry.

On the other hand, the present invention provides a compound of formula (2).

[Formula 2]

In Formula 2, Y is C ₁ ~ C ₆ Alkyl group, C ₂ ~ C ₆ Alkenyl group, OH, CO ₂ H, SO ₃ H, or PO ₃ H; Z is a pyrrolinyl group, an imidazole group, a pyrazole group, a pyridinyl group, a pyrimidinyl group, an indolinyl group, a quinolinyl group, or a benzotriazole group; n is an integer of 5 to 500.

The compound of Formula 2 may be prepared by a conventional method in the art according to Scheme 1 below.

Scheme 1

Hereinafter, the present invention will be described in detail with reference to the following Examples. However, the following examples are merely to illustrate the present invention and the present invention is not limited by the following examples.

Example 1

Example 1-1. Preparation of New Compound

A quinaldyl chloride solution was prepared by dissolving 9.6 g (50 mmol) of quinaldyl chloride in 40 ml of chloroform.

Meanwhile, 60 g (50 mmol) of polyethylene glycol (ethylene oxide 22.3 mol) was added to 80 ml of chloroform, and while stirring, 7.7 ml (55 mmol) of triethyl amine and 7.5 mg (5 mmol) of 2-dimethyl amino pyridine were added thereto. The temperature was adjusted to 4 ° C. or less. Here, while the quinaldyl chloride solution prepared above was slowly added dropwise, the reaction was performed by stirring at 8 ° C. for 1 hour. (See Scheme 2)

Scheme 2

NMR and Mass Spectroscopy analyzes were performed on the compounds obtained above to confirm that the compound of Formula 3 (Q-PEG) was prepared (see FIGS. 2 and 3).

(3)

Example 1-2. Preparation of CMP Slurry

2% by weight of silica, 1% by weight of hydrogen peroxide, 0.5% by weight of glycine, 0.5% by weight of ethanol, and 0.1% by weight of the compound of Formula 3 (Q-PEG) were added to a polypropylene bottle, and the pH was adjusted to 9 after adding water. After adjusting to bring the total weight to 100% by weight, the mixture was stirred at high speed for 10 minutes to prepare a CMP slurry.

Example 2

Example 2-1. Preparation of New Compound

A compound of Chemical Formula 4 was prepared in the same manner as in Example 1-1, except that 40 g (40 mmol) of Methoxypolyethylene glycol (ethylene mol 22 mol) was used instead of polyethylene glycol.

At this time, NMR and Mass Spectroscopy analysis of the obtained compound was confirmed that the compound of Formula 4 (Q-MPEG) was prepared (see FIGS. 4 and 5).

[Formula 4]

Example 2-2. Preparation of CMP Slurry

2% by weight of silica, 1% by weight of hydrogen peroxide, 1% by weight of glycine, and 0.1% by weight of the compound of Formula 4 (Q-MPEG) were added to a polypropylene bottle, and after adding water, the pH was adjusted to 9.1 and the total weight was increased. The mixture was brought to 100% by weight and then stirred for 10 minutes at high speed to prepare a CMP slurry.

Comparative Examples 1 and 2

As shown in Table 1 below, a CMP slurry was prepared in the same manner as in Example 1-2, except that the abrasive, oxidant, complexing agent, corrosion inhibitor, and pH were changed.

Experimental Example: Evaluation of Polishing Speed

Using the CMP slurry prepared in Examples 1 and 2 and Comparative Examples 1 and 2, respectively, polishing was carried out for 1 minute under the following conditions, and then the removal rate was determined by measuring the thickness change of the to-be-polished surface generated by polishing. The results are shown in Table 1 below.

[Polishing condition]

Polishing equipment: UNIPLA 210 (Doosan DND)

Pad: IC1000 / SubaIV Stacked (Rodel)

Platen Speed: 24 rpm

Spindle speed: 100 rpm

Wafer Pressure: 3 psi

Retainer Ring Pressure: 4 psi

Slurry Flow Rate: 200 ml / min

[Polishing target]

8 inch copper wafer deposited with PVD by PVD

8-inch tantalum wafer deposited 3000 Å by PVD

8 inch SiO ₂ with 7000 μs deposited by PETEOS wafer

[evaluation]

The thickness measurement of the metal film was calculated by the following formula after measuring the sheet resistance of each film using LEI1510 Rs Mapping (LEI).

