Abrasive tool making process – material – or composition – With inorganic material – Metal or metal oxide
Reexamination Certificate
2000-04-21
2001-05-29
Marcantoni, Paul (Department: 1755)
Abrasive tool making process, material, or composition
With inorganic material
Metal or metal oxide
C501S152000
Reexamination Certificate
active
06238450
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to ceria powders and particularly to ceria powders of the type used to fine and polish optical and semiconductor surfaces.
It is well known that in order to produce a satisfactory optical surface, it is necessary that the surface be free of scratches and have as low an R
a
as possible. This R
a
measurement is the average distance between the highest and lowest points on the surface perpendicular to the plane of the glass sheet being polished. Thus, accepting that the surface will not be totally flat at the submicron scale, it is a measure of the variation between highest and lowest points. Clearly the lower the figure the better for optical clarity and freedom from distortion.
In polishing processes a slurry of abrasive particles in a liquid medium, (usually water-based), is placed in contact with the surface to be polished and a pad is caused to move across the surface in predetermined patterns so as to cause the abrasive in the slurry to polish the surface. The present invention relates to ceria particles useful in such processes.
Various slurry formulations have been proposed in the art. U.S. Pat. No. 4,576,612 produces its slurry in situ in controlled amounts by providing a pad with a surface layer comprising the abrasive particles in a resin which gradually dissolves during use to liberate the polishing particles. The particles declared to be useful include cerium oxide (“ceria”), zirconium oxide (“zirconia”) and iron oxide.
EP 608 730-A1 describes an abrasive slurry for polishing a surface in an optical element which comprises an abrasive selected from alumina, glass, diamond dust, carborundum, tungsten carbide, silicon carbide or boron nitride with particle sizes up to one micron.
U.S. Pat. No. 5,693,239 describes an aqueous slurry for polishing and planarizing a metallic workpiece which comprises submicron particles of alpha alumina together with other softer form of alumina or amorphous silica.
U.S. Pat. No. 5,804,513 teaches a planarization slurry comprising ceria and alumina or silica, where the particle sizes are 2 microns or less and the ceria component is from 5 to 40% of the total weight.
U.S. Pat. No. 4,106,915 teaches a method of polishing glass using abrasive particles dispersed in a cured, unsaturated polyester resin wherein the abrasive can be ceria.
U.S. Pat. No. 4,601,755 teaches ceria-based glass polishing compositions incorporating a rare earth pyrosilicate.
A considerable amount of art also exists in the related field of slurry formulations for chemical mechanical planarization, (“CMP”), or polishing of semiconductor substrates and again, these commonly employ the same abrasives with variations in components of the dispersion medium.
The use of ceria in such applications is therefore well known and widely practiced. The ceria is commercially obtainable in reasonably small particle sizes of a little over a micron but there is always a drive to ever more perfect surfaces and this propels a move to finer ceria particle sizes still. There is however an associated problem with this move in that the ceria tends to be sensitive to agglomeration and such agglomerates can cause scratching of the surface being polished. Thus commercially available ceria tends to a reach a limiting value of around 10 A for surface roughness, (R
a
), when used in a slurry form to polish glass surfaces.
The present invention is based on the discovery of a form of ceria with unique characteristics that leads it to be highly successful in resisting agglomeration and polishing to R
a
levels of 5 Å or even lower and a method by which such ceria can be obtained.
DESCRIPTION OF THE INVENTION
The present invention provides a ceria powder that has been milled at a pH of from 9 to 12.5 to comminute a powder with a BET surface area below 10 m2/gm until the BET surface area is increased by at least 5 m
2
/gm.
The present invention also provides a polishing slurry comprising ceria particles with a BET specific surface area of at least 10 m
2
/gm and a positive ionic surface charge at pH levels of 4 or lower when the pH is adjusted from such levels to basic values of 9 or higher. The ceria particles comprising such powders are essentially agglomerate-free by which is meant that the ceria is free of agglomerate particles that will not re-disperse upon stirring and that are larger than 5 microns in size.
The BET specific surface area is determined by the Brunauer/Emmett/Teller method described in
The Journal of the American Chemical Society,
60, page 309 (February 1938). The ionic surface charge is measured in terms of the random accoustophoretic mobility by the technique described in
Langmuir,
1988, 4, pp 611-626.
Ceria as commercially available is often present in amounts that are only 50% or more of cerium oxide, with the balance being made up primarily of rare earth metal oxides such as lanthana, neodymia and praseodymia. The preferred ceria used in the present invention comprises 70% ceria with about 30% of lanthana and is commercially available under the trade name CEROX® 1650. This product is available from Optical Manufacturers, Limited. This product has a nominal average particle size of 1.3 micrometers though in fact there were significant numbers of aggregates and a wide variation in particle size before comminution. A form of ceria with a higher actual ceria content is available from the same supplier under the trade mark Opaline®.
The ceria abrasive particles may be present along with other abrasive particles such as alpha alumina, gamma alumina, silica, zirconia and the like. Such particles however would need to be of the same order of particle size as the ceria to avoid scratching. Preferably the ceria according to the invention should represent at least 5% such as at least 10 or 20% and more preferably at least 50% of the abrasive particle weight in any such mixture.
When a slurry of the novel ceria powder of the invention is used to polish glass, after an induction period in which very little material is removed, a consistent and effective rate of material removal is achieved along with a final surface having a roughness of less than 10 Å and preferably from 3 to 7 Å. The reason for this unusual behavior is not fully understood.
The invention further comprises a process for producing ceria which maintains a positive ionic surface charge at pH values when titrated from acid values of about 4 or lower to a pH of at least 10 which comprises subjecting ceria comprising agglomerated ultimate particles and an average particle size of greater than one micron to a mechano-chemical treatment, which comprises milling a slurry of the particles using low-purity alumina or zirconia milling media and a pH of from 9 to 12.5, until an essentially de-agglomerated product with a BET surface area of at least 10 m
2
/gm is obtained. The pH at which the de-agglomeration occurs is preferably from 10 to 12.5 and the time required may be from seconds up to 15 days depending on the equipment used and the degree of de-agglomeration required. Conventional vibratory mills such as a Sweco mill may require seven days or more but an attrition mill can accomplish the same degree of comminution in a matter of seconds.
Usually very satisfactory results can be obtained, starting from commercially available ceria such as “CEROX” 160. This product typically has an average particle size of about 1.3 micrometers and comprises many agglomerates that are substantially larger than that. The higher quality ceria sold under the trade name “OPALINE” can also be used in the formulations of the invention after receiving the mechano-chemical milling treatment disclosed herein. It does not however appear to retain the positive surface charge during titration from acid to basic values referred to above for reasons that are not entirely clear.
The de-agglomeration is carried out using milling media selected from low purity alumina and zirconia. By “low-purity alumina” it is intended that the alpha alumina content is from about 85-90% by weight, an
Garg Ajay K.
Khaund Arup
Tanikella Brahmanandam V.
Bennett David
Marcantoni Paul
Saint-Gobain Industrial Ceramics, Inc.
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