Correcting for global space charge by positive ion generation

Abstract

In proposed high throughput electron beam projection systems there are surprisingly serious problems due to the first and third order lens action of the global space charge of the beam. Computer simulations [e.g., Takahashi et al., J. Vac. Sci. Technol. B 19, 2572 (2001)] show it may be possible to eliminate the deleterious effects of global space charge in these systems by introducing positive ions into the optical column. We have checked these claims experimentally on a scaled system designed to enhance global space charge effects. In our system, global space charge produces a first order focus shift of 5.4 mm and a field curvature of 2.2 mm. Introducing 4 mTorr of argon corrects the focus shift to 1 mm and reduces the field curvature to 1.3 mm, and we observed similar results with 7 mTorr of hydrogen. Introducing helium had no effect. Our experiment provides no significant information on stochastic Coulomb effects, although simulations indicate that these have little effect (e.g., Takahashi, ibid.).