Advanced die-to-database inspection technique for embedded attenuated phase shift mask

We have developed a deep ultraviolet die-to-database inspection system MC-3500 for the 130 nm node and beyond. The system involves comparison of a mask image scanned at a wavelength of 257 nm and a reference data calculated from complex amplitude of objects with 200 Mpixel/s throughput. An approximation of Hopkins formulation in the region of a large partial coherence factor gives a nonlinear filtering scheme for embedded attenuated phase shift mask (EPSM). The nonlinear filtering was evaluated for intensity fidelity of the reference image using comparison with Hopkins formulation. Evaluation results show that phase optimization is effective in eliminating excessive image ringing and the minimum feature size to maintain fidelity is found to be over 3 pixels. The technique is promising in applications to high-transmission EPSM.