A maskless ion projection lithography technique called maskless. microion beam reduction lithography (MMRL) has been developed at Lawrence Berkeley National Laboratory. As a candidate for a next generation lithography system, MMRL is designed to achieve sub-70 nm resolution. By adding a 100 mum diam limiting aperture at the focal plane of the ion beam, an aperture angle of 0.56 mrad [corresponding to numerical aperture (NA) = 5.6 x 10(-4)] at the image plane has been obtained, Small NA can decrease the geometrical and axial chromatic aberrations of the MMRL system. Thus resolution better than 150 nm has been obtained. Simulation results indicate that the resolution can be further improved by decreasing the aperture diameter and minimizing the limiting aperture induced aberrations. Meanwhile, the single-shot exposure time can still be as low as 193 ms at NA approximate to 10(-5) if the high brightness ion source design is employed in the MMRL system. (C) 2003 American Vacuum Society.