The invention relates to a method for positioning a wafer (3) with a reference mark (6) in a vacuum processing unit with a transport chamber containing a transport device (2, 20, 21) for moving the wafers (3) in a plane to a process chamber arranged on said chamber and a single sensor (1), arranged within the transport chamber before the process chamber for recording the position of the wafer (3) by means of recording the edge thereof at a first detection point (4) and a second detection point (5), such that the actual position of the wafer (12) with a known wafer diameter can be determined with electronic analysis of both measured detection points (4, 5) and the transport device (2, 20, 21) guides the wafer (3) to a desired set position. The wafer (3) is aligned in a given position on the transport device (2, 20, 21) in relation to the reference marks (6) thereof and the projection of the reference marks (6) determines a non-permitted zone (22) along a direction of movement on the wafer (3) and hence defines a free zone on the remainder of the wafer (3). The sensor (1) is arranged in the transport chamber such as to guarantee that the non-permitted zone (22) is not scanned and the sensor (1) can thus only record the circular art of the wafer edge and not parts of the reference mark (6).