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  Polishing composition and polishing methodUSPTO Application #: 20070021040Title: Polishing composition and polishing method Abstract: A first polishing composition includes abrasive grains and an iodine compound and has a pH of 6 or more. The first polishing composition can suitably polish the Si [0001] plane of a single crystal silicon carbide substrate. A second polishing composition includes an iodine compound and has a pH of 8 or less. The second polishing composition can suitably polish the C [000-1] plane of a single crystal silicon carbide substrate. A third polishing composition includes abrasive grains and an iodine compound and has a pH of 6 to 8, inclusive. The third polishing composition can suitably polish each of the Si [0001] and C [000-1] planes of a single crystal silicon carbide substrate. (end of abstract) Agent: Vidas, Arrett & Steinkraus, P.A. - Minnetonka, MN, US Inventors: Kenji Kawata, Kazutoshi Hotta USPTO Applicaton #: 20070021040 - Class: 451041000 (USPTO) Related Patent Categories: Abrading, Abrading Process, Glass Or Stone Abrading The Patent Description & Claims data below is from USPTO Patent Application 20070021040. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] The present invention relates to a polishing composition for use in polishing an object formed of single crystal semiconductor, in particular an object formed of single crystal silicon carbide, such as a substrate of single crystal hexagonal silicon carbide, and the polishing method using the polishing composition. [0002] A substrate of single crystal silicon carbide, such as a single crystal 4H--SiC or 6H--SiC substrate is normally polished by a preliminary step for polishing the substrate surface with a slurry containing abrasive grains of diamond, and finishing step for polishing the preliminarily polished surface. The finishing step removes an amorphous polishing-modified layer developing on the substrate surface in the preliminary step and, at the same time, flattens the exposed surface into a crystal plane as a result of removal of the polishing-modified layer. A polishing composition containing colloidal silica and used at a pH of 7 to 10, disclosed by Japanese Laid-Open Patent Publication No. 2004-299018, is one of the known polishing compositions for the finishing polishing step. This composition, however, involves problems resulting from needing a very long polishing time, because of its poor capability of polishing crystal planes. [0003] A substrate of single crystal 4H--SiC or 6H--SiC as single crystal hexagonal silicon carbide has two planes of different orientation, Si [0001] and C [000-1] planes. Oxidation and etching proceed at a lower rate on the Si [0001] plane of single crystal 4H--SiC or 6H--SiC substrate than on other planes, e.g., C [000-1] plane. Therefore, it is difficult for a conventional method to polish the single crystal Si [0001] plane at a high removal rate. Moreover, the polishing composition disclosed by the above patent publication also involves problems of easily causing surface defects, e.g., pit, when applied to the C [000-1] plane of single crystal 4H--SiC or 6H--SiC substrate. [0004] Still more, a single crystal hexagonal silicon carbide substrate must be polished separately for the Si [0001] and C [000-1] planes, which have very different chemical and mechanical properties, with two different polishing compositions, each suitable for each plane. However, there are demands for polishing compositions which can singularly polish these planes to improve work efficiency. SUMMARY OF THE INVENTION [0005] A first object of the present invention is to provide a polishing composition which can suitably polish the Si [0001] plane, and a polishing method using the composition. A second object of the present invention is to provide a polishing composition which can suitably polish the C [000-1] plane, and a polishing method using the composition. A third object of the present invention is to provide a polishing composition which can suitably polish each of the Si [0001] and C [000-1] planes singularly, and a polishing method using the composition. [0006] To achieve the foregoing objectives and in accordance with a first aspect of the present invention, a polishing composition including abrasive grains and an iodine compound and having a pH of 6 or more is provided. [0007] In accordance with a second aspect of the present invention, a polishing composition including an iodine compound and having a pH of 8 or less is provided. [0008] In accordance with a third aspect of the present invention, a polishing composition including abrasive grains and an iodine compound and having a pH of 6 to 8, inclusive is provided. [0009] In accordance with a fifth aspect of the present invention, a method for polishing an object formed of single crystal silicon carbide is provided. The method includes: preparing any one of the polishing composition; and polishing the object using the prepared polishing composition. [0010] Other aspects and advantages of the invention will become apparent from the following description, illustrating by way of example the principles of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0011] The first and second embodiments of the present invention will be described below. [0012] The polishing composition of the first embodiment is produced by incorporating colloidal silica with a given quantity of iodate or periodate, and, as required, diluting the resulting mixture with water. It is therefore essentially composed of colloidal silica working as abrasive grains, iodate or periodate as an iodine compound and water. [0013] The polishing composition of the second embodiment is produced by incorporating colloidal silica with a given quantity of iodic or periodic acid and an alkali, and, as required, diluting the resulting mixture with water. It is therefore essentially composed of colloidal silica working as abrasive grains, an iodic or periodic