INFLUENCE OF COULOMB INTERACTIONS ON CHOICE OF MAGNIFICATION, APERTURE SIZE, AND SOURCE BRIGHTNESS IN A 2 LENS FOCUSED ION-BEAM COLUMN

Focused ion beam columns with liquid metal ion sources should be optim ized to give the highest possible current in a probe of given dimensio n. A procedure for this optimization is presented, which includes the influence of Coulomb interactions between the particles in the beam. T he column is first optimized without including the Coulomb effects and it is found that the optimized magnification and aperture size are in dependent of the gun brightness. The magnification approaches a consta nt value in the high current domain, where the gun lens aberrations se riously influence the probe size. The Coulomb effects can be character ized by pointing out a domain in the probe current-probe size plane wh ich is inaccessible, no matter how high the brightness of the gun may be. The approximate position of this inaccessible area can be found wi th a relatively small number of calculations. It is then demonstrated how the beam current as a new function of probe size can be reoptimize d by the choice of magnification and aperture size. Coulomb effects tu rn out to be dominating the probe size except for very small probes an d for very large probes. (C) 1996 American Vacuum Society.