In this report, we propose palladium oxide (PdO) as an absorber material for an EUV mask that can print line-and-space patterns with a half pitch down to 14 nm. In our simulations, because of its low refractive index (n = 0.8634) and high extinction coefficient (k = 0.0536), an attenuated phase shift mask with a very thin (∼20 nm) PdO absorber can provide an EUV contrast as high as 88% at a 14 nm half pitch under dipole illumination. This results in a very limited horizontal–vertical critical dimension bias (≤2.6 nm) and a sufficiently high normalized image log slope (≥2.78) down to a 14 nm half pitch.
In extreme ultraviolet lithography, the photon shot noise effect is a main cause of low-quality imaging characteristics such as line edge roughness and critical dimension (CD) nonuniformity. In this study, the stochastic imaging property of an attenuated phase-shift mask (PSM) was evaluated, and the results showed that “informative” photons from the first order diffraction are essential for mitigating the photon shot noise effect. This structure exhibits a reflectivity of ∼6% at the absorber stack and a phase shift of 180° at 13.5 nm wavelength. The improved stochastic patterning properties of the PSM were compared with those of a conventional binary intensity mask.
The coherent scattering microscopy/in-situ accelerated contamination system (CSM/ICS) is a developmental metrology tool designed to analyze the impact of carbon contamination on the imaging performance. It was installed at 11B EUVL beam-line of the Pohang Accelerator Laboratory (PAL). Monochromatized 13.5 nm wavelength beam with Mo/Si multilayer mirrors and zirconium filters was used. The CSM/ICS is composed of the CSM for measuring imaging properties and the ICS for implementing acceleration of carbon contamination. The CSM has been proposed as an actinic inspection technique that records the coherent diffraction pattern from the EUV mask and reconstructs its aerial image using a phase retrieval algorithm. To improve the CSM measurement accuracy, optical and electrical noises of main chamber were minimized. The background noise level measured by CCD camera was approximately 8.5 counts (3 sigma) when the EUV beam was off. Actinic CD measurement repeatability was <1 A (3 sigma) at 17.5 nm line and space pattern. The influence of carbon contamination on the imaging properties can be analyzed by transferring EUV mask to CSM imaging center position after executing carbon contamination without a fine alignment system. We also installed photodiode and ellipsometry for in-situ reflectivity and thickness measurement. This paper describes optical design and system performance observed during the first phase of integration, including CSM imaging performance and carbon contamination analysis results.
The authors evaluated the feasibility of using coherent scattering microscopy (CSM) as an actinic metrology tool by employing it to determine the critical dimension (CD) and normalized image log-slope (NILS) values of contaminated extreme ultraviolet (EUV) masks. CSM was as effective as CD scanning electron microscopy (CD-SEM) in measuring the CD values of clean EUV masks in the case of vertical patterns (nonshadowing effect); however, only the CSM could detect shadowing effect for horizontal patterns resulting in smaller clear mask CD values. Owing to weak interaction between the low-density contaminant layer and EUV radiation, the CSM-based CD measurements were not as affected by contamination as were those made using CD-SEM. Furthermore, CSM could be used to determine the NILS values under illumination conditions corresponding to a high-volume manufacturing tool.
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