Layout optimization method to equalize the best-focus position of different patterns

Abstract

In optical lithography for microchip manufacturing, it is important that the focal ranges of all patterns in the layout be closely aligned in order to maximize a common process window. In practice, large pattern-dependent variations in the position of the best focus are observed, which have been traced back to phase errors induced on the image-forming beams by scattering from mask topography. We show that this degradation mechanism can be exploited as a source of corrective phase shift, allowing pattern-dependent focus shifts to be controlled purely by changing the details of the mask layout, without requiring a significant change in the mask-making process. Phase distortions in the imaging beams are corrected by the optimized insertion of orthogonally oriented subresolution jogs into existing edges in the layout, thereby introducing a tailored scatter contribution whose quadrature component has the opposite sign from that of the primary edge.