Nodular Silica Sol and Method of Producing the Same

Abstract

A novel nodular silica sol adapted to use as a polishing material for polishing, for instance, CMP.The nodular silica sol has a ratio of an average particle diameter (r) measured by the dynamic light scattering method versus a particle diameter (r′) converted to that of an equivalent sphere computed from an average specific surface area measured by means of the nitrogen absorption method (r / r′, referred to as “association ratio”) in a range from 1.2 to 10, the particle diameter (r′) in a range from 5 to 200 nm, and the specific surface area in a range from 13 to 550 m2 / g. The nodular silica particles have heterogeneous forms, and contents of Ca and Mg contained in the nodular silica particles are below 1000 ppm against SiO2 respectively.

Description

FIELD OF THE INVENTION[0001]The present invention relates to a method of producing a nodular silica sol which is a preferable polishing material.BACKGROUND OF THE INVENTION[0002]In production of substrates each with a semiconductor integrated circuit thereon, irregularities or steps are generated when a circuit is formed on a silicon wafer with metal such as copper, and therefore metal portions of the circuit are preferentially removed by polishing to remove the irregularities or steps in a surface. Furthermore, irregularities are generated when aluminum wiring is provided on a silicon wafer and an oxide film of silica or the like as an insulating film is provided, and the irregularities are averaged by polishing the oxide film. In the process of polishing a substrate as described above, it is generally required that the polished surface does not include any step nor irregularity and is flat and smooth without any micro blemish or the like, and that the polishing speed is high.[0003...

AI Technical Summary

Benefits of technology

[0026]An object of the present invention is to enable production of a nodular silica sol with high purity extremely little containing impurities such as Na, K, Cl, Cu, Ni, and Fe by combining a step of producing the nodular silica sol with a step of improving purity of the silica sol.

[0044]With the production method according to the present invention, now it is possible to easily obtain a nodular silica sol of particles having heterogeneous forms. When the nodular silica sol as described above is used as a polishing material, an excellent polishing performance is provided. Furthermore, with another production method according to the present invention, it is possible to obtain a highly purified nodular silica sol of particles having heterogeneous forms containing only an extremely small amount of impurities.

Problems solved by technology

Furthermore, irregularities are generated when aluminum wiring is provided on a silicon wafer and an oxide film of silica or the like as an insulating film is provided, and the irregularities are averaged by polishing the oxide film.

Recently, in association with the needs for size reduction and higher performance of electric and electronic products, a degree of integrated in semiconductor devices has been becoming increasingly higher, but, for instance, when there remain impurities such as Na or K in a separating layer of a transistor, the desired performance may not be realized or some failures may occur.

Especially, when Na is deposited on a surface of a polished semiconductor substrate or of an oxide film, Na having high dispersibility may be trapped in defects of the oxide film or the like.

In this case, even when a circuit is formed on the semiconductor substrate, sometimes an insulation failure or a short circuit may occur, and furthermore the dielectric constant may drop.

With the spherical particles as described above, however, when a portion above the common plane is polished, the metal for a circuit within a wiring trench under concave sections may disadvantageously be polished down to a level under the common plane, and the problem is generally referred to as dishing.

When the dishing (excessive polishing) as described above occurs, there occur such problems as that a thickness of the wiring diminishes and the wiring resistance becomes larger, or that the flatness of an insulating film formed on the polished surface becomes lower, so that there is the need for suppressing the dishing.

In the methods described above, there are the several problems, for instance, that stability of the obtained aqueous silica is not sufficient, or that an alkoxy group (OR group) remains, the density of obtained particles is low, and therefore a sufficient polishing speed can not be obtained when the silica sol is used as a polishing material.

However, since many types of metals such as Al, Ti, Fe, Mg, and Ca are contained as impurities in the glass sand, alkali metals (represented by Na) remain much in the obtained aqueous solution in proportion to an amount of the metal ions, so that the aqueous solution is not suited for use as a polishing material for a semiconductor substrate.

Such metals as Al and Na can be reduced to some extend by processing the aqueous solution with an acid or a chelating agent, but the purity is not sufficient as a polishing material for a semiconductor substrate.

Also high purity silica powder is used in place of the glass sand, but the high purity silica is expensive, and it take disadvantageously long time in solving the high purity silica with an alkali.

In this method, impurities are removed (leaching) by adding a strong acid in the stage of active silicic acid, but there is a problem concerning stability of the active silicic acid after leaching, which easily causes gelatination of the sol.

Also in this method, there are the same problems like those recognized in the invention disclosed in the Japanese Patent Laid-Open Publication No.

Namely in this invention, ion exchange is performed through multiple steps, so that the production efficiency is disadvantageously low.

In addition, the chelate type, ion exchange resin has strong selectivity to tervalent ions, and it is difficult to reuse the resin repeatedly, and when it is tried to reuse the chelate type ion exchange resin, a large amount of reclaiming agent is disadvantageously required.

When a chelating agent is used to capture metal ions, it is necessary to repeat cleaning with a ultrafiltration film for removing the remaining chelating agent and the chelating agent having capture metal ions, so that the production efficiency is rather low, and even when cleaning is repeated, it is difficult to completely remove the chelating agent, and the remaining chelating agent may give negative effects to the polishing performance.