Claims for Patent: 4,833,790

Summary for Patent: 4,833,790

Title:	Method and system for locating and positioning circular workpieces
Abstract:	A system for locating and positioning wafers includes a wafer shuttle, a spindle, and a position sensor. The wafer shuttle retrieves wafers from a storage location, typically a wafer cassette, and transports the wafers to the spindle. The wafers are then incrementally rotated on the spindle, and the distance between the center of rotation and the periphery of the wafer along a linear path is measured. By using three such measurements, the distance and angle with which the center of the wafer is offset from the center of rotation may be calculated. The wafer can then be centered on the spindle by rotation and translated a proper distance by the wafer shuttle. Usually, the wafers will be further rotationally oriented so that the crystal lattice lies in a desired direction.
Inventor(s):	Spencer; Robert M. (San Juan Capistrano, CA), Lada; Christopher O. (Palo Alto, CA)
Assignee:	Lam Research (Fremont, CA)
Application Number:	07/048,194
Patent Claims:	1. A system for positioning a circular workpiece, said system comprising: means for conveying the workpiece along a linear path; means for rotating the workpiece about a center of rotation located on the linear path; means for measuring the distance along the linear path between the center of rotation and the periphery of the workpiece; and means for determining the offset between the center of rotation and the center of the workpiece based on the measured distance between the center of rotation and the periphery of the workpiece at at least three different angles of rotation. 2. A system as in claim 1, wherein the means for determining the offset includes a computer and an interface between the computer and the means for conveying, the means for rotating, and the means for measuring. 3. A system for positioning a circular workpiece, said system comprising: a cassette capable of holding a plurality of horizontal workpieces arranged in a vertical stack; means for selecting elevating the cassette; an arm; means for reciprocating the arm along a linear path, with the arm extending into the cassette at one end of the path; means on the arm for detachably securing the workpiece; means proximate the linear path remote from the cassette for rotating the workpiece about a center of rotation on the linear path; means for measuring the distance along the linear path between the center of rotation and the periphery of the workpiece; and means for determining the offset between the center of rotation and the center of the workpiece based on the measured distance between the center of rotation and the periphery of the workpiece at at least three different angles of rotation. 4. A system as in claim 3, wherein the means for reciprocating the arm includes at least one guide rod disposed parallel to the linear path, and means for translating the arm along the guide rod. 5. A system as in claim 3, wherein the means for detachably securing the workpiece includes a vacuum port at a remote end of the arm. 6. A system as in claim 3, wherein the means for rotating the workpiece includes a spindle and the arm includes a J-shaped end which circumscribes the spindle at a preselected position along the linear path. 7. A system for positioning a circular workpiece, said system comprising: means for conveying the workpiece along a linear path; a spindle located proximate the path; means on the spindle for detachably securing the workpiece; means for selectively elevating and rotating the spindle, whereby the workpiece may be secured, raised from the conveying means, and rotated through a desired angle; means for measuring the distance along a line between the center of rotation and the periphery of the workpiece; and means for determining the offset between the center of rotation and the center of the workpiece baesd on the measured distance between the center of rotation and the periphery of the workpiece at at least three different angles of rotation. 8. A system as in claim 7, wherein the center of rotation is located on the linear path. 9. A system as in claim 7, wherein the distance between the center of rotation and the periphery of the workpiece is measured along the linear path. 10. A system as in claim 7, wherein the means for detachably securing the workpiece includes a vacuum port. 11. A system as in claim 7, wherein the spindle is a cylindrical shaft having a vacuum port at one end. 12. A system as in claim 11, wherein the means for selectively elevating and rotating the spindle includes a controllable motor connected to rotate the spindle. 13. A system for positioning a circular workpiece, said system comprising: means for conveying the workpiece along a linear path to a first location; means at said first location for raising the workpiece from the means for conveying and for rotating the workpiece about a center of rotation; a position sensor; means for reciprocating a position sensor along at least a portion of the linear path, whereby the distance between the center of rotation and the periphery of the workpiece may be measured; and means for determining the angular deviation (.alpha.) between the linear path and a line drawn through the center of the workpiece and the center of rotation and the linear distance (l) between the center of the workpiece and the center of rotation based on the measured distance between the center of rotation and the periphery of the workpiece at at least three different angles of rotation. 14. A system as in claim 13, wherein the position sensor is an optical sensor. 15. A system as in claim 13, wherein the means for reciprocating the position sensor includes a drive screw and a carriage mounted on the drive screw. 16. A system as in claim 13, further including a controllable motor connected to rotate the drive screw. 17. A method for positioning a circular workpiece, said method comprising: incrementally rotating the workpiece about a fixed center of rotation where the center of rotation and the center of the workpiece are not coincident; measuring the distance between the center of rotation and the periphery of the workpiece at at least three different angles of rotation; and determining angular deviation (.alpha.) between an arbitrary baseline drawn through the center of rotation and a line drawn through the center of the workpiece and the center of rotation and the linear distance (l) between the center of the workpiece and the center of rotation, based on said measured distances; and positioning the workpice by rotating through angle .alpha. and translating through distance l so that the center of the workpiece and the center of rotation are aligned. 18. A method as in claim 17, further including the following steps which are performed after the center of the workpiece and center of rotation are aligned: rotating the workpiece to detect discontinuities in its circular periphery; and rotationally positioning the workpiece so that the discontinuities are rotationally aligned in a preselected pattern. 19. A method as in claim 17, wherein measurement between the center of rotation and the periphery of the workpiece is repeated to confirm the determination of angular deviation .alpha. and distance l. 20. A method as in claim 17, wherein the angular deviation .alpha. is determined by the following algorithm: ##EQU3## wherein: r.sub.1 is the distance between the center of rotation and a point on the periphery of the workpiece when the workpiece is rotated by angle .theta..sub.1 from the baseline; r.sub.2 is the distance between the center of rotation and a point on the periphery of the workpiece when the workpiece is rotated by angle .theta..sub.2 from the baseline; r.sub.3 is the distance between the center of rotation and a point on the periphery of the workpiece when the workpiece is rotated by angle .theta..sub.3 from the baseline; and 21. A method as in claim 20, wherein the length l of offset is determined by the following algorithm: ##EQU4##