Hyper-NA EUV lithography: an imaging perspective

Abstract

To print ever smaller features at high contrast projection lithography technology has evolved to shorter wavelength light and larger NA. After enabling the EUV wavelength, the industry is looking into increasing the NA. This study aims to identify EUV specific challenges regarding NA scaling beyond 0.55. We study if EUV imaging can still work at this higher NA and whether specific changes to the mask stack are required. At NA's much higher than 0.55, new effects like polarization will play a role, and larger impact of ultimate mask resolution and material interactions is expected. Already at NA 0.55, a small contrast loss is predicted due to the use of unpolarized light in the scanner. Further increasing the NA will enhance the contrast loss. We study these polarization effects in detail and assess their impact quantitatively for a set of generic building blocks. In addition, the larger incidence angles on mask when the NA increases above 0.55, will further enhance the M3D effects forcing additional mask changes. To enable proper imaging at high incidence angles, new mask architectures, that include changes in the EUV mask absorber and multilayer, will have to be tested using rigorous simulations.

Best focus alignment through pitch strategies for hyper-NA EUV lithography

“…We arbitrarily chose the values of threshold and wafer CD target as mentioned above; however, later in this paper, it will demonstrate that SRG is effective for different values other than these wafer CD target and threshold. We opted for NA 0.75 for this simulation considering that a new type of multilayer structure might be required for NA above 0.85 5 . The most basic full leaf shaped illumination source fixed for 16nm pitch was used, and a high-reflective Low-n mask model 8 was employed in this simulation.…”

“…Figure 15 (a) shows the NILS as a function of the main features pitch with varying SRG size, and (b) the maximum, minimum, and average NILS for various pitch ranging from 15.5nm to 63nm as a function of SRG size. To see only the effect from SRG, below 15nm pitch was excluded as the NILS degradation at that pitch region is mostly driven by the polarization effect because the un-polarized dipole illumination source is used in this work 5 . The recovery of NILS in the isolated pitch range can be observed with the introduction of SRG and 3nm SRG shows a stable NILS across the pitch.…”

“…After enabling the extreme-ultraviolet (EUV) wavelength, the next stage for EUVL is to increase the NA beyond 0.55. While recent studies have focused intensively on imaging perspective 5 , this paper will primarily delve into research concerning depth of focus (DoF) and best focus (BF) variation. DoF can be approximated by inserting a value for lambda, NA, and k2 which represents the process dependent constants.…”