Analytic study of gratings patterned by evanescent near field optical lithography

Simulation results which investigate the near field of conducting gratings are presented to show some of the major issues affecting evanescent near fi eld optical lithography (ENFOL), namely ultimate resolution, depth of field , exposure variations due to edge enhancements, and resonant diffraction. U ltimate resolution down to 10 nm for grating structures is predicted, indep endent of illumination wavelength. The depth of field reduces approximately linearly as the feature size reduces in the evanescent regime. Variations in mask profile were investigated by modeling the radii of curvature of mas k conductors from 1-10 nm. Strict mask profile control is shown to be impor tant to avoid exposure variations due to the increasing zeroth transmitted order with increasing radii. A diffraction resonance occurs when the gratin g pitch matches the wavelength for a transverse magnetic excited grating. T he cut off of the ii diffracted orders coincides with a plasmon resonance a nd a strong, frequency doubled interference pattern is produced. To avoid s uch resonant conditions, standard ENFOL requires a low coherence source and /or strongly absorbing resists. However, this near field interference offer s the possibility of frequency-doubled interferometric replication of quasi periodic structures, with strong intensity enhancement at the expense of re duced depth of field. Overall, the key to successful evanescent lithography is restricting the lithography to a depth in which high contrast is availa ble with good process latitude due to the presence of sufficient numbers of diffracted orders of sufficient strength. (C) 2000 American Vacuum Society . [S0734-211X(00)09106-X].