Design and Evaluation of an Electron Objective Lens System with Two Lenses and Two Deflectors

The fundamental aberration coefficients of an objective lens system with two lenses and two deflectors for electron beam lithography system were investigated. First, we designed a two-deflector objective lens system by using computer simulations. The parameters of two deflectors (positions, strength, rotation angles) were optimized to compensate the transverse coma and transverse chromatic aberrations and the landing angle. The deflection aberration at the corner of the 2-mm field was almost the same as that at the center and was reduced by 35% compared with a one-deflector system. Next, all fundamental aberration coefficients, including the five geometrical aberration coefficients (spherical, transverse coma, astigmatism, field curvature, and deflection distortion), and two chromatic aberration coefficients (axial and transverse) were independently measured by using a practical objective lens system. The measured aberration coefficients agreed well with the simulated values. This agreement shows that the two deflectors successfully reduced the aberrations. It is concluded that this measurement is indispensable for designing and evaluating an electron objective lens system.