DESCRIPTION AGENT FOR INCREASING SELECTION RATIO OF POLISHING RATES FIELD OF THE INVENTION The present invention relates to an agent for increasing a selection ratio of a polishing rate of a silicon nitride film to that of a silicon oxide film (simply referred to as"agent for increasing a selection ratio of the polishing rates"in some cases). The present invention also relates to a polishing composition containing the agent for increasing the selection ratio of the polishing rates, a process for increasing a selection ratio of a polishing rate of a silicon nitride film to that of a silicon oxide film using the polishing composition, and a fixed abrasive polishing tool having a polishing member containing abrasive grains and a resin, a polishing process of a semiconductor substrate using the above- mentioned polishing composition, and a fixed abrasive polishing tool having a polishing member containing abrasive grains and a resin, a process for manufacturing a semiconductor device using the above-mentioned polishing process, and a polishing apparatus employing the above-mentioned polishing process.

BACKGROUND OF THE INVENTION In recent years, with the advancement of the high integration of semiconductor devices, circuit interconnections have become finer and the distance between wires has become shorter. Especially in a case of a photolithography performed at a linear width of 0.5 Rm or less, surfaces on which pattern images are to be focused by a stepper are required to be as flat as

possible because a focal depth becomes shallower. As one means of subjecting a surface of the semiconductor substrate to planarization, chemical mechanical polishing (CMP) has been known.

An apparatus (CMP apparatus) for carrying out the chemical mechanical polishing is provided with a polishing table having a polishing pad attached thereon and a top ring. An object to be polished is then inserted between the polishing table and the top ring, and pressed against the polishing table by means of the top ring with feeding a polishing liquid to the surface of the polishing pad, whereby polishing is carried out to give a surface of the object to be polished with a flat mirror finish.

The CMP apparatus as described above is, for instance, used in a shallow trench isolation forming process for forming a transistor circuit in a lowest layer of a semiconductor device. Figure 1 shows an example of a process for forming a transistor. A silicon oxide film 3 (in general, Si02) and a silicon nitride film 4 (in general Si3N4) are sequentially laminated on a silicon substrate 1 (0) in an element formation region 2. Using the resulting laminated film as a mask, the silicon substrate 1 is subjected to dry etching (RIE : Reactive Ion Etching) to form a shallow trench 5 (shallow groove), i. e. an element isolation region (0).

The obtained silicon (Si) substrate surface (surface formed by the silicon substrate 1, the silicon oxide film 3 and the silicon nitride film 4) exposed within the trench 5 is thermally oxidized to form a thin film made of silicon oxide (not illustrated in the figure). Thereafter, a silicon oxide film 6, which is an insulation material, is embedded into this trench 5 by CVD (oxo). Since excess silicon oxide film remains on an outer peripheral surface of the trench 5, the excess silicon oxide film is removed by polishing to expose the surface of the

silicon nitride film 4 (@)). Subsequently, the silicon nitride film 4 is removed by wet etching (ou). A transistor 7 is formed in the element formation region 2 via the groove (trench 5) of the silicon oxide film formed as described above ((ß)).

Among the above steps, the polishing step by CMP is the above- mentioned stage (!). The purposes of this polishing step is to completely remove an excess silicon oxide film 6 formed on the silicon nitride film 4 and to polish the silicon nitride film 4 to a given film thickness. The remaining silicon nitride film 4 is removed by an etching treatment with hot phosphate in the next step after the CMP. Since the etching treatment is generally time-controlled, the following problem may arise unless the thickness of the silicon nitride film 4 after the CMP is uniform. For instance, in a case where the silicon nitride film 4 is too thick after the CMP, residue of the silicon nitride film is generated even after the etching treatment for a given period of time, and conversely in a case where the silicon nitride film 4 is too thin, damaging of the silicon substrate 1, the base material, is caused by excess etching.

A problem in a case where the silicon nitride film is polished to a given film thickness is easy progress of dishing of the silicon oxide film in the trench portion. The amount of dishing relates to a ratio of a polishing rate of the silicon nitride film to that of the silicon oxide film. In general, the greater the ratio of the polishing rates, the larger the amount of dishing. For instance, the ratio of the polishing rates in a case where ceria is used as fixed abrasives with feeding purified water to the fixed abrasives satisfies the relationship in which a ratio of the polishing rate of a silicon nitride film to the polishing rate of a silicon oxide film is less than 1/6. Therefore, dishing is likely to progress in this region because the silicon oxide film is more easily polished than the silicon nitride film.

