A large area deflection system with very low aberration

Abstract

A new deflector for electron or ion beam lithography is described. It combines the functions of focusing and deflection in a single unit. Focusing is accomplished by means of a uniform magnetic field oriented parallel with the optic axis of the system. A uniform electric field oriented perpendicular to the magnetic field provides the deflection capability. The optical properties, including the deflected spot size, are found from an exact solution to the nonrelativistic equations of motion. The third and fifth order aberrations are also derived. To third order, the aberration is identical with that of the uniform magnetic field alone, with no additional contribution from the electric field. This means that no growth of the spot occurs with deflection, and the beam size and shape are uniform over the entire deflection area. The system is telecentric, in that the beam intercepts the target plane at normal incidence. The focal surface is planar, and there is no distortion to third order. There is also no transverse chromatic aberration to this approximation. A practical implementation is proposed, based on a solenoidal field and a cylindrical deflector. The optical properties are analyzed numerically in the presence of electric and magnetic fringing fields, as occur with a conductive target plane and a system of finite length. A test case with a 20 kV beam, a 1.0 cm deflection area, and a 10 mrad aperture semiangle are considered. The maximum spot size with dynamic focus and astigmatism corrections is 0.17 μ. An energy spread of 1.0 eV is assumed, although much larger spreads are tolerable. This makes the system attractive for ion beam work. For a 1 cm square deflection area, the system offers an increase in addressable resolution elements by a factor of 50 over a more conventional electrostatic octupole deflector.