Shape and size variations during nanopatterning of photoresist using near-field scanning optical microscope

The shape and size variations of the nanopatterns produced on a positive photoresist using a near-field scanning optical microscope (NSOM) are investigated with respect to the process variables. A cantilever-type nanoprobe having a 100 nm aperture at the apex of the pyramidal tip is used with the NSOM and a He-Cd laser at a wavelength of 442 nm as the illumination source. Patterning characteristics are examined for different laser beam power at the entrance side of the aperture (Pin), scan speed of the piezo stage (V), repeated scanning over the same pattern, and operation modes of the NSOM (DC and AC modes). The pattern size remained almost the same for equal linear energy density. Pattern size decreased for lower laser beam power and greater scan speed, leading to a minimum pattern width of around 50 nm at Pin=1.2 μW and V=12 μm/s. Direct writing of an arbitrary pattern with a line width of about 150 nm was demonstrated to verify the feasibility of this technique for nanomask fabrication.