Therefore, in order to polish the silicon nitride film to a given thickness with suppressing the amount of dishing, it is necessary that the ratio of the polishing rate of the silicon nitride film to that of the silicon oxide film is increased, i. e. the ratio of the polishing rate of the silicon nitride film to that of the silicon oxide film approximates 1: 1.

In addition, another application example of the CMP apparatus to a step for manufacturing a semiconductor device includes a step of forming a gate electrode, which is a part of the process for forming a transistor. For instance, in the stage 6 of Figure 1, it is necessary to form a gate electrode during the course of the formation of the transistor 7 in the element formation region 2. In some cases, the newly laminated silicon nitride film and silicon oxide film are simultaneously polished by means of CMP even during the course of the formation process to completely remove the upper surface portion of the silicon nitride film. In this case, it is also necessary to polish the silicon nitride film with suppressing the amount of dishing in the same manner as the STI polishing.

Therefore, it is necessary that the ratio of the polishing rate of the silicon nitride film to that of the silicon oxide film approximates 1: 1.

Therefore, a process using a polishing composition containing silica having a particle size of 10 nm or less and phosphoric acid or a phosphoric acid derivative has been known as a process for increasing a polishing rate of a silicon nitride film to a polishing rate of a silicon oxide film (see Japanese Patent Laid-Open No. Hei 11-176773). In the process using this polishing composition, however, the polishing rate is slow and an increase in the ratio of the polishing rates is not sufficient.

In addition, Japanese Patent Laid-Open No. 2000-173955 discloses a

process for subjecting a semiconductor substrate to planarization with a grinding wheel containing ceria abrasive grains, and a working liquid containing a dispersant. The purpose of this process is to subject the silicon oxide film to planarization with suppressing aggregation of the abrasive grains and not lowering the polishing rates by using an alkaline working liquid containing a dispersant such as ammonium polyacrylate. However, this publication does not mention that the planarization is carried out by increasing the polishing rate of the silicon nitride film to the polishing rate of the silicon oxide film.

SUMMARY OF THE INVENTION The present invention relates to: (1) an agent for increasing a selection ratio of polishing rates, wherein the agent comprises an organic cationic compound, which increases a ratio of a polishing rate of a silicon nitride film to a polishing rate of a silicon oxide film, wherein the agent is provided as a component of a polishing composition used together with a fixed abrasive polishing tool comprising a polishing member comprising abrasive grains and a resin; (2) a polishing composition which increases a ratio of a polishing rate of a silicon nitride film to a polishing rate of a silicon oxide film, wherein the polishing composition comprises the agent, and wherein the polishing composition is used together with a fixed abrasive polishing tool comprising a polishing member comprising abrasive grains and a resin.

(3) a process for increasing a ratio of a polishing rate of a silicon nitride film to a polishing rate of a silicon oxide film, comprising the step of applying the polishing composition of the above (2) to a fixed abrasive polishing tool

comprising a polishing member comprising abrasive grains and a resin; (4) a polishing process of a semiconductor substrate comprising the steps of: feeding the polishing composition of the above (2) to a surface of a fixed abrasive polishing tool comprising a polishing member comprising abrasive grains and a resin, and polishing the semiconductor substrate with the fixed abrasive polishing tool, wherein a ratio of a polishing rate of a silicon nitride film to a polishing rate of a silicon oxide film is increased, thereby subjecting the semiconductor substrate to planarization; and (5) a process of manufacturing a semiconductor device comprising the steps of: feeding the polishing composition of the above (2) to a surface of a fixed abrasive polishing tool comprising a polishing member comprising abrasive grains and a resin, and polishing a semiconductor substrate with the fixed abrasive polishing tool, wherein a ratio of a polishing rate of a silicon nitride film to a polishing rate of a silicon oxide film is increased, thereby subjecting the semiconductor substrate to planarization; and (6) a polishing apparatus comprising: a means of holding a semiconductor substrate which is an object to be polished; a polishing table; a fixed abrasive polishing tool comprising a polishing member comprising abrasive grains and a resin, the tool being mounted on the polishing table;

a dressing means for generating free abrasive grains from fixed abrasives in the fixed abrasive polishing tool; a means capable of varying rotational speeds for adjusting each of the rotational speeds of the fixed abrasives and the substrate; a means capable of varying pressure for adjusting a pressure of a substrate to the fixed abrasives; and a means of feeding the polishing composition of the above (2) to the surface of the fixed abrasives.

