Following resist exposure with scanning probes, latent images are often evident in the resist surface. For both SAL601 negative resist and PMMA positive resist, the exposed regions may be raised 2-5 nm above the unexposed regions. Below we show the SAL601 resist topography after patterning with an exposure dose of 1000 nC/cm. Here the exposed areas are raised about 4.5 nm over the unexposed regions. This image was taken before the post exposure bake (PEB). The thickness of the lines was even greater after development, suggesting additional resist swelling during the PEB and/or development steps.
SAL601 is a chemically amplified resist. During electron beam exposure, an acid is generated in the resist which causes cross-linking during a subsequent PEB step. SPL exposure can itself causes the cross linking of the SAL601 matrix polymer. In fact, no PEB step is necessary to create features with SPL. This is likely due to the high area dose delivered during SPL. It may also be an indication of the polymer’s greater sensitivity to low energy electrons. This latent image formed in the resist film during exposure can be imaged with the AFM, suggesting the feasibility of in situ monitoring of the lithography process or aligning with previously written features.
