Exposure of ultrathin polymer resists with the scanning tunneling microscope

Abstract

The scanning tunneling microscope (STM) can be used to generate very narrow beams of low-energy electrons with extremely high current density. Because of this, it shows promise as a tool for lithography. We have previously demonstrated the capability of the STM in nanometer-scale lithography. Here we present new results using resist films prepared by the Langmuir–Blodgett (LB) technique, in which the polymer monolayer is spread on the water surface and is then transferred layer by layer onto the substrate. We have exposed both poly(methyl methacrylate) (PMMA) and poly(vinyl cinnamate). Patterns have been obtained in 5-monolayer (4.5 nm) and 17-monolayer (14 nm) LB PMMA films with linewidths down to 30 nm. For 17-monolayer PMMA, we have observed successful pattern transfer into the Cr layer for both polarities of tip-to-target voltage; smaller linewidths are observed for negative bias (target negative with respect to tip) than for positive bias (target positive with respect to tip). This is in agreement with our computer model, which predicts a smaller beam diameter for negative bias than for positive bias. For poly(vinyl cinnamate), patterns have been obtained in 11-monolayer (10 nm) films with linewidths down to 70 nm in the resist; the patterns transferred successfully into the Cr layer. Occasionally, with both materials, we have observed a ‘‘ghost image’’ of the pattern displaced from the primary image by 0.2–1 μm, with a reduced linewidth. We believe this is caused by two sharp protrusions on the tip, which may be separated by a comparable distance, acting as electron emission or collection sites.