JP2020107838A - Semiconductor polishing composition - Google Patents

Abstract

To provide a semiconductor polishing composition which enables reduction in abrasion-originating defect (PID) without worsening a haze.SOLUTION: A semiconductor polishing composition comprises silica, a hydroxyethyl cellulose, water, a basic compound, and a plurality of kinds of polyhydric alcohol. The plurality of kinds of polyhydric alcohol include polyhydric alcohol of which the characteristic value A represented by the formula (1) below is 1.37 or more, and polyhydric alcohol of which the characteristic value A is less than 1.37. (1) Characteristic value A=(1+cos(t2))/(1+cos(t1)), where t1 is a contact angle of 1 mass%-aqueous solution of polyhydric alcohol with a bare silicon substrate, and t2 is a contact angle of the 1 mass%-aqueous solution of polyhydric alcohol with the silicon substrate covered with a silicon oxide film.SELECTED DRAWING: Figure 2

Description

The present invention relates to a polishing composition.

Polishing of a semiconductor substrate by CMP achieves highly accurate smoothing and flattening by performing multi-step polishing of three steps or four steps. The polishing composition used in the final polishing step performed in the final stage is required to have a property of reducing polishing-induced defects (PID) and haze (surface haze).

The polishing composition used in the final polishing step of a semiconductor substrate generally contains abrasive grains, a basic compound, and a water-soluble polymer. The abrasive grains have a polishing function. The water-soluble polymer often plays a role of protecting the semiconductor substrate during or after polishing from foreign matters by adsorbing to the semiconductor substrate and wetting the substrate surface. Further, the polishing composition contains various additives for the purpose of reducing the number of PIDs in addition to the above compounds.

For example, in WO 2017/069253 (Patent Document 1 below), abrasive grains and at least one water-soluble polymer selected from vinyl alcohol resins having a 1,2-diol structural unit A polishing composition containing a polyhydric alcohol and an alkali compound is disclosed.

International Publication No. 2017/069253

In recent years, with the progress of miniaturization of design rules of semiconductor devices, stricter management is required for PID and haze on the surface of a semiconductor substrate. On the other hand, the role of the additive of the semiconductor polishing composition has not been fully clarified. Conventionally, selection of an additive to be added to a semiconductor polishing composition and reduction of PID and haze due to the selection have largely depended on trial and error.

An object of the present invention is to provide a semiconductor polishing composition capable of reducing PID without deteriorating the haze of a semiconductor after polishing.

A semiconductor polishing composition according to an embodiment of the present invention contains silica, hydroxyethyl cellulose, water, a basic compound, and a plurality of types of polyhydric alcohols. The plurality of types of polyhydric alcohols include a polyhydric alcohol having a characteristic value A represented by the following formula (1) of 1.37 or more and a polyhydric alcohol having a characteristic value A of less than 1.37. ..
Characteristic value A=(1+cos(t2))/(1+cos(t1)) (1)
t1: Contact angle of 1% by mass aqueous solution of polyhydric alcohol with bare silicon substrate
t2: Contact angle of a 1% by mass aqueous solution of polyhydric alcohol with a silicon substrate coated with a silicon oxide film

According to the present invention, PID can be reduced without deteriorating the haze of a semiconductor substrate after polishing.

FIG. 1 is a diagram for explaining the contact angle. FIG. 2 is a diagram for explaining a state during polishing using the polishing composition. FIG. 3 is a diagram showing the characteristic value A of PO derivative (1), EO derivative (1), PO derivative (2), EO derivative (2) and water in Table 1.

The present inventor has conducted various studies in order to clarify the properties of additives effective for reducing PID without deteriorating haze. In the study, we focused on both the ease of adsorption to silica abrasive grains and the ease of adsorption to semiconductor substrates among the properties of the additive. A characteristic value A represented by the following formula (1) was introduced as an index showing the properties of both surfaces.
Characteristic value A=(1+cos(t2))/(1+cos(t1)) (1)
In the above formula (1), t1 is the contact angle of the 1% aqueous solution of polyhydric alcohol with the bare silicon substrate, and t2 is the contact angle with the silicon substrate coated with the silicon oxide film.

