Classification of EUV masks based on the ratio of the complex refractive index k/(1-n)

Hiroyoshi Tanabe

doi:10.1117/12.2601875

12 October 2021 Classification of EUV masks based on the ratio of the complex refractive index k/(1-n)

Hiroyoshi Tanabe

Proceedings Volume 11854, International Conference on Extreme Ultraviolet Lithography 2021; 1185416 (2021) https://doi.org/10.1117/12.2601875
Event: SPIE Photomask Technology + EUV Lithography, 2021, Online Only

Abstract

Optical masks are classified based on the transmittance of the absorber. The optical density of binary masks is over three. This means that the transmittance is less than 0.1 %. Long experience in developing optical phase-shift masks (PSMs) has shown that the optimum transmittance of PSMs is 5~15 %. High transmittance PSMs are avoided due to the unwanted sidelobe printing. In the same way, EUV masks can be classified based on the reflectance of the absorber. Assuming the phase-shift value of the absorber to be π, the reflectance relative to ML is calculated by exp(-2πk/(1-n)), where n+ik is the complex refractive index of the absorber. Therefore, EUV masks are classified by the ratio of the complex refractive index k/(1-n). We classify EUV masks into four categories, i) binary mask: 1 < k/(1-n), ii) low-R PSM: 0.5 < k/(1-n) < 1, iii) PSM: 0.3 < k/(1-n) < 0.5, and iv) high-R PSM: k/(1-n) < 0.3. Conventional TaN absorber is a low-R PSM where k/(1-n) = 0.58. It is not a binary mask as it requires black borders. The benefit of PSMs is that they reduce dose. The drawback of PSMs is that they increase edge placement error (EPE). Especially in high NA optics, the error cannot be compensated by illumination optimization. On the other hand, binary masks reduce EPE at the expense of increased dose. There is a tradeoff between dose and EPE. The optimum absorber selection depends on the pattern layout. In CH applications, PSMs are favorable to reduce dose. Binary masks are suitable for L/S applications to suppress EPE.

Conference Presentation

Citation Download Citation

Hiroyoshi Tanabe "Classification of EUV masks based on the ratio of the complex refractive index k/(1-n)", Proc. SPIE 11854, International Conference on Extreme Ultraviolet Lithography 2021, 1185416 (12 October 2021); https://doi.org/10.1117/12.2601875

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