Patent ID: 8283629

Claim:
An ion beam column including a mass filter, comprising: an ion source for providing ions of different charge-to-mass ratios; a first lens for first forming the ions from the ion source into a beam along a first axis; a first mass filter stage that provides a first beam deflection away from the first axis, the first beam deflection depending on the charge-to-mass ratio of each ion in the beam, the first axis being a symmetry axis of the first mass filter stage; a rotational symmetry axis, the rotational symmetry axis being oriented perpendicular to both the first axis and the first beam deflection, the rotational symmetry axis being offset parallel to the first beam deflection; a second mass filter stage positioned 180 degrees around the rotational symmetry axis, a symmetry axis of the second mass filter stage being a second axis, the second mass filter stage providing a second beam deflection opposite to that of the first beam deflection, the combination of the deflections of the first and second mass filter stages being equal to the displacement between the first and second axes for one of the ions of different charge-to-mass ratios exiting the second mass filter stage; a mass separation aperture for passing less than all the ions from the second mass filter stage, the mass separation aperture being centered on the second axis; and a second lens receiving ions from the second mass filter and focusing those ions onto a substrate surface, the second mass filter stage substantially canceling the chromatic aberration from the first mass filter stage at the substrate surface.