The characteristic value A indicates that the larger the value, the higher the affinity for silicon oxide, and the smaller the value, the higher the affinity for silicon. The polishing performance of semiconductor substrates was examined for various polishing compositions containing silica as abrasive grains, a basic compound, and a water-soluble polymer, as well as additives having various characteristic values A.

As a result of trial and error, the inventor used hydroxyethyl cellulose (HEC) as the water-soluble polymer, and then used a combination of a polyhydric alcohol easily adsorbed by silicon and a polyhydric alcohol easily adsorbed by silicon oxide in the polishing composition. It has been found that the addition thereof can reduce the PID without deteriorating the haze in semiconductor polishing.

The present invention has been completed based on this finding. Hereinafter, the semiconductor polishing composition according to one embodiment of the present invention will be described in detail.

A semiconductor polishing composition according to an embodiment of the present invention contains silica, hydroxyethyl cellulose, water, a basic compound, and a plurality of types of polyhydric alcohols. The plurality of types of polyhydric alcohols include a polyhydric alcohol having a characteristic value A represented by the following formula (1) of 1.37 or more and a polyhydric alcohol having a characteristic value A of less than 1.37. ..
Characteristic value A=(1+cos(t2))/(1+cos(t1)) (1)
t1: Contact angle of 1% by mass aqueous solution of polyhydric alcohol with bare silicon substrate
t2: Contact angle of a 1% by mass aqueous solution of polyhydric alcohol with a silicon substrate coated with a silicon oxide film

The semiconductor polishing composition contains, in addition to silica and a basic compound, HEC, a polyhydric alcohol having a plurality of kinds of characteristic values A of 1.37 or more, and a polyhydric alcohol having a characteristic value A of less than 1.37. Including. That is, this semiconductor polishing composition is used for a polyhydric alcohol having a higher affinity for silicon oxide (a polyhydric alcohol having a characteristic value of 1.37 or more) than a water molecule which is a solvent, and a water molecule which is a solvent. By comparison, both of the polyhydric alcohols, which have a higher affinity for silicon, are included. A combination of these plural kinds of polyhydric alcohols and HEC provides a semiconductor polishing composition capable of reducing PID without deteriorating the haze of the surface of the semiconductor substrate to be polished.

This mechanism is speculated as follows. FIG. 1 is a diagram for explaining a contact angle of a polyhydric alcohol with a bare silicon substrate and a silicon oxide film. When the contact angle t1 of the polyhydric alcohol with the bare silicon substrate is large, that is, the affinity with the bare silicon substrate is low, cos(t1) becomes small. On the contrary, when the contact angle t1 of the polyhydric alcohol with the bare silicon substrate is small and the affinity is high, cos(t1) becomes large. Similarly, when the contact angle t2 of the polyhydric alcohol with the silicon oxide film is large, that is, when the affinity with the silicon oxide film is low, cos(t2) becomes small. On the contrary, if the contact angle t2 of the polyhydric alcohol with the silicon oxide film is small and the affinity is high, cos(t2) becomes large. Therefore, if the characteristic value A=(1+cos(t2))/(1+cos(t1)) is large, it indicates that the affinity for silicon oxide is high, and if the value is small, the affinity for silicon is high. Show high.

FIG. 2 is a diagram for explaining a state of polishing a semiconductor substrate (silicon substrate) using a polishing composition. As shown in FIG. 2, in water as a solvent, a polyhydric alcohol having a characteristic A of 1.37 or more is more likely to be adsorbed on silica (silicon oxide) as abrasive grains than on a silicon substrate. A polyhydric alcohol having a characteristic A of less than 1.37 is more easily adsorbed on a silicon substrate than silica. Hydroxyethyl cellulose is easily adsorbed on both silica and silicon substrates. Due to the balance of the easiness of adsorption of these hydroxyethyl cellulose and two kinds of polyhydric alcohols on silica and silicon substrates, polishing of the silicon substrate surface by silica and protection of the silicon substrate surface of hydroxyethyl cellulose function in an appropriate balance. Conceivable. As a result, it is considered that the effect that the PID can be reduced without deteriorating the haze is obtained.

Silica is blended in the polishing composition as abrasive grains. As silica, those commonly used in this field can be used, and for example, colloidal silica, fumed silica and the like can be used. The particle size of the abrasive grains is not particularly limited, but for example, those having a secondary average particle size of 30 to 150 nm can be used.

