JP2008072094A - Polishing composition for semiconductor wafer, production method thereof, and polishing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polishing composition for a semiconductor wafer not easily resulting in particle contamination, particularly "leaving of polishing particle" on the flat surface such as a wafer at the time of polishing the flat surface and edge of device wafer, etc. <P>SOLUTION: A colloidal silica is prepared from an active silicic acid aqueous solution obtained by removing alkali from an alkali silicate aqueous solution and a quaternary ammonium base, and is stabilized with a quaternary ammonium base. The polishing composition for semiconductor wafers contains no alkali metals. The polishing composition contains a buffer solution that is a combination of a weak acid having a pKa from 8.0 to 12.5 at 25°C (pKa is a logarithm of the reciprocal of acid dissociation constant) and a quaternary ammonium base, and exhibits a buffer action in the range from pH8 to pH11 at 25°C. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention polishes the planar and edge portions of a semiconductor wafer such as a semiconductor device substrate on which a silicon film or a metal film, oxide film, nitride film, etc. (hereinafter referred to as a metal film) is formed. The present invention relates to a wafer polishing composition to be applied and a method for producing the same. Furthermore, the present invention relates to a processing method for performing mirror processing of the plane and edge portions of a semiconductor wafer using the wafer polishing composition.

Electronic parts such as IC, LSI, or VLSI using semiconductor materials such as silicon single crystal as raw materials write many fine electric circuits on a wafer obtained by slicing a silicon or other compound semiconductor single crystal ingot into a thin disk shape. It is manufactured based on the divided piece-like semiconductor element chip. The wafer sliced from the ingot is processed into a mirror wafer having a mirror-finished flat surface and edge surface through processes of lapping, etching, and polishing (hereinafter also referred to as polishing). In the wafer process, a fine electrical circuit is formed on the mirror-finished surface in the subsequent device process. Currently, from the viewpoint of increasing the speed of LSI, the wiring material is more resistant than the conventional Al. In low Cu, the insulating film between wiring prevents the diffusion of Cu into the low dielectric constant film from the silicon oxide film to the low dielectric constant film having a lower dielectric constant, and between Cu and the low dielectric constant film. Therefore, the process is moving to a wiring formation process having a structure through a barrier film of tantalum or tantalum nitride. In order to form such a wiring structure and achieve high integration, a polishing process is frequently performed repeatedly for flattening an interlayer insulating film, forming a metal connection part (plug) between upper and lower wirings of a multilayer wiring, and forming a buried wiring. In this flat surface polishing, a semiconductor wafer is placed on a surface plate on which a polishing cloth made of a synthetic resin foam or a suede-like synthetic leather is stretched, and a polishing composition solution is quantitatively supplied while pressing and rotating. However, the method of processing is common.
The edge surface is in a state where the above metal film or the like is irregularly deposited. Until the wafer is divided into semiconductor element chips, the wafer is subjected to a process such as conveyance with the edge portion supported while maintaining the original disk shape. If the outer peripheral side edge of the wafer has an irregular structure at the time of transfer, micro-breakage occurs due to contact with the transfer device, and fine particles are generated. The fine particles generated in the subsequent processes are dissipated and contaminate the surface that has been subjected to precision processing, greatly affecting the product yield and quality. In order to prevent this fine particle contamination, it is necessary to perform mirror polishing of the edge portion of the semiconductor wafer after the formation of the metal film or the like.

The above-mentioned edge polishing is performed by polishing silica or the like while pressing the edge portion of a semiconductor wafer on a polishing machine in which a polishing cloth made of synthetic resin foam, synthetic leather or nonwoven fabric is attached to the surface of the polishing cloth support. This is achieved by rotating a polishing pad support and / or a wafer while supplying a polishing composition solution containing abrasive grains as a main component. As abrasive grains of the polishing composition used at this time, colloidal silica equivalent to that used for edge polishing of silicon wafers, fumed silica, ceria, alumina, etc. used for planar polishing of device wafers have been proposed. Yes. In particular, colloidal silica and fumed silica are fine particles, and are attracting attention because they can easily obtain a smooth mirror surface.
Such a polishing composition is also referred to as a “slurry” and may be described as such below.

  A polishing composition mainly composed of silica abrasive grains is generally a solution containing an alkali component, and the principle of processing is chemical action by the alkali component, specifically, the surface of a silicon oxide film, a metal film or the like. This is a combination of the erosion action and the mechanical polishing action of silica abrasive grains. Specifically, a thin soft erosion layer is formed on the surface of a workpiece such as a wafer by the erosion action of an alkali component. It is presumed to be a mechanism for removing the eroded layer by the mechanical polishing action of fine abrasive grains, and it is considered that the processing proceeds by repeating this process. After polishing the workpiece, a cleaning step is performed to remove silica abrasive grains and alkaline liquid from the processed surface and the edge portion.

In this cleaning process, a problem that abrasive grains remain on the wafer surface has been pointed out. Although the remaining abrasive grains on the wafer surface can be greatly improved by the polishing conditions and the cleaning method, on the other hand, the problem has not been solved due to a significant decrease in the polishing rate and complication of the cleaning method.
Furthermore, miniaturization of device wiring is becoming more and more noticeable year by year. According to the International Technology Roadmap for Semiconductors, the target wiring width for devices is 90 nm in 2004, 65 nm in 2007, and 2010. 50nm, 2013 35nm are shown. As the device wiring width becomes finer, the surface of the semiconductor wafer needs to be more clean after polishing. The abrasive used for polishing the semiconductor wafer contains abrasive grains having a particle diameter of about several tens of nanometers as described above. Conventionally, the abrasive grain remaining on the surface of the semiconductor wafer has not been a major problem because the grain size of the abrasive grains is sufficiently small relative to the wiring width. However, due to miniaturization of the device wiring, the particle size of the abrasive grains and the device wiring width are almost the same size, and the remaining abrasive grains on the surface of the semiconductor wafer cause a malfunction of the device, which is a serious problem. ing.

