Design of electron-beam scanning systems using the moving objective lens

Abstract

Some considerations and results of designing air-core scanning systems comprising round lenses and saddle-type deflection coils are presented. If the deflection coils are cosine distributed, the third-order deflection aberrations are similar to those of a round magnetic lens. This round-lens-type deflection aberrations can be eliminated or reduced by using the Moving Objective Lens (MOL) and predeflection. As is seen from the first-order Taylor-series expansion, a round lens whose potential is φ (x,y,z) can be moved in the x direction by superposing the deflection field ∂φ/∂x. Such a MOL is placed before the image plane and a predeflection coil placed at the object position deflects the beam into the center of the moved lens. Then, a particular aberration such as coma or astigmatism can be eliminated by properly arranging this predeflection. In particular, if coma is eliminated by the predeflection, astigmatism and field curvature are at the stationary point with respect to the predeflection, which is of importance from the design tolerance point of view. A practical scanning system which may be used in microfabrication has been designed. The deflection coils used are half-inch vidicon printed coils which are of best geometrical precision available. A found system produces at the corner of 5×5 mm deflection field with 0.005 rad semiaperture and 1 in 104 beam-voltage ripple, a total aberration disk of 0.35 μm, of which field curvature is 0.30 μm and chromatic aberrations are 0.10 μm.