acid as an iodine compound, alkali as a pH adjuster and water. In other words, the polishing composition of the second embodiment differs from that of the first embodiment in that it contains an iodic or periodic acid as an iodine compound in place of iodate or periodate, and an alkali. [0014] The polishing compositions of the first and second embodiments are used for polishing a single crystal 4H--SiC or 6H--SiC substrate, more specifically the Si [0001] plane of a single crystal 4H--SiC or 6H--SiC substrate. [0015] Each of the polishing compositions of the first and second embodiments may fail to exhibit a sufficient polishing capability when it contains colloidal silica at below 0.05% by mass, or more specifically below 0.1% by mass, or even more specifically below 1% by mass. Therefore, it preferably contains colloidal silica at 0.05% by mass or more, more preferably 0.1% by mass or more, most preferably 1% by mass or more, in order to achieve a higher removal rate. On the other hand, it is uneconomical when it contains colloidal silica at above 50% by mass, or more specifically 48% by mass, or even more specifically 45% by mass, because colloidal silica with a higher content than commercial colloidal silica is needed for production of the polishing composition. Therefore, each of the polishing compositions of the first and second embodiments contains colloidal silica preferably at 50% by mass or less, more preferably 48% by mass or less, most preferably 45% by mass or less. [0016] Colloidal silica having an average primary particle size below 5 nm, or more specifically 15 nm, or even more specifically more below 25 nm does not have a sufficient polishing capability for the Si [0001] plane. Therefore, colloidal silica for each of the polishing compositions of the first and second embodiments preferably has an average primary particle size of 5 nm or more, more preferably 15 nm or more, most preferably 25 nm or more, for the composition to achieve a higher removal rate. On the other hand, colloidal silica having an average primary particle size above 120 nm, or more specifically above 100 nm, or even more specifically above 85 nm has to be incorporated at a fairly high content in each of the polishing compositions of the first and second embodiments, for the composition to exhibit a sufficient polishing capability. Therefore, it preferably has an average primary particle size of 120 nm or less, more preferably 100 nm or less, most preferably 85 nm or less, in order to reduce cost of the polishing composition. The average primary particle size of colloidal silica may be determined from the relative surface area measured by, e.g., the BET method. [0017] The iodine compound incorporated in the polishing composition of the first embodiment to improve its removal rate may be an iodate or periodate, preferably the latter, more preferably sodium metaperiodate (NaIO.sub.4). A periodate is preferable to an iodate, because of its higher redox potential and higher oxidizing power. Sodium metaperiodate is preferable to any other periodate, because of its wider availability. [0018] The iodine compound incorporated in the polishing composition of the second embodiment to improve its removal rate may be an iodic or periodic acid, preferably the latter, more preferably orthoperiodic acid (H.sub.5IO.sub.6). A periodic acid is preferable to an iodic acid, because of its higher redox potential and higher oxidizing power. Orthoperiodic acid is preferable to any other periodic acid, because of its wider availability. [0019] Each of the polishing compositions of the first and second embodiments may fail to polish the Si [0001] plane at sufficiently high removal rate when it contains an iodine compound at below 0.1 g/L, or more specifically below 0.5 g/L, or even more specifically below 1 g/L. Therefore, it preferably contains an iodine compound at 0.1 g/L or more, more preferably 0.5 g/L or more, most preferably 1 g/L or more, in order to achieve a higher removal rate. On the other hand, when each of the polishing compositions of the first and second embodiments contains an iodine compound at above 500 g/L, or more specifically above 250 g/L, or even more specifically above 100 g/L, it may deteriorate a polishing pad faster. Moreover, a precipitate may be formed in the polishing composition when the iodine compound is an iodate or periodate. In order to avoid these problems, each of the polishing compositions of the first and second embodiments contains an iodine compound preferably at 500 g/L or less, more preferably 250 g/L or less, most preferably 100 g/L or less. [0020] An alkali to be incorporated in the polishing composition of the second embodiment is preferably a lithium compound, e.g., lithium hydroxide (LiOH) or an inorganic lithium salt, or ammonia (NH.sub.3), more preferably lithium hydroxide or ammonia to make the composition have a good storage stability. Specifically, the inorganic lithium salt includes chloride, lithium carbonate, lithium nitrate, lithium sulfate and lithium phosphate, of which lithium chloride and lithium carbonate are more available. [0021] When the polishing composition of the second embodiment contains an alkali at below 0.1 g/L, or more specifically below 0.5 g/L, or even more specifically below 1 g/L, it may have deteriorated polishing capability for the Si [0001] plane, because of insufficient pH level of the composition. Therefore, it preferably contains an alkali at 0.1 g/L or more, more preferably 0.5 g/L or more, most preferably 1 g/L or more, in order to achieve a higher removal rate. On the other hand, when it contains an alkali at above 20 g/L, or more specifically above 15 g/L, or even more specifically above 10 g/L, it may have deteriorated storage stability. Therefore, it contains an alkali preferably at 20 g/L or less, more preferably 15 g/L or less, most preferably 10 g/L or less, in order to have good storage stability. Continue reading... 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