Abrasive tool making process – material – or composition – With inorganic material – Clay – silica – or silicate
Reexamination Certificate
2000-07-14
2001-06-19
Marcheschi, Michael (Department: 1755)
Abrasive tool making process, material, or composition
With inorganic material
Clay, silica, or silicate
C106S003000, C423S335000, C516S086000
Reexamination Certificate
active
06248144
ABSTRACT:
The present invention relates to a process for producing a polishing composition. More particularly, it relates to a process for producing a polishing composition suitable for planarization in production of semiconductor devices, especially for planarization of insulating interlayers.
Progress of so-called high-tech products including computers has been remarkable in recent years, and parts to be used for such products, such as ULSI, have been developed for integration and high speed, year after year. Along with such progress, the design rule for semiconductor devices has been progressively refined year after year, the depth of focus in a process for producing devices tends to be shallow, and planarization required for the pattern-forming surface tends to be increasingly severe. Further, to cope with an increase in resistance of the wiring due to refinement of the wiring, multilayer devices with shorter wiring have been employed. However, the irregularity in the surface level of a pattern formed has become problematic as an obstacle to multilayer devices.
Surface planarization for elimination of the irregularity is necessary in production of refined multilayer devices, and for planarization, spin-on-glass, resist-etch-batch and other planarization methods have been used.
These methods can achieve regional planarization but have difficulty in achieving global planarization required for advanced devices. Therefore, planarization is dominantly carried out by chemical mechanical polishing (hereinafter referred to as CMP), which is the combination of mechanical polishing and chemical polishing.
For CMP, a suspension (a polishing compositions, generally called a slurry) containing abrasive grains and some chemical components in water is usually used, and as the abrasive grains, alumina, ceria, silica or the like is usually used. Among them, alumina and ceria are likely to precipitate in polishing compositions due to their large specific gravities and therefore have a problem in dispersibility. They are also likely to form aggregates in polishing compositions and have a drawback that they often leave many polishing defects (hereinafter referred to as scratches) on the surface of a polished object.
Silica is classified as colloidal silica, fumed silica and others according to its production process. Colloidal silica is generally produced by growing particles of ultra-fine colloidal silica obtained by ion exchange of sodium silicate or by acid or alkali hydrolysis of an alkoxysilane. Colloidal silica produced by these wet methods is usually available as a slurry containing primary particles or secondary particles dispersed in water. Therefore, colloidal silica shows excellent dispersibility as abrasive grains in polishing compositions but imposes a limit on the mechanical stock removal rate. On the other hand, fumed silica is obtained by burning silicon tetrachloride, hydrogen and oxygen. Fumed silica obtained by the vapor phase method is in the form of secondary particles having a three-dimensional chain structure made of several to dozens of concatenated primary particles and is characterized by a relatively low metal impurity content. When fumed silica is dispersed in water in the process for producing a polishing composition, it does not completely disintegrate into individual primary particles and therefore can realize an excellent stock removal rate in mechanical polishing.
Some polishing compositions using fumed silica as mentioned above have been proposed, for example, by the inventions disclosed in JP-A-3-60420 and JP-A-3-50112.
The aqueous colloidal dispersion of fumed silica disclosed in JP-A-3-60420 (hereinafter referred to as prior example 1) is an aqueous colloid dispersion containing fumed silica, an acid and a stabilizer and contains at least about 40 wt % of fumed silica dispersed in water, from 0.025 to 0.05 wt %, based on the fumed silica, of an acid and such an amount of a stabilizer as to raise the pH of the dispersion to about 7.0-12.0.
The aqueous colloidal dispersion of fumed silica disclosed in JP-A-3-50112 (hereinafter referred to as prior example 2) is a stable and non-dilatant aqueous colloid dispersion containing no stabilizer and about 35 wt % of fumed silica dispersed in water.
Both prior art examples 1 and 2 are aimed at providing low-viscosity and non-dilatant polishing compositions with excellent dispersity which are stable enough not to gel in at least 1 day.
The present inventors tried preparing polishing compositions using fumed silica, by following prior examples 1 and 2 mentioned above, under the conditions recommended therein.
However, in any cases, fumed silica aggregated in the course of their production, and the resulting compositions turned into gels in extreme cases and were unsuitable for production of semiconductor devices. They tried again under conditions which suppress aggregation and gelation, namely, at the minimum silica concentrations in prior examples 1 and 2, but the polishing compositions obtained underwent agglomeration of fumed silica or gelation immediately after their preparation and were found to be unfavorable upon polishing due to formation of many scratches and instability in the stock removal rate.
The present inventors speculate on the reason for the instability as follows.
Because fumed silica is usually in the form of secondary particles having a chain structure with relatively hydrophobic surfaces, it can not accept water immediatily. However, if it is kept in contact with water, hydroxyl groups form on the surfaces and turn the surfaces hydrophilic. However, after the adjustment to water, the hydroxyl groups thus formed join (presumably by hydrogen bonding or reductive dehydration), and gelation eventually occurs. Therefore, polishing compositions prepared by following prior examples 1 and 2 mentioned above certainly can keep stable dispersity for a short time but can hardly keep it for a long time. In short, in the inventions as prior examples 1 and 2 mentioned above, due to the lack of a step for stabilizing the surfaces of fumed silica as abrasive grains under acidic conditions, the instable state of the surfaces presumably promotes the gelation of polishing compositions.
The present invention has been made to solve the above-mentioned problems and aims at providing a process for polishing compositions with excellent polishing performance which keep fumed silica as abrasive grains dispersed for a long time.
Under the above-mentioned circumstances, the present inventors conducted extensive research and have found out the necessity of a technique which creates surface conditions which suppress aggregation of fumed silica or gelation of polishing compositions for long time stable dispersion of fumed silica, and established the technique.
Namely, the present invention provides a process for producing a polishing composition, which comprises adding fumed silica to water preliminarily adjusted to pH 2-4 to a concentration of 40 to 60 wt % under high shearing force, adjusting the viscosity to 2-10000 cps with water, stirring the mixture under low shearing force for at least 5 minutes, adding water to a fumed silica concentration of 10-38 wt %, and adding a basic substance under vigorous stirring to a pH of 9-12.
The process for producing a polishing composition of the present invention is preferably characterized in that the polishing composition is used for planarization of the surface of semiconductor devices.
Further, the process for producing a polishing composition of the present invention is preferably characterized in that hydrochloric acid is used as an acidic substance for the pH adjustment to 2-4.
The process for producing a polishing composition of the present invention is preferably characterized also in that the fumed silica has a specific surface area of from 70 to 110 m
2
/g and a bulk density of at least 70 g/L and is dispersed in the water.
The process for producing a polishing composition of the present invention is preferably characterized further in that potassium hydroxide or ammonium hydroxide is
Ina Katsuyoshi
Tamai Kazusei
Fujimi Incorporated
Marcheschi Michael
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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