Spatial frequency analysis of optical lithography resolution enhancement techniques

A consistent frequency-space analysis of the effects of optical lithography resolution enhancement techniques including optical proximity correction, off-axis illumination, phase-shift masks, and imaging interferometric lithography is presented. The improvements for each of these techniques are directly related to the enhanced frequency-space coverage afforded. Optical proximity correction extends the frequency coverage out to ∼1.2–1.3×NA/λ where NA is the optical system numerical aperture and λ the exposure wavelength enabling κ1’s of ∼0.45 in the context of the usual Rayleigh resolution equation CD=κ1λ/NA. There are many possible configurations for off-axis illumination. For a quadrupole oriented at 45° to the (x, y) pattern axes, the maximum spatial frequency is extended to 2NA/λ or κ1∼0.43. Adding pupil plane filters to ensure a uniform modulation transfer function and orienting the quadrupole along the pattern axes allows extension to frequencies of 2NA/λ or κ1∼0.3. Phase shifts at the mask plane emphasize the high frequency image components by increasing the importance of the quadratic imaging terms and allow frequencies to 2NA/λ and κ1’s out to ∼0.35. Imaging interferometric lithography further extends the frequency coverage out to either (1+NA)/λ or 3NA/λ depending on the details with corresponding κ1 s of ∼0.23–0.2.