[Copper film thickness (Å)] = [Copper film resistivity (Ω / cm) ÷ Sheet resistance (Ω / square (□))] ⅹ10 ⁸

[Thickness of tantalum film (Å)] = [Copper film resistivity (Ω / cm) ÷ sheet resistance (Ω / square (□))] ⅹ10 ⁸

The thickness of the TEOS film was measured using Nanospec 6100 (Nanometeics).

Experimental Example 2: Evaluation of Etch Rate

In 30 ml of the CMP slurry prepared in Examples 1 to 2 and Comparative Examples 1 to 2, the wafer obtained by cutting 1500 nm of Cu layer deposited by PVD (Physical Vapor Deposition) into 2 X 2 cm 2 was immersed for 30 minutes. Then, by measuring the changed weight of the wafer to determine the etching rate of Cu, the results are shown in Table 1 below.

Experimental Example 3: Measurement of dishing amount

Using the CMP slurry prepared in Examples 1 and 2 and Comparative Examples 1 and 2, respectively, polishing was performed for 1 minute under the same polishing conditions as Experimental Example 1, and at this time, the patterns of all surfaces were exposed. Polishing was performed by cutting for 10 seconds until it was.

[Polishing target]

D4: Patterned wafer having a wiring area density of 90% (line width of 90 μm, space 10 μm) (MIT854: manufactured by ATDF Corporation)

BK: Pattern wafer of 50% of wiring area density (line width 50 µm, space 50 µm) (MIT854: manufactured by ATDF)

[evaluation]

The amount of dishing was measured using a Dektek 8 (Veeco) surface profiler.

[Table 1]

Example 1 Example 2 Comparative Example 1 Comparative Example 2 Abrasive (wt%) Silica (2) Silica (2) Silica (3) Silica (3) Oxidizing agent (wt%) H ₂ O ₂ (1) H ₂ O ₂ (1) H ₂ O ₂ (2) H ₂ O ₂ (1) Complexing agent (wt%) Glycine (0.5) Glycine (0.5) Oxalic acid (1) Glycine (0.5) Corrosion Inhibitor (wt%) Q-PEG (0.1) Q-MPEG (0.1) BTA (0.01) - Etc EtOH (0.5) - - - pH 9 9.3 9.3 9.3 Polishing Speed
(Å / min)
Cu 5523 5878 2315 5892 Ta 88 82 214 143 SiO ₂ 43 36 46 53 Copper etching speed (Å / min) 10 16 35 225 Dishing volume
(nm) D4 38 30 121 223 BK 21 16 66 154

As a result, the CMP slurry of Comparative Example 1 using BTA (benzotriazole), which is a conventional corrosion inhibitor, has a lower copper etching rate and dishing amount than the CMP slurry of Comparative Example 2, which does not use any corrosion inhibitor. In spite of using, the polishing rate was significantly reduced.

On the other hand, according to the present invention, the CMP slurries of Examples 1 and 2, which contain a compound having a nitrogen-containing aromatic group and a polyethylene oxide group as corrosion inhibitors, have a significantly higher copper etching rate and dishing amount than those of the CMP slurries of Comparative Examples 1 and 2. The results were reduced. In addition, despite the use of a larger amount of corrosion inhibitor than Comparative Example 1, the polishing rate was comparable to that of Comparative Example 2.

From this, when using a compound having a nitrogen-containing aromatic group and a polyethylene oxide group as a corrosion inhibitor according to the present invention, it can be seen that the dishing of the wiring layer, erosion can be minimized without lowering the polishing rate.

1 is a schematic diagram of a polishing process according to the present invention.

2 is a result of NMR analysis of the Q-PEG compound prepared in Example 1-1.

3 is a mass analysis result of the Q-PEG compound prepared in Example 1-1. At this time, the measured value is the value of [M + Na] ⁺ .

4 is a result of NMR analysis of the Q-MPEG compound prepared in Example 2-1.

5 is a mass analysis result of the Q-MPEG compound prepared in Example 2-1. At this time, the measured value is the value of [M + Na] ⁺ .

