JP7750652B2 - polishing composition - Google Patents

Description

The present invention relates to a polishing composition for Si wafers.

Ultra-precision processing is an extremely important technology in the manufacture of semiconductor products. With the recent trend toward miniaturization of LSI devices, the demands for wafer surface roughness and flatness after precision polishing are becoming increasingly stringent. Chemical mechanical polishing (CMP) is used for precision polishing. A variety of abrasives have been proposed for use in polishing compositions for CMP.

JP 2005-262413 A describes the use of colloidal silica, with an average primary particle size of 1 nm or more and less than 40 nm and a zeta potential of -15 to 30 mV, as the abrasive in a polishing composition. This polishing composition reduces nano-scratches and is suitable for polishing precision component substrates such as memory hard disk substrates and semiconductor substrates.

JP 2017-197590 A discloses a polishing composition for polishing an object having an oxide film and a nitride film. This polishing composition has a pH of 2.0 or higher and contains silica that exhibits a positive zeta potential. This polishing composition can increase the polishing rate of an oxide film while suppressing the polishing rate of a nitride film.

Japanese Patent Application Laid-Open No. 2005-262413 JP 2017-197590 A

The inventors investigated the composition of polishing compositions used in polishing Si wafers. Through their investigations, the inventors discovered that surface roughness can be reduced by giving the polishing composition a composition specific to polishing Si wafers.

The object of the present invention is to provide a polishing composition that can reduce surface roughness when polishing Si wafers.

A Si wafer polishing composition according to one embodiment of the present invention is used to polish Si wafers. The Si wafer polishing composition contains silica, a pH adjuster, and water. The pH is 1.0 to 6.9, and silanol groups on the surface of the silica are substituted with functional groups containing amino or carboxy groups.

The present invention provides a polishing composition for Si wafers that can reduce surface roughness during polishing of Si wafers.

FIG. 1 shows an example of silica in which the silanol groups on the surface are substituted with functional groups containing amino groups. FIG. 2 shows an example of silica in which the silanol groups on the surface are substituted with functional groups containing carboxy groups.

The inventors investigated the composition of a polishing composition suitable for polishing Si wafers. In their investigations, the inventors used silica as the abrasive grains and attempted to modify the surface by substituting functional groups for the silanol groups on the silica surface. After further investigations, the inventors discovered that surface roughness can be reduced by making the polishing composition acidic and by configuring it so that the abrasive grains are silica that has been surface-modified with functional groups containing amino or carboxy groups.

A Si wafer polishing composition according to one embodiment of the present invention is intended for polishing Si wafers. The Si wafer polishing composition contains silica, a pH adjuster, and water. The Si wafer polishing composition has a pH of 1.0 to 6.9, and silanol groups on the surface of the silica are substituted with functional groups containing amino or carboxy groups. By making the polishing composition acidic and substituting silanol groups on the surface of the silica with functional groups containing amino or carboxy groups, the surface roughness of the Si wafer can be reduced.

In the above-mentioned Si wafer polishing composition, the silanol groups on the surface of the silica may be substituted with functional groups containing amino groups. Alternatively, the silanol groups on the surface of the silica may be substituted with functional groups containing carboxy groups.

The mechanism by which the surface roughness of the Si wafer is reduced is not clear, but it is thought to be as follows. By making the polishing composition acidic and substituting the silanol groups on the silica surface with functional groups containing amino or carboxy groups, the silanol groups on the silica surface are substituted with functional groups that carry a positive charge under acidic conditions. This is thought to make it easier for the zeta potential of the silica to fall within an appropriate range from the perspective of reducing surface roughness. As a result, it is thought that the surface roughness of the Si wafer is reduced.

A method for polishing a Si wafer and a method for manufacturing a Si wafer including the polishing method are also included in embodiments of the present invention. The method for polishing a Si wafer includes a step of polishing the Si wafer with a polishing composition disposed between the Si wafer and a polishing pad. The polishing composition contains abrasive grains, a pH adjuster, and water, and has a pH in the range of 1.0 to 6.9. The abrasive grains have a positive zeta potential under conditions of a pH of 1.0 to 6.9. The product of the zeta potential of the abrasive grains and the surface zeta potential of the Si wafer (parameter A) is negative (A<0), and the product of the zeta potential of the abrasive grains and the surface zeta potential of the polishing pad (parameter B) is -50 or greater (B≧-50). This polishing method can reduce the surface roughness of the Si wafer.

