Fine particles of metal oxides have been used in various applications. For example, cerium oxide is used a material of a polishing slurry, a catalyst, an ultraviolet screening agent or the like, cobalt oxide as a material of a capacitor, a varistor, a secondary cell or the like, nickel oxide as a material of ferrite or the like, and titanium oxide as a material of a photocatalyst, a pigment or the like.
Particularly, fine particles of cerium oxide are rapidly spreading as an abrasive for precision polishing of semiconductor integrated circuits. The average size of such fine particles of cerium oxide used as an abrasive for precision polishing is generally in the range of several nanometers to several hundreds nanometers. Various methods have been proposed to obtain such fine particles of cerium oxide.
First, ammonium carbonate or ammonium hydrogencarbonate is added to a cerium salt solution such as an aqueous solution of cerium nitrate, to yield cerium carbonate precipitates. Then, the precipitates are washed, filtered, dried and heated to give cerium oxide. The heating temperature should be 400° C. or more to thermally decompose cerium carbonate. The size of the cerium oxide particles thus obtained is not significantly different from the size of the cerium carbonate particles. For example, when cerium carbonate in the form of a plate crystal aggregate having an average particle diameter of several tens nanometers is heated at 700° C., cerium oxide particles in the form of a plate crystal aggregate having an average particle diameter of several tens nanometers are obtained. Then, the resulting cerium oxide is dry-milled with a jet mill or wet-milled with a bead mill, to make it particles microparticulated in the range of several nanometers to several hundreds nanometers.
In this method, however, considerable labor is required for milling, and coarse cerium oxide particles may remain depending on the capability of the mill. When milling is continued for a long time, parts of the mill may be worn to increase the possibility of contamination of the resulting polishing slurry with abrasion powder. Coarse cerium oxide particles and abrasion powder are not preferable because they cause polishing scratches.
There is also a method of obtaining precipitates of cerium oxalate by adding oxalic acid to a cerium salt solution such as an aqueous solution of cerium nitrate. In this method, cerium oxalate is also heated to give cerium oxide which is then microparticulated by milling, so there is a possibility of generation of polishing scratches for the same reason as described above.
There is also a method which comprises optimizing the concentration and reaction temperature of an aqueous solution of cerium nitrate and an aqueous solution of ammonium hydrogen carbonate to form fine precipitates of cerium carbonate and heating the precipitates, whereby spherical cerium oxide having an average particle diameter of 50 nm or less is obtained without milling (see Japanese Patent Application Laid-Open No. 2004-107186). In this method, however, the precipitates are so fine as to easily retain ammonium, thus requiring long time for washing. In addition, the precipitates are so fine as to easily retain water as well, thus requiring long time for drying too. When the heating time is high, the cerium oxide particles are so fine that a part of them may be calcinated to form coarse cerium oxide particles.
There is also a method which comprises heating cerium carbonate in water to give fine precipitates of cerium monooxycarbonate, filtering the precipitates and drying them at 300° C. or more, followed by milling to give cerium oxide particles free of coarse particles (see Japanese Patent Application Laid-Open No. 2005-126253). In this method, however, the step of heating cerium carbonate in water is 2 to 48 hours, and the step of precipitating cerium monooxycarbonate is 5 to 96 hours, and thus the treatment requires a long time.