The content of silica is not particularly limited, but is, for example, 0.10 to 20 mass% of the entire polishing composition. The content of silica is preferably as small as possible from the viewpoint of reducing polishing scratches and residual foreign matter on the silicon wafer after polishing. On the other hand, when the polishing composition does not contain silica at all, the oxide film on the surface of the silicon wafer cannot be removed. The polishing composition is used after being diluted 10 to 40 times during polishing. The polishing composition according to the present embodiment is preferably diluted and used so that the concentration of abrasive grains is 100 to 5000 ppm (mass ppm; the same applies hereinafter). The higher the concentration of the abrasive grains, the smaller the defects and the haze tend to be. The lower limit of the concentration of the abrasive grains after dilution is preferably 1000 ppm, more preferably 2000 ppm. The upper limit of the concentration of the abrasive grains after dilution is preferably 4000 ppm, more preferably 3000 ppm.

The basic compound efficiently reacts with the wafer surface and contributes to the polishing characteristics of chemical mechanical polishing (CMP). The basic compound is, for example, an amine compound or an inorganic alkaline compound.

The amine compound is, for example, a primary amine, a secondary amine, a tertiary amine, a quaternary ammonium and its hydroxide, or a heterocyclic amine. Specifically, ammonia, tetramethylammonium hydroxide (TMAH), tetraethylammonium hydroxide (TEAH), tetrabutylammonium hydroxide (TBAH), methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, hexylamine, Cyclohexylamine, ethylenediamine, hexamethylenediamine, diethylenetriamine (DETA), triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine, monoethanolamine, diethanolamine, triethanolamine, N-(β-aminoethyl)ethanolamine, anhydrous piperazine , Piperazine hexahydrate, 1-(2-aminoethyl)piperazine, N-methylpiperazine, piperazine hydrochloride, guanidine carbonate and the like.

Examples of the inorganic alkali compound include alkali metal hydroxides, alkali metal salts, alkaline earth metal hydroxides and alkaline earth metal salts. Specific examples of the inorganic alkaline compound include potassium hydroxide, sodium hydroxide, potassium hydrogen carbonate, potassium carbonate, sodium hydrogen carbonate, sodium carbonate and the like.

The above basic compounds may be used alone or in a mixture of two or more. Among the above-mentioned basic compounds, alkali metal hydroxides, alkali metal salts, ammonia, ammonium hydroxide, amines, ammonium salts, and quaternary ammonium hydroxides are particularly preferable.

The content of the basic compound (when two or more kinds are contained, the total amount thereof) is not particularly limited, but for example, in a mass ratio with the abrasive grains, abrasive grains: basic compound = 1:0.001 to 1:0. .10. The polishing composition according to the present embodiment is preferably used by diluting it so that the concentration of the basic compound is 5 to 200 ppm.

Although the molecular weight of hydroxyethyl cellulose and the content thereof are not particularly limited, for example, the molecular weight is 500,000 or more and 1,500,000 or less, and the ratio of the mass% of the hydroxyethyl cellulose to the mass% of the abrasive grains is 0.0075 or more and 0.025 or more. The following may be mentioned. The molecular weight of hydroxyethyl cellulose is a weight average molecular weight measured by GFC (Gel Filtration Chromatography) method.

The polishing composition contains water. The content of water is not particularly limited and may be appropriately mixed. As will be described later, the water is prepared by adjusting the polishing composition as a high-concentration liquid having a higher concentration than the desired concentration at the time of use, and when diluted and used at the time of use, it is used as a diluent at the time of dilution. You may mix.

The polishing composition contains multiple types of polyhydric alcohols. Polyhydric alcohol is an alcohol containing two or more hydroxy groups in one molecule. The plurality of types of polyhydric alcohols include polyhydric alcohols having a characteristic value A represented by the above formula (1) of 1.37 or more and polyhydric alcohols having the characteristic value A of less than 1.37. The contact angle t1 and the contact angle t2 of the polyhydric alcohol in the above formula (1) are values measured by the measuring method shown in Examples described later. The types of polyhydric alcohol contained in the polishing composition are not limited to two types. For example, three or more polyhydric alcohols may be contained in the polishing composition.