In the above polishing method, the abrasive grains may be silica, and the silanol groups on the surface of the silica may be substituted with functional groups including amino or carboxy groups. This allows the zeta potential of the abrasive grains, the relationship between the zeta potential of the abrasive grains and the Si wafer (A), and the relationship between the abrasive grains and the polishing pad (B) to be set within the above ranges with a simple configuration.

The polishing pad may be made of polyurethane. This makes it easier to set the relationship (B) between the abrasive grains and the polishing pad within the above range.

(silica)
The silica as abrasive grains contained in the polishing composition of this embodiment may be, for example, colloidal silica. In each silica particle, at least a portion of the silanol groups on the surface are substituted with functional groups containing amino groups or carboxy groups. In other words, the silica is surface-modified with functional groups containing amino groups or carboxy groups. In the polishing composition, the functional groups are fixed to the silica.

The functional groups that are substituted onto the silanol groups on the silica surface are positively charged under acidic conditions, i.e., pH conditions of 1.0 to 6.9. This is thought to make the abrasive properties of silica, including its zeta potential, favorable from the standpoint of polishing performance in acidic polishing compositions.

FIG. 1 shows an example of silica in which silanol groups on the surface have been substituted with functional groups containing amino groups. In the example shown in FIG. 1, some of the silanol groups on the surface of the silica have been substituted with functional groups 10. The functional groups 10 include amino groups 11. The amino groups 11 are functional groups formed by removing hydrogen from ammonia, primary amines, or secondary amines. R1 in FIG. 1 represents the bond between Si and the amino groups 20. R1 may be, for example, an alkylene group, a dialkylamino group, or a derivative thereof. For example, the functional group 10 may be a methylamino group, an ethylamino group, a propylamino group, an N-2-(aminoethyl)-3-aminopropyl group, or an N-2-(aminoethyl)-3-aminopropylmethyl group. R2 and R3 are hydrogen or substituents of the amino groups. As an example, the amino groups 11 may be -NH3 ⁺ .

Figure 2 shows an example of silica in which the silanol groups on the surface have been substituted with functional groups containing carboxy groups. In the example shown in Figure 2, some of the silanol groups on the silica surface have been substituted with functional groups 10. The functional groups 10 contain carboxy groups 12. R4 in Figure 2 is a bonding chain between Si and the carboxy groups 12. R4 may be, for example, an alkylene group such as a propylene group. For example, the functional group 10 may be a formic acid (methanoic acid) group, an acetic acid (ethanoic acid) group, a propionic acid (propanoic acid) group, or a butyric acid (butanoic acid) group.

The silanol groups on the silica surface can be replaced with functional groups, for example, using a silane coupling agent. The type of functional group may be one or more. For example, the silica may be subjected to a single-layer treatment by silane coupling, in which the silanol groups are replaced with one type of functional group. Alternatively, the silanol groups may be replaced with functional groups by a multi-layer treatment in which a single-layer silane coupling treatment is performed, followed by another silane coupling treatment.

The content of silica abrasive grains is not particularly limited, but is, for example, 0.10 to 20 mass% of the total polishing composition. It is preferable to keep the abrasive grain content as low as possible from the perspective of reducing polishing scratches and foreign matter remaining on the silicon wafer after polishing. On the other hand, if the polishing composition does not contain any abrasive grains, it will be unable to remove, for example, the oxide film on the silicon wafer surface. The polishing composition is diluted 10 to 45 times before use during polishing. The polishing composition of this embodiment is preferably diluted so that the abrasive grain concentration is 20 to 20,000 ppm (ppm by mass; the same applies hereinafter). A higher abrasive grain concentration tends to reduce microdefects and haze. The lower limit of the abrasive grain concentration after dilution is preferably 1,000 ppm, more preferably 2,000 ppm. The upper limit of the abrasive grain concentration after dilution is preferably 15,000 ppm, more preferably 10,000 ppm.

(pH adjuster)
The pH of the polishing composition is adjusted to 1.0 to 6.9 by the pH adjuster contained in the polishing composition. The pH of the polishing composition is preferably 1.0 to 6.0, more preferably 1.0 to 5.5. The pH adjuster used is acidic. The pH adjuster is not limited to a specific one, but may be, for example, at least one acid selected from inorganic acids and organic acids.

