Impact of lens aberrations on phase-shifting masks

Abstract

The application of phase-shifting mask (PSM) technology in a manufacturing environment will ultimately be decided by whether this technology provides sufficient enhancement to warrant the additional costs. Initial experimental results on flat wafers and in the center of the field have demonstrated that for select feature types and layouts there is considerable improvement in the process margin when PSM are employed. However, these studies did not include the impact of residual lens aberrations in the optical exposure system. This study examines the effect of lens aberrations on the application of attenuating PSM through both SPLAT-generated (simulation of projection lens aberrations via TCCs) simulation and direct aerial image monitoring using latent image metrology. To benchmark the results obtained, all the work has been compared against a binary intensity mask (BIM). The simulation study concentrated on the three major Siedel aberrations detected in current i-line steppers, namely, coma, astigmatism, and spherical aberrations. The SPLAT-generated aerial images with these selected aberrations degrade the process latitude for both BIM and attenuated PSM; however, the initial enhancement provided by the phase-shifting technique made the attenuated PSM more tolerant of the aberration. As the feature size increased and the incremental process latitude gained by the phase-shifting technique decreased, the magnitude of the degradation with the two mask types became comparable. In summary, while PSM exposed on steppers will still maintain their performance superiority over BIM, the real improvement in process margin will suffer depending upon the feature type and layout.