Abrading – Abrading process
Reexamination Certificate
2000-05-04
2001-12-25
Hail, III, Joseph J. (Department: 3723)
Abrading
Abrading process
C451S035000, C451S041000, C451S113000
Reexamination Certificate
active
06332829
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to methods and devices for polishing and, in particular, to polishing of surfaces with magnetic polishing fluids (MPF).
BACKGROUND OF THE INVENTION
There exist, known in the art, systems for polishing optical lenses. Some of these systems utilize a magnetorheological polishing substance known as a polishing slurry. Typically, the slurry is a mixture of magnetorheological compound, abrasive particles, and stabilizers.
When not acted upon by magnetic forces, the slurry is commonly in a liquid state. However, once acted upon by such a force, the slurry becomes much more viscous, pushing the abrasive particles to the surface of the liquid. This more viscous slurry, with the abrasives protruding from the surface, is then utilized as a polishing tool for abrading and polishing a work piece surface. Kardonsky et al. in U.S. Pat. Nos. 5,677,948 and 5,449,313 describes such a system.
In order for the magnetorheological polishing device to be most efficient when being used as a polishing tool, it should be hard enough to apply sufficient force to firmly press the abrasive particles to the surface of the work piece. The polishing devices utilized in prior art systems acquire viscous, almost plasticized, properties known as Bingham properties, under the influence of magnetic forces. At this point, the device is hard enough to be used as a polishing tool. However, prior art devices reach this fully developed Bingham state only once, at the onset of the polishing movement,
Thereafter, once the movement of the work piece relative to the slurry commences, the slurry no longer sustains the Bingham properties, and the slurry loses the plasticized properties. Hence, although they become more viscous, they ultimately remain in the liquid state. Thus, frequently the liquid does not have sufficient force to push the submerged abrasive particles firmly against the polishing surface and, consequently, the abrasives do not efficiently abrade the work piece.
Additionally, polishing of the work piece is carried out in stages. At any given time a small surface area is polished. This area is defined by the size of a zone, known as the polishing zone, which is small relative to the size of the work piece. Thus the work piece is polished zone by zone. This approach hinders the achievement of uniform polishing across the entire surface of the work piece. Non-uniform work pieces, such as silicon wafers, present a potential problem in devices such as semi-conductors.
An additional problem is the non-uniformity of the magnetic field, which affects the affectivity of the polishing zone. The magnetic field on the magnet's edge is almost an order of magnitude higher than that at the center of the magnet. Therefore, the visco-plastic properties of the slurry in the polishing zone are non-uniform, contributing to non-uniform polishing of the surface.
SUMMARY
It is an objective to provide an improved device for polishing of planar surfaces.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a system for polishing a surface. The system includes an MPF and one or more magnetic means in contact with the MPF. The one or more magnetic means react upon the MPF to plasticize it, whereby the plasticized material is then used to polish the surface. Preferably the MPF intermittently and repeatedly contacts with the surface for polishing.
The system preferably also includes a means for intermittently magnetizing the one or more magnetic means. Additionally, the one or more magnetic means may be one or more permanent magnets placed in alternating directions. The means for intermittently magnetizing may be a shuttle for intermittently acting upon the one or more magnetic means, causing the magnetic means to provide a magnetic field when acted upon.
Alternatively, the one or more magnetic means may be one or more electro-magnets, and these electromagnets may be driven by a controlled current or an alternating current (AC).
The system may further include a cylinder, wherein the one or more magnetic means are laid along the longitudinal axis of the cylinder, which is rotatable along the longitudinal axis, and the plasticized material covers the cylinder. The one or more magnetic means are either flush with, protrude from, or are recessed within the outer surface of the cylinder. The magnetic means may have a spiral pattern.
The system may further include a vessel holding a pool of the polishing material, wherein a lower portion of the cylinder sits in the pool. The cylinder may further include an array of tubes containing a supply of the polishing material for secreting onto the outer surface of the cylinder. Additionally, the system may include a trimmer.
The system may include either a wire mesh or a metal-wool blanket covering the cylinder. The blanket includes ferromagnetic materials or non-metal materials.
Alternatively, the cylinder may include a ferro-magnetic mandrel, inserted co-axially into the center of the cylinder, wherein an outer surface of the mandrel contacts with the magnetic means, causing the magnetic means to change magnetic field upon contact, or the cylinder may include an array of electromagnets.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a method for polishing a planar surface. The method includes bringing two or more magnetic means into contact with a MPF, thereby plasticizing the polishing material. Then a planar side of the surface contacts with the plasticized polishing material, so that the plasticized polishing material polishes the surface. Upon contact with the surface, the plasticized MPF liquidizes. After termination of the contact with the surface, the liquidized polishing material solidifies. The surface is moved, and the above steps are repeated a multiplicity of times until the surface is polished.
REFERENCES:
patent: 5404680 (1995-04-01), Mizuguchi et al.
patent: 5931718 (1999-08-01), Komanduri et al.
Eitan Pearl Latzer & Cohen-Zedek
Hail III Joseph J.
MPM Ltd.
Thomas David B.
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