In the polishing composition of the present embodiment, the compounding amount (content of mass%) of each kind of polyhydric alcohols of plural kinds is, for example, the compounding amount of hydroxyethyl cellulose (content of mass%). ) Less than. Moreover, the compounding amount (content of mass%) of the plurality of kinds of polyhydric alcohols is also smaller than the compounding amount (content of mass%) of hydroxyethyl cellulose as an example. In addition, as another example, the compounding amount (content of mass%) of the plurality of kinds of polyhydric alcohols may be larger than the compounding amount (content of mass%) of hydroxyethyl cellulose. The content of the polyhydric alcohol is not particularly limited, but may be, for example, 0.0100 to 0.1 mass% of the entire polishing composition.

Examples of the polyhydric alcohol contained in the polishing composition include, for example, a glycoside (glycoside) in which a sugar and an organic compound other than sugar are glycoside-bonded, a polyhydric alcohol alkylene oxide adduct in which an alkylene oxide is added to a polyhydric alcohol. , Polyhydric alcohol fatty acid ester in which fatty acid and polyhydric alcohol are ester-bonded, glycerin, propylene glycol, polyethylene glycol and the like. Examples of the glycoside include an alkylene oxide derivative of methyl glucoside represented by the following formula (2). Examples of the polyhydric alcohol alkylene oxide adduct include alkylene oxide adducts such as glycerin, pentaerythritol, and ethylene glycol. Examples of the polyhydric alcohol fatty acid ester include glycerin fatty acid ester, sorbitol fatty acid ester, sorbitan fatty acid ester, sucrose fatty acid ester and the like.

Examples of the alkylene oxide derivative of methyl glucoside include polyoxyethylene methyl glucoside and polyoxypropylene methyl glucoside. For example, polyoxyethylene methyl glucoside may be contained in the polishing composition as the polyhydric alcohol having the characteristic value A of water which is the characteristic value A of water, that is, 1.37 or more. Further, polyoxypropylene methyl glucoside may be contained in the polishing composition as a polyhydric alcohol having the characteristic value A of less than 1.37.

Examples of the polyhydric alcohol contained in the polishing composition include polyglycerin derivatives. The polyglycerin derivative is a compound in which a functional group is introduced into polyglycerin having an average degree of polymerization of glycerin of 3 or more. Examples of the polyglycerin derivative include polyoxyalkylene polyglyceryl ether and the like. Examples of the polyoxyalkylene polyglyceryl ether include polyoxyethylene polyglyceryl ether and polyoxypropylene polyglyceryl ether. For example, polyoxyethylene polyglyceryl ether may be contained in the polishing composition as a polyhydric alcohol having the characteristic value A of 1.37 or more. Further, polyoxypropylene polyglyceryl ether may be contained in the polishing composition as a polyhydric alcohol having the characteristic value A of less than 1.37.

The polishing composition according to the present embodiment is preferably used by diluting it so that the concentration of the polyhydric alcohol is 50 to 500 mass ppm, more preferably 100 to 400 mass ppm. The molecular weight and the content of the polyhydric alcohol added to the polishing composition are not particularly limited, but for example, a polyhydric alcohol having a molecular weight of 300 to 1500, preferably 450 to 1000, and more preferably 500 to 800 is used. Two or more types can be included in the polishing composition. The molecular weight of polyhydric alcohol is a weight average molecular weight.

The polishing composition according to the present embodiment may further contain a pH adjuster. The pH of the polishing composition according to the present embodiment is preferably 8.0 to 12.0.

In addition to the above, the polishing composition according to the present embodiment may optionally contain a compounding agent generally known in the field of polishing compositions.

The polishing composition according to the present embodiment is prepared by appropriately mixing silica, a basic compound, hydroxyethyl cellulose, plural kinds of polyhydric alcohols and other compounding materials, and adding water. Alternatively, the polishing composition according to the present embodiment is prepared by sequentially mixing silica, a basic compound, hydroxyethyl cellulose, plural kinds of polyhydric alcohol and other compounding materials in water. As a means for mixing these components, a means that is commonly used in the technical field of polishing compositions, such as a homogenizer and ultrasonic waves, is used.

The polishing composition described above is diluted with water so as to have an appropriate concentration and then used for polishing a semiconductor wafer.

The polishing composition according to the present embodiment can be particularly preferably used for finish polishing of a silicon wafer.

