Position difference detecting device and method thereof

摘要

An R- theta robot moves a wafer between chambers according to an instruction issued by a robot controller. An optical sensor detects two edge positions of the wafer which stands still in a hand of the R- theta robot when the R- theta robot is in a predetermined stationary state. The robot controller obtains the length and the middle position of a chord of the wafer based on the two edge positions. This data is compared with the data of the length and the middle position of the chord which should be obtained when the wafer is placed in a reference position in the hand of the R- theta robot, so that a position difference of the wafer to be moved from the reference position is detected. The robot controller controls the operations of the R- theta robot in order to correct the detected position difference.