Sub-30 nm lithography with near-field scanning optical microscope combined with femtosecond laser

We report direct laser writing of lithography patterns with a feature width of 20 +/- 5 nm on thin photoresist film by combining a double-frequency femtosecond laser and a near-field scanning optical microscope. The obtained feature size is much smaller than the laser wavelength (lambda) and the aperture diameter (d) with a resolution of lambda/20 and d/2, respectively. The lithography patterns were analyzed with an atomic force microscope and a scanning electron microscope. The effects of laser energy and writing speed on the feature size were investigated. The underlying physical mechanism was also discussed.