Hereinafter, the present invention will be described more specifically with reference to Examples. The invention is not limited to these examples.

[Polishing Example 1]
The polishing compositions of Examples 1 and 2 and Comparative Examples 1 to 4 shown in Table 1 were prepared.

The contents in Table 1 are all contents after dilution. As the abrasive grains, colloidal silica having an average secondary particle diameter of 70 nm was used. “NH ₄ OH” represents an aqueous ammonia solution.

The PO derivative (1) is polyoxypropylene methyl glucoside. The EO derivative (1) is polyoxyethylene methyl glucoside.

The PO derivative (2) is polyoxypropylene polyglyceryl ether (average molecular weight 750). The EO derivative (2) is polyoxyethylene polyglyceryl ether (average molecular weight 750).

FIG. 3 is a diagram showing the characteristic value A of PO derivative (1), EO derivative (1), PO derivative (2), EO derivative (2) and water in Table 1. The characteristic value A in FIG. 3 was calculated using the above equation (1). The contact angles t1 and t2 in the above formula (1) were measured using a contact angle measuring device (Drop Master DM500 manufactured by Kyowa Interface Science Co., Ltd.).

The contact angle t1 (θ2 in FIG. 3) is 1% by mass of an aqueous solution of polyhydric alcohol (PO derivative (1), EO derivative (1), PO derivative (2), EO derivative (2)) on a bare silicon substrate. The attached state was photographed with a camera, and the contact angle between the substrate and the aqueous solution was calculated using the θ/2 method. As the bare silicon substrate, a silicon substrate with a natural oxide film was immersed in a 0.5% hydrofluoric acid aqueous solution for 1 minute to remove the oxide film, and the contact angle was measured. That is, after removing the oxide film of the silicon substrate with a natural oxide film, a polyhydric alcohol aqueous solution was quickly attached to the surface of the silicon substrate, and the contact angle with the bare silicon substrate was measured.

The contact angle t2 (θ1 in FIG. 3) was determined by preparing an aqueous solution of 1% by mass of polyhydric alcohol (PO derivative (1), EO derivative (1), PO derivative (2), EO derivative (2)), and The state of being attached to the surface of the silicon substrate with an oxide film was photographed with a camera, and the contact angle between the substrate and the aqueous solution was calculated using the θ/2 method.

Details of the method for measuring the contact angle are as follows. Approximately 10 ml of the polyhydric alcohol aqueous solution was filled in the syringe, the polyhydric alcohol aqueous solution was ejected from the injection needle and attached to the substrate, and the image taken with a camera was taken 1 second after the droplet and the needle tip were separated. And the contact angle was calculated.

The water-soluble polymer "HEC" is hydroxyethyl cellulose. As the modified PVA, a modified PVA having an estimated degree of polymerization of 450 and a degree of saponification of 99.0 mol% or more was used.

A 12-inch silicon wafer was polished using the polishing compositions of these Examples and Comparative Examples. A silicon wafer having a conductivity type of P type and a resistivity of 0.1 Ωcm or more and less than 100 Ωcm was used. The polished surface was the <100> surface. As the polishing device, a single-sided polishing device (diameter of platen φ31.5 mm) was used. A nap pad was used as the polishing pad. The polishing composition was supplied at a supply rate of 600 mL/min. The rotation speed of the surface plate was 52 rpm, the rotation speed of the carrier was 50 rpm, and the polishing load was 16 kPa, and polishing was performed for 3 minutes. Before polishing with the polishing compositions of Examples and Comparative Examples, preliminary polishing was performed for 3 minutes using a polishing slurry Nanopure (registered trademark) NP7050S (manufactured by Nitta Haas Co., Ltd.).

The micro defects and haze of the silicon wafer after polishing were measured. The minute defects were measured using a wafer surface inspection device MAGICS M5640 (manufactured by Lasertec). The measurement conditions were D37 mV, and the haze was measured with a wafer inspection device Surfscan SP2 manufactured by KLA-Tencor.

