Method and apparatus for automatically positioning a workpiece relative to a scanning field or mask

摘要

A method for automatically positioning a workpiece having at least one marking, specifically a wafer for integrated circuits which is to be processed in a charged-particle beam apparatus, relative to a scanning field or mask. In the method, a scanning beam scans the workpiece along the line and a marking signal is generated when the marking is reached by the beam. This marking signal controls a device which generates a control signal in response to the marking signal which is proportional to the deviation of the marking position from a reference position and which drives a device for correcting the position of the workpiece relative to the scanning field or mask. The improvement of the invention comprises the steps of scanning the beam across the workpiece successively in opposite directions at the same velocity through a forward sweep and a return sweep of the beam; integrating a positive reference voltage in an integrator during the forward sweep of the beam from the beginning of the line scanned until the marking on the workpiece is reached by the beam; integrating a negative reference voltage having the same magnitude as the positive reference voltage in the integrator during the return sweep of the beam from the end of the line scanned by the beam until the marking on the workpiece is reached by the beam; and generating an output control signal by means of the integrator after a full forward and return sweep of the scanning beam over the workpiece for correcting the position of the workpiece relative to the scanning field or mask. A method using digital scanning and apparatus for carrying out the methods are also disclosed.