Method and apparatus for correcting chromatic aberration in charged particle beams

A method for compensating for chromatic aberration in a particle beam, the aberration occurring at a point of deflection of the beam in a selected transverse direction, as a result of energy differences of the particles, the method comprising the steps of:

• determining the location of a point of compensation upstream of the point of deflection;
• and
• applying a compensating deflection force to the beam in the selected transverse direction at the point of compensation, such that the compensating force applied to each particle is proportional to its relative phase in longitudinal phase space at the point of compensation, but is proportional to particle energy when the particles reach the point of deflection, whereby the compensation deflection force compensates for the chromatic aberration that would otherwise be present at the point of deflection;
• and wherein the point of compensation is spaced upstream of the point of deflection by a distance that exceeds n cycles in longitudinal phase space by onefourth of a cycle, where n is zero or a positive integer, and is equal to an integral number of half-cycles in transverse phase space.

Apparatus for producing a particle beam free of chromatic aberration in all transverse directions, the apparatus comprising:

• first deflection means located at a point of deflection, for deflecting the beam in a selected transverse direction for purposes of beam steering or focusing;
• and
• second deflection means for compensating for chromatic aberration caused by the first deflection means, the second deflection means providing a compensating deflection force proportional to the amount of deflection applied in the first deflection means, and having an effect at the first deflection means proportional to the energy of each deflected particle;
• and wherein the longitudinal spacing between the first and second deflection means exceeds n cycles in longitudinal phase space by one-fourth of a cycle, where n is zero or a positive integer, and is also equal to an integral number of half-cycles in transverse phase space.