Abstract

Cylindrically symmetric electrostatic lenses are used to focus ions to delineate integrated circuit features in microlithography. In the image projection configuration, ions from a submillimeter source are accelerated, transmitted through a stencil mask that contains the circuit pattern information, and focused onto a target wafer. Electrostatic lens design is computationally intensive. We have developed a refractive analog that enables use of faster optical design codes to treat the electrostatic problem. We employ a series of plano-convex lenses of vanishing thickness and high refractive index, with the convex surfaces described by eighth-order polynomials. The index varies between lenses with local voltage. In a test problem, a 12-cm diam mask was imaged by a single, 75-kV, 48-cm diam lens onto a target plane 200 cm away with nominal image magnification of — 1/6. The analog employed 400 refractive lenses. Meridional rays were traced through field points, and Coddington’s equations and a primary lateral color formalism were used to determine image quality. The analog calculation was performed rapidly on a PC; the trajectory code employed a supercomputer.

© 1989 Optical Society of America