Abstract
A near-field scanning optical microscopy (NSOM) and a double-frequency femtosecond laser (400
nm, 100
fs) were applied to push the optical resolution further down to sub-50
nm on thin UV photoresist. A 20-nm feature size can be obtained. It is at a resolution of
λ/20 (
λ: laser wavelength) and
a/2 (
a: NSOM probe aperture diameter), respectively. It is proved that laser power and exposure time can affect feature size of lithography patterns. In this paper, the effect of probe-to-sample distance on dot-pattern features is studied, and different dot-pattern shapes are generated: dumbbell-dot, ellipsoid-dot and circle-dot. The simulated light field spatial distributions across the nano-aperture based on Bethe–Bouwkamp model is found to agree with experimental results very well.