BRIEF DESCRIPTION OF THE DRAWING Figure 1 is an explanatory schematic view showing a stepwise process of shallow trench isolation forming process for forming a transistor circuit in a lowest layer of a semiconductor device by CMP.

The reference numerals in Figure 1 are as follows.

1 is a silicon substrate, 2 is an element formation region, 3 is a silicon oxide film, 4 is a silicon nitride film, 5 is a trench, 6 is a silicon oxide film, and 7 is a transistor.

DETAILED DESCRIPTION OF THE INVENTION All publications cited herein are hereby incorporated by reference.

The present invention relates to an agent for increasing a selection ratio of a polishing rate of a silicon nitride film to that of a silicon oxide film stably and at a low cost. The present invention also relates to a polishing composition containing the agent for increasing the selection ratio of the polishing rates, a process for increasing a selection ratio of a polishing rate of a silicon nitride film

to that of a silicon oxide film using the polishing composition, and a fixed abrasive polishing tool having a polishing member containing abrasive grains and a resin, a polishing process of a semiconductor substrate using the polishing composition, and a fixed abrasive polishing tool, a process for manufacturing a semiconductor device using the above-mentioned polishing process, and a polishing apparatus employing the above-mentioned polishing process.

These and other advantages of the present invention will be apparent from the following description.

In the present invention, the term"increasing a selection ratio of polishing rates"means increasing a ratio of a polishing rate of a silicon nitride film to a polishing rate of a silicon oxide film, which is hereinafter also referred to as"a selection ratio. "The term may be also referred hereinafter to as"increasing a selection ratio. "Therefore, the term"agent for increasing a selection ratio of polishing rates"of the present invention (hereinafter simply referred to as"an agent for increasing a selection ratio") means an agent having an action of increasing a ratio of a polishing rate of a silicon nitride film to a polishing rate of a silicon oxide film, specifically an effect of reducing a polishing rate of a silicon oxide film without dramatically reducing a polishing rate of a silicon nitride film by containing the agent for increasing a selection ratio in polishing composition and using the agent together with a fixed abrasive polishing tool including a polishing member containing abrasive grains and a resin. Therefore, when the agent for increasing a selection ratio of the present invention is used, the polishing rate of a silicon oxide film is 0.6 or less, preferably 0.5 or less, more preferably 0.4 or less, even more preferably 0.3 or less, even more preferably 0.2 or less, on the basis of the polishing rate of a silicon oxide film (relative

polishing rate) when polishing is carried out with the fixed abrasive polishing tool including a polishing member containing abrasive grains and a resin and purified water. In addition, the polishing rate of a silicon nitride film is 0.3 or more, preferably 0.4 or more, more preferably 0.5 or more, even more preferably 0.6 or more, even more preferably 0. 8 or more, on the basis of the polishing rate of a silicon nitride film (relative polishing rate) when polishing is carried out with the fixed abrasive polishing tool including a polishing member containing abrasive grains and a resin and purified water. In addition, the polishing rate of a silicon nitride film is preferably 4 or less, more preferably 2 or less, even more preferably 1.5 or less. In addition, the ratio of the polishing rates which is increased by the agent for increasing a selection ratio, i. e. (polishing rate of silicon nitride film)/ (polishing rate of silicon oxide film) is 0.2 or more, preferably 0.3 or more, even more preferably 0.4 or more. Also, the ratio of the polishing rates is preferably 1.5 or less, more preferably 1.3 or less, even more preferably 1.1 or less. The polishing rate of each of the films is determined according to a method described in Examples set forth below.

The organic cationic compound used in the present invention acts as an agent for increasing a selection ratio which increases a ratio of a polishing rate of a silicon nitride film to a polishing rate of a silicon oxide film, by containing the organic cationic compound in the polishing composition used together with the fixed abrasive polishing tool including a polishing member containing abrasive grains, especially, ceria abrasive grains, and a resin.

The organic cationic compound is preferably a nitrogen-containing compound, more preferably a compound having amino group or quaternary ammonium group. The molecular weight of the organic cationic compound is

preferably from 30 to 10000, more preferably from 30 to 1000, even more preferably from 30 to 200, even more preferably from 40 to 120, from the viewpoint of the water solubility.

