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POLISHING ABRASIVE GRAINS,
POLISHING AGENT AND POLISHING
BACKGROUND OF THE INVENTION 5
1) Field of the Invention
The present invention relates to polishing abrasive grains, a polishing agent and a polishing method for use in polishing the surfaces of substrates, particularly semiconductor substrates, so called "bare wafers", and more particularly 10 semiconductor device substrates. The term "semiconductor device substrates" used herein means substrates with a deposited thin film or films including a thin oxide film and a thin metallic film and/or a printed wiring thereon.
2) Description of the Related Art 15 According to the conventional polishing method,
polishing, typically mirror polishing, of semiconductor device substrate surface is carried out by first supplying a polishing agent onto a polishing cloth-mounted table, and then pressing a substrate mounted on a support plate onto the turning or oscillating table with an appropriate pressing force, thereby polishing and flattening the substrate surface. Then, the polished substrate surface is cleaned to remove grains, particularly polishing abrasive grains of the polishing agent remaining on the substrate surface.
To remove metallic contamination, for example, due to metallic impurities originating from the polishing equipment, polishing cloth, polishing agent, etc. from the polished substrate surface after the polishing, at least fol- 3Q lowing 4 steps are required:
1) Mechanical cleaning by brushing to remove fine grains remaining on the polished substrate surface after the polishing,
2) Cleaning with an alkaline solution to further com- 35 pletely remove the fine grains from the polished substrate surface,
3) Cleaning with an acid solution or chelating agent to remove metallic contamination from the polished substrate surface, and 40
4) Precleaning of the polished substrate surface for successive steps.
In the above-mentioned conventional polishing method, the polished substrate must be subjected to at least said four cleaning steps 1) to 4) to remove metallic contamination 45 from the polished substrate surface. In some cases, each of the cleaning steps must be carried out repeatedly, depending upon cleaning conditions, with the result of much increased cleaning runs. Sometimes, cleaning with a very dilute acid solution may be required, as in a case of polished semicon- 50 ductor device substrates with printed aluminum (alloy) wirings on the surfaces, where such problems as corrosion of the wirings are always involved with the result of inevitable yield deterioration.
Without the cleaning with a cleaning solution having an 55 adverse corrosive effect on the printed aluminum wirings to avoid the corrosion of wirings, on other hand, another problem such as failure to remove the metallic contamination caused mainly by the remaining polishing abrasive grains from the substrate surface will be involved. 60
U.S. Pat. No. 4,968,381 discloses a polishing method using a polishing agent containing at least one polar compound having hydrophilic and hydrophobic groups selected from the group consisting of alcohols, ketones, ethers, esters and amides, such as diethyleneglycol diethylether or dieth- 65 yleneglycol dimethyl ether as an additive at the final phase of the polishing process.
JP-A 2-275629 discloses a polishing method for rinsepolishing a semiconductor device substrate with a diluted solution of an oxidizing agent such as sodium peroxide, sodium chlorate, hydrogen peroxide, ozone, etc. just before the completion of polishing, thereby making the semiconductor device substrate surface hydrophilic.
JP-A 7-235519 discloses a polishing method using a liquid mixture of hydrobromic acid, glycerine and water as a polishing agent, thereby making the semiconductor device substrate surface hydrophilic after the final polishing.
JP-A 8-22970 discloses a polishing method using a polishing agent containing an organic compound having at least one hydrophilic group such as carboxyl group, sulfor group, etc. and a molecular weight of at least 100, such as polymeric ammonium polycarboxylate and polymeric ammonium polysulfonate, thereby preventing dishing.
Furthermore, high purity amorphous silica fine powder produced by sol-gel process using alkoxysilane as starting material [e.g. Quatron colloid (PL series), trademark of products made by Fuso Siltech Co., Ltd., Japan], and synthetic silica fine powder showing a lipophilic property upon alkyl modification [e.g. Quatron water-repellent fine powder (WR series), trademark of products made by Fuso Siltech K.K., Japan] are now commercially available. Uses of PL series include, for example, not only a polishing agent for semiconductor device substrates but also fillers for various coatings and synthetic glass materials.
SUMMARY OF THE INVENTION
An object of the present invention is to provide polishing abrasive grains, a polishing agent and a polishing method, capable of removing polishing abrasive grains remaining on the surface of a polished substrate to a satisfactory degree by simple cleaning using an aqueous cleaning solution comprising pure water after polishing the substrate surface.
In one aspect, the present invention provides polishing abrasive grains.