  Conventionally, various polishing compositions have been proposed for mirror polishing of semiconductor wafers. For example, Patent Document 1 discloses colloidal silica containing sodium carbonate and an oxidizing agent. Patent Document 2 discloses colloidal silica containing ethylenediamine. Patent Document 3 describes the use of silica particles having a bowl-like shape. Patent Document 4 discloses a device wafer polishing method using an aqueous solution containing ethylene / diamine / pyrocatechol and silica fine powder. Patent Document 5 discloses a method for polishing a semiconductor wafer using an aqueous solution containing fine powders of glycine, hydrogen peroxide, benzotriazole and silica. Patent Document 6 discloses an abrasive in which fumed silica having an average particle diameter of 5 to 30 nm is dispersed in an aqueous KOH solution and a method for producing the same. Patent Document 7 describes a polishing slurry of colloidal silica from which sodium has been removed by cation exchange, and has proposed the addition of an amine as a polishing accelerator and the addition of a quaternary ammonium salt as a bactericidal agent. Patent Document 8 describes the use of specific amines. In Patent Document 9, colloidal silica is produced using tetramethylammonium hydroxide in place of sodium hydroxide as an alkali agent used in the particle growth process of colloidal silica, and is substantially free of sodium. Purity colloidal silica is described. Patent Document 10 discloses a buffer solution having a buffering action between pH 8.7 and 10.6 by adding a combination of either a weak acid and a strong base, a weak acid and a weak base, or a weak acid and a weak base. A conditioned silicon oxide colloidal solution is described. Patent Document 11 describes a polishing composition having a buffering action in which an alkali component and an acid component are added, and using quaternary ammonium as the alkali component.

  When colloidal silica is used as in Patent Document 1 and Patent Document 2, there is a problem of impurities. Colloidal silica is manufactured from sodium silicate as a raw material, and contains a relatively large amount of alkali metal such as sodium, and is a material in which abrasive grains are likely to remain. The silica particles having a bowl-like shape of Patent Document 3 are manufactured using an organosilicon compound as a raw material and are excellent in that they are high purity and do not contain an alkali metal. The disadvantage is that the speed is low. Patent Documents 4 and 5 are excellent in that they do not contain an alkali metal. However, since they use silica fine powder, fumed silica is used, and the polishing speed is high, but scratches occur on the polishing surface. It becomes easy. Patent Document 6 is a slurry using fumed silica, which has a high polishing rate but tends to generate scratches on the polished surface and uses a KOH aqueous solution, and is not a suitable material. In the low sodium colloidal silica described in Patent Document 7, as specified on page 7 of the same document, the polishing accelerator is an amine, and the quaternary ammonium salt is added in a small amount as a bactericidal agent having a polishing promoting effect. Yes. In the examples, the use of aminoethylethanolamine and piperazine as the amine is described. Recently, amines have been found to cause metal contamination of wafers, particularly copper contamination, due to their metal chelate-forming action. In the same document, KOH is used for pH adjustment, and reduction of the amount of sodium is an issue. Patent Document 8 describes the risk of wafer contamination by aminoethylethanolamine. The colloidal silica described in Patent Document 9 is a very preferable abrasive because sodium does not exist in the aqueous phase, the particle surface, and the inside of the particle. However, with only quaternary ammonium hydroxide, the pH variation during polishing is large, and the pH drop due to carbon dioxide in the atmosphere is also large, so a stable polishing rate cannot be obtained.

  Comparing the polishing process of the edge part of the semiconductor wafer and the polishing process of the flat part of the semiconductor wafer, the former is shorter in time for the polishing cloth to contact the edge part than the latter, so the pressure applied to the processing surface is high, In addition, the linear velocity of the polishing cloth with respect to the processed surface is also increased. That is, it can be said that the polishing process of the edge portion is a very severe condition as compared with the planar polishing. The edge of the semiconductor wafer has a very rough surface. Under such processing conditions, a sufficient polishing rate and surface roughness cannot be obtained even if a conventional semiconductor wafer planar polishing composition using fumed silica is used.

JP-A-62-101034, page 5 JP-A-2-146732 Patent Claim Japanese Patent Laid-Open No. 11-60232, page 2 JP-A-6-53313, page 3 JP-A-8-83780, page 5 Japanese Patent Application Laid-Open No. Hei 9-193004 Japanese Patent Application Laid-Open No. 3-202269 Patent Page 7 Japanese Patent Application Laid-Open No. 2002-105440, page 2 JP 2003-89786 A Japanese Patent Application Laid-Open No. 11-302634 JP, 2000-80349, A Claims

  An object of the present invention is to provide a composition for mirror polishing of a planar surface and an edge portion of a semiconductor wafer, in which a good surface roughness is obtained while suppressing residual abrasive grains generated on the surface of the semiconductor wafer and maintaining a high polishing rate, and It is in providing the manufacturing method. Furthermore, another object of the present invention is to provide a method for mirror polishing of the planar and edge portions of a semiconductor wafer using the polishing composition.

  The present inventors are colloidal silica that does not contain an alkali metal and is stabilized by a quaternary ammonium base, and the logarithmic value (pKa) of the reciprocal of the acid dissociation constant at 25 ° C. is 8.0 to 12.5. By using a semiconductor wafer polishing composition comprising a buffer solution in which a weak acid and a quaternary ammonium base are combined and having a buffering action at 25 ° C. between pH 8-11, the plane and edges of the semiconductor wafer are obtained. The inventors have found that the mirror polishing of the portion can be effectively performed, and have completed the present invention.

