20 December 2001 Damage resistant and low-stress Si-based multilayer mirrors
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Applications of multilayer mirrors for extreme ultraviolet lithography (EUVL) require not only a high normal incidence reflectivity but also a long lifetime and minimal residual stress. A serious problem of Mo-Si multilayers is the structural instability in the case of localized absorption of in- and outband radiation from the EUV source followed by the degradation of the multilayer. A number os solutions have been envisaged in the past, including the use of compound materials (MoSi2Si) as well as the use of C barrier layers. We focused our interest on two Si-based systems: Mo/Si and Mo2C/Si multilayer mirrors. The mirrors were designed for normal incidence reflectivity at about 13 nm wavelength and were deposited by dc magnetron sputtering. Maximum normal incidence reflectivities of 68.4%12.8 nm for Mo/Si multilayer mirrors and 66.8%12.8 nm for Mo2C/Si have been achieved. Investigating the thermal stability of the multilayers in the temperature range from 300 degree(s)C to 500 degree(s)C we found that the reflectivity of Mo/Si mirrors is drastically decreasing after annealing above 300 degree(s)C, whereas the Mo2C/Si multilayers show a superior stability up to 400 degree(s)C...500 degree(s)C. Another problem of EUV multilayer mirrors is the large residual compressive stress (-400 to - 500 MPa), which causes undesirable distortion of the substrate figure. The reduction of residual stress of Mo/Si and Mo2C/Si multilayers with annealing has been investigated. Using a slow thermal annealing (1 degree(s)C/min), it is possible to reduce the stress from -520 MPa to zero by heating the Mo/Si samples up to 310 degree(s)C. However, this results ina reflectivity drop of about 3...4%. On the other hand one can reduce the stress of a Mo2C/Si multilayer from -490 MPa to zero by annealing without a considerable reflectivity drop.
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Torsten Feigl, Sergey A. Yulin, Thomas Kuhlmann, Norbert Kaiser, "Damage resistant and low-stress Si-based multilayer mirrors", Proc. SPIE 4506, Soft X-Ray and EUV Imaging Systems II, (20 December 2001); doi: 10.1117/12.450958; https://doi.org/10.1117/12.450958

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