Focused ion beam system and method

An ion beam system for modifying a sample or workpiece surface, the system comprising:

• an ion source for generating ions;
• an ion beam focusing column configured to direct ions received from the ion source to form an ion beam and focus the ion beam towards a target area;
• and
• a sample stage for receiving the sample or workpiece to be modified and for positioning the sample or workpiece surface at the target area,
• wherein the ion beam focusing column comprises:
• an aperture plate having an elongate non-circular beam limiting aperture configured to limit the extent of the ion beam passing therethrough;
• and
• a multipole aberration compensator configured to apply a four-fold astigmatism to the ion beam to compensate for spherical aberration produced by one or more lenses in the ion beam focusing column;
• wherein the ion beam system further comprises:
• a beam deflector system configured to deflect the ion beam and move the ion beam in a first scan direction across the target area, and the elongate non-circular beam limiting aperture is arranged with its major dimension aligned such that the ion beam major dimension at the target area is parallel to the first scan direction, and the beam deflector system is further configured to move the ion beam across the target area, for modifying the sample or workpiece surface, with a leading edge having a width corresponding to the minor dimension;
• and
• wherein the four-fold astigmatism is aligned with the major dimension of the elongate non-circular beam limiting aperture to compensate for third order spherical aberration of the focused ion beam in the minor dimension at the sample or workpiece surface.