Design of Mask Stack for High NA EUV Lithography

Abstract: Extreme ultraviolet lithography (EUV) is a next-generation lithography technology using 13.5 nm wave-length indicdent light. For EUV system employing numerical aperture (NA) 0.5 and a demagnification of M = 4, the incident EUV light is in a range of 9° ± asin(0.5/4), that is, 9° ± 7.2°. This requires a re-design of EUV masks, to reflect the light over the above-mentioned incident angle range.

By using genetic algorithm (GA) module in Dr.LITHO, a software developed by Fraunhofer IISGB, Erlangen, we develop several bilayer stacks which exhibit relativly high and uniform reflectivity curves. By using mulit-objective genetic algorithm module in Dr.LITHO, we compute and plot the pareto fronts of average reflectivity rate and the reciprocal of standard derivation (SD) of the reflectivity rate, which are obtained from different mask options.

The imaging performance of EUV binary masks is investigated by employing DR.LITHO. For dense line feature, semi-dense line feature, contact hole feature, and line's end features, the aerial images are plotted, the thresholds, normalized image log-slope (NILS) vaules, Depth of Focus (DOF) range and other lithographic process criteria are computed along both X- and Y-orientations. High reflectivity rate results in high threshold, small SD and large DOF range are correlated.

The imaging performance for EUV systems employing NA 0.5 is compared with that for NA 0.33 systems. Larger NA leads to larger NILS values, but smaller DOF ranges.