EUV Pupil Rotation Impact on Resolution

Inconsistent illumination across the slit

The optics used for illuminating masks in EUV lithography systems is quite complex and it really is not discussed enough [1]. Constraints on the illumination system affect the ability to maintain productivity while improving resolution. Improving resolution involves shrinking pitch, which restricts how much of the pupil can be used, i.e., the pupil fill. When the pupil fill is below 20%, light is excluded, essentially by being absorbed by the illumination system [2]. However, besides pupil fill, there is also the issue of pupil rotation [3]. A field lens with a toroidal surface reflects light so that a homogeneous intensity distribution fills the exposure field, which necessarily rotates the plane of incidence through the arc-shaped exposure field. The range of rotation is +/-30 degrees [4]. Evidence of rotation of the plane of incidence is shown in the horizontal-vertical bias across slit [5] as well as the pattern shift across slit [6].

Using only a small portion of the arc-shaped field to limit the degree of rotation [7] wastes the unused portion of the exposure field, and will mean more exposure stops per wafer, causing throughput to suffer. For example, restricting to only +/- 15 degrees of rotation halves the used exposure slit width, requiring twice as many reticle and wafer scans, as well as stage steps in between. Thus, we must expect to use the whole width of the exposure field, and evaluate the resulting impact on achieving higher resolution.

Let’s now consider the implications for a couple of key application cases for EUV lithography.

30 nm pitch (0.33 NA)

The Intel 4 [8] and Intel 3 [9] minimum metal pitch is 30 nm. However, it will not be recommended to use EUV direct print for this resolution.

Figure 1. +/-30 degrees rotation removes 20% of the pupil from being used for 30 nm pitch imaging, leaving only 13%. This is less than the 20% minimum to avoid system absorption.

As Figure 1 indicates, only 13% of the pupil is able to be used for 30 nm pitch imaging across slit. Since this results in system absorption, the throughput will take a hit, as the source power is effectively reduced significantly.

For pitches smaller than 30 nm, it gets even worse. For 24 nm pitch, for example, the maximum allowed pupil fill is below 20% even without considering rotation.

Instead of direct print, multipatterning [e.g., 10-12] would be necessary with the 0.33 NA system for this case.

Honeycomb patterns (Hexapole illumination)

Honeycomb patterns requiring hexapole illumination are encountered in the storage node landing locations in DRAM. This illumination mode is very sensitive to pupil rotation since the angular width of each pole is quite narrow; +/- 15 degrees tolerance can already be estimated to be impossible assuming the angular distance between poles to be equal to the angular width of the poles. As a result, as shown in Figure 2, for example, a 36 nm center-to-center honeycomb array is impacted by the cross-slit pupil rotation.

Figure 2. Pupil rotation of hexapole illumination for storage node landing locations of 11nm-class DRAM. The hexapole illumination is essentially rotated out of place, beyond the tight angle tolerance.

Fortunately, like the line-space case above, there are known alternate means for producing the honeycomb patterns, even with a single DUV mask exposure, such as self-aligned spacer patterning [13] or directed self-assembly [14].

High-NA Status

The most recent update on the High-NA (0.55 NA) EUV system [15] indicates that the cross-slit pupil rotation is addressed with a new illuminator design. The degree to which it is eliminated would be confirmed by measuring the horizontal-vertical CD and pattern shift difference across slit.

References

[1] EUV Lithography's Pupil Fill Tradeoff: Defocus Tolerance vs. Throughput (substack.com)

[2] M. van de Kerkhof, H. Jasper, L. Levasier, R. Peeters, R van Es, J-W. Bosker, A. Zdravkov, E. Lenderink, F. Evangelista, P. Broman, B. Bilski, T. Last, "Enabling sub-10nm node lithography: presenting the NXE:3400B EUV scanner," Proc. SPIE 10143, 101430D (2017).

[3] M. Antoni, W. Singer, J. Schultz, J. Wangler, I. Escudero-Sanz, B. Kruizinga, “Illumination optics design for EUV lithography,” Proc. SPIE 4146, 25 (2000).

[4] M. van den Kerkhof, A. Klein, P. Vermeulen, T. van der Woord, I. Donmez, G. Salmaso, R. Maas, “High-transmission EUV pellicles supporting >400W source power,” Proc. SPIE 12051, 120510B (2022).

[5] G. McIntyre, C-S. Koay, M. Burkhardt, H. Mizuno, O. Wood, “Modeling and Experiments of Non-Telecentric Thick Mask Effects for EUV Lithography,” Proc. SPIE 7271, 72711C (2009).

[6] M. Sugawara, G. R. McIntyre, “Assessment of CD and pattern position error caused by non-flat surface of mask and chuck,” 2007 International EUVL Symposium.

[7] A Forbidden Pitch Combination at Advanced Lithography Nodes

[8] B. Sell et al., “Intel 4 CMOS Technology Featuring Advanced FinFET Transistors optimized for High Density and High-Performance Computing,” 2022 Symposium on VLSI Technology and Circuits.

[9] W. Hafez et al., “An Intel 3 Advanced FinFET Platform Technology for High Performance Computing and SOC Product Applications,” 2024 Symposium on VLSI Technology and Circuits.

[10] Multipatterning Reduction with Gridded Cuts and Vias (linkedin.com); Easing of Multipatterning with Gridded Cuts and Vias | by Frederick T. Chen | Oct, 2024 | Medium

[11] Simplifying Multipatterning with Gridded Cuts and Vias (youtube.com)

[12] BEOL Mask Reduction Using Spacer-Defined Vias and Cuts (linkedin.com); BEOL Mask Reduction Using Spacer-Defined Vias and Cuts - SemiWiki

[13] J. M. Park, Y. S. Hwang, S. -W. Kim, S. Y. Han, J. S. Park, J. Kim, J. W. Seo, B. S. Kim, S. H. Shin, C. H. Cho, S. W. Nam, H. S. Hong, K. P. Lee, G. Y. Jin, ES Jung, “20nm DRAM: A new beginning of another revolution,” IEDM 2015.

[14] H-C. Wu, M-C. Liao, E. Hirahara, T. Iwaki, “Wet etch process for high resolution DSA patterning for advanced node DRAM,” Proc. SPIE 12957, 1295721 (2024).

[15] J. van Schoot et al., J. Micro/Nanopattern. Mater. Metrol. 24, 011009 (2024).