Patent ID: 6884554

Claim:
A method comprising: projecting a mask pattern through a projection lens within a leveling sensor on a semiconductor wafer to form a first image field, such that under the projection lens a second image field is formed identical to the first image field; projecting on corners of the first image field a plurality of first beams; projecting on corners of the second image field a plurality of second beams; determining a first leveling tilt of a semiconductor wafer having an asymmetrical topography using the leveling sensor, based on the plurality of first beams and the plurality of second beams; rotating the semiconductor wafer; determining a second leveling tilt of the semiconductor wafer using the leveling sensor, based on the plurality of first beams and the plurality of second beams on the corners of the first and the second image fields after the semiconductor wafer has been rotated; averaging the first leveling tilt and the second leveling tilt to yield an average leveling tilt; and, compensating for the average leveling tilt.