Perspectives and tradeoffs of absorber materials for high NA EUV lithography

Abstract

Next-generation extreme ultraviolet (EUV) systems with numerical apertures of 0.55 have the potential to provide sub-8-nm half-pitch resolution. The increased importance of stochastic effects at smaller feature sizes places further demands on scanner and mask to provide high contrast images. We use rigorous mask diffraction and imaging simulation to understand the impact of the EUV mask absorber and to identify the most appropriate optical parameters for high NA EUV imaging. Simulations of various use cases and material options indicate two main types of solutions: high extinction materials, especially for lines spaces, and low refractive index materials that can provide phase shift mask solutions. EUV phase masks behave very different from phase shift masks for DUV. Carefully designed low refractive index materials and masks can open up a new path toward high contrast edge printing. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Investigation of waveguide modes in EUV mask absorbers

“…The investigations of this work are triggered by the observation of waveguide-like modes in the simulated near-field of EUV masks. 16 Waveguiding effects in EUV were also observed in modeling studies on EUV lithography with an NA of 0.99 21 and in coherent diffractive imaging using EUV light. 22 This work perceives the patterned mask absorber as a waveguide.…”

“…The imaging effects are investigated for vertical onedimensional features (space). Vertical spaces are more critical than horizontal features 13,16 and have less propagating waveguide modes due to lower demagnification (4×) compared with horizontal (8×). The waveguide approach can be used to explain effects that far-field diffraction (Fourier) analysis or the simplified geometrical shadowing approach cannot explain.…”

“…This expected performance of high k materials can be explained by shadowing effects of the mask. 11,16 However, shadowing arguments cannot explain the increasing NILS for thicker absorbers for the n ¼ 0.88 binary mask [orange, Fig. 24(a)].…”

“…Various forms of hybrid mask models have been developed to enable an explicit consideration of double diffraction of EUV light at the absorber and to separate the contributions of the absorber and the multilayer to the imaging characteristics. [7][8][9] In addition to such models, the analysis of diffraction orders, [10][11][12] specific investigations of multilayer contributions, 13,14 and the study of images from separate parts of the source 15,16 help to explain mask 3D (M3D) effects and to devise mitigation strategies. 17 Various modeling studies have been performed to identify the most appropriate absorber materials for EUV masks.…”

“…17 Various modeling studies have been performed to identify the most appropriate absorber materials for EUV masks. 11,[15][16][17][18][19][20] These studies also include attenuated phase shift mask (AttPSM) for EUV. The results of these studies suggest the usage of absorbers with high extinction coefficient k and/or with low refractive index n. Changing the phase shift from π to 1.2π was suggested for AttPSM.…”