A focused ion beam optical column with microwave ion source used formicrofabrication of cells is described in this paper. Both analytic calculation of beamdiameter in the column and numerical simulation of space- charge-induced beambroadening were made

and main factors influencing beam size and methods ofreducing the beam spot were found. By this system

a 25keV N+ beam of about 20 mdiameter with 148nA current was finally achieved when the aperture diameter was0. 1mm.