Optimizing NA and sigma for subhalf-micrometer lithography

Abstract

The effects of numerical aperture (NA) and partial coherence ((sigma) ) on lithographic performance have been investigated. A deep-UV exposure system with a variable objective NA from 0.53 to 0.35 was used to obtain experimental results. The illuminator system has a variable NA from 0.39 to 0.16. The variable illuminator allows a partial coherence of 0.74 to 0.31 for the 0.53 objective NA and a partial coherence greater than one for the 0.35 objective NA. Experimental measurements have been performed using both negative XP89-131 photoresist and positive APEX-E. These results show that the realizable depth of focus (DOF) for a given NA does not always follow the Rayleigh equation or simple contrast threshold simulation models. Maximum DOF for 0.35 dense line/space features imaged in APEX-E was obtained with an NA of 0.40 while maximum DOF for 0.25 micrometers features was obtained with the full 0.53 NA. Both results differ from simulation and Rayleigh's equation. Partial coherence has also been found to affect DOF. Higher partial coherence values lead to higher DOF in both positive and negative photoresist. A second effect of variable partial coherence is the effect on the critical dimension (CD) split between dense and isolated lines. For example, 0.35 micrometers features imaged in APEX-E with 0.53 NA exhibit a 5 nm dense/iso CD split with (sigma) equals 0.74, but the dense/iso split with (sigma) equals 0.44 is 70 nm.