Abrasive tool making process – material – or composition – With inorganic material – Metal or metal oxide
Reexamination Certificate
1999-08-10
2001-04-24
Marcheschi, Michael (Department: 1755)
Abrasive tool making process, material, or composition
With inorganic material
Metal or metal oxide
C051S307000, C106S003000
Reexamination Certificate
active
06221118
ABSTRACT:
TECHNICAL FIELD
This invention relates to a cerium oxide abrasive and a method of polishing substrates.
BACKGROUND ART
In semiconductor device fabrication processes, colloidal silica type abrasives have commonly been studied as chemomechanical abrasives for smoothing inorganic insulating film layers such as SiO
2
insulating films formed by a process such as plasma-assisted CVD (chemical vapor deposition) or low-pressure CVD. Colloidal silica type abrasives are produced by growing silica particles into grains by, e.g., thermal decomposition of silicon tetrachloride, followed by pH-adjustment with an alkali solution containing non alkali metal, such as aqueous ammonia. Such abrasives, however, have a technical problem of a low polishing rate which prevents them being put into practical use, because the inorganic insulating films can not be polished at a sufficiently high polishing rate.
Meanwhile, cerium oxide abrasives are used in glass-surface polishing for photomasks. The cerium oxide abrasives are useful for finish mirror polishing because they have a hardness lower than silica particles and alumina particles and hence the polishing surface is very difficult to scratch. Also, cerium oxide has a chemically active nature and is known as a strong oxidizing agent. Making the most of this advantage, it is useful to apply the cerium oxide in chemomechanical abrasives for insulating films. However, if the cerium oxide abrasives used in glass-surface polishing for photomasks are used in the polishing of inorganic insulating films as they are, they have so large a primary particle diameter that the insulating film surface may come to have polish scratches which are visually perceivable.
DISCLOSURE OF THE INVENTION
The present invention provides a cerium oxide abrasive that can polish the surfaces of objects such as SiO
2
insulating films without causing scratches and at a high rate, and also provides a method of polishing substrates.
The cerium oxide abrasive of the present invention comprises a slurry comprising cerium oxide particles whose primary particles have a median diameter of from 30 nm to 250 nm and slurry particles have a median diameter of from 150 nm to 600 nm; the cerium oxide particles being dispersed in a medium.
The cerium oxide abrasive of the present invention may also comprise a slurry made up of cerium oxide particles whose primary particles have a median diameter of from 100 nm to 250 nm and slurry particles have a median diameter of from 150 nm to 350 nm; the cerium oxide particles being dispersed in a medium.
In the above cerium oxide particles, the primary particles may preferably have a maximum diameter of 600 nm or smaller and a primary-particle diameter of from 10 nm to 600 nm.
The cerium oxide abrasive of the present invention may still further comprise a slurry made up of cerium oxide particles whose primary particles have a median diameter of from 30 nm to 70 nm and slurry particles have a median diameter of from 250 nm to 600 nm; the cerium oxide particles being dispersed in a medium.
The above cerium oxide particles may preferably have a primary-particle diameter of from 10 nm to 100 nm.
In the cerium oxide abrasive of the present invention, the cerium oxide particles may preferably have a maximum particle diameter of 3,000 nm or smaller.
Water may be used as the medium, and at least one dispersant selected from a water-soluble organic high polymer, a water-soluble anionic surface-active agent, a water-soluble nonionic surface-active agent and a water-soluble amine may be used, of which ammonium polyacrylate is preferred.
As the cerium oxide particles, cerium oxide obtained by firing cerium carbonate may preferably be used.
The cerium oxide abrasive of the present invention can polish a given substrate for semiconductor chips or the like, on which silica films have been formed.
BEST MODES FOR PRACTICING THE INVENTION
The cerium oxide is commonly obtained by firing a cerium compound such as cerium carbonate, cerium sulfate or cerium oxalate. SiO
2
insulating films formed by TEOS-CVD, for example, can be polished at a higher rate as abrasives have a larger primary-particle diameter and a lower crystal strain, i.e., have better crystallinity, but tend to be prone to polish scratches. Accordingly, the cerium oxide particles used in the present invention are produced without making their crystallinity so high. Also, since the abrasive may be used to polish semiconductor chips, its content of alkali metals and halogens may preferably be controlled to be 1 ppm or less.
The abrasive of the present invention has a high purity, and does not contain more than 1 ppm of Na, K, Si, Mg, Ca, Zr, Ti, Ni, Cr and Fe each and more than 10 ppm of Al.
In the present invention, firing may be employed as a process for producing the cerium oxide particles. In particular, low-temperature firing is preferred, which can make the crystallinity as low as possible in order to produce particles that do not cause polish scratches. Since the cerium compounds have an oxidation temperature of 300° C., they may preferably be fired at a temperature of from 600° C. to 900° C.
The cerium carbonate may preferably be fired at a temperature of from 600° C. to 900° C. for 5 to 300 minutes in an oxidative atmosphere of oxygen gas or the like.
The cerium oxide obtained by firing may be pulverized by dry-process pulverization such as jet milling or by wet-process pulverization such as bead milling. The jet milling is described in, e.g., KAGAKU KOGYO RONBUNSHU (Chemical Industry Papers), Vol. 6, No. 5 (1980), pages 527-532. Cerium oxide obtained by firing was pulverized by dry-process pulverization such as jet milling, whereupon a pulverization residue was seen to occur.
The slurry of cerium oxide in the present invention is obtained by dispersion-treating an aqueous solution containing cerium oxide particles produced in the manner described above or a composition comprising cerium oxide particles collected from this aqueous solution, water and optionally a dispersant. Here, the cerium oxide particles may preferably be used in a concentration ranging, but not particularly limited to, from 0.1 to 10% by weight in view of readiness to handle suspensions. As the dispersing agent, it may include, as those containing no metal ions, water-soluble organic high polymers such as acrylic polymers and ammonium salts thereof, methacrylic polymers and ammonium salts thereof, and polyvinyl alcohol, water-soluble anionic surface-active agents such as ammonium lauryl sulfate and ammonium polyoxyethylene lauryl ether sulfate, water-soluble nonionic surface-active agents such as polyoxyethylene lauryl ether and polyethylene glycol monostearate, and water-soluble amines such as monoethanolamine and diethanolamine.
Ammonium polyacrylate, in particular, ammonium polyacrylate having weight-average molecular weight of from 5,000 to 20,000 is preferred. Any of these dispersing agents may preferably be added in an amount ranging from
0
.
01
part by weight to 5 parts by weight based on 100 parts by weight of the cerium oxide particles in view of the dispersibility and anti-sedimentation properties of particles in the slurry. In order to improve its dispersion effect, the dispersing agent may preferably be put in a dispersion machine simultaneously with the particles at the time of dispersion treatment.
These cerium oxide particles may be dispersed in water by dispersion treatment using a conventional agitator, and besides by using a homogenizer, an ultrasonic dispersion machine or a ball mill. Particularly for dispersing the cerium oxide particles as fine particles of 1 &mgr;m or smaller, it is preferable to use wet-process dispersion machines such as a ball mill, a vibration ball mill, a satellite ball mill and a media agitating mill. In a case where the slurry should be made more highly alkaline, an alkaline substance containing no metal ions, such as aqueous ammonia, may be added during the dispersion treatment or after the treatment.
The cerium oxide abrasive of the present invention may be used as it is in the fo
Ashizawa Toranosuke
Kurata Yasushi
Matsuzawa Jun
Ootuki Yuuto
Tanno Kiyohito
Hitachi Chemical Company Ltd.
Marcheschi Michael
Pennie & Edmonds LLP
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