Optimization of the depth of focus based on the analysis of the diffraction orders in the pupil plane

The evolution of the depth of focus has been studied using a simple geometrical analysis of the diffraction orders in the pupil plane. This analysis is based on the observation that the diffraction orders participating to the interactions must be symmetrical with respect to the optical axis. The center of the gravity of the areas of the different diffraction orders captured by the pupil is evaluated as a function of pitch, numerical aperture and wavelength. Analytical expressions have been derived. They give which illumination settings (partial coherence) optimise the DOF for conventional, annular and quadrupole illuminations. The impact of the diffraction orders on the evolution of the DOF through pitch curve was also studied. The appearance of a peak in the DOF versus pitch curve, for annular and quadrupole cases is caused by the perfect symmetry between the diffracted waves interacting together. [Copyright &y& Elsevier]