Abrading – Precision device or process - or with condition responsive... – Computer controlled
Reexamination Certificate
1998-12-15
2001-05-01
Scherbel, David A. (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
Computer controlled
C451S006000, C451S043000, C451S044000
Reexamination Certificate
active
06224459
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns apparatus and methods for inspecting circular workpieces such as disc-like wafers of semi conductor material, prior to and after machining. The invention is applicable to apparatus and methods of grinding the edge of such a workpiece, and assists in placement of the wafer on a grinding chuck, adjustment of machining tools, and checking the accuracy of the machined edge.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 4,638,601 describes a system for rotating a circular disc in a first station so as to align the disc in a particular manner so that it can then be transferred linearly without rotation into a second station for mounting on a vacuum chuck for machining. In the first station the disc is rotated so as to determine any eccentricity of the disc relative to the centre about which it is rotated, and this eccentricity is then aligned with the linear transfer path. Having achieved this alignment the centre about which the disc was rotating and the geometric centre of the disc are now aligned with the transfer axis. By so aligning the disc, any error in placement at the first station (which typically can be of the order of a few millimeters) as between the geometric centre and the actual axis of rotation in the first station, is converted into an adjustment in the linear shift along the path from the one station to the other. This enables the disc to be centred accurately at the second station so that the geometric centre of the disc is aligned with the axis of rotation of the vacuum chuck in the second station, thereby enabling accurate grinding of the edge of the disc by a grinding wheel.
There are many situations in which this type of placement is insufficient, since it does not allow discs to be positioned at the second station so as to accommodate all desired forms of rotation and grinding. Thus for example it may be desirable actually to mount the disc eccentrically relative so as to accommodate a damaged region of the periphery of the disc. By locating the disc so that a damaged region is at a greater radius from the geometric centre than is the undamaged remainder of the periphery and rotating the disc about the eccentric axis, a new disc is formed, albeit slightly smaller than the original disc, but in which the damaged region is ground away in the process. The eccentric axis about which the disc is rotated in the second station eventually, on completion of the grinding becomes the geometric centre of the new disc, formed by the grinding process.
Where an alignment method is employed such as disclosed in U.S. Pat. No. 4,638,601, there are only 2 chances in 360 that a damaged peripheral region will lie at one end or the other of the diameter of the original disc which has to be aligned with the transfer path in the prior art method.
It is and object of the invention to provide a workpiece alignment and shifting system which enables a disc to be mounted in the second station for rotation about any chosen axis.
It is to be understood that, the invention is not limited to situations in which the edge of the disc is damaged, but can of course be employed in all situations, provided (as intended), it is possible to mount the disc for rotation about any axis (including the geometric centre of the disc).
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention a workpiece alignment and shifting system for moving circular workpieces from a first inspection station to a second machining station, comprises workpiece holding means for attachment to the face of a disc workpiece to move the latter from one position to another, robotic means controlling the position of the workpiece holding means and adapted to move in at least two orthogonal directions, both of which are parallel to the plane of the disc when it occupies the first station, means for rotating the disc in the inspection station and in the machining station, inspection means for determining the position of the geometric centre of the disc as it is rotated in the first station, and computing means for calculating from data delivered by the inspection means the two shifts required along the orthogonal directions of movement of the robotic means, to move the geometric centre of the disc to a second desired position whose co-ordinates are known.
Preferably the computing means includes memory means for storing the said co-ordinates.
Preferably the means for rotating the disc workpiece in each of the first and second stations comprises a vacuum chuck and motor means associated therewith.
Preferably the workpiece holding means is vacuum operated.
Typically the co-ordinated are those of the centre of rotation of the workpiece holder or chuck in the said second station.
Preferably the computing means is adapted to compute the translation needed to shift the disc workpiece from the first station along one, or other, or all three, of three orthogonal axes, so as to position the disc with its geometric centre coincident with the axis of rotation of the chuck in the second station, or with the said geometric centre of the disc displaced from the axis of rotation of the chuck by a desired known distance and angular orientation.
By displacing the geometric centre of the disc relative to the axis of rotation of the second chuck, so a particular eccentricity, equal to the said displacement and initial orientation will be introduced into the rotation of the disc, thereby enabling it to be ground so as to shift the geometric centre to a new position determined by the said displacement.
In a preferred embodiment of the invention a three axis robot is employed to pick and place disc-like wafers of semi-conductor material for edge grinding. Movement along two of the robot axes, X and Z is used to control the centering of the wafer on a vacuum chuck in the second grinding station.
In one method according the invention the wafer is places in the inspection station so that its geometric centre aligns approximately with the axis of rotation of the inspection station, the wafer is rotated around the inspection station axis and two dimensions X
1
and Z
1
are computed, these being the distances, measured parallel to the X and Z axes, through which the geometric centre of the wafer needs to be shifted to coincide with the centre of rotation of the inspection station, and using the robot, the wafer is then transferred to a grinding chuck and is positioned thereon by making compensations in the X and Z directions so as to position the (now) known geometric centre of the wafer accurately relative to the axis of rotation of the grinding chuck.
This method does not require the wafer to be rotated so that the line joining the axis of rotation of the inspection station to the geometric centre of the wafer is parallel to the robot X axis, in order to make the compensation, as is required in the method described in U.S. Pat. No. 4,638,601.
Since it is now possible to shift the wafer so as to move the geometric centre thereon to any desired position relative to the axis of rotation of the grinding chuck, the X and Z adjustments incorporated in the transfer from the first inspection station to the second grinding station chuck, allows a “best fit” compromise between centering the wafer on the grinding chuck and unnecessarily removing material from the notch. Furthermore where damage is identified around the periphery of the wafer, the method allows the deliberate eccentric placement of the wafer on the grinding station chuck so that where peripheral damage has occurred as during slicing, the wafer can be re-ground about a new centre, so as to remove the unwanted damaged peripheral region. No such facility exists in general with a single axis shifting system as described in the aforementioned US Patent.
It is of course possible to rotate the disc to any desired orientation by rotating the vacuum chuck in the inspection station and transferring the disc at that orientation to the workhead.
According to another aspect of the invention, the exact distances measured parallel to the X and Z axes between
Falkner Dermot Robert
Morantz Paul Martin Howard
Pierse Michael George
Stocker Mark Andrew
Lee Mann Smith McWilliams Sweeney & Ohlson
McDonald Shantese
Scherbel David A.
Unova U.K. Limited
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