High-NA EUV lithography targets 14 nm pitch as its ultimate application [1]. However, at 18 nm pitch and below, the polarization plays a significant role in imaging [1-3].

Polarization by EUV Multilayers

It has already been established that the multilayer coatings used in EUV mirrors have partially polarizing properties [4]. This is in fact, related to the Brewster's angle [2], where the TE polarization (E-field perpendicular to the plane of reflection) is reflected more predominantly than the TM polarization (E-field within the plane of reflection).

Figure 1. The EUV multilayer can be tuned to reflect maximally at targeted angles. However, for each case, the TE polarization reflects more than the TM polarization. This becomes more obvious for larger angles.

Impact on High-NA EUV Imaging

As implied by Figure 1 and earlier work [4], even the currently operating 0.33 NA EUV systems already have partially polarized imaging. The polarization is predominantly perpendicular to the plane of reflections. However, at the pitches in use on the 0.33 NA machines, the polarization impact is not significant. It only becomes significant at pitches of 18 nm and below [3], which is where High-NA (0.55 NA) systems are targeted. Smaller pitches require interfering waves at larger angles, which practically result in reduced resolution for the TM polarization.

Figure 2. At smaller pitches, the angle is larger, which degrades imaging for the TM polarization but does not affect the TE polarization.

For the 16 nm pitch, for example, the peak-to-valley difference is smaller for the TM polarization (Figure 3), since the normalized image log-slope (NILS), the key image quality metric, is too low. Hence, the lines perpendicular to the plane of reflection (these are the horizontal lines) will have better images than the lines parallel to the plane of reflection (these are the vertical lines).

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Figure 3. 16 nm pitch at 13.5 nm wavelength has unsatisfactory imaging (NILS<2) for the TM polarization.

ASML CTO Martin van den Brink had recently mentioned [5] the possibility of even higher NA, which is being explored by the company. However, he is doubtful of going beyond the 0.55 NA. This would not be surprising, since at higher NA, even larger angles result in even worse degradation of the TM polarization, preventing the satisfactory imaging of sub-10 nm vertical lines.

References

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

[2] L. Bilalaj, H. Mesilhy, A. Erdmann, "Simulation study on EUV multilayer polarization effects," Proc. SPIE 11875, 118750L (2021).

[3] F. Chen, Polarization by Reflection in EUV Lithography Systems, https://www.youtube.com/watch?v=agMx-nuL_Qg

[4] T. Jota, R. Chipman, "Polarization aberrations induced by graded multilayer coatings in EUV lithography scanners," Proc. SPIE 9776, 977617 (2016).

[5] P. van Gerven, https://bits-chips.nl/artikel/hyper-na-after-high-na-asml-cto-van-den-brink-isnt-convinced/