That is, the first invention of the present invention is a colloidal silica stabilized by a quaternary ammonium base produced by an active silicate aqueous solution obtained by removing alkali from an alkali silicate aqueous solution and a quaternary ammonium base, It contains a buffer solution containing a weak acid and a quaternary ammonium base having no alkali metal, a reciprocal of the acid dissociation constant at 25 ° C. (pKa) of 8.0 to 12.5, and a pH of 8 to 11 at 25 ° C. It is a semiconductor wafer polishing composition characterized by having a buffering action between.
The colloidal silica stabilized with a quaternary ammonium base preferably contains non-spherical silica particles.
The polishing composition is preferably an aqueous dispersion having a silica concentration of 2 to 50% by weight based on the entire colloidal solution.
The polishing composition preferably has an electrical conductivity at 25 ° C. of 15 mS / m or more per 1% by weight of silica particles.
The conductivity is preferably adjusted such that the conductivity at 25 ° C. is 15 mS / m or more per 1% by weight of silica particles by having a strong acid and a salt of a quaternary ammonium base.
The strong acid and quaternary ammonium salt is preferably quaternary ammonium sulfate, quaternary ammonium nitrate or quaternary ammonium fluoride.
The anion constituting the weak acid is preferably a carbonate ion and / or a bicarbonate ion, and the quaternary ammonium is preferably a choline ion, a tetramethylammonium ion, a tetraethylammonium ion or a mixture thereof. Choline is a common name for trimethyl (hydroxyethyl) ammonium.
Moreover, it is preferable that the average particle diameter by the BET method of the silica particle of the colloidal silica in the said semiconductor wafer polishing composition is 10 nm-200 nm.

  In the second aspect of the present invention, diluted sodium silicate is brought into contact with a cation exchange resin, sodium ions are removed to prepare an aqueous active silicic acid solution, and then a quaternary ammonium hydroxide base is added to the active silicic acid aqueous solution. The colloidal particles are grown by heating at a pH of 8 to 11 and concentrated by ultrafiltration to prepare a colloidal silica containing 10 to 60% by weight of silica and containing no alkali metal. The colloidal silica is used for polishing. A method for producing a composition for polishing a semiconductor wafer, comprising preparing a silica concentration of 2 to 50% by weight and simultaneously adding a weak acid and a quaternary ammonium base to obtain a buffer composition.

The third invention of the present invention is to press the semiconductor wafer against the upper and lower surfaces of the semiconductor wafer plane or a rotatable surface plate having a polishing cloth affixed on one side, and supply the polishing composition, A polishing method is characterized in that the semiconductor wafer is rotated to polish the flat surface of the semiconductor wafer.
The fourth invention of the present invention is to supply the polishing composition to a polishing apparatus having a drum-shaped polishing member having a polishing cloth affixed to the surface or a polishing member having an arcuate work surface, A polishing method comprising polishing an edge portion of a semiconductor wafer while pressing the edge portion of the semiconductor wafer and rotating the polishing member and / or the semiconductor wafer.

  When the polishing composition according to the present invention is used, an excellent effect is obtained that it is difficult to cause particle contamination, in particular, residual abrasive grains (hereinafter referred to as “abrasive residual grains”) on a flat surface in polishing a semiconductor wafer or the like. The “abrasive grain residue” is a state in which the abrasive grain component of the polishing composition adheres to the planar portion of the wafer during polishing, and the abrasive grains remain on the planar portion even after cleaning. According to the present invention, a polishing composition having an excellent polishing power and its durability in mirror polishing of a wafer has been solved by solving abrasive grains remaining on a flat surface, which has been relatively insufficient in the past. The effect on related industries is extremely large.

It is important that the polishing composition of the present invention is a colloidal silica which does not contain an alkali metal in the aqueous phase, the surface of the silica particles and the inside of the silica particles and is stabilized by a quaternary ammonium base. Colloidal silica stabilized with commercially available sodium is generally 20 to 50 wt% silica (SiO ₂ ) component and 0.1 to 0.3 wt% Na ₂ O component (0.07 to 0.07 in terms of Na). 0.22% by weight). When the amount of Na is expressed per silica, it contains 0.2 to 0.7% by weight per silica. In general, the colloidal silica having a larger particle diameter has a smaller amount of Na.

A little explanation about "stabilization". For example, in a state where silica particles are dispersed in pure water, there are silanol groups on the particle surface, and the outside is only water molecules. Since the particles vibrate and move due to Brownian motion, the particles collide with each other, dehydration condensation occurs between the silanol groups, the particles are connected, and as the connection expands, the viscosity of the colloid increases, eventually It becomes a gel. On the other hand, for example, when the silica particles are dispersed in a dilute aqueous sodium hydroxide solution having a pH of about 9, hydrated sodium cations exist outside the silanol groups on the surface of the particles, and the particles have an anionic charge, outside the hydration phase OH ^- ions are present in close proximity, further there will be water molecules on the outside. When the silica particle surface has such a constrained phase, a repulsive force is generated between the particles, and collision and connection of the particles do not occur. This is called “stabilization”.

  Colloidal silica stabilized with sodium hydroxide contains Na in the aqueous phase, the surface of the silica particles, and the inside of the silica particles. Na in the silica particles is 0.1 to 0.5% by weight per silica. Na on the surface of the aqueous phase and silica particles can be removed by bringing colloidal silica into contact with a proton-type cation exchange resin, but a portion of Na inside the silica particles is at a rate of several months at room temperature. It gradually moves to the particle surface and is observed as a change in pH. The aqueous phase and the Na on the silica particle surface again exist.

  On the other hand, trace amounts of sodium are also present in colloidal silica stabilized with a quaternary ammonium base as in the present invention. The diluted sodium silicate is brought into contact with a cation exchange resin to remove sodium ions to prepare an active silicic acid aqueous solution. However, sodium ions cannot be completely removed and are present in a trace amount in the active silicic acid aqueous solution. Usually, the amount of Na is 50 ppm or less per silica by weight. In the present invention, this amount of Na is acceptable.

