Lift-off metallization using poly(methyl methacrylate) exposed with a scanning tunneling microscope

Abstract

The scanning tunneling microscope (STM) is a unique tool for ultrahigh resolution, ultralow voltage electron beam lithography. In a previous paper we described the formation of metallic patterns by a ‘‘contamination’’ process followed by sputter etching. Here we report the formation of 22-nm-wide metal lines by STM exposure of poly(methyl methacrylate) (PMMA) resist followed by liftoff. Resist exposure and liftoff have also been achieved with reverse polarity, where the electrons are emitted from the resist sample and accelerated towards the tip. Results include the first working device made with a STM, a thin-film resistor 2 μ long and 120 nm in width. In addition to lithographic applications, studying resists in this regime may offer new insights on the chemical process of resist exposure and the role played by secondary electrons in exposing resist. Our results indicate that PMMA only works as a negative resist for energies above 25 eV; below this value it appears that the electrons cannot cause sufficient crosslinking to render the resist insoluble in the usual solvents.