The compound having amino group may be a compound having one or more amino groups in one molecule. The number of amino groups in the compound is preferably from 1 to 20, more preferably from 1 to 10, even more preferably from 1 to 5, even more preferably from 1 to 3, from the viewpoint of water solubility. The ratio of carbon atoms to nitrogen atoms (C/N ratio) contained in one molecule of the compound having amino group is preferably from 1 to 20, more preferably from 1 to 10, even more preferably from 1 to 6, even more preferably from 1 to 4, from the viewpoint of water solubility. In addition, the compound having amino group includes monoamines such as primary amines, secondary amines and tertiary amines, and polyamines. The compound can contain a functional group other than amino group, for instance, OH group, ether group, or the like.

As the monoamine, preferable are those having from 1 to 20 carbon atoms, more preferably from 1 to 10 carbon atoms, even more preferably from 1 to 6 carbon atoms, even more preferably from 1 to 4 carbon atoms, from the viewpoint of water solubility. Specific examples of the monoamine include primary amines, such as methylamine, ethylamine, propylamine, isopropylamine, butylamine, isobutylamine, sec-butylamine, tert-butylamine, pentylamine, hexylamine, octylamine, laurylamine and stearylamine; secondary amines, such as dimethylamine, ethylmethylamine, diethylamine, methylpropylamine, isopropylmethylamine, ethylpropylamine, butylmethylamine, butylethylamine, dipropylamine, diisopropylamine, dibutylamine, diisobutylamine and

ethylhexylamine; and tertiary amines such as trimethylamine, dimethylethylamine, diethylmethylamine, dimethylisopropylamine, butyldimethylamine, triethylamine, and diisopropylethylamine.

As the polyamine, preferable are those having from 1 to 30 carbon atoms, more preferably from 2 to 20 carbon atoms, even more preferably from 2 to 15 carbon atoms, even more preferably from 2 to 10 carbon atoms, from the viewpoint of water solubility. Specific examples of the polyamine include a diamine such as ethylenediamine, 1,2-diaminopropane, 1,3-diaminopropane, 1, 4-diaminobutane, N, N-dimethylethylenediamine, N-ethylethylenediamine, N-methyl-1, 3-propanediamine, 1,5-diaminopentane, 2, 2-dimethyl-1, 3-propanediamine, N, N-dimethyl-1, 3-propanediamine, N-isopropylethylenediamine, N, N-diethylethylenediamine, hexamethylenediamine, 1,2-diaminocyclohexane, 1,4-diaminocyclohexane, N, N, N', N'-tetramethylethylenediamine, 1,7-diaminoheptane, N, N, N', N'-tetramethyl-1, 2-propanediamine, N, N, N', N'-tetramethyl-1, 3-propanediamine, N, N, 2, 2-tetramethyl-1, 3-propanediamine, octamethylenediamine, N, N, N', N'-tetramethyl-1, 3-butanediamine, N, N, N', N'-tetramethyl-1, 4-butanediamine and N, N, N', N'-tetramethyl-1, 6-hexanediamine, and a polyamine having three or more amino groups in its molecule, such as diethylenetriamine, N- (3-aminopropyl)-1, 3-propanediamine, 3,3'-diamino-N-methyldipropylamine, spermidine, N, N, N', N', N"-pentamethyldiethylenetriamine, 3,3'-iminobis (N, N-dimethylpropylamine), bis (hexamethylene) triamine, triethylenetetramine, N, N'-bis (3-aminopropyl) ethylenediamine and

tetraethylenepentamine.