A method for producing colloidal silica stabilized with a quaternary ammonium base is described below.
First, as the alkali silicate aqueous solution used as a raw material, a sodium silicate aqueous solution usually called water glass (water glass No. 1 to No. 4 etc.) is preferably used. This is relatively inexpensive and can be easily obtained. In addition, considering a product for semiconductor use that dislikes Na ions, an aqueous potassium silicate solution is also suitable as a raw material. There is also a method of preparing an alkali silicate aqueous solution by dissolving a solid alkali metal silicate in water. Alkali metasilicates are produced through a crystallization process, and therefore some of them have few impurities. The aqueous alkali silicate solution is diluted with water as necessary.
The diluted alkali silicate aqueous solution is brought into contact with a cation exchange resin to produce an active silicate aqueous solution. The cation exchange resin used in the present invention can be appropriately selected from known ones and is not particularly limited. The contact step of the alkali silicate aqueous solution and the cation exchange resin is, for example, diluting the alkali silicate aqueous solution with water to a silica concentration of 3 to 10% by weight, then contacting the H-type strongly acidic cation exchange resin for dealkalization, and if necessary It can be carried out by contacting with an OH type strongly basic anion exchange resin to deanion. By this step, an active silicic acid aqueous solution is prepared. There have been various proposals for details of the contact conditions, and any known conditions can be adopted in the present invention.

  Next, a colloidal particle growth step is performed. In this growth step, a quaternary ammonium base is used instead of a conventional alkali metal hydroxide. As the quaternary ammonium base, those described above are used. In this growth step, a conventional operation is performed. For example, for the growth of colloidal particles, a quaternary ammonium base can be added so as to have a pH of 8 or more, and the mixture can be heated to 60 to 240 ° C. Above 100 ° C, hydrothermal treatment is performed using an autoclave. The higher the temperature, the larger the particle size. In addition, there is a method in which activated silica is added to a seed sol having a pH of 8 or higher and a temperature of 60 to 240 ° C. by using a build-up method. Build-up is generally performed at an atmospheric pressure of 80 to 100 ° C. Regardless of which method is used, particle growth is performed so that the particle diameter of silica is 10 to 200 nm. The dispersed state of the particles may be monodispersed or secondary agglomerated, and can be properly used depending on the application. The shape of the particles may be true spherical or non-spherical, and can be properly used depending on the application. Unlike conventional production methods using alkali metal hydroxides, non-spherical particles can be easily produced by particle growth using a quaternary ammonium base.

Next, silica is concentrated, but evaporative concentration of water may be used, but ultrafiltration is advantageous in terms of energy.
The ultrafiltration membrane used when concentrating silica by ultrafiltration will be described. Separation to which an ultrafiltration membrane is applied has target particles of 1 nm to several microns, but since dissolved polymer substances are also targeted, filtration accuracy is expressed in the nanometer range as a fractional molecular weight. In the present invention, an ultrafiltration membrane having a molecular weight cut-off of 15000 or less can be preferably used. When a film in this range is used, particles of 1 nm or more can be separated. More preferably, an ultrafiltration membrane having a molecular weight cutoff of 3000 to 15000 is used. If the membrane is less than 3000, the filtration resistance is too large and the treatment time becomes long and uneconomical, and if it exceeds 15000, the degree of purification becomes low. The material of the membrane includes polysulfone, polyacrylonitrile, sintered metal, ceramic, carbon and the like. Any of them can be used, but polysulfone is easy to use because of its heat resistance and filtration speed. There are spiral, tubular, and hollow fiber types that can be used, but the hollow fiber type is compact and easy to use. In addition, when the ultrafiltration process also serves to wash out and remove metal impurities, work to increase the removal rate by further washing out and removing by adding pure water after reaching the target concentration, if necessary. Can also be done. In this step, it is preferable to concentrate so that the silica concentration is 10 to 60% by weight, particularly 20 to 50% by weight.

Further, a purification step using an ion exchange resin can be added as needed before or after the ultrafiltration step. For example, impurities and alkali metals mixed in the particle growth process can be removed by contacting with an H-type strongly acidic cation exchange resin, and purified by deanion by contacting with an OH-type strongly basic anion exchange resin. Therefore, further high purity can be achieved.
As described above, high-purity colloidal silica having a silica particle size of 10 to 200 nm and a silica concentration of 10 to 60% by weight is obtained.

A colloidal silica that does not contain an alkali metal and is stabilized by a quaternary ammonium base is combined with a weak acid having a logarithmic value (pKa) of the reciprocal of the acid dissociation constant at 25 ° C. of 8.0 to 12.5 and a quaternary ammonium base. The semiconductor wafer polishing composition is prepared by adding and mixing the buffer solution so that the pH is 8 to 11 at 25 ° C. and having a buffer action between pH 8 and 11.
The quaternary ammonium base is preferably choline ion, tetramethylammonium ion, tetraethylammonium ion or a mixture thereof. As the other quaternary ammonium base, a quaternary ammonium ion composed of an alkyl group having 4 or less carbon atoms or a hydroxyalkyl group having 4 or less carbon atoms is preferable. As the alkyl group, a methyl group, an ethyl group, a propyl group, or a butyl group is preferable. The hydroxyalkyl group is a hydroxymethyl group, a hydroxyethyl group, a hydroxypropyl group, or a hydroxybutyl group. Specifically, tetrapropylammonium ion, tetrabutylammonium ion, methyltrihydroxyethylammonium ion, triethyl (hydroxyethyl) ammonium ion, etc. are easily available and preferable.
Further, as other quaternary ammonium bases, benzyltrimethylammonium ion, phenyltrimethylammonium ion and the like are easily available and preferable.
The quaternary ammonium base has different corrosiveness and polishing performance with respect to the wafer depending on the type of organic group, and also has different abrasive cleaning properties. Therefore, it is preferable to select and use a combination of a plurality of quaternary ammonium bases.