According to the results shown in Table 1, Example 1 containing both the PO derivative (1) and EO derivative (1) was Comparative Examples 1 and 2 containing only one of the PO derivative (1) and the EO derivative (1). The number of defects is reduced as compared with. It can be considered that the haze of Example 1 is not deteriorated. In Example 3 containing both the EO derivative (2) and PO derivative (2), the number of defects and the haze were reduced as compared with Comparative Example 4 containing only the EO derivative (2). In Examples 1 and 2, when the polyhydric alcohol having the characteristic value A of 1.37 (=the characteristic value A of water) or more and the polyhydric alcohol having the characteristic value A of less than 1.37 were combined. Applicable In Comparative Example 3 containing the EO derivative (2) and the EO derivative (1), the number of defects and the haze are deteriorated. Comparative Example 3 corresponds to a case where two types of polyhydric alcohol having a characteristic value A of 1.37 (=characteristic value A of water) or more are included. From these results, by adding both the polyhydric alcohol that is easily adsorbed by the silicon substrate and the polyhydric alcohol that is easily adsorbed by the silica abrasive grains to the polishing composition, without deteriorating the haze, polishing-induced defects ( It was found that the PID) can be reduced.

As shown in FIG. 3, the PO derivative (1) and the PO derivative (2) have a characteristic value A of water or less, and the EO derivative (1) and the EO derivative (2) have a characteristic value A of water or more. It turns out that From this, it is assumed that in the bare silicon polishing environment, the PO derivative (1) and the PO derivative (2) are adsorbed on the bare silicon in preference to the water molecules that are present in large amounts as solvents. On the other hand, it is presumed that the EO derivative (1) and the EO derivative (2) are preferentially adsorbed to silicon oxide (that is, silica abrasive grains in the polishing environment) in preference to water molecules. Due to the presence of both the polyhydric alcohol having the characteristic value A of water or more and the polyhydric alcohol having the characteristic value A of less than water, both bare silicon and silica abrasive particles are adsorbed and protected by the polyhydric alcohol, and the damage to the wafer is reduced. Therefore, it can be inferred that the PID is decreased due to this.

The polishing composition of Comparative Example 5 is the polishing composition of Example 2 in which HEC is replaced with modified PVA. In Comparative Example 5, deterioration of haze is seen. From this, the method of reducing PID by using a combination of a plurality of kinds of polyhydric alcohols distinguished by the characteristic value A as an additive is particularly effective when hydroxyethyl cellulose is used as the water-soluble polymer. all right. It was also found that the number of defects and haze can be reduced by combining hydroxyethyl cellulose, a polyhydric alcohol having a characteristic value A of 1.37 or more, and a polyhydric alcohol having a characteristic value A of less than 1.37.

The reason for this is not clear, but it is presumed as follows. Modified PVA has a weaker adsorptive power to bare syrincon than hydroxyethyl cellulose. Use of a polyhydric alcohol having a characteristic value A of water less than 1.37 (polyhydric alcohol having a high affinity for bare silicon) also inhibits the adsorption of the modified PVA itself on the bare silicon. Therefore, it is presumed that the haze was deteriorated because the protection by the water-soluble polymer was weakened. On the other hand, when hydroxyethyl cellulose is used together with a polyhydric alcohol having a characteristic value A of less than 1.37, its adsorption on a silicon substrate is not hindered, and the action of polyhydric alcohol suppresses the deterioration of Bayes. However, it is presumed that the PID is being reduced.

The embodiments of the present invention have been described above. The above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments without departing from the spirit thereof.

Claims (3)

Silica,
Hydroxyethyl cellulose,
water and,
A basic compound,
Including multiple types of polyhydric alcohol,
The plurality of types of polyhydric alcohols include a polyhydric alcohol having a characteristic value A represented by the following formula (1) of 1.37 or more and a polyhydric alcohol having a characteristic value A of less than 1.37. A composition for polishing a semiconductor.
Characteristic value A=(1+cos(t2))/(1+cos(t1)) (1)
t1: Contact angle of 1% by mass aqueous solution of polyhydric alcohol with bare silicon substrate
t2: Contact angle of a 1% by mass aqueous solution of polyhydric alcohol with a silicon substrate coated with a silicon oxide film The semiconductor polishing composition according to claim 1, wherein at least one of the plurality of polyhydric alcohols is an alkylene oxide derivative of methyl glucoside. The semiconductor polishing composition according to claim 1, wherein at least one of the plurality of kinds of polyhydric alcohols is polyoxyalkylene polyglyceryl ether.

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