Interaction of pattern orientation and lens quality on CD and overlay errors

As product error budgets continue to shrink with decreasing ground rule, more attention is being paid to the effects of exposure tool lens aberrations. Interaction of the reticle pattern with the lens can cause both image placement shifts as well as variations of critical dimension across the exposure field. A particularly subtle effect is the interaction of the reticle pattern orientation with lens aberrations. It can be shown that there is often a large difference in aberration-driven errors for patterns oriented differently relative to the lens axis. This paper develops the physical model behind this phenomenon as a function of pattern aspect ratio and orientation. Specific examples are given in reference to line width control and overlay for typical DRAM patterns, using both simulation and experimental data. In addition, optimization schemes for pattern orientation are explored, as well as implications for practical implementation on exposure tools. We also show that pattern orientation can be optimized on a level-by-level basis to provide great benefit in CD and overlay error performance.