The inorganic acid may be, for example, at least one acid selected from hydrochloric acid, sulfuric acid, phosphoric acid, and nitric acid. Examples of organic acids include carboxylic acids, ascorbic acid, sulfonic acids, acidic phosphate esters, phosphonic acids, ethylene oxide adducts of alkylamines, partial esters of polyhydric alcohols, and carboxylic acid amides.

Examples of carboxylic acids include monocarboxylic acids, dicarboxylic acids, tricarboxylic acids, and aromatic carboxylic acids.

Examples of carboxylic acid amides include ethylenediaminetetraacetic acid, sodium ethylenediaminetetraacetate, nitrilotriacetic acid, sodium nitrilotriacetate, ammonium nitrilotriacetate, hydroxyethylethylenediaminetriacetic acid, sodium hydroxyethylethylenediaminetriacetate, diethylenetriaminepentaacetic acid, sodium diethylenetriaminepentaacetate, triethylenetetraminehexaacetic acid, and sodium triethylenetetraminehexaacetate.

Examples of phosphonic acids include 2-aminoethylphosphonic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, aminotri(methylenephosphonic acid), ethylenediaminetetrakis(methylenephosphonic acid), diethylenetriaminepenta(methylenephosphonic acid), ethane-1,1-diphosphonic acid, ethane-1,1,2-triphosphonic acid, ethane-1-hydroxy-1,1-diphosphonic acid, ethane-1-hydroxy-1,1,2-triphosphonic acid, ethane-1,2-dicarboxy-1,2-diphosphonic acid, methanehydroxyphosphonic acid, 2-phosphonobutane-1,2-dicarboxylic acid, 1-phosphonobutane-2,3,4-tricarboxylic acid, and α-methylphosphonosuccinic acid.

The polishing composition according to this embodiment may contain any of the ingredients generally known in the field of polishing compositions, in addition to those mentioned above. For example, the polishing composition may further contain at least one of a water-soluble polymer, a surfactant, and a chelating agent.

Water-soluble polymers adsorb to the surface of workpieces such as semiconductor wafers, modifying the surface. This improves polishing uniformity and reduces surface roughness. Examples of water-soluble polymers that can be used include, but are not limited to, celluloses such as hydroxyethyl cellulose (HEC), hydroxyethyl methyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, cellulose acetate, and methyl cellulose; vinyl polymers such as polyvinyl alcohol (PVA), modified PVA (polyvinyl alcohol derivative), and polyvinylpyrrolidone (PVP); glycosides; and other polyhydric alcohols. Polyhydric alcohols are alcohols containing two or more hydroxy groups per molecule. Examples of glycosides include alkylene oxide derivatives of methyl glucoside. Examples of alkylene oxide derivatives of methyl glucoside include polyoxyethylene methyl glucoside and polyoxypropylene methyl glucoside.

The polishing composition of this embodiment is prepared by appropriately mixing silica, pH adjuster, and other compounding ingredients and adding water. Alternatively, the polishing composition of this embodiment can be prepared by sequentially mixing abrasive grains, pH adjuster, and other compounding ingredients with water. These components can be mixed using means commonly used in the technical field of polishing compositions, such as a homogenizer or ultrasonic waves.

The polishing composition described above is diluted with water to an appropriate concentration and then used to polish semiconductor wafers. The polishing composition can be suitably used to polish Si wafers (silicon substrates).

The present invention will be explained in more detail below using examples. The present invention is not limited to these examples.

Slurries containing the abrasive grains, pH adjuster, and water shown in Table 1 below were prepared. Polishing was performed using these slurries under the following conditions. For each slurry, the pH of the slurry, the abrasive grains, the zeta potential of the Si wafer and polishing pad, parameters A and B, and surface roughness Sa were measured.

(Slurry (Polishing Composition))
The abrasive grains A, B, C and D in Table 1 are as follows:
Abrasive grain A is colloidal silica (product name: Quattron PL-3-C (Fuso Chemical Co., Ltd.)) in which the silanol groups on the surface have been substituted with functional groups containing amino groups (propylamino groups). The propylamino groups that have substituted the silanol groups in Abrasive grain A are functional groups that take on a positive charge in acidic conditions.
Abrasive B is colloidal silica (product name: Quattron PL-3 (Fuso Chemical Co., Ltd.)) in which the silanol groups on the surface have not been substituted with functional groups.
Abrasive C is colloidal silica (product name: Quattron PL-3-D (Fuso Chemical Co., Ltd.)) in which the silanol groups on the surface have been substituted with functional groups that do not contain either amino or carboxyl groups. The functional groups that have substituted the silanol groups in Abrasive C are functional groups that are negatively charged in acidic conditions.
Abrasive D is colloidal silica (product name: Quattron PL-3-P (Fuso Chemical Co., Ltd.)) in which the silanol groups on the surface have been substituted with functional groups containing carboxyl groups (acetate groups). The acetate groups that have substituted the silanol groups in Abrasive D are functional groups that are positively charged in acidic conditions.