In addition, as the amine having OH group or OH groups and the amine having ether group, preferable are those having from 1 to 30 carbon atoms, more preferably from 2 to 20 carbon atoms, even more preferably from 2 to 15 carbon atoms, even more preferably from 2 to 10 carbon atoms, from the viewpoint of water solubility. Specific examples thereof include an amine having OH group or OH groups, such as monoethanolamine, 1-aminopropanol, 3-aminopropanol, 2-methylaminoethanol, 2-amino-l-butanol, 2-amino-2-methyl-1-propanol, N, N-diethylhydroxyamine, N, N-dimethylethanolamine, 2-ethylaminoethanol, 1-(dimethylamino)-2-propanol, 3-dimethylamino-1-propanol, 2- (isopropylamino) ethanol, 2- (butylamino) ethanol, 2- (tert-butylamino) ethanol, N, N-diethylethanolamine, 2-dimethylamino-2-methyl-1-propanol, 2- (diisopropylamino) ethanol, 2- (dibutylamino) ethanol, 6-dimethylamino-1-hexanol, diethanolamine, 2-amino-2-methylpropanediol, N-methyldiethanolamine, diisopropanolamine, 2- [2- (dimethylamino) ethoxy] ethanol, N-ethyldiethanolamine, N-butyldiethanolamine, triisopropanolamine, triethanolamine and 2- (2-aminoethylamino) ethanol; and an amine having ether group, such as 2-methoxyethylamine, 2-amino-1-methoxypropane, 3-methoxypropylamine, 3-ethoxypropylamine, 3-isopropoxypropylamine, bis (2-methoxyethyl) amine, 2, 2'- (ethylenedioxy) bis (ethylamine) and 4, 7, 10-trioxa-1, 13-tridecanediamine.

Other amines include polymeric amines such as polyethyleneimines, polyvinylamines and polyallylamines.

Furthermore, the organic cationic compound includes a heterocyclic ring- containing compound containing nitrogen atom, such as piperidine, piperazine,

pyrrolidine, pyridine, pyrazine, pyrrole, triethylenediamine, morpholine, 2-aminopyridine, 3-amino-1, 2,4-triazole, and the like.

The compound having the quaternary ammonium group may have one or more quaternary ammonium groups in one molecule, and the number of quaternary ammonium groups is preferably from 1 to 20, more preferably from 1 to 10, even more preferably from 1 to 5, even more preferably from 1 to 3, from the viewpoint of water solubility. The number of carbon atoms of the compound having quaternary ammonium group is preferably from 4 to 20, more preferably from 4 to 15, even more preferably from 4 to 10, even more preferably from 4 to 7, from the viewpoint of water solubility. The ratio of carbon atoms to nitrogen atoms (C/N ratio) contained in one molecule of the compound having quaternary ammonium group is preferably from 1 to 20, more preferably from 1 to 10, even more preferably from 1 to 6, even more preferably from 1 to 4, from the viewpoint of water solubility.

Specific examples of the compound having quaternary ammonium include each of hydroxides, methoxides, fluorides, chlorides, bromides, iodides, sulfates, methylsulfates, nitrates, dimethyl phosphates, formates and acetates of tetramethylammonium, tetraethylammonium, tetrapropylammonium, tetrabutylammonium, ethyltrimethylammonium, propyltrimethylammonium, butyltrimethylammonium, hexyltrimethylammonium, octyltrimethylammonium, phenyltrimethylammonium, benzyltrimethylammonium, benzyltriethylammonium, methyltributylammonium, N- (2-hydroxypropyl)-N, N, N-trimethylammonium, N-hydroxyethyl-N-hydroxypropyl-N, N-dimethylammonium and N- (hydroxyethyl)-N, N, N-trimethylammonium.

Among them, more preferable are methylamine, ethylamine, propylamine, isopropylamine, butylamine, dimethylamine, diethylamine, dipropylamine, diisopropylamine, ethylenediamine, 1,2-diaminopropane, 1,3-diaminopropane, 1, 4-diaminobutane, N, N-dimethylethylenediamine, N-ethylethylenediamine, N-methyl-1, 3-propanediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, monoethanolamine, 6-dimethylamino-1-hexanol, diethanolamine, triethanolamine, and each of hydroxides, methoxides, fluorides, chlorides, bromides, iodides, sulfates, methylsulfates, nitrates, dimethyl phosphates, formates and acetates of tetramethylammonium, ethyltrimethylammonium, propyltrimethylammonium, butyltrimethylammonium, N- (2-hydroxypropyl)-N, N, N-trimethylammonium, N-hydroxyethyl-N-hydroxypropyl-N, N-dimethylammonium and N- (hydroxyethyl)-N, N, N-trimethylammonium, from the viewpoint of the water solubility and from the viewpoint of the increase in selection ratio.

Among them, ethylamine, propylamine, isopropylamine, butylamine and ethylenediamine are even more preferable.

