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We have used the MET5 exposure system using “dark field” lithography, where a small σ=0.1 source is wholly contained in the 30% Central Obscuration of the 0.5NA mirror optics. One goal of this paper is to quantify and explain the superior image contrast of dark field lithography over normal dipole imagery. We demonstrate that almost ideal grating images can be obtained over the pitch range from 15 to 25nm. With the x-polarized source, gratings with Horizontal lines (TE polarized) have the best image contrast, while Vertical lines (TM polarized) have lesser contrast, showing less contrast as pitch decreases. By comparing lines of different orientation, the impact of polarization on lithography can be assessed. At the 16nm pitch, the experimental data showed roughly 20% improvement of the LWR metric for TE over TM. Besides the image contrast, we also calculate the effective contrast Ceff by folding in a Gaussian resist blur. These calculations highlight the need to find resist processes with small blur, e.g. with σ < 3nm. Exposure latitude scales with Ceff, as does LWR, LCDU and stochastic defect levels. Therefore optimization of Ceff, at the small pitches needed for production, is of high importance. We have also looked at dense arrays of bright spots produced with dark field imaging, which can produce either dense contact holes or dense pillars depending on the resist process tone. Our experimental results used a negative tone Metal Oxide resist process to print pillar arrays with pitches of 22, 24 and 26nm. Our experiments, and most of our simulations, were done with a standard EUV mask using a Ta-based absorber. Additional simulations explored the use of alternative absorber materials which can increase the dark field image intensity. For example, 25nm thick Ru absorber can more than double the image intensity relative to Ta absorber. The MET5 dark field litho imaging method is well-suited for testing resist processes in advance of the high NA tool availability. Both dense line gratings and dense hole/pillar array images can be imaged with good image contrast. However, pure dark field imaging is not capable of producing all the patterns needed for production, such as the larger pitch structures needed for overlay and alignment marks. On the other hand, there is a kind of “partial” dark field imagery that is very promising for production imaging with the high NA tool. Allowing Source Mask Optimization (SMO) software to include source points within the obscured part of the pupil, i.e. “dark field source points”, implements this capability in a very natural way and seems attractive for High Volume Manufacturing (HMV) applications with the high NA tool in the near future.
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