The present polishing abrasive grains are characterized in that the grains have a hydrophilic surface. Absolute value of surface potential (zeta potential) as an indicator of surface hydrophilic property is in a range of 0 to 50 mV when measured at pH 7.
Method for imparting a surface hydrophilic property to the polishing abrasive grains is not particularly limited, and preferably includes a method for terminating at least a portion, preferably at least 80%, and more preferably the whole of extremities of polishing abrasive grains with hydrophilic functional groups.
The present polishing abrasive grains are preferably composed of oxide of at least one metal species selected from the metal group consisting of light metals, transition metals and rare earth metals, and at least a portion, preferably at least 80%, more and preferably the whole of whose extremities of the polishing abrasive grains has hydrophilic functional groups. The term "light metals" used herein means metals with a small specific gravity such as alkali metals, alkaline earth metals, aluminum, beryllium, magnesium, etc. The term "oxide of metal" used herein means an oxide of preferably aluminum, zirconium, manganese, titanium, cerium, calcium, barium or copper. The term "hydrophilic functional groups" used herein means groups containing such an atom as oxygen, nitrogen, sulfur or the like, including, for example, hydroxyl groups, carboxyl groups, amino groups, carbonyl groups, sulfo groups, etc., and preferably hydroxyl groups.
The present polishing abrasive grains terminated with hydrophilic groups, for example, hydroxyl groups, at the
extremities can be prepared by a plasma irradiation method and a sol-gel process (process for preparing an oxide hydrate sol by hydrolyzing a metal alkoxide) known to those skilled in the art using a metal alkoxide containing the relevant metal species as a starting material, among which the sol-gel 5 process is preferable.
Metal alkoxide for use in the sol-gel process is not particularly limited, so long as it is an alcohol ester of a metal species selected from the above-mentioned metal group, but preferably the number of carbon atoms in the 1° alkoxy group is 1 to 4 from the viewpoint of the ease of hydrolysis. Process for preparing a metal alkoxide is not particularly limited, and includes well known processes to those skilled in the art such as processes for reaction of simple metals, metal oxides, metal hydroxides or metal 15 halides with alcohol. A method for terminating the polishing abrasive grains with functional groups such as carboxyl groups, amino groups, carbonyl groups sulfo groups, etc. at the extremities is also not particularly limited, and methods well known to those skilled in the art are available. 20
To prevent metallic contamination, preferably the metal alkoxide is a high purity metal alkoxide containing no more than 10 ppm, preferably no more than 1 ppm, and more preferably no more than 0.1 ppm of other metal species than the relevant metal species. 25
The larger the grain size of polishing abrasive grains, the more improved the polishing rate, but the polishing abrasive grains having larger grain sizes than the necessary sizes may damage the substrate. Thus, the grain sizes are usually in a range of 1 to 10,000 nm, preferably 10 to 100 nm, and more preferably 20 to 50 nm.
In another aspect, the present invention provides a polishing agent.
The present polishing agent comprises the above- 35 mentioned polishing abrasive grains and a solvent. The solvent may contain a dispersion agent such as well known surfactants, etc. or electrolytic ionized water, etc., if required, as well as deionized water. The dispersion agent such as a surfactant, etc. can be contained at a concentration 40 of no more than 10% by weight, preferably no more than 3% by weight, and more preferably 1 to 3% by weight, on the basis of total weight of the solvent. Ranges of pH of the polishing agent is not particularly limited and can be selected appropriately in view of the desired use. In case of 45 polishing the deposited oxide film of a semiconductor device silicon substrate, pH is 8 to 13, preferably 8 to 12, and more preferably 10 to 12, where pH can be adjusted by adding an amine such as monoethylamine, isopropylamine or the like, KOH, ammonia or the like thereto. In case of polishing the 50 deposited metallic film of the semiconductor device silicon substrate, pH is 2 to 6, preferably 2 to 5, and more preferably 3 to 4, where pH can be adjusted by adding iron nitrate, copper sulfate or the like thereto.
It is preferable that polishing abrasive grains are uni- 55 formly dispersed in the polishing agent. For example, the polishing abrasive grains can be brought into a colloidal state or in a uniformly dispersed state by adding a dispersion agent such as a surfactant, etc. thereto, as mentioned above.
The lower the viscosity of a polishing agent, the harder to 60 uniformly disperse the polishing abrasive grains, whereas the higher the viscosity, the harder to polish the semiconductor device silicon substrate while maintaining uniform thickness of the oxide film or metallic film of the semiconductor device silicone substrate. Thus, the viscosity, when 65 measured at 25° C. by the method specified JIS Z 8803 using a rotation viscometer in a dispersion of 1% by weight of
polishing abrasive grains in a solvent, is usually 1 to 10 mpa's, preferably 1 to 5 mPa's, and more preferably 1 to 2 mpa's.
