Characteristics of focused beam spots using negative ion beams from a compact surface plasma source and merits for new applications

Abstract

The characteristics of H− beams from a compact Penning-type surface plasma source have been studied with the goal of evaluating their merits for improved focused ion beam (FIB) systems. As a proof-of-the principles experiment to demonstrate the beam quality a simple, prototype source was developed without any special cooling arrangement of the electrodes. The source is operated in a pulsed mode with a maximum duty factor of 1%; and the operation can be extended to the dc mode by using water cooling. The beam brightness is estimated to be ∼0.5×105 A cm2 sr−1 at a beam energy of 7 kV, and the energy spread of the beam for angular beam intensity of ∼40 mA/sr is ⩽3 eV. The beam at an energy of 7 keV has been focused by a lens with a magnification factor of M∼0.1, and the spot size is determined by scanning a Si knife edge over the beam as well as by poly(methylmethacrylate) resist exposures. A focused spot, with diameter of ∼6 μm and current density of ∼10 mA/cm2, has been achieved. The dependence of the spot characteristics with the beam intensity from the source has been studied. The results suggest that a submicron size spot with a current density of ⩾1 A/cm2 can be generated using these beams for M∼0.01. The source, with its simple, robust technology, seems an attractive choice for FIB applications.