Abrading – Precision device or process - or with condition responsive... – Computer controlled
Reexamination Certificate
2001-08-28
2004-09-28
Hail, III, Joseph J. (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
Computer controlled
C451S047000, C451S056000, C451S147000, C451S161000, C451S900000
Reexamination Certificate
active
06796877
ABSTRACT:
The present invention generally relates to a polishing machine and method for abrading or polishing a workpiece.
The present invention also relates to a tool and an abrasive cup for fitment to a tool for the use in abrading or polishing a workpiece.
The abrading or polishing of the surface of a workpiece is a technique which has applications in many different fields including the production of semi-conductor devices and optical components. The requirement is to provide a surface which has a particular surface contour and a particular surface finish i.e. smoothness. In the field of optical polishing there are two different techniques, one technique uses a tool for polishing which has a size comparable with that of the size of the workpiece. The limitation of this technique is that the tool is designed for a specific workpiece and this cannot be used universally. In order to reduce this limitation, an active lap has been developed as disclosed in GB 2163076 wherein the pressure distribution over the workpiece can be varied in order to differentially abrade or polish.
In the second technique the tool is substantially smaller than the workpiece and is moved across the workpiece in order to carry out the abrading or polishing process. Such a technique is described in U.S. Pat. No. 4,128,968. In this technique two pads are maintained in contact with the surface of the workpiece and are relatively rotated and moved in a spiralling path around the surface of the workpiece. Another such technique disclosed in WO97/00155 uses a tool which has a flexible working surface so that the effective area of contact with the workpiece can be controlled. This provides the benefit that the area being polished at any one time during the polishing cycle can be controlled.
In these prior art techniques, the tool is usually spun around an axis normal to the workpiece. A limitation to this technique is that on the axis the relative movement is zero and thus the removal or ablation rate is zero. Thus the use of such a tool having such a removal prof lee Bakes it difficult to achieve a desired target profile using an automatic polishing or abrading technique.
In the technique of WO97/00155, the angle of attack of the tool to the workpiece is variable using an arrangement which provides a “virtual pivot” on the surface of the workpiece. This has the benefit of ensuring when the tool is tilted there is no lateral or vertical movement of the centre of the tool in contact with the workpiece. However, the disclosed mechanical arrangement is complex and large.
U.S. Pat. No. 4,958,463 discloses a different technique in which a resilient working member is rotated about its axis parallel to the surface of the workpiece to provide relative lateral motion between the working member and the surface of the workpiece. The working member is held to rotate in a mounting member. The mounting member mounting the working member is also rotated perpendicularly to the surface of the workpiece. While this technique does not suffer from the lack of removal of material in the centre of the area of contact, it requires a complex arrangement involving the use of two motors to provide the two axes of rotation.
In accordance with a first aspect of the present invention there is provided a method and apparatus for abrading or polishing a workpiece. A workpiece is held on a holding surface of the and a head having a surface for abrading or polishing the workpiece is moved across the workpiece in a figuring pattern in order to polish or abrade the workpiece. In addition to the figuring movement of the head, the face of the head carrying out the abrading or polishing is arranged to move in a direction laterally on the surface of the workpiece by inclining and rotating the head. The direction of lateral movement of the face is rotated by moving the head to positions which are a precession of the inclined head about a precession axis normal to the workpiece surface.
Thus in accordance with this aspect of the present invention, instead of having a removal profile which is zero at the centre of the area of contact of the tool, it is possible to simply incline the head to use a face which is moved laterally relative to the workpiece, thus providing a non-axially symmetric removal profile at any one instance in time. An axially symmetric removal profile is however an advantage when deterministic automatic polishing is required. In order to make the average removal profile over a period of time symmetric, the head is moved to positions precessed relative to the surface of the workpiece such that the direction of lateral relative movement of the face of the tool rotates. Thus any pattern generated in the surface of the workpiece by the lateral movement at an instance in time will be generated at a number of rotational angles thus reducing the defects and producing an axially symmetric profile.
Another advantage of the technique of this aspect of the invention is that the movement of the abrading face is self-wetting. The cooling/lubricating fluid or slurry used between the abrading surface and the workpiece will be carried under the tool by the polishing action. In contrast, in the prior art techniques which use axial rotation of a tool normal to the surface of the workpiece, the cooling/lubricating fluid will tend to move to the circumference of the polishing area by centrifugal force.
This aspect of the present invention is applicable to any form of inclined rotating tool which can provide relative lateral movement between the workpiece and the abrading face and which can be precessed to rotate the direction of lateral movement e.g. an axially rotatable conical shaped tool.
In order to achieve averaging, preferably the precession takes place throughout at least 360 degrees. This can be achieved by incrementing the precession. Preferably such increments should be over more than one precession cycle. In one embodiment the increment in the precession is not an integer division of 360 degrees so that the direction of relative lateral motion is different for each cycle. In another embodiment the increments are symmetric about the 360° precession cycle.
In a preferred embodiment, the face comprises a compliant bulbous portion extending from the head. As the compliant portion is rotated about its inclined axis the bulbous portion forms an area of contact with the workpiece wherein there is relative lateral movement.
The abrading face can comprise a cloth or pitch on to which an abraded loaded slurry e.g. a diamond paste is placed. Alternatively, a bound abrasive can be used which is bound to the abrading face. When such a bound abrasive is used, only a cooling/lubricating fluid is required.
A second aspect of the present invention provides a polishing or abrading machine or method wherein a head carrying a face for abrading or polishing is held by a mechanical arrangement to allow for figuring of a workpiece. The mechanical arrangement includes a tilting mechanism arranged to tilt the head about a pivot to enable tilting relative to the workpiece. In this way not only can the head follow the contours of the workpiece but also it can be tilted to either follow the surface or be inclined at a required angle to the surface. Because the pivot is not on the workpiece when the head is tilted there is a displacement of the face of the head across, to or away from the workpiece. This is compensated for by either calculating or looking-up the required compensation values to control the mechanical arrangement to take up the displacements in the lateral and vertical directions.
This aspect of this present invention differs from the prior art disclosed in WO97/00155 in that a far simpler pivoting arrangement can be provided for example by use of an orthogonal arcuate track arrangement allowing tilting in any solid angle. In order to enable the use of a such a simple pivoting arrangement however, the displacement of the polishing face must be compensated for by a controller which determines the solid tilt angle and compensates the lateral and vertical displacem
Bingham Richard
Riley David Charles
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Hail III Joseph J.
Ojini Anthony
University College London
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