In yet another aspect, the present invention provides a polishing method.
The present polishing method comprises a first step of polishing a substrate surface with the above mentioned polishing agent and a second step of cleaning the polished substrate surface from the first step with an aqueous cleaning solution, thereby removing remaining polishing abrasive grains from the polished substrate surface.
Polishing temperature is usually in a range of 10° to 70° C, preferably 20° to 50° C, and more preferably 20° to 40° C. When polishing is carried out in polishing equipment by pressing a substrate onto polishing cloth mounted on a turning table, the table can be turned at 10 to 200 r/min, preferably 20 to 80 r/min, and more preferably 20 to 50 r/min; the pressing force to the substrate can be 1 to 15 psi (6895 to 103421 Pa), preferably 3 to 10 psi (20684 to 68948 Pa), and more preferably 3 to 7 psi (20684 to 48263 Pa); and the feed rate of polishing agent is 0.1 to 1.01/min, preferably 0.1 to 1.5 1/min, more preferably 0.2 to 0.3 1/ min.
Polishing rate of a oxide film, as determined under standard polishing conditions (i.e. polishing temperature: 25° C; table turning speed: 30 r/min; and pressing force: 7 psi (48263 Pa)), is 100 to 300 nm/min, preferably 150 to 250 nm/min.
Polishing rate of a metallic film, as determined standard polishing conditions (i.e. polishing temperature: 25° C; table turning speed: 50 r/min and pressing force: 4 psi (27579 Pa)), is 150 to 400 nm/min, preferably 250 to 400 nm/min.
Cleaning solution is not particularly limited, so long as it is an aqueous cleaning solution. For example, an aqueous solution comprising ammonia, hydrogen peroxide and pure water usually in a weight ratio of ammonia:hydrogen peroxide:water=l-2:l-5:4^10. The cleaning solution may further contain a chemical such as a chelating agent, e.g. citric acid, in a proportion of 2 to 30% by weight, preferably 2 to 10% by weight, and more preferably 5 to 10% by weight on the basis of total weight of the cleaning solution. Cleaning temperature is not particularly limited and is, for example, in a range of 70° to 90° C.
The present polishing abrasive grains, polishing agent and polishing method are applicable to polishing of semiconductor device substrates, particularly semiconductor device silicon substrates.
The present polishing agent contains polishing abrasive grains having hydrophilic surfaces, particularly those terminated with hydrophilic functional groups at the extremities. Thus, the present polishing agent is very compatible with the aqueous cleaning solution comprising pure water used in the cleaning step following the polishing, and thus can be effectively transferred in to the cleaning solution from the surfaces of polished substrates by simple cleaning thereby facilitating and ensuring removal of the remaining polishing abrasive grains.
For example, when most widely used semiconductor device silicon substrates are to be polished with a polishing agent containing the conventional cerium oxide polishing abrasive grains with no hydrophilic extremities and then cleaned with an aqueous cleaning solution, no electrochemical repulsion force occurs at all between the polished substrates and the polishing abrasive grains remaining on the surfaces of the polished semiconductor device silicon substrates in the cleaning solution due to the material difference
between the substrates and the polishing abrasive grains, and thus the polished substrates cannot be cleaned to a satisfactory degree and the polishing abrasive grains still remain even considerably after the cleaning. In the present invention, on the other hand, cerium oxide polishing abra- 5 sive grains with hydrophilic extremities are used. That is, the present polishing abrasive grains have a good hydrophilic property as mentioned above, and thus have a good compatibility with an aqueous cleaning solution. Accordingly, the remaining polishing abrasive grains can be readily 10 removed from the polished substrate surfaces with the aqueous cleaning solution and the polished substrate surfaces can be cleaned to a satisfactory degree. Such effects of the present invention can be obtained not only with cerium oxide but also with oxides of metal species selected from the 15 metal group consisting of light metals, transition metals, rare earth metals, etc., so long as they have a hydrophilic surface as mentioned above. According to the present invention, a polishing agent having good polishability and cleaning susceptibility can be obtained with the polishing agent 20 containing a hydrophilic surface, selected from oxides of the above-mentioned metal group.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows typical structural formulas in process of 25 forming polishing abrasive grains according to a first embodiment of the present invention.
FIG. 2 is a schematic flow sheet showing the polishing step and the cleaning step for semiconductor device silicon substrates according to the first embodiment of the present invention.