  The phenomenon that the silica particles of colloidal silica that have been removed from the aqueous phase and the surface of the silica particles and stabilized with a quaternary ammonium base are difficult to stick to the wafer surface was first revealed by the present inventors. The mechanism can be estimated as follows. First, in the case of colloidal silica stabilized with sodium hydroxide, the silica particles and the wafer are accompanied by a slight evaporation of water during the lapse of a short time while the polishing slurry remains attached to the polished wafer surface. Sodium hydroxide corrodes the surface metal (or metal oxide), and silica and metal hydroxide are bonded. The bonding may be due to fusion between the particle surface and the metal hydroxide surface, or electrostatic bonding due to the negative charge of silica and the positive charge of the metal hydroxide surface.

  On the other hand, in the case of colloidal silica stabilized with a quaternary ammonium base, quaternary ammonium ions are present on the surface of the silica particles, and quaternary ammonium ions are also present on the wafer surface. The alkyl group of is exposed. The repulsive force between the alkyl groups prevents the silica particles from sticking to the wafer surface. In the field of metal anticorrosion, quaternary ammonium bases and amines are treated as inhibitors (rust inhibitors). Nitrogen atoms in the molecule are adsorbed on the metal surface, and the alkyl group side faces the liquid phase surface. It is said that it forms an aqueous phase and exhibits an anticorrosive action. It is considered that a similar anticorrosive action is also exhibited on the wafer surface.

  The polishing composition of the present invention is preferably an aqueous dispersion having a silica concentration of 2 to 50% by weight based on the entire composition. From the viewpoint of further improving the polishing power of the polishing composition, the concentration of silica is more preferably 10 to 25% by weight.

  Furthermore, in the present invention, it is preferable to maintain the pH of the entire solution in the range of 8 to 11 in order to maintain a stable polishing force during actual polishing. When the pH is less than 8, the polishing rate decreases and may be out of the practical range. On the other hand, when the pH exceeds 11, the etching at the portion other than the polishing portion becomes too strong, and the silica particles start to aggregate, so that the stability of the polishing composition is lowered, which may be out of the practical range.

  Furthermore, it is preferable that this pH does not change easily due to possible external conditions such as friction, heat, contact with outside air, or mixing with other components. Particularly in edge polishing, the polishing composition is used as a circulating flow. That is, the polishing composition supplied from the slurry tank to the polishing site is used in a manner of returning to the slurry tank. The polishing composition containing only the alkali agent is lowered in pH in a short time during use. This is due to dissolution of the object to be polished and mixing of cleaning water, and fluctuations in the polishing rate caused by fluctuations in pH tend to cause overpolishing due to insufficient polishing or excessive polishing.

In order to keep the pH of the polishing composition of the present invention constant, a weak acid having a logarithmic value (pKa) of the reciprocal of the acid dissociation constant at 25 ° C. of 8.0 to 12.5, and a strong quaternary ammonium base are preferably used. The buffer solution composition is preferably combined. Also in this case, it is preferable to have a buffering action between pH 8-11. The phrase “having a buffering action when the pH is between 8 and 11” means that the pH after the polishing composition of the present invention is diluted 100 times with water is between 8 and 11.
The anion constituting the weak acid is carbonate ion and / or bicarbonate ion, and the cation constituting the quaternary ammonium strong base is at least one of choline ion, tetramethylammonium ion or tetraethylammonium ion. Is preferred. As the other quaternary ammonium ions, those mentioned above are used.
Therefore, in the present invention, it is preferable that the polishing composition itself is a liquid having a strong so-called buffering action, which has a small range of pH change with respect to changes in external conditions. In order to form a buffer solution, a weak acid and a strong quaternary ammonium base having a logarithmic value (pKa) of the reciprocal of the acid dissociation constant (Ka) at 25 ° C. in the range of 8.0 to 12.5 are used. That's fine. When the logarithmic value (pKa) of the reciprocal of the acid dissociation constant at 25 ° C. is less than 8.0, it is not preferable because it is necessary to add a large amount of a weak acid and a strong base in order to increase the pH. If the logarithmic value (pKa) of the reciprocal of the acid dissociation constant at 25 ° C. is larger than 12.5, it is not preferable because it is difficult to form a buffer solution having a large buffering action that stabilizes the pH in the range of 8-11.

  In the present invention, the weak acid used for forming the polishing composition solution having a buffering action is preferably carbonic acid (pKa = 6.35) or hydrogen carbonate (pKa = 10.33), and boric acid (pKa = 9. 24), phosphoric acid (pKa = 2.15, 7.20, 12.35), water-soluble organic acids, and the like, or a mixture thereof. In addition, a quaternary ammonium hydroxide is used as the strong base. The buffer solution described in the present invention refers to a solution formed by the combination described above, in which a weak acid is dissociated as ions having different valences, or a dissociated state and an undissociated state coexist. It is characterized by little change in pH even when a small amount of acid or base is mixed.

