FOCUSED ION-BEAM OPTICAL-COLUMN DESIGN AND CONSIDERATION ON MINIMUM ATTAINABLE BEAM SIZE

We discussed a two lens optical system focused ion beam (FIB) employin g a Ga-emitter (LMIS), whose acceleration voltage V-acc is ranged 30-1 00 kV, which will be used for milling and secondary ion microscope obs ervation purposes. On such an optical system, we investigated the rela tion between minimum obtainable beam diameter and chromatic aberration coefficient against V-acc, and studied the possibility of improving F IB resolution by increasing the V-acc. The beam diameter is mainly det ermined by the size of a Gaussian image and the chromatic aberration, especially that of objective lens, if the beam current is very low. Fr om this fact, we showed that the magnification optimization method whi ch is one of the optical optimization methods can be greatly simplifie d at a lower beam current region. Using this simplified method, we sum marized a guide line for evaluating an V-acc value from the standpoint of realizing a finer beam. Also given is information useful for desig ning a FIB column in consideration of optimized optical column design for realizing the; optimized magnification. Finally, calculating the a ttainable minimum beam diameter and optimum column length against V-ac c values, we discussed the limitation of beam diameter by increasing t he V-acc. (C) 1998 American Vacuum Society.