Analytic study of gratings patterned by evanescent near field optical lithography

Abstract

Simulation results which investigate the near field of conducting gratings are presented to show some of the major issues affecting evanescent near field optical lithography (ENFOL), namely ultimate resolution, depth of field, exposure variations due to edge enhancements, and resonant diffraction. Ultimate resolution down to 10 nm for grating structures is predicted, independent 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 mask conductors from 1–10 nm. Strict mask profile control is shown to be important to avoid exposure variations due to the increasing zeroth transmitted order with increasing radii. A diffraction resonance occurs when the grating pitch matches the wavelength for a transverse magnetic excited grating. The cut off of the ±1 diffracted orders coincides with a plasmon resonance and a strong, frequency doubled interference pattern is produced. To avoid such resonant conditions, standard ENFOL requires a low coherence source and/or strongly absorbing resists. However, this near field interference offers the possibility of frequency-doubled interferometric replication of quasiperiodic structures, with strong intensity enhancement at the expense of reduced depth of field. Overall, the key to successful evanescent lithography is restricting the lithography to a depth in which high contrast is available with good process latitude due to the presence of sufficient numbers of diffracted orders of sufficient strength.