In the present invention, the polishing process speed can be remarkably improved by increasing the conductivity of the polishing composition solution. The conductivity is a numerical value indicating the ease of passing electricity in the liquid, and is an inverse value of the electric resistance value. In the present invention, the numerical value of conductivity (milli · Siemens) is shown as a numerical value converted to 1% by weight of silica. In the present invention, further preferably equal to preferably against improvement of polishing rate if the conductivity at 25 ° C. is 15mS / m / 1% -SiO ₂ or more, 20mS / m / 1% -SiO 2 or more. Since the addition of salts reduces the stability of the colloid, there is an upper limit for the addition. The upper limit varies depending particle size of silica is approximately 60mS / m / 1% -SiO ₂ approximately.
As described above, this processing is an application of the chemical action of alkali, which is its component, specifically, the erodibility to a workpiece such as a silicon oxide film or a metal film. That is, a thin soft erosion layer is formed on the surface of a workpiece such as a wafer due to the corrosiveness of alkali. The processing proceeds by removing the thin layer by the mechanical action of fine abrasive grains. The erosion of the metal film is a reaction in which the metal is oxidized, and the metal surface receives electrons from the solution in contact and moves to the solution as metal hydroxide ions. In order for this donation of electrons to proceed quickly, the conductivity of the solution needs to be high.

There are the following two methods for increasing the conductivity. One is a method of increasing the concentration of the buffer solution, and the other is a method of adding salts. In order to increase the concentration of the buffer solution, it is only necessary to increase the concentration without changing the molar ratio of acid to base. The salt used in the method of adding salts is composed of a combination of an acid and a base, but the acid may be either a strong acid or a weak acid, and a mineral acid and an organic acid can be used, or a mixture thereof may be used. . A water-soluble quaternary ammonium hydroxide is used as the base. When adding in a combination of a weak acid and a strong base, a strong acid and a weak base, a weak acid and a weak base, the pH of the buffer solution may be changed. The above two methods may be used in combination.
The salt of the strong acid and the quaternary ammonium base is preferably at least one of quaternary ammonium sulfate, quaternary ammonium nitrate, or quaternary ammonium fluoride. The cation constituting the strong quaternary ammonium base is preferably at least one of choline ion, tetramethylammonium ion or tetraethylammonium ion. As the other quaternary ammonium ions, those mentioned above are used.

  The average particle diameter of colloidal silica silica particles by BET method is preferably 10 nm to 200 nm, preferably 20 nm to 120 nm. The average particle diameter according to the BET method referred to here is an average primary particle diameter calculated from the specific surface area in terms of a true sphere based on the following formula, by measuring the specific surface area of powdered colloidal silica by the nitrogen adsorption BET method.

2720 / specific surface area (m ² / g) = average primary particle diameter (nm) calculated in terms of true sphere

  Further, the polishing composition of the present invention preferably contains a chelating agent that forms a water-insoluble chelate compound with copper. For example, as the chelating agent, known compounds such as azoles such as benzotriazole, quinoline derivatives such as quinolinol and quinaldic acid are preferable. As described above, a chelating agent that forms a water-soluble chelate compound with copper such as ethanolamine is not preferred.

  In order to improve the physical properties of the polishing composition of the present invention, a surfactant, a dispersant, an antifoaming agent, an anti-settling agent and the like can be used in combination. Examples of surfactants, dispersants, antifoaming agents, and antisettling agents include water-soluble organic substances and inorganic layered compounds. Moreover, although the polishing composition of the present invention is an aqueous solution, an organic solvent may be added. The polishing composition of the present invention may be prepared by mixing other polishing agents such as colloidal alumina, colloidal ceria, colloidal zirconia, a base, an additive, water and the like during polishing.

  As described above, in the production of the polishing composition of the present invention, colloidal silica having a silica concentration of 10 to 60% by weight is prepared, the above buffer solution is added to the colloidal silica, the pH is adjusted, and the silica concentration Adjust. Furthermore, in order to adjust the silica concentration and / or the conductivity in the polishing composition of the present invention, an aqueous solution of the above-described salts may be added. In addition, it is preferable to add deionized water or the like as necessary to obtain the polishing composition of the present invention.

Next, a method for polishing a semiconductor wafer using the polishing composition of the present invention will be described.
In the case of surface polishing, the polishing surface of the work piece is pressed against a rotatable surface plate having a polishing cloth made of synthetic resin foam or suede-like synthetic leather on the upper or lower surface or one surface, and silicon oxide fine particles are washed with water. While the polishing composition dispersed in is quantitatively supplied, the surface plate and / or the workpiece are rotated to polish the polished surface of the workpiece. The planar polishing processing machine used in the present invention is an apparatus shown in, for example, SH-24 single-side polishing apparatus, FAM-20B double-side polishing apparatus, etc. manufactured by Speedfam.

  In the case of edge polishing, the workpiece (workpiece) is generally applied to a polishing machine in which a polishing cloth made of synthetic resin foam, synthetic leather or nonwoven fabric is attached to the surface of a rotatable polishing cloth support. The edge portion of a silicon wafer or the like subjected to beveling (chamfering) is inclined and pressed while being rotated, and the edge portion is polished while supplying the polishing composition. The edge polishing processing machine used in the present invention is, for example, as shown in an EP-IV type edge polishing apparatus manufactured by Speedfam Co., Ltd., a rotatable polishing cloth support having a polishing cloth affixed to the surface, and a workpiece A gripping part that grips, rotates, and tilts at an arbitrary angle, presses an edge part of the work attached to the gripping part against the polishing pad support, and supplies the polishing composition of the present invention to the work Both the polishing cloth support and the polishing cloth support are rotated to perform mirror polishing of the edge portion of the workpiece. That is, polishing is performed while dripping the polishing composition of the present invention onto a processed portion by pressing the workpiece at a certain angle while rotating the workpiece onto a polishing cloth support that gradually changes in position while rotating. To do. The method for polishing a semiconductor wafer using the polishing composition of the present invention will be described in detail in the following examples. The apparatus is not limited to the above description, and any apparatus described in, for example, Japanese Patent Application Laid-Open Nos. 2000-317788 and 2002-36079 can be used.

  Next, the semiconductor wafer polishing composition of the present invention and the polishing method using the same will be specifically described with reference to examples and comparative examples. However, the present invention is not limited to these examples. Absent.