Claims (15)

Abrasives; Oxidizing agents; Complex formers; Corrosion inhibitors; And CMP slurry comprising water, The corrosion inhibitor comprises a compound having a nitrogen-containing aromatic group and a polyethylene oxide group at the same time CMP slurry. The CMP slurry according to claim 1, wherein the compound is a compound represented by the following Chemical Formula 1: [Formula 1]

In Formula 1, X is a nitrogen (N) containing aromatic group; Y is a C ₁ to C ₆ alkyl group, a C ₂ to C ₆ alkenyl group, OH, CO ₂ H, SO ₃ H, or PO ₃ H; n is an integer of 5 to 500. The method of claim 1, wherein the nitrogen-containing aromatic group is selected from the group consisting of pyrrolinyl group, imidazole group, pyrazole group, pyridinyl group, pyrimidinyl group, indolinyl group, quinolinyl group, and benzotriazole group CMP slurry. The CMP slurry of claim 1, wherein the weight average molecular weight of the compound is 200 to 1,000,000. The CMP slurry according to claim 1, wherein the corrosion inhibitor is contained in an amount of 0.001 to 2% by weight. The CMP slurry of claim 1, wherein the abrasive is selected from the group consisting of metal oxides, organic particles, and organic-inorganic composite particles. The metal oxide of claim 6, wherein the metal oxide is selected from the group consisting of silica (SiO ₂ ), alumina (Al ₂ O ₃ ), ceria (CeO ₂ ), zirconia (ZrO ₂ ), titania (titanium oxide), and zeolite. Phosphorus CMP Slurry. The method of claim 6, wherein the organic particles are (i) polystyrene, styrene copolymer, poly (meth) acrylate, (meth) acrylate copolymer, polyvinyl chloride, polyamide, polycarbonate, and polyimide CMP slurry characterized in that the particles of the core / shell structure selected from the group consisting of, or (ii) at least one polymer selected from the group (iii) comprises a core, a shell, or both. The CMP slurry according to claim 6, wherein the metal oxide has a primary particle size of 10 to 200 nm, and the organic particles have a primary particle size of 10 to 500 nm. The method of claim 1, wherein the oxidizing agent is hydrogen peroxide, ammonium persulfate (APS), potassium persulfate (KPS), hypochlorous acid (HOCl), potassium dichromate, potassium permanganate, iron nitrate, potassium ferricyanide, potassium periodate, CMP slurry characterized by selected from the group consisting of sodium hypochlorite (NaOCl), vanadium trioxide, potassium bromate (KBrO ₃ ), peracetic acid, perbenzoic acid and tert-butylhyperperoxide. The CMP slurry of claim 1, wherein the complex forming agent is selected from the group consisting of an amino acid compound, an amine compound, and a carboxylic acid compound. The method of claim 1, wherein the complexing agent is alanine, glycine, cystine, histidine, asparagine, guanidine, Tryptophan, hydrazine, ethylenediamine, 1,2-diaminocyclohexane, diaminopropionic acid, 1,2-diaminopropane, 1,3-diaminopropane, diaminopropanol, maleic acid, malic acid, tartaric acid, citric acid, mal Lonic acid, phthalic acid, acetic acid, lactic acid, oxalic acid, 2,3-pyridinedicarboxylic acid, ascorbic acid, aspartic acid, 3,5-pyrazoledicarboxylic acid, quinald acid, pyridinecarboxylic acid and salts thereof CMP slurry. The CMP slurry according to claim 1, wherein the CMP slurry is a copper wiring forming slurry. A chemical mechanical polishing (CMP) method, characterized in that the metal film, oxide film, insulating film, or metal wiring is planarized using the CMP slurry of any one of claims 1 to 13. A compound having a structure of Formula 2 [Formula 2]

In Formula 2, Y is C ₁ ~ C ₆ Alkyl group, C ₂ ~ C ₆ Alkenyl group, OH, CO ₂ H, SO ₃ H, or PO ₃ H; Z is a pyrrolinyl group, an imidazole group, a pyrazole group, a pyridinyl group, a pyrimidinyl group, an indolinyl group, a quinolinyl group, or a benzotriazole group; n is an integer of 5 to 500.

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