The polishing composition of the present invention contains the above- mentioned agent for increasing a selection ratio. The amount of the agent for increasing a selection ratio is preferably 0. 001% by weight or more, more preferably 0.003% by weight or more, even more preferably 0.005% by weight or more, even more preferably 0.01% by weight or more, of the entire amount of the polishing composition, from the viewpoint of selectively controlling the polishing rate of a silicon oxide film. In addition, the content of the agent is preferably 20% by weight or less, more preferably 5% by weight or less, even more preferably 3% by weight or less, even more preferably 1% by weight or

less, of the entire amount of the polishing composition, from the viewpoint of maintaining the polishing rate of a silicon nitride film. From both viewpoints, the content of the agent is preferably from 0.001 to 20% by weight, more preferably from 0.003 to 5% by weight, even more preferably from 0.005 to 3% by weight, even more preferably from 0.01 to 1% by weight, of the entire amount of the polishing composition.

It is preferable that the polishing composition of the present invention further contains a pH adjusting agent.

As the pH adjusting agent, a basic compound such as an aqueous ammonia or potassium hydroxide; and an acidic compound such as an inorganic acid such as sulfuric acid, hydrochloric acid, nitric acid or phosphoric acid, a monocarboxylic acid such as formic acid, acetic acid, propionic acid or benzoic acid, a polycarboxylic acid such as oxalic acid, malonic acid, succinic acid, glutamic acid, adipic acid, maleic acid, fumaric acid, phthalic acid or trimellitic acid, a hydroxycarboxylic acid, such as glycolic acid, citric acid, lactic acid, malic acid, gluconic acid or tartaric acid, a phosphonic acid such as methylphosphonic acid, 1-hydroxyethylidene-1, 1-diphosphonic acid or aminotri (methylenephosphonic acid), a polyaminocarboxylic acid such as ethylenediaminetetraacetic acid, nitrilotriacetic acid or diethylenetriaminepentaacetic acid can be used. It is preferable that these pH adjusting agents have high water solubility. From this viewpoint, the basic compound is preferably potassium hydroxide or an aqueous ammonia, and the acidic compound is preferably an inorganic acid, acetic acid or glycolic acid. In addition, the content of the pH adjusting agent is preferably from 0.001 to 30% by weight, more preferably from 0.01 to 20% by weight, even more preferably

from 0.1 to 10% by weight, of the entire amount of the polishing composition, from the viewpoint of increasing the selection ratio.

The balance of the polishing composition of the present invention is water.

The content of water is preferably from 50 to 99.999% by weight, more preferably from 75 to 99.995% by weight, even more preferably from 85 to 99.995% by weight, of the entire amount of the polishing composition, from the viewpoint of increasing the selection ratio.

In addition, there can be added to the polishing composition of the present invention other components such as a disinfectant or an antibacterial agent including tetramethylammonium chloride, tetraethylammonium chloride, tetramethylammnoium hydroxide, tetraethylammonium hydroxide, benzalkonium chloride or benzethonium chloride as occasion demands.

The concentration of each component of the above-mentioned polishing composition is preferable concentration when polishing, or the concentration may be the concentration during the preparation of the composition. In some cases, the composition is prepared as a concentrate, which is diluted upon use.

The pH of the polishing composition is preferably from 2 to 9, more preferably from 2 to 8, even more preferably 2 to 7, even more preferably from 2 to 6, even more preferably from 3 to 6, even more preferably from 4 to 6, from the viewpoint of selectively controlling the polishing rate of a silicon oxide film.

The fixed abrasive polishing tool including a polishing member containing abrasive grains and a resin, which is used in the present invention, is obtained by holding fine abrasive grains in the resin and solidifying into a form of pad. Since this pad is more rigid than a polyurethane-based polishing pad which has been generally used, this pad is excellent for step height properties

such as suppression of dishing. The fixed abrasive polishing tool used in the present invention includes a fixed abrasive pad in which abrasive grains having an average particle size of 0.2 Rm or less are bonded with a resin material. As the resin material, a thermoplastic resin or a thermosetting resin can be used, and the thermoplastic resin is even more preferable. In addition, it is desirable that the abrasive grains are those having a positively charged surface potential within a pH range of 2 to 9. Specific examples of the abrasive grains include those made of ceria, alumina, zirconia and the like, those made of ceria is even more preferable.