(Example 1)
<Example of production of colloidal silica raw material A>
520 kg of JIS No. 3 sodium silicate (SiO ₂ : 28.8 wt%, Na ₂ O: 9.7 wt%, H ₂ O: 61.5 wt%) was added to 2810 kg of deionized water and mixed uniformly to obtain a silica concentration of 4 A 5 wt% diluted sodium silicate was prepared. This diluted sodium silicate was passed through a 1000 liter column of H-type strongly acidic cation exchange resin (Amberlite IR120B manufactured by Organo Co., Ltd.) regenerated with hydrochloric acid in advance to remove the alkali, and the silica concentration was 3.7% by weight and pH 2.9. 3800 kg of active silicic acid was obtained. This active silicic acid had Na and K contents of 80 ppm and 5 ppm per silica, respectively. Next, the build-up method was taken to grow colloidal particles. That is, 20% tetramethylammonium hydroxide aqueous solution was added to 580 kg of the obtained active silicic acid with stirring to adjust the pH to 8.7, maintained at 95 ° C. for 1 hour, and the remaining 3220 kg of active silicic acid was added over 6 hours. did. During the addition, a 20% tetramethylammonium hydroxide aqueous solution was added to maintain the pH at 10, and the temperature was also maintained at 95 ° C. After completion of the addition, the mixture was aged at 95 ° C. for 1 hour and allowed to cool. Subsequently, pressure filtration was performed by pump circulation using a hollow fiber type ultrafiltration membrane having a molecular weight cut off of 6000 (Microsa UF module SIP-1013 manufactured by Asahi Kasei Co., Ltd.) to concentrate to a silica concentration of 31% by weight. About 475 kg of colloidal silica was recovered. The colloidal silica had a silica particle size of 15 nm and Na and K contents per silica of 13 ppm and 1.2 ppm, respectively. A TEM photograph of this colloidal silica is shown in FIG. The silica particles in FIG. 1 are a mixture of spherical particles and “saddle-shaped” or “V-shaped” non-spherical particles in which several spheres are connected. In TEM, colloidal silica particles had a minor axis of about 20 nm and a major axis of about 50 nm.

<Production example of additive A>
37.5 Kg of pure water is added to 37.5 Kg of pure water to make 75 Kg of diluted sulfuric acid. To this diluted sulfuric acid, 265 Kg of 25% tetramethylammonium hydroxide aqueous solution is added dropwise to neutralize to pH 7 to give tetrasulfate. 340 kg of aqueous methylammonium solution was prepared.
<Production example of additive B>
Carbon dioxide gas was blown into 164 kg of 25% tetramethylammonium hydroxide aqueous solution under strong stirring, and neutralized to pH 8.4 to prepare 184.2 kg of 33% tetramethylammonium hydrogen carbonate aqueous solution. To this, 149.1 kg of a 25% tetramethylammonium hydroxide aqueous solution was added and mixed to prepare 333.3 kg of a mixed tetramethylammonium solution for a buffer composition.
In the additive B, tetramethylammonium hydrogen carbonate is a salt that is a combination of carbonic acid (pKa = 10.33) as a weak acid and a strong base, and is the buffer solution of the present invention. Additive B is an additive for increasing electrical conductivity.

(Example 2)
<Preparation of colloidal silica with pH buffer composition>
The additive A and additive B were added to 17 Kg of colloidal silica prepared by the preparation method of Example 1 so as to have the levels shown in Table 1, and mixed for 24 hours. Thus, colloidal silica having a pH buffering action and a silica concentration of 30% was prepared. Three types of colloidal silica were abbreviated as C-1, C-2, and C-3, respectively, and their properties are shown in Table 2. In the table, “Na (ppm / SiO ₂ )” represents the sodium concentration per silica. In the table, the conductivity “mS / m / 1% -SiO ₂ ” is a value obtained by measuring the conductivity of each colloidal silica using a conductivity meter and dividing the measured value by the silica concentration.

(Example 3)
The colloidal silica of Example 1 and Example 2 was diluted with pure water so as to have the silica concentration of each level shown in Table 2, and the following polishing test was conducted. The results are shown in Table 2.

<Polishing test>
A polishing experiment of an 8-inch silicon wafer with a poly-Si film was performed by the above-described method.
The wafer edge polishing apparatus and polishing conditions used in the present invention are as follows.
Polishing apparatus: EPF-200X type edge polishing apparatus manufactured by Speed Fam Co., Ltd. Wafer rotation speed: 2000 times / minute Polishing time: 60 seconds / sheet Polishing composition flow rate: 3 L / minute Polishing cloth: suba400 (manufactured by Nitta Haas) )
Weight: 40 N / unit 10 wafers were polished continuously, and the following evaluation test was performed on the 10th wafer.

After edge polishing is completed, pure water is used instead of the polishing composition to wash away the polishing composition, the wafer is removed from the polishing apparatus, brush scrub cleaning is performed using 1% aqueous ammonia and pure water, and nitrogen blowing is performed. Spin drying was carried out.
About the wafer obtained above, the number of particles of 0.15 μm or more adhering to the surface was measured by SEM and a laser light scattering surface inspection apparatus.
In addition, the haze and pits generated on the polished surface and the unfinished edge polishing caused by incomplete edge polishing were visually observed under a condenser lamp and observed with an optical microscope at a magnification of 800 times over the entire circumference of the workpiece. did. The polishing rate was determined from the weight difference between the device wafers before and after polishing.

