High-resolution, low-energy beams by means of mirror optics

Abstract

Electron and ion beam systems with low beam landing energies are needed for many important applications, such as microcircuit inspection and charge storage devices. In this paper, therefore, we have analyzed focusing and deflection systems with strong electrostatic retarding fields immediately in front of the final target plane, with beam landing energies as low as 50 eV. A simple theory is presented, which proves that such strong retarding fields dramatically reduce all the aberration coefficients, including the deflection aberration coefficients as well as the on-axis ones. The overall spot size (for given beam current) inevitably increases as the beam is retarded, due to the increased aperture angle and increased fractional energy spread at the image plane. Nevertheless, on account of the great reduction in the aberration coefficients, the degradation of spot size at low beam landing energies turns out to be much less severe than might intuitively be expected. Our electron optical design software has been enhanced to compute accurately the aberrations of such retarding field systems. This software has been used to verify the predictions of our theory. A practical retarding field system with 50-eV beam landing energy has been built and tested, and its experimental performance confirms our computer analysis. The design has been significantly further improved with the aid of our programs. The computed results show that, using a thermal-field emission source with 1-eV energy spread, the proposed final design should be capable of producing a 111-nA beam of <0.3-μm diameter throughout a 10×10 mm2 deflection field, at a final landing energy of 50 eV.