Sub-resolution grating as an option for mask 3D effect mitigation in anamorphic imaging system

Abstract

Background Projection lithography technology has been developed to allow the use of shorter wavelength light and to increase numerical aperture (NA) from 0.33 to 0.55. After enabling extreme-ultraviolet (EUV) wavelengths, to keep up with the scaling trends, the industry would now again like to increase the NA. As the depth of focus (DoF) is inversely proportional to the square of NA, in hyper NA (> 0.55) EUV lithography (EUVL), we anticipate that the total available DoF in the lithography process would be further limited.

Aim We aim to improve the imaging performance for a wide range of lines and space pitches by minimizing best focus (BF) variations generated from different pitches on an anamorphic EUV mask.

Approach Sub-resolution grating (SRG) is proposed to address the increasingly complex design of sub-resolution assisted feature (SRAF), especially in high-NA or hyper-NA regimes where SRAF insertion becomes challenging. We first identify how the mask 3D (M3D) effect-induced BF variation through pitch behaves according to changes in the pattern orientation and mask tone for hyper NA EUVL. We study how various focus shift mitigation strategies can be combined to align the best foci and enhance the image contrast for hyper NA EUVL.

Results Simulation results indicate that for increased NA, BF variations due to M3D effects for vertical lines deteriorate more significantly than for horizontal lines. As mitigation strategies, we presented diverse solutions in the mask and illumination space, leading to the achievement of well-aligned BF with enhanced image contrast for a broad pitch range and various feature types. By using a phase-less binary mask that has an EUV refractive index of n≅1 and a high EUV extinction coefficient k, the phase offset-induced BF variation through pitch can be mitigated, which could be a favorable option for hyper NA where overlapping DoF becomes crucial. Illumination source optimization in conjunction with aberration injection can correct pole-to-pole offset. SRG in the mask design addresses BF variation through pitch and improves normalized image log slope (NILS) for various patterns, wafer critical dimension targets, and a wide range of exposure dose processes using the most simple and straightforward method. This technique could be one of the best complementary techniques for a high-reflective attenuated phase shift mask also known as a low-n mask.

Conclusions We identified that SRG aligns BF for various patterns with different pitches, and this can be applied to all absorber thicknesses. In addition, we identified that SRG even improves NILS at certain absorber thicknesses.