Validation of imaging benefits of dual monopole exposures

Dual monopole exposure has been proposed1 as a way to improve imaging performance in EUV lithography by reducing the loss of contrast from image fading caused by the image shifts for the two poles of a dipole source. Simulations showed that the imaging advantages can be significant, with more than 15% image contrast improvements predicted. This paper presents experimental wafer data to demonstrate and verify the predicted advantages. The observed imaging enhancements include: • 18% better NILS (Normalized Image Log-Slope) for 28nm pitch patterns. • Better tip-to-tip patterns enabling gaps as much as 3nm smaller than normal patterning. Tip-to-tip LCDU and exposure latitude were improved at the same time, for better overall T2T capability. • Best focus offsets between three pitches: P28, P56 and P96 was reduced from 30nm range to nearly zero. • Smaller LWR (Line-Width Roughness), as much as 20% depending on pitch. • Reduced micro-bridging defects in the "stochastic cliffs" of narrow trenches, as large as a 40X defect density reduction for narrow trenches with 82nm pitch. Our experimental results validate the substantial imaging advantages seen by initial simulations. In addition to verifying dual monopole imaging advantages, our experiments have measured the pole-to-pole image shift δx_P2P, an important parameter relating to image contrast which has never been measured before. This important parameter depends on the detailed mask structure as well as the specific shape and location of the source poles. Our measured δx_P2P was consistent with simulated expectations.