Initially, an ion beam is formed as an elongated shape incident on a wafer, where the shape has a length along a first axis longer than a diameter of the wafer, and a width along a second axis shorter than the diameter of the wafer. Then, a center of the wafer is moved along a scan path intersecting the ion beam at a movement velocity, and the wafer is rotated around at a rotation velocity simultaneously. During the simultaneous movement and rotation, the wafer is totally overlapped with the ion beam along the first axis when the wafer intersects with the ion beam, and the rotation velocity is at most a few times of the movement velocity. Both the movement velocity and the rotation velocity can be a constant or have a velocity profile relative to a position of the ion beam across the wafer.