DESIGN OF A HIGH-CURRENT-DENSITY FOCUSED-ION-BEAM OPTICAL-SYSTEM WITH THE AID OF A CHROMATIC ABERRATION FORMULA

A 30-kV high-current-density focused-ion-beam optical system using a liquid metal ion source, has been designed for micromachining under the following restrictions: (1) all inter-electrode voltages are less than the acceleration voltage; (2) the sample and deflectors are not biased; (3) working distance is longer than 20 mm; and (4) the beam has an intermediate crossover. The design guidelines were obtained from an analysis of the chromatic aberration formula. The system has a tetrode condenser lens (zoom lens) and an einzel objective lens, each of which is made up of thin Butler-type lens electrodes. Both of the lenses are used in the deceleration mode. Calculated beam current density of the system is 1-10 A/cm2 in a wide range of beam diameters of 0.04-2-mu-m.