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Lithography
Posted on by Fred Chen

A Perfect Storm for EUV Lithography

A Perfect Storm for EUV Lithography
by Fred Chen on 04-03-2025 at 6:00 am
Categories: Lithography

Key Takeaways

  • EUV lithography faces challenges from electron blur, stochastics, and polarization as pitch sizes decrease.
  • The loss of contrast due to electron blur is around 50%, significantly affecting stochastic fluctuations.
  • Polarization effects are becoming a growing concern, contributing to image degradation.

Electron blur, stochastics, and now polarization, are all becoming stronger influences in EUV lithography as pitch continues to shrink

As EUV lithography continues to evolve, targeting smaller and smaller pitches, new physical limitations continue to emerge as formidable obstacles. While stochastic effects have long been recognized as a critical challenge [1,2], and electron blur more recently has been considered in depth [3], polarization effects [4,5] are now becoming a growing concern in image degradation. As the industry moves beyond 2nm node, these influences create a perfect storm that threatens the quality of EUV-printed features. Loss of contrast from blur and polarization make it more likely for stochastic fluctuations to cross the printing threshold [3].

Figure 1 shows the combined effects of polarization, blur, and stochastics for 18 nm pitch as expected on a 0.55 NA EUV lithography system. Dipole-induced fading [6] is ignored as a relatively minor effect. There is a 14% loss of contrast if unpolarized light is assumed [5], but electron blur has a more significant impact (~50% loss of contrast) in aggravating stochastic electron behavior in the image. The total loss of contrast is obtained by multiplying the contrast reduction from polarization by the contrast reduction from electron blur.

P18 polarization blur and stochastics
Figure 1. 9 nm half-pitch image as projected by a 0.55 NA 13.5 nm wavelength EUV lithography system. No dipole-induced image fading [6] is included. The assumed electron blur is shown on the right. The stochastic electron density plot in the center assumes unpolarized light (50% TE, 50% TM) [5]. A 20 nm thick metal oxide resist (20/um absorption) was assumed.
The edge “roughness” is severe enough to count as being defective. The probability of a stochastic fluctuation crossing the printing threshold is not negligible. As pitch decreases, we should expect this to grow worse, due to the more severe impact of electron blur [3] as well as the loss of contrast for unpolarized light [4,5] (Figure 2).

P18 P16 P14 electron stochastic cross section
Figure 2. Reduction of image contrast worsens with smaller pitch. The stochastic fluctuations in electron density also grow correspondingly more severe. Aside from pitch, the same assumptions were used as in Figure 1.

Note that even for the 14 nm pitch case, the 23% loss of contrast from going from TE-polarized to unpolarized is still less than the loss of contrast from electron blur (~60%). As pitch continues to decrease, the polarization contribution will grow, along with the increasing impact from blur. As noted in the examples considered above, although polarization is recognized within the lithography community as a growing concern, the contrast reduction from electron blur is still more significant. Therefore, we must expect any useful analysis of EUV feature printability and stochastic image fluctuations to include a realistic electron blur model.

References

[1] P. de Bisschop, “Stochastic effects in EUV lithography: random, local CD variability, and printing failures,” J. Micro/Nanolith. MEMS MOEMS 16, 041013 (2017).

[2] F. Chen, Stochastic Effects Blur the Resolution Limit of EUV Lithography.

[3] F. Chen, A Realistic Electron Blur Function Shape for EUV Resist Modeling.

[4] F. Chen, The Significance of Polarization in EUV Lithography.

[5] H. J. Levinson, “High-NA EUV lithography: current status and outlook for the future,” Jpn. J. Appl. Phys. 61 SD0803 (2022).

[6] T. A. Brunner, J. G. Santaclara, G. Bottiglieri, C. Anderson, P. Naulleau, “EUV dark field lithography: extreme resolution by blocking 0th order,” Proc. SPIE 11609, 1160906 (2021).

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Also Read:

Variable Cell Height Track Pitch Scaling Beyond Lithography

A Realistic Electron Blur Function Shape for EUV Resist Modeling

Powering the Future: How Engineered Substrates and Material Innovation Drive the Semiconductor Revolution

Rethinking Multipatterning for 2nm Node

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