Feasibility Study of Multiple-Beam Scanning Electron Microscopy for Defect Inspection

Abstract

Multiple beams are formed around an optical axis and scanned on a specimen. Secondary electrons (SEs) emitted from the scanned points are accelerated by an electrostatic objective lens and deflected by an E cross B (E×B) beam separator to a secondary-electron detector. By scanning over an 80 nm line & space pattern with four beams, Scanning Electron Microscopy (SEM) images are obtained. From these SEM images it is seen that there is no problem with cross-talk from the neighboring beams in multiple-beam SEM where there are plural detectors. The emittance and brightness of the electron gun with a LaB6 spherical cathode are measured to be 163 mrad µm and 1.5×105 A/cm2sr, respectively, for an emission current of 450 µA and a beam energy of 4.5 keV. A simulation is performed for a combination lens, with a lens, the magnetic gap of which is formed at the specimen side, and an electrostatic lens with an electrode to which a positive high voltage is applied. Based on these electron gun characteristics and simulated aberration characteristics, 8-columns by 8-rows multiple beams with a 160 nA total beam current and a 50 nm resolution are expected. A defect inspection with equivalent pixel frequencies of 4 GHz is expected. This pixel frequency is 10 times larger than that of a commercial available system, and then the multiple-beam SEM is a most promising system for a next-generation inspection.