Direct-Write Ion Lithography

An ion column utilizing mass-separated silicon ions at 150 kV (300 keV Si(++)) is an optimum choice for direct-write ion lithography. Since liquid metal ion source technology requires the use of copper or gold alloys of silicon to produce silicon ions, a mass separator is incorporated into the focusing column to remove the undesired ion species from the beam. Exposed depth measurements in PMMA were made using unseparated flood and focused ion beams from LM sources. The range of silicon ions in resist at 200 keV is greater than 0.5 micrometers. The resist was less sensitive to the high current density (1 A/sq. cm) focused beam than to the low current density (1 microAmps/sq. cm) flood beam. Repolymerization of the resist occurred at a dose approximately 80-90% of that required for maximum range due to the heavy gold or copper ions. The problems of range loss, repolymerization, and stray dc magnetic fields are also reasons for using a mass-separated column. Although more complex than a single accelerating lens, a high-resolution focusing column utilizing an ExB separator is feasible. Care will be taken in designing the separator to minimize astigmatism. (Author)