Electron-optical optimization for Gaussian, high-current, high-dose columns

This article demonstrates the electron-optical optimization of a high-current, high-dose column operating at 10 keV. The goal is to increase the available dose to the resist, which requires increasing the current density to more than 800 A/cm(2). Our calculations use the MEBS Ltd. BOERSCH program. We model the complete column as a set of thin lenses, separated by field-free drift spaces. Monte Carlo simulation propagates discrete bunches of electrons through the column, taking into account the mutual repulsion between pairs of electrons. Gaussian spot size and current density in the column were calculated far three beam currents: 314, 75, and 35 nA. The results show that to achieve a higher current density, it is necessary to change the electron gun and the column. Specifically, a low aberration gun and a significantly shorter column are required. At 10 keV, the column performance at high beam currents (similar to 300 nA) is almost entirely dominated by electron-electron interactions. Major improvements in gun performance only yield minor improvement in total column performance, and a significant decrease in column length is required to reduce the electron-electron interactions enough to meet the design specification. A new, low aberration gun operating in an accelerating lens mode gun was chosen. A major effort was focused on redesigning the middle column to reduce electron-electron interactions; The combination of the low-aberration electrostatic gun and a shorter middle column provides the key improvement in the current density (dose) performance of the column. Results of experimental measurements on the optimized columns are in good agreement with the presented Monte Carlo simulations. (C) 1998 American Vacuum Society. [S0734-211X(98) 14306-8].