EUV multilayer mirrors

The main optical elements used in EUV optics are based on multilayer (ML) film reflectors. As stated above, at 13.5 nm, a single-surface reflector made of any material has very low reflectivity. For instance, at normal incidence, the reflectivity of molybdenum is about 10 at 13.5 nm (see Fig. 14.3), which corresponds to an electric field amplitude reflectivity of about 3%. By constructing a smooth substrate, comprising a multilayer stack of alternating thin (a few nanometers) layers of molybdenum and silicon, with negligible photon absorption, it is possible to 

Practically all materials have very short attenuation lengths in the EUV range this significantly limits the reflectivity of actual multilayer film reflectors. Maximizing reflectivity at each interface and minimizing absorption of the spacer material makes the choice of the materials to he used for this purpose extremely 

From the above equation, it can be seen that having low component absorption and maximizing the difference between the real part of the refractive indices of the two components provide the overall maximum reflectivity for the multilayer. 

5-nm wavelength, silieon and molybdenum make an excellent pair of materials for an EUV multilayer mirror. The real part of their refractive indices is quite different in the EUV spectral region in addition, molybdenum has the lowest absorption at EUV relative to other alternative metals with reasonable single interface reflectivