A variable energy focused ion beam system for i n s i t u microfabrication

The combination of molecular-beam epitaxy with focused ion beam (FIB) technology offers the opportunity for generating true three-dimensional ultrasubmicron structures. One problem, however, is that conventional FIB systems employ landing energies of ∼50 keV and up, and so the spatial extent of damage by the beam, exceeds both the radius of the beam and the thickness of layers frequently grown. We have investigated focusing of beams of ions of energies down to 25 eV by employing retarding field optics. Modeling of the optics indicates that space charge becomes serious at currents above 1 nA for Ga+ ions of 50-eV energy but that submicron beams of such ions are possible at lower currents. An experimental column was built and largely confirmed the predictions. Beam diameters were measured to be within a factor of 2 of the predicted value with a value of 0.95 μm for a 1-nA beam of Ga+ ions of landing energy of 25 eV.