STOCHASTIC COULOMB INTERACTIONS IN ION PROJECTION LITHOGRAPHY SYSTEMSWITH ABERRATION-BROADENED CROSSOVER

The allowable current in ion beam projection lithography is limited by stochastic Coulomb interactions. Theories for the dependence of these interactions on the parameters of the system, i.e., length, beam ener gy, print size, crossover diameter, etc. are thus important to guide t he design of future machines. All existing theories on blur from stoch astic Coulomb interaction assume a homogeneous current distribution in the crossover, where a large part of the blur is produced. However, i n actual lithography machines, the crossover may be substantially broa dened by the spherical aberration of the ion lenses, thus giving a ver y inhomogeneous current distribution in the crossover. In most analyti cal theories, the stochastic blur is independent of the radius of the homogeneously field crossover r(c) for very small crossovers, and rela tes to r(c) as (1/r(c))(1/2) or (1/r(c))(1/3) for larger ones. If aber ration broadening could do the same, the effect of broadening the cros sover from r(c) = 1.5 to 50 mu m would decrease the blur by a factor a s much as 6, without the negative effect of increasing the curvature-o f-field blur. We find physical arguments that such a large factor is i mpossible, Monte Carlo simulations first of all show that the blur red uction is larger near the edges of the image field than in the center, near the optical axis. For the simulated model system, the reduction on axis can be increased to a factor 2 at very large aberration coeffi cients. (C) 1997 American Vacuum Society.