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 δxP2P, 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 δxP2P was consistent with simulated expectations.
Image contrast is an important measure of image quality, especially for EUV lithography where high contrast can mitigate stochastic process fluctuations1 such as Line-Edge Roughness (LER), Line Width Roughness (LWR), Local CD Uniformity (LCDU) and stochastic defects. In this paper, several aspects of image contrast for EUV lithography are discussed. We will look at some of the fundamental mechanisms which degrade image contrast, including resist blur from acid diffusion. We assume the imaging of dense line/space images with k1<0.5. This assumption guarantees that the image is a simple sinusoid, and implies that the exposure latitude is proportional to the image contrast. We then consider various methods to experimentally measure image contrast. The classic measure would be to measure the exposure latitude from a Focus/Exposure Matrix (FEM) wafer. The exposure latitude is generally giving a contrast measure which is averaged over some portion of the wafer. In this paper, we propose the use of MEEF (Mask Error Enhancement Factor) targets to track contrast at specific locations on the wafer. In principle, one could measure many points across the image field (or even across the whole wafer) to map out the spatial variation of contrast, i.e. a contrast map. The paper includes experimental contrast measurements that relate to stage fading, source shape and resist blur from different processes. We will also briefly look at fading from the Pole-to-Pole image shift, and how to mitigate this with a novel exposure method called Dual Monopole.
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