Experimental results demonstrated that the EUV low-n mask improves both LCDU and dose for dense features. For semi-isolated and isolated features best-focus shifts through pitch can occur with the low-n mask as well as with the Tabased mask. In this paper, we show how mask-3D induced phase offsets between diffraction orders lead to best focus shifts for dense and (semi-) isolated features. Subsequently we investigate what are the options to mitigate best focus shifts. First, when the pitch is large enough, sub-resolution assist features (SRAFs) can significantly improve contrast and reduce best-focus shifts. Second, optimizing the target and mask bias, is very effective for focus-shift mitigation. Especially the low-n mask shows a strong response to target and mask bias. Third, using bright field imaging instead of dark-field imaging generally also results in smaller best focus shifts through pitch. Using source-mask optimization (SMO) examples through pitch, we show how these mitigation strategies are effectively applied to through pitch use cases. We find NILS DOF improvements up to 65% with the low-n mask and optimized retargeting.
The low-n attenuated phase-shift mask can strongly improve extreme ultraviolet imaging performance; it enhances contrast by mask 3D mitigation and a phase-shift effect while simultaneously reducing the required exposure dose. The latter happens because the low-n mask gives optimum contrast at more open mask bias values than its Ta-based counterpart. Here, we experimentally verify the imaging physics of the low-n mask. We show that optimum exposure latitude (EL) with the low-n mask is obtained at more open mask bias values compared to the Ta-based reference mask. This leads to dose reductions exceeding 30% for pitch 38-nm regular contact holes (CHs). Initial local critical dimension uniformity (LCDU) data for hexagonal CHs pitch 38 and 40 nm show 15% LCDU improvement with the low-n mask compared to the Ta-based reference. A 16-nm dense lines show a substantial EL increase and dose reduction with the low-n mask compared to the Ta-based case; this can be even further improved by combining the novel mask absorber with asymmetric illumination. As the low-n masks studied here have absolute reflectivities in the range 8% to 15%, side-lobe printing should be carefully monitored. Initial experimental data for pitch 120-nm CHs and simulations on P32 metal clips, show no signs of side-lobe printing. Careful monitoring of stochastic side-lobe printing for various use cases is recommended.
Simulations on attenuated phase-shift masks (att PSM) for EUV have shown that these novel mask absorbers can strongly boost optical contrast. The optimum EUV imaging mask does not only need to balance the diffraction order amplitudes (as in DUV imaging), it also needs to mitigate the strong mask 3D effects that are present in EUV lithography. The latter is very important and strongly relies on material properties. Here, we present an overall progress update on our att PSM work, including the first experimental lithography results on an EUV att PSM test mask and guidelines needed for optimum performance from diffraction point of view.
The purpose of pellicles is to protect reticles from particle contamination, thus reducing the number of defects and increasing yield. In this paper we show how recent progress in pellicle technology has succeeded in solving the main challenges in imaging with EUV pellicles. We demonstrate this using the recent results of imaging tests in scanner, EUV reflectivity measurements, and lifetime testing. EUV light reflectivity of pellicles is one of the effects that have negatively impacted imaging with pellicles in the past. Light reflected from pellicles leads to the overexposure of neighboring fields in the corners and edges. Tests with pellicles produced using a new process show EUV reflectivity within specification of 0.04%, and measured impact on critical dimension in the corners below 0.15nm for multiple pellicles. Lifetime performance was tested by exposing up to 3000 wafers with a pellicle while periodically assessing the stability of imaging metrics. The lithometrics studies include: critical dimension (CD) and critical dimension uniformity (CDU), and contrast (via line width roughness). DoseMapper, which is an EUV scanner application developed to improve CDU, was applied during the lifetime test. Here we show that it can successfully reduce the pellicle-induced CDU and CDU over lifetime (previously shown to be dominated by pellicle EUV transmission drift). Our results using DoseMapper show that whilst intrafield CDU 3sigma increases over lifetime, it stays comfortably within the 1.1nm NXE3400 ATP specification using DoseMapper.
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