Method for in-cassette wafer center determination

DETAILED DESCRIPTION It is therefore an advantage of the present invention to provide an improved method and apparatus for reading an OCR or similar indicial mark.
It is a further advantage of the present invention to provide a method and apparatus for reading an indicial mark that requires less space within a clean room environment than that required in conventional systems.
It is another advantage of the present invention to provide a method and apparatus for reading an indicial mark in fewer steps than that required in conventional systems.
It is a further advantage of the present invention to provide an improved method and apparatus for positioning a wafer support tool under the center of a wafer.
It is a still further advantage of the present invention to identify the location of the center of a wafer without having to extract the wafer from a cassette in which the wafer is seated.
It is another advantage of the present invention to center a wafer support tool under a wafer prior to extracting the wafer from the cassette to prevent contact of the wafer with the cassette upon wafer extraction.
These and other advantages are accomplished by the present invention which is one embodiment relates to a system for centering an end effector of a wafer transfer robot with respect to a wafer while the wafer is seated in a cassette.
The wafer handling robot may thereafter extract the wafer, align the known center of the wafer with the known axis of rotation of the wafer handling robot, and rotate the wafer until an OCR or other similar indicial mark is located under an image reading camera.
In order to center the end effector of the wafer handling robot with respect to a Wafer to be extracted from the cassette, the relative position between the end effector and the wafer must first be determined.
For this purpose, an embodiment of the present invention includes a sensor system having a pair of sensor units.
The sensor units are spaced apart along an arc generally perpendicular to the angle of approach of the end effector toward the cassette