In this paper we show experimental verification of the feasibility of printing pitch 40x70nm hexagonal holes using EUV single patterning. We show that at a local CDU (LCDU) of 2.7nm and an exposure dose of 54 mJ/cm2 a defect rate smaller than 7x10-9 is observed. This result was enabled by optimization of the illumination source and improvements in the resist. Resist selection identified multiple candidates that show a promising LCDU performance and optimization of the processing conditions resulted in improved performance. Experimental validation of the defect performance was done using HMI eP5 on the baseline process. Assessment of the LCDU performance for EUV single expose at pitches beyond 40x70nm, showed promising results.
We continue our work on the physics of mask-topography-induced phase effects in imaging using extreme ultraviolet (EUV) lithography, and specifically how these effects can be mitigated by alternative mask absorbers. We present a semianalytical model to calculate the mask-topography-induced phase offset and study its trend throughout the entire material space at 13.5-nm wavelength. We demonstrate that the model is in good agreement with 3D rigorous simulations. Using the model, we explain why the previously demonstrated phase shift close to 1.2π works optimally for EUV imaging. We show a low refractive index mask absorber (n < 0.91) is crucial for good mask 3D mitigation. We demonstrate the importance of mask bias and incident angle for imaging with an optimized attenuated phase-shift mask (PSM), which makes good source-mask optimization indispensable. We present the lithographic performance of alternative mask absorbers including a high-k mask, and a low- and high-transmission attenuated PSM for a few basic use cases, confirming the lithographic gain that can be obtained by mask-absorber optimization.
Alternative reticles have the potential to improve EPE for low-k1 EUV lithography on multiple aspects, by reducing mask 3D effects and improving optical contrast. We study the application of high-k masks and attenuated phase-shift masks at diffraction level and show that mitigation of mask 3D effects, such as contrast fading, is crucial for both good performance of both alternative-reticle types. We present optimum embodiments for both mask types. We find that the optimum attenuated phase-shift mask (PSM) results in a phase shift of 1.2 π. The extra 0.2 π phase shift required for the EUV mask compared to its DUV counterpart is needed to compensate the strong mask 3D effects; the 1.2 π phase shift is crucial for good performance at small pitch and was found for all 3 materials studied in this work: Ru, Pd, and Mo. We show that our Rubased attenuated PSM embodiment results in a strong gain in normalized image log slope (NILS). <30% NILS gain can be achieved compared to a Ta-based reference mask. To demonstrate the generic applicability of the mask, we show NILS gain using the same attenuated PSM embodiment for different use cases for 0.33 and 0.55-NA EUV lithography, including regular contacts, DRAM patterns, and contacts through pitch. We show that the optimum mask-type choice is application dependent and present our recommendations in a mask-decision tree. We discuss the implications of using new reticle absorbers for scanner integration.
EUV lithography is being used at relatively high-k1 Rayleigh factors. Advancing EUV to smaller resolution requires several technological advancements. The EUV reticle is a strong contributor that limits current EUV imaging performance. Improvements with advanced mask types are required to reduce mask 3D effects and to improve image contrast. This will enable low-k1 resolution with reduced stochastic defect rates. In this paper we discuss what the requirements of high-k absorber masks and attenuated phase shift masks are to achieve optimal imaging performance. Recommendations on the mask stack composition and the application of mask types to different use cases are based on the physical understanding of the mask diffraction spectrum.
Mitigation of mask 3D effects is essential for EUV imaging of high resolution features. The 3D EUV masks give rise to phase effects over the diffracted orders and potentially distort the image on the wafer. These phase effects may reduce contrast, result in pattern shifts and result in best focus variations on wafer. Two variations on the current absorber are investigated to their impact on reduction of M3D effects and impact on image quality. Use of high-k absorber materials allows for thinner masks to be used and helps to reduce averse M3D effects. Attenuated phase shift masks work by allowing a higher optical transmission while giving a phase shift to the transmitted light, which further improves image contrast on wafer and also enables thinner absorbers to be used. Attenuated PSM absorbers show a stronger variation in imaging performance through incidence angle onto the reticle. It has been shown that this results in a variation in imaging performance for varying features and pitches. Specifically of interest is how NILS through focus is influenced by the different absorbers. Phase shift masks show better performance for NILS through focus on contact holes, and high-k masks work well for dense lines.
Due to the high energy of extreme ultraviolet (EUV) photons, stochastic effects become more important at a constant dose when compared with deep ultraviolet exposures. Photoresists are used to transfer information from the aerial image into physical features and play an important role in the transduction of these stochastic effects. Recently, metal-oxide-based nonchemically amplified resists (non-CARs) have attracted a lot of attention. We study how the properties of these non-CARs impact the local critical dimension uniformity (LCDU) of a regular contact hole array printed with EUV lithography using Monte Carlo simulations and an analytical model. We benchmark both the simulations and the analytical model to experimental data, and then use the flexibility of both methods to systematically investigate the role of microscopic resist properties in the final LCDU. It is found that metal-oxide clusters should be <1 nm in diameter, otherwise granularity will have a significant contribution to LCDU. When varying resist properties to change the resist dose-to-size, we find that the LCDU scaling with dose depends on how the resist is modified. After performing an overall sensitivity analysis to identify the optimum scaling of LCDU with dose, we find a scaling of dose − 0.5 when the development threshold is modified, and a scaling of dose − 0.33 when core radius or the quantum efficiency is changed.