IMPROVEMENT OF ELECTROSTATIC LENSES FOR ION BEAM LITHOGRAPHY.

A systematic investigation of a large number of practically feasible axial potential distributions was carried out with a subsequent reconstruction of the electrode systems producing such distributions. By using parametrized analytical functions and different curve-fitting techniques it was possible to find many potential distributions with both small spherical and chromatic aberrations. A cubic spline curve-fitting algorithm proved to be especially effective. The reconstruction of the electrode systems resulted in several new electrostatic ion lenses with excellent optical properties. As an example, a four-electrode electrostatic lens is presented with a voltage ratio of V//m//a//x/V//m//i//n equals 9, spherical aberration coefficient C//s//0/f//0 equals 0. 92 and chromatic aberration coefficient C//c//0/f//0 equals 0. 55 (both coefficients are referred to the object, related to the object side focal length and calculated for the case of infinite magnification). With an acceptance angle alpha //0 equals 5 mrad and magnification M equals 2. 58 a 10. 5 nm aberration disk diameter can be achieved. If alpha //0 equals 2 mrad, the disk diameter is 3 nm.