Chromatic and spherical aberration correction in axially symmetric electrostatic lenses

High performance focused ion beam systems utilizing liquid metal ion sources are performance limited due to the chromatic aberration of the optical elements and the finite energy spread of the ion source. Some concepts are presented to reduce the chromatic and spherical aberrations utilizing axially symmetric electrostatic optical elements. Unlike paraxial optics, in off-axis optics the individual aberrations of each lens element interact. This allows the total system aberrations to be minimized by optimizing lens parameters. Fifth order raytracing is used to determine the off-axis aberrations of a two lens system. The aberrations are then calculated as a function of the inter-lens spacing and are shown to have distinct minima. By optimizing the lens acceptance angle and the lens spacing off-axis chromatic aberration and the geometric aberrations (including spherical aberration) may be reduced to values below that achievable using paraxial optics at the same beam current. Novel coaxial optical lens designs are presented which also offers the possibility of chromatic aberration correction and may provide improved performance over conventional optics for special applications.