It is well known from 193nm simulation studies that accounting for the electromagnetic (EM) interaction between the incident light and the mask become more important as the mask geometry shrinks. In particular this transition occurs when the size of the mask pattern becomes comparable to the wavelength of light.Early simulation work in EUV lithography indicated rigorous EM calculations are required to predict the subtle effects associated with the mask absorber shadowing effect. These calculations generally show that non-normal mask incidence creates several problems, including HV-bias, slit position dependent bias, and slit position dependent pattern shift. These results are surprising because the mask sizes studied are much larger than the 13.5nm wavelength. If approximate methods could be used rather than the rigorous EM calculations then EUV simulations would be much faster and more accessible.In this study, rigorous EM simulation results are compared with a Kirchoff approximation. The results show that Kirchoff simulations can mimic the shadowing effect with a simple mask bias. It is also found that the pattern shift effect is an artifact caused by a misinterpretation of the rigorous simulation results. With proper biasing depending on the pattern orientation and field position, simple Kirchoff simulation can be used. Thus Any MBOPC tool currently available can handle EUV proximity correction with minor modifications.
EUV lithography for resolutions below 8 nm half pitch requires the numerical aperture (NA) of the projection lens to be significantly larger than the current state-of-the-art 0.33NA. In order to be economically viable, a throughput in the range of 100 wafers per hour is needed.As a result of the increased NA, the incidence angles of the light rays at the mask increase significantly. Consequently the shadowing deteriorates the aerial image contrast to unacceptably low values at the current 4x magnification.The only solution to reduce the angular range at the mask is to increase the magnification. Simulations show that we have to double the magnification to 8x in order to overcome the shadowing effects. Assuming that the mask infrastructure will not change the mask form factor, this would inevitably lead to a field size that is a quarter of the field size of current 0.33NA step & scan systems. This would reduce the throughput of the high-NA scanner to a value significantly below 100 wafers per hour unless additional measures are taken.This paper presents an anamorphic step & scan system capable to print fields that are half the field size of the current full field. The anamorphic system has the potential to achieve a throughput in excess of 150 wafers per hour by increasing the transmission of the optics as well as increasing the acceleration of the wafer-and mask stage. This makes it an economically viable lithography solution. Furthermore, the simulated imaging behavior of the system is demonstrated and its impact on the rest of the lithographic system is discussed.
In this paper we introduce new source-mask co-optimization (SMO) capabilities for EUV with specific support of the details of imaging with NXE:33x0 scanners. New algorithms have been developed that fully exploit the adjustability of the light distribution inside the NXE:33x0 flexible illuminator, FlexPupil. The fast NXE M3D+ model accurately predicts the reflective 3D mask effects and enables novel pupil symmetries and mask defocus optimization. This mitigates the H-V bias, Bossung tilt, and pattern shift caused by shadowing and non-telecentricity, and reduces the sensitivity to flare. New pupil optimization flows will be shown. The optimized pupils are fully compliant with NXE:33x0 scanner specifications. We will demonstrate enhanced imaging performance of this NXE specific SMO on 7 nm node logic cut masks and show benefits up to 20% improved CD uniformity, and a reduction in the maximum pattern shifts.
In this paper, a broader understanding of the so called Mask Error Factor (MEF) will be described. MEF is defined as the ratio of the measured CD range on the wafer and the expected CD range due to the reticle. As a result, the MEF plays a very important role in the final CD range as observed on the wafer. It will be shown that the MEF can be controlled by Numerical Aperture (NA), illuminator settings, process conditions and resist type. Since the optimum conditions for Depth of Focus (DoF) will usually be found at different settings than the optimum for MEF, DoF has to be balanced with MEF in order to achieve a minimized CD uniformity over a required focus range.We will show experimental results for different illumination settings (e.g. quadrupole versus annular illumination). Results for contact holes, isolated lines and dense lines and spaces will be presented. Smaller resolutions, especially line widths below the exposure wavelength, will yield a higher MEF. In turn, decreasing reticle CD's yield reticles where CD uniformity is judged to have worse reticle quality. This stresses the importance of understanding and being able to control the MEF.
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