Abrasive tool making process – material – or composition – With inorganic material – Metal or metal oxide
Reexamination Certificate
2000-05-26
2002-11-12
Marcheschi, Michael (Department: 1755)
Abrasive tool making process, material, or composition
With inorganic material
Metal or metal oxide
C106S003000, C438S692000, C438S693000, C451S036000
Reexamination Certificate
active
06478836
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a cerium oxide slurry for polishing, and more particularly, to a cerium oxide slurry for polishing a glass article such as a photomask or a lens, or an insulating film during a step in the manufacture of a semiconductor device. The cerium oxide slurry of the present invention provides a high polishing rate with a finished surface having very few defects. The present invention also relates to a process for preparing the cerium oxide slurry and a process for polishing with the slurry.
BACKGROUND OF THE INVENTION
In the field of manufacturing semiconductor devices, polishing technique has been proposed and earnestly studied in order to address a variety of issues such as demand for attaining a focus depth in a photolithography step, which is required along with enhancement of the degree of integration of a semiconductor device and increase in the number of layers in a multi-layer device.
Application of a polishing technique to manufacture of a semiconductor device has been developed furthest in relation to a step of planarizing an insulating film. In this polishing, an alkaline suspension obtained through dispersion of fumed silica in water is predominantly used as an abrasive.
Meanwhile, a cerium oxide abrasive has been practically used for polishing a glass product such as a photomask or a lens, and application of the cerium oxide abrasive is proposed for planarizing of an insulating film made of a silicon-dioxide-based material which is substantially equivalent to glass.
Japanese unexamined Patent Publication (kokai) No. 5-326469 discloses a technique for polishing an insulating film by use of an abrasive composition containing cerium oxide. It also describes that the technique has enabled planarization of steps based on the configuration of polysilicon or other wiring or interconnection, and a maximum size of cerium oxide particles is preferably 4 &mgr;m or less, in consideration of minimizing generation of flaws.
Japanese Unexamined Patent Publication (kokai) No. 6-216096 discloses that use of high-purity cerium oxide containing a trace element other than Ce and O in an amount of 100 ppm or less is advantageous for preventing contamination of a wafer.
Japanese Patent No. 2592401 discloses polishing of an insulating film with abrasive grains comprising, in predetermined amounts, cerium oxide “OPALINE” having a particle size of 300-500 nm, fumed silica, and precipitated silica, to thereby provide excellent surface flatness.
Japanese Unexamined Patent Publication (Kohyo) No. 8-501768 discloses that sub-micron cerium oxide particles are obtained through a process comprising two steps: (a) forming an aqueous solution comprising a water-soluble trivalent cerium salt and an oxidizing agent and (b) aging the solution for four hours or longer, the solution being maintained in a liquid state.
Japanese Unexamined Patent Publication (kokai) No. 8-153696 discloses that an organic or inorganic insulating film is polished with cerium oxide particles having a crystalite size of 30 nm or less or 60 nm or more while the pH of a abrasive solution is controlled.
Japanese Unexamined Patent Publication (kokai) No. 9-82667 discloses an abrasive composition comprising a plurality of cerium oxide particle grains having average crystallite sizes that differ from one another.
Japanese Unexamined Patent Publication (kokai) No. 8-134435 discloses that an abrasive used in a manufacturing step of a semiconductor device, which comprises cerium oxide having an average primary-particle size of 0.1 &mgr;m or less as measured under an SEM (scanning electron microscope).
Japanese Patent No. 2746861 discloses a process for producing ultramicro particles of cerium oxide single crystal having a particle size of 10-80 nm, which ultramicro particles can be used in manufacture of a semiconductor device.
Japanese Unexamined Patent Publication (kokai) No. 8-3541 discloses an abrasive composition for precise polishing which comprises an alkaline ceric oxide sol containing an organic acid having two or more carboxyl groups. The average particle size, as measured through a dynamic light scattering method, must fall within the range of 2 nm to 200 nm.
Japanese unexamined Patent Publication (kokai) No. 8-81218 discloses an aqueous dispersion of ceric oxide particles having an average particle size, as measured by use of an apparatus for measuring particle size distribution based on centrifugal sedimentation, of 0.03-5 &mgr;m, as well as a process for producing the dispersion, The dispersion is also applicable to manufacture of a semiconductor device.
An article in “
Denshi Zairyo
(Electronic Material)” 1997, May, p. 113 and on discloses basic polishing performance of cerium oxide having an average particle size of 0.5 &mgr;m, as measured through a laser diffraction method.
As described above, an abrasive comprising cerium oxide finding a possible use in planarizing an insulating film has been studied extensively. However, practical use thereof in this field has not yet been attained. This is due to difficulty in simultaneously achieving minimization of defects on a finished surface and a high polishing rate of an insulating film typically formed of a silicon dioxide film.
Production processes for an abrasive comprising cerium oxide for polishing an insulating film in manufacture of a semiconductor device that have been studied are roughly divided into two types. One is a firing process which comprises firing a cerium compound such as cerium carbonate or cerium oxalate so as to produce cerium oxide, and typically crushing the resultant cerium oxide to make the particle size appropriate for use as an abrasive. The other is a wet synthesis process which comprises mixing an aqueous solution of a water-soluble cerium compound such as cerium nitrate and an aqueous alkaline solution such as aqueous ammonia to thereby produce a gelatinous slurry containing cerium hydroxide and typically aging the resultant slurry at 80-300° C.
The thus-produced conventional cerium oxide slurry has a conductivity in the deaerated state (in this specification a conductivity in the deaerated state is referred to simply as “conductivity”) of 400 &mgr;S/cm or more (the letter “S” stands for the unit “Siemens”), typically 600 &mgr;S/cm or more, when the slurry has a concentration of 10 wt. %. Since the correlation between the conductivity of a slurry and the polishing rate is not recognized, it is difficult to simultaneously achieve minimization of defects on a finished surface and a high polishing rate of an insulating film typically formed of a silicon dioxide film, as described above.
The conductivity of a slurry increases with the concentration of an ionic substance contained in the slurry, and therefore, conductivity serves as an index for the concentration of the ionic substance. A conventional cerium oxide slurry contains ionic impurities originating from a cerium oxide source, and ionic impurities are by-produced during the wet synthesis process. Such impurities elevate the conductivity of a slurry. In addition, a variety of ionic substances, such as a dispersant and a pH-adjusting agent, are typically added to a slurry for polishing, and these additives further elevate the conductivity of the slurry.
In view of the foregoing, an object of the present invention is to provide a cerium oxide slurry for polishing which simultaneously achieves a high polishing rate and minimization of defects on a finished surface after polishing to a level allowing practical use.
DISCLOSURE OF THE INVENTION
The present inventors have conducted earnest studies in order to solve the above-described problems, and have found that the problems can be solved through a decrease in the concentration of an ionic substance contained in a slurry; i.e., lowering of the conductivity of the slurry.
Accordingly, the present invention includes the following:
(1) A cerium oxide slurry for polishing, which slurry comprises cerium oxide dispersed in water, wherein the slurry has a conductivity of about 30 c &mgr;S/cm or less when the
Ichikawa Kagetaka
Kido Takanori
Sanbayashi Masayuki
Tsujino Fumio
Marcheschi Michael
Showa Denko K.K.
Sughrue & Mion, PLLC
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