FIG. 3 shows typical structural formulas in process of forming polishing abrasive grains according to a second embodiment of the present invention. 35
FIG. 4 shows typical structural formulas in process of forming polishing abrasive grains according to a third embodiment of the present invention.
FIG. 5 is a schematic flow sheet showing a polishing step and another cleaning step of semiconductor device silicon 40 substrates according to a further embodiment of the present invention.
DETAILED DESCRIPTION OF THE
The present polishing abrasive grains, polishing agent and polishing method will be described in detail below, referring to preferable embodiments, Example and drawings, which are not intended to limit the present invention. First Embodiment 50
The present invention will be explained below according to a first embodiment, where a semiconductor device silicon substrate is polished with a polishing agent comprising cerium oxide polishing abrasive grains having a hydrophilic surface as metal oxide polishing abrasive grains. 55
FIG. 1 shows structural formulas in process of forming polishing abrasive grains for the polishing agent according to the first embodiment.
To form the polishing abrasive grains, tetraethoxycerium as a starting material is subjected to a water-addition poly- 60 condensation reaction in the presence of a specific catalyst to form a cerium (Ce) network through oxygen atoms and allow the cerium network to grow into a larger network while maintaining the same structure as normal crystal structure of cerium oxide and finally into abrasive grains of 65 cerium oxide. The resulting abrasive grains in this state still contain a large amount of ethoxy groups and can react with
remaining unreacted starting material, resulting in formation of grain size-uncontrollable gel. By subjecting the abrasive grains to distillation at about 100° C. while adding water thereto, the abrasive grains are terminated with hydroxyl groups at the extremities, whereby ultimate polishing abrasive grains having a hydrophilic surface can be obtained. By this treatment, the polishing abrasive grains can be terminated with hydroxyl groups typical of hydrophilic groups at the extremities to such a degree as to make the polishing abrasive grains hydrophilic. Sometimes, "—O—" bonds may be formed by dehydration of adjacent two hydroxyl groups, and such abrasive grains with "—O—" bonds are within the scope of the present invention, if such "—O—" bonds are in such a proportion as not to give any influence on the required hydrophilic property of the abrasive grain surfaces. This will be applicable to second and third embodiments of the present invention which follow. A polishing agent comprising such polishing abrasive grains having a hydrophilic surface can serve to polish substrate surfaces, typically, semiconductor device silicon substrate surfaces.
In the formation of the present polishing abrasive grains, the degree of polymerization (grain size of polishing abrasive grains) can be adjusted by controlling the wateraddition polycondensation reaction time. For example, the grain size of the polishing abrasive grains can be adjusted in a range of about 10 to about 10,000 nm.
Then, semiconductor device substrate surfaces are flattened by chemico-mechanical polishing (CMP), using the above-mentioned polishing agent, as shown in FIG. 2.
As shown in FIG. 2(a), a semiconductor device silicon substrate 1 is transferred from a substrate carrier 2 to a support plate 3, where the substrate 1 is supported on the support plate 3. Then, as shown in FIG. 2(b), the abovementioned polishing agent is supplied from a polishing agent feed means 4 onto polishing cloth 5. The semiconductor device silicon substrate 1 is pressed onto the polishing cloth mounted on a turning table 6 while supplying the polishing agent thereto to polish the surface of semiconductor device silicon substrate 1 with a pressing force for about 2 to about 5 minutes, thereby flattening the substrate surface.
Then, as shown in FIG. 2(c), the substrate carrier 2 encasing polished semiconductor device silicon substrates 1 is dipped in a cleaning tank 7 filled with a cleaning solution to clean the surfaces of polished semiconductor device substrates 1.
The present polishing agent used in the polishing step contains cerium oxide polishing abrasive grains having hydroxyl groups typical of the hydrophilic functional groups terminated on the extremities. Thus, the present polish agent has a good compatibility with an aqueous cleaning solution comprising pure water to be used in the cleaning step after the polishing. Accordingly, the cerium oxide polishing abrasive grains remaining on the surfaces of polished semiconductor device silicon substrates can be readily and assuredly removed from the surfaces with the aqueous cleaning solution. A polishing agent comprising cerium oxide polishing abrasive grains with no hydrophilic surface typical of conventional metal oxide polishing abrasive grains cannot be removed to a satisfactory degree from the polished semiconductor device silicon substrate surfaces in the cleaning step, because of material difference between the cerium oxide polishing abrasive grains and the semiconductor device silicon substrates. On the other hand, the present polishing agent having a distinguished hydrophilic property can have good compatibility with and cleaning susceptibility to the aqueous cleaning solution even if cerium oxide is used as a metal oxide for the polishing abrasive grains.