METHOD AND APPARATUS FOR PREALIGNING WAFERS IN A WAFER SORTING SYSTEM

A wafer sorting system utilizing optical character recognition and optical "gate sensors" to determine the orientation of the wafers for prealigning. It is envisioned that the machine will be installed in conjunction with a computer controller and multiple cassette stations. A first end of a transfer arm of a robot is equipped with an end effector to transfer individual wafers. The end effector of the transfer arm is extendable and retractable to select and remove the desired wafer from its cassette, and to transfer it to the prealigner or a target cassette in its proper orientation. The end effector includes a vacuum pickup and a sensor that enables detection of presence or absence of wafers in the cassette, and any misaligned wafers in the cassette. Further gate sensors are mounted on the prealigner to accomplish the orientation function prior to the wafer being placed on the prealigner chuck. Multiple perimeter points are used to determine the position of the center of the wafer. The optical character recognition capability of the system then allows the individual wafers to be identified and to be sorted as desired.