In the present invention, by using the fixed abrasive polishing tool and a polishing composition containing the agent for increasing the selection ratio, there are exhibited some excellent effects that the ratio of the polishing rate of the silicon nitride film to the polishing rate of the silicon oxide film can be increased stably and at a low cost. Although not wanting to be limited by theory, the reasons for exhibiting such effects are presumably due to the fact that the surface of the silicon oxide film is negatively charged and the surface of the silicon nitride film is positively charged under ordinary polishing conditions (pH being in a neutral to acidic range), so that the agent for increasing a selection ratio, which is the above-mentioned organic cationic compound, is selectively absorbed to the surface of the silicon oxide film, thereby suppressing the polishing rate of the silicon oxide film. Therefore, the present invention provides a process for increasing a ratio of a polishing rate of a silicon nitride film to a polishing rate of a silicon oxide film comprising the step of applying a polishing composition containing the above-mentioned agent for increasing a selection ratio to the fixed abrasive polishing tool.

In addition, a polishing process of the present invention comprises the steps of: feeding the polishing composition of the present invention to a surface of the above-mentioned fixed abrasive polishing tool, and polishing the semiconductor substrate with the above-mentioned fixed abrasive polishing tool, wherein a ratio of a polishing rate of a silicon nitride film to a polishing rate of a silicon oxide film is increased, thereby subjecting the semiconductor substrate to planarization.

The polishing process of the present invention includes, for instance, a process comprising pressing a semiconductor substrate against polishing platens, to which a fixed abrasive polishing tool is attached and moving the polishing platens or the substrate with feeding a polishing composition of the present invention to the surface of the above-mentioned polishing pad.

By using the polishing process of the present invention, the ratio of a polishing rate of a silicon nitride film to a polishing rate of a silicon oxide film can be increased, whereby there are exhibited some effects that a polished substrate having a very small step height after the removal of the silicon nitride film can be obtained stably and at a low cost. Therefore, the semiconductor substrate can be subjected to planarization.

In the present invention, by applying the above-mentioned polishing process to a process for manufacturing a semiconductor device, a semiconductor device having excellent planarization property can be manufactured. Therefore, the present invention relates to a process for manufacturing a semiconductor device. An example of applying the polishing process to a process for

manufacturing a semiconductor device includes a process including the steps of removing an excessive silicon oxide film formed on the silicon nitride film in a shallow trench isolation step by means of a conventional chemical mechanical polishing, and thereafter polishing a silicon nitride film to a given film thickness by chemical mechanical polishing. Alternatively, an example includes a process including the step of simultaneously polishing a silicon nitride film and a silicon oxide film, thereby completely removing an upper surface portion of the silicon nitride film in the step of forming a gate electrode, which is a part of the process for forming a transistor.

As the silicon nitride film in the present invention, a low pressure CVD film, a normal pressure CVD film or the like can be used. In addition, as the silicon oxide film, a plasma TEOS film, a normal pressure CVD film, a thermal oxide film or the like can be used. Here, there are no limitations on the properties such as thickness of the silicon nitride film and the silicon oxide film.

It is desired that a polishing apparatus of the present invention which is used in the polishing process for a semiconductor substrate or in the process for manufacturing a semiconductor device, contains: a means of holding a semiconductor substrate which is an object to be polished; a polishing table; a fixed abrasive polishing tool including a polishing member comprising abrasive grains and a resin, the tool being mounted on the polishing table; a dressing means for generating free abrasive grains from fixed abrasives in the above-mentioned fixed abrasive polishing tool; a means capable of varying rotational speeds for adjusting each of the

rotational speeds of the above-mentioned fixed abrasives and the substrate; a means capable of varying pressure for adjusting a pressure of a substrate to the fixed abrasives; and a means of feeding the above-mentioned polishing composition to the surface of the fixed abrasives.

EXAMPLES The following examples further describe and demonstrate embodiments of the present invention. The examples are given solely for the purposes of illustration and are not to be construed as limitations of the present invention.

The unit"% by weight"in the following examples refers to a ratio to an entire amount of a polishing composition. Also, each of the conditions for polishing carried out in Examples are described hereinbelow.

(D Substrates to Be Polished In the present examples, an 8-inch (20.3 cm) silicon substrate to which a silicon oxide film was deposited on its surface by plasma TEOS in a thickness of 10000A (1000 nm) and an 8-inch silicon substrate to which a silicon nitride film was deposited on its surface by low-pressure CVD in a thickness of 3000A (300 nm) were used as substrates to be polished.

