Optical proximity correction for intermediate-pitch features using sub-resolution scattering bars

Using our original scattering bar implementation as a starting point, we identify two major discontinuities in the CD versus pitch curve. The first discontinuity occurs at the pitch where two standard scattering bars will no longer fit between features at the proper optical distance and must be replaced by a single shared bar. The second discontinuity occurs at the pitch where a single shared bar no longer fits between features at the proper optical distance, and must be deleted. We propose several techniques that can be used in the pitch range around these discontinuities that allow us to decompose them into a finer series of smaller discontinuities, in effect smoothing out the curve. We introduce the concepts of “placement elasticity” and “optical weight modulation,” and two new classes of scattering bars: thin scattering bars and binary halftone scattering bars. We combine all of these elements in a hybrid optical proximity correction strategy, recommending actual dimensions for i-line, KrF (248 nm) and ArF (193 nm) reticles. We report both simulated and actual results for an i-line embodiment of this approach, demonstrating a greater than 50% improvement in overlapped depth-of-focus for both on-axis and off-axis illumination.