Techniques for wafer prealignment and sensing edge positions

Abstract

A wafer prealignment apparatus comprising a CCD linear sensor and a wafer prealignment method which facilitate the sensing of wafer presence, improvement in precision of position sensing, and edge sensing adapted to the material of the wafer even of the material is transparent or not. A photodiode () is provided in a vicinity of the first pixel from the center shaft of the wafer rotation means of a CCD linear sensor () to detect a wafer () on the basis of the luminous energy value. The time from the issuance of a measurement command to the sensor () to that of an ROG signal for transmitting the detection timing of the sensor () is measured to correct the angular error of a measurement position due to the rotation during this measurement. For sensing an edge position of the wafer (), the scan start point of the sensor () is set on the opposite side of the center shaft of the wafer rotation means. When the wafer is transparent a sense signal is binarized to set its change point as the edge position. When the wafer is not transparent, information on a position lastly sensed as an edge is judged to be correct so that a particle may not be misrecognized as the edge.