( Polishing Conditions As the polishing device, a rotary polishing device commercially available from Ebara Corporation under the trade name of EPO-222 in which a polishing table and a polishing head (wafer carrier) are in relative movement to each other

was used. As a polishing pad with a polishing member containing abrasive grains, the one described below was used. Also, a polishing load was 350 hPa, and a flow rate of the polishing composition was 200 mL/min. The number of rotations of a disc was 11 r/min, and the number of rotations of a polishing work (head) was 21 r/min, the disc and the polishing work (head) being rotated in the same direction. A polishing time was 2 minutes.

) Calculation of Relative Polishing Rates The polishing rate was calculated by obtaining an average value of a change in thickness of the polishing film of each of substrates to be polished before and after polishing (measurement locations: 81 locations in the diameter direction of the wafer, edge exclusion: 5 mm), and dividing the obtained value by a polishing time. The amount of change in the film thickness was calculated from a film thickness determined with a light interference film thickness gauge commercially available from KLA-Tencor under the trade name of UV-1050.

The polishing rates of the obtained silicon nitride film and silicon oxide film were expressed as a relative polishing rate on the basis of a polishing rate of a silicon oxide film of Comparative Example 1, i. e. a case where the polishing composition as purified water.

03Calculation of Selection Ratio of Polishing Rates The selection ratio of polishing rates, i. e. selection ratio of relative polishing rates, was calculated by dividing a relative polishing rate of a silicon nitride film by a relative polishing rate of a silicon oxide film.

(S) Preparation of Fixed Abrasive Polishing Tool The fixed abrasive polishing tool used in Examples was obtained by molding a mixture of ceria abrasive grains (average particle size: about 0.2 ; jim) and a thermoplastic resin under given pressure and temperature conditions.

6Preparation of Agent for Increasing Selection Ratio Two-hundred and twenty grams of a 2N aqueous hydrochloric acid and 255 g of ion-exchanged water were added to 25 g of propylamine, to give a 5% aqueous propylamine solution (agent A for increasing selection ratio) with a pH of about 8.

Ninety-eight grams of a 2N aqueous hydrochloric acid and 377 g of ion- exchanged water were added to 25 g of octylamine, to give a 5% aqueous octylamine solution (agent B for increasing selection ratio) with a pH of about 7.5.

(2) Preparation of Polishing Compositions The amount 1.3 g of the agent A for increasing selection ratio, and a 0.1 N aqueous hydrochloric acid and ion-exchanged water were added so as to have a pH of 4.0, to give 1000 g of a polishing composition of Example 1.

The same procedures as in Example 1 were carried out for Examples 2 to 8 and Comparative Example 1 except for preparing each of the compositions as shown in Table 1.

Examples 1 to 8 and Comparative Example 1 Using each of the above polishing compositions, polishing of a silicon nitride film and a silicon oxide film was carried out under the polishing conditions as mentioned above. The results are shown in Table 1.

Table 1 Ex. No. Agent for Increasing pH Relative Polishing Rate Selection Selection Ratio Ratio* Kind Content Silicon Silicon (% by wt. ) Nitride Film Oxide Film 1 A 0.13 4.0 0.13 0.28 0.46 2 A 0.13 5.0 0.13 0.40 0.33 3 A 0.13 8.3 0.10 0.50 0.20 4 A 1.3 5.0 0.12 0.40 0.30 5 A 1.3 6.0 0.10 0.29 0.34 6 B 0.28 4.0 0.06 0.15 0.40 7 B 0.28 5.0 0.06 0.24 0.25 8 B 0.28 6.0 0.06 0.25 0.24 Comp. None-7. 0 0.15 1.0 0.15 Ex. 1 * : (Relative Polishing Rate of <BR> <BR> (Selection Ratio) = silicon Nitride Film) <BR> <BR> <BR> (Relative Polishing Rate of Silicon Oxide Film) It can be seen from the results in Table 1 that the polishing compositions of Examples 1 to 8 using the agents for increasing a selection ratio of the present invention show increase in the selection ratio of polishing rates as compared to the polishing composition of Comparative Example 1.

By using a fixed abrasive polishing tool including a polishing member containing abrasive grains and a resin and the polishing composition of the present invention containing an agent for increasing a selection ratio of polishing

rates, the polishing rate of the silicon nitride film relative to the polishing rate of the silicon oxide film can be increased.

The present invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.