Determination of acid diffusion rate in a chemically amplified resist with scanning tunneling microscope lithography

Abstract

A scanning tunneling microscope (STM) was employed to investigate properties of the chemically amplified negative electron beam resist MICROPOSIT■ SAL-601■, from Shipley Company. The usual processing sequence of the resist includes a postexposure bake (PEB) to promote the cross-linking action of the acid catalyst. However, it has been proposed that PEB can induce thermally activated diffusion of the acid catalyst as well. Using the STM to generate a spatially localized beam of low kinetic energy electrons, we have investigated the effects of exposure dose and voltage and PEB time on linewidth in thin (50 nm) resist films. Even with no PEB, insoluble resist features were observable. Energy calculations do not support significant local heating. Therefore, cross-linking must be due to the electron exposure. Dose and PEB times under 10 min have only a small effect on linewidth. From the variation of feature size with PEB time, an upper limit to the diffusion coefficient was measured to be 0.3 nm2/s. This value is well below what has been previously reported. The process latitude of exposure with a 50 kV electron beam has also been investigated. Both dose and PEB conditions had a much larger effect on feature size for the high energy lithography.