Hydrogen and Phosphorus Ion Beam Exposure Characteristics of PMMA

Hydrogen and phosphorus ion beam exposure characteristics of PMMA are investigated as functions of dose and acceleration energy. PMMA shows both positive and negative exposure characteristics depending on the ion dose. A hydrogen ion dose of is required to reveal positive exposure characteristics with a contrast of 3, which indicates that PMMA is about two orders of magnitude more sensitive to ions than to electron beam. More than a ion dose affords negative exposure characteristics with a contrast of about 1. The positive exposure characteristics of PMMA are mainly determined by the amount of energy deposited per unit volume. On the other hand, the negative exposure characteristics are mainly determined by the total energy that the surface region receives. The projected ranges and projected deviations of hydrogen and phosphorus ions in PMMA are experimentally determined from the exposure characteristics and are about one order of magnitude smaller than the calculated results. Patterns as narrow as 1 μm on a stencil mask are neatly replicated in a PMMA layer by a 180 keV hydrogen ion beam, and demonstrate the applicability of ion beam lithography to the submicrometer VLSI fabrication.