Direct-Write Ion Lithography

For a practical ion-lithography column of 150 kV, the light elements Li, Be, B, C, and Si were found to be the most promising for resist exposure. With the exception of Li, these elements emit doubly charged ions when used on a liquid metal (LM) ion source. Lithium, although highly reactive, can be operated as a single-element species, while the others must be used in alloys. Mass spectra for alloys of Be-Au, Si-Au, and B-Pt were obtained and showed significant fractions of the desired ion species. A new LM ion source structure (needle and heater assembly) was tested and found to improve the stability of the Be-Au ion emission. This ion source structure appears to be suitable for all elements and alloys considered for resist exposure. The results show that 0.5 micrometers of resist can be exposed with Be+ ions at an energy of 60 to 80 keV while the same exposed depth requires 175 to 200 keV for Si+ ions. Focused-beam exposures in PMMA were made using the Be40-Au60 alloy as the ion source material. Since the focused beam was not mass separated, the gold was implanted along with the beryllium. Because of the short range of the gold ions, polymerization of the resist occurred at doses greater than approx. 9 x 10 to the 12th power Be(++)/sq cm. The low Be(++) dose at 55 kV is not sufficient for maximum range, and most of the exposed depth is due to the combination of Be+ and Be(++) ions.