(polishing conditions)
Polishing device: Single-sided polishing machine Polishing pad: Polyurethane pad (representative physical properties: hardness 83; JIS-A, compressibility 2.2%, density 0.40 g/cm ³ ) (manufactured by Nitta DuPont Co., Ltd.)
Flow rate: 300mL/min
Surface pressure: 150 (gf/cm ² )
Wafer to be polished: 8" silicon wafer

(Measurement of Zeta Potential)
The zeta potential of the slurry was measured using a ZETASIZER NANO (manufactured by Malvern) as a measuring device.
The zeta potential of the Si wafer and the pad was measured using an ELSZ-2 (manufactured by Otsuka Electronics Co., Ltd.) as a measuring device.

(Calculation of parameters)
Parameter A: Zeta potential of abrasive grains × Zeta potential of silicon wafer Parameter B: Zeta potential of abrasive grains × Zeta potential of polishing pad

(Conditions for measuring surface roughness Sa)
Measuring device: Contour GT-X (manufactured by Bruker)
Measurement field of view: 177 μm x 133 μm
Sa is calculated using the arithmetic mean height (ISO 25178).

(Evaluation (Slurry Composition))
The results shown in Table 1 show that Examples 1 to 3, which have silica as abrasive grains in which surface silanol groups have been substituted with functional groups containing amino or carboxy groups and have an acidic pH of 1.0 to 6.9, have reduced surface roughness compared to Comparative Examples 1 and 2, which have silica as abrasive grains in which surface silanol groups have not been substituted or have been substituted with functional groups containing neither amino nor carboxy groups and have an acidic pH. Furthermore, Examples 1 to 3 also have reduced surface roughness compared to Comparative Examples 3 and 4, which have silica as abrasive grains in which surface silanol groups have been substituted with functional groups containing amino groups but have an alkaline pH of 7.0 or higher. These results demonstrate that the surface roughness of Si wafers can be reduced by configuring a slurry containing silica as abrasive grains in which surface silanol groups have been substituted with functional groups containing amino or carboxy groups and having an acidic pH. Furthermore, the silanol groups on the surface of abrasive grains A and D are substituted with functional groups that are positively charged in acidic conditions, while the silanol groups on the surface of abrasive grains C are substituted with functional groups that are negatively charged in acidic conditions. This shows that the surface roughness of Si wafers can be reduced by using silica abrasive grains whose surface silanol groups have been substituted with functional groups that are positively charged in acidic conditions in a slurry having an acidic pH.

(Evaluation (Zeta Potential Configuration))
Furthermore, in the results shown in Table 1, Examples 1 to 3, in which the surface roughness was reduced, all satisfied the conditions that the zeta potential of the abrasive grains was positive, A<0, and B≧−50. In contrast, Comparative Examples 1 to 4, in which the surface roughness was relatively large, did not satisfy one or more of the above three conditions. From this, it was found that the surface roughness can be reduced when the abrasive grains have a positive zeta potential at a pH of 1.0 to 6.9, the product of the zeta potential of the abrasive grains and the surface zeta potential of the Si wafer is negative (A<0), and the product of the zeta potential of the abrasive grains and the surface zeta potential of the polishing pad is −50 or greater (B≧−50). It was also found that the above zeta potential conditions are more easily met by using silica abrasive grains in which the surface silanol groups are substituted with functional groups containing amino or carboxy groups and by setting the pH to an acidic level.

The above describes an embodiment of the present invention. The above-described embodiment is merely an example for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and it is possible to appropriately modify the above-described embodiment within the scope of the spirit of the present invention.

Claims (1)

A polishing composition for Si wafers to be polished, comprising:
Silica and
A pH adjuster;
and water,
pH is 1.0 to 6.9,
A polishing composition for Si wafers , wherein silanol groups on the surface of the silica are substituted with functional groups containing carboxy groups.