(Comparative Example 1)
<Colloidal silica D-1>
3333 g of the additive B was added to 128 kg of general-purpose sodium-stabilized colloidal silica (silica doll 40: silica concentration 40.4 wt%, average particle size 18 nm, sodium content 4000 ppm) and mixed for 24 hours. Thus, colloidal silica having a pH buffering action and having a silica concentration of 39% and a pH of 10.4, that is, a polishing composition was prepared. The polishing composition had an electrical conductivity of 691 mS / m, and was 17.7 mS / m / 1% -SiO ₂ divided by the silica concentration.
Using this polishing composition, the same polishing test as in Example 1 was performed, and the results are shown in Table 2.

Example 4
The colloidal silica of Example 1 and Example 2 was diluted with pure water so as to have the silica concentration of each level shown in Table 3, and the following polishing test was conducted. The results are shown in Table 3.
<Polishing test>
Polishing experiments were performed by the method described above. As the silicon wafer, an 8-inch etched silicon wafer having a resistivity of 0.01 Ω · cm, a crystal orientation <100>, and a conductivity type P-type manufactured by the CZ method was used.
The wafer polishing apparatus and polishing conditions used in the present invention are as follows.
The polishing conditions were mirror polishing by the following method.
Polishing apparatus: SH-24 type, speed fam Co., Ltd. Surface plate rotation speed: 70 RPM
Pressure plate rotation speed: 50 RPM
Polishing cloth: SUBA400 (made by Nitta Haas)
Load: 150 g / cm ² Polishing composition flow rate: 80 ml / min Polishing time: 10 minutes After the surface polishing is completed, the polishing composition is washed away by flowing pure water instead of the polishing composition, and the wafer is removed from the polishing apparatus. After brush scrub cleaning with 1% aqueous ammonia and pure water, spin drying was performed while nitrogen blowing was performed. About the wafer obtained above, the number of particles of 0.15 μm or more adhering to the surface was measured by SEM and a laser light scattering surface inspection apparatus. The polishing rate was determined from the weight difference between the silicon wafers before and after polishing. The polished surface was evaluated by observing the state of haze and pits with the naked eye under a condensing lamp.

  As is apparent from the results shown in the examples of Tables 2 and 3, experiments were conducted in which the edge portion was processed by circulating the polishing composition using a polishing composition free of sodium and planar mirror polishing. In the experiment, flat abrasive grains remained very little, and satisfactory results were obtained in terms of both the polishing rate and the edge surface condition. On the other hand, as shown in the comparative example, the polishing composition that does not remove sodium has a large amount of abrasive grains remaining on the surface, resulting in an expected defect in semiconductor performance.

2 is a TEM photograph of the colloidal silica of the present invention obtained in Example 1. FIG. 3 is a TEM photograph of colloidal silica used in Comparative Example 1.

Claims (11)

  Colloidal silica stabilized by an aqueous silicate solution obtained by removing alkali from an aqueous solution of silicate and a quaternary ammonium base, which is stabilized by a quaternary ammonium base, which does not contain an alkali metal and is acid dissociated at 25 ° C. A logarithmic value (pKa) of a reciprocal of a constant includes a buffer solution in which a weak acid and a quaternary ammonium base of 8.0 to 12.5 are combined, and has a buffering action at 25 ° C. between pH 8 and 11. A semiconductor wafer polishing composition.   The composition for polishing a semiconductor wafer according to claim 1, wherein the colloidal silica stabilized with a quaternary ammonium base contains non-spherical silica particles.   The composition for polishing a semiconductor wafer according to claim 1 or 2, wherein the composition is an aqueous dispersion having a silica concentration of 2 to 50% by weight based on the entire colloidal solution.   The composition for polishing a semiconductor wafer according to any one of claims 1 to 3, wherein the electrical conductivity at 25 ° C is 15 mS / m or more per 1% by weight of silica particles.   5. The composition for polishing a semiconductor wafer according to claim 4, wherein the composition at 25 ° C. has a conductivity of 15 mS / m or more per 1% by weight of silica particles by having a salt of a strong acid and a quaternary ammonium base.   The composition for polishing a semiconductor wafer according to claim 5, wherein the salt of the strong acid and the quaternary ammonium base is quaternary ammonium sulfate, quaternary ammonium nitrate, or quaternary ammonium fluoride.   2. The anion constituting the weak acid is a carbonate ion and / or a hydrogen carbonate ion, and the quaternary ammonium base is a choline ion, a tetramethylammonium ion, a tetraethylammonium ion or a mixture thereof. 3. The semiconductor wafer polishing composition according to 2 or 2.   The composition for polishing a semiconductor wafer according to any one of claims 1 to 7, wherein the silica particles of the colloidal silica have an average particle diameter of 10 nm to 200 nm by BET method.   A method for producing a composition for polishing a semiconductor wafer according to claim 1 or 2, wherein diluted sodium silicate is contacted with a cation exchange resin, sodium ions are removed to prepare an active silicic acid aqueous solution, and then active silicic acid aqueous solution Colloidal silica containing no alkali metal having a silica concentration of 10 to 60% by weight is prepared by adding quaternary ammonium base and heating to pH 8 to 11 to grow colloidal particles and concentrating by ultrafiltration. A method for producing a composition for polishing a semiconductor wafer, comprising preparing the colloidal silica to a polishing silica concentration of 2 to 50% by weight and simultaneously adding a weak acid and a quaternary ammonium base so as to obtain a buffer composition.   The surface plate and / or the semiconductor wafer is rotated while pressing the semiconductor wafer against the upper and lower surfaces or a rotatable surface plate having a polishing cloth affixed on one side, and supplying the polishing composition according to claim 1. A polishing method comprising polishing a flat surface of a semiconductor wafer.   While supplying the polishing composition according to claim 1 to a polishing apparatus having a drum-shaped polishing member having a polishing cloth affixed on its surface or a polishing member having an arcuate work surface, A polishing method comprising polishing an edge portion of a semiconductor wafer while pressing the edge portion and rotating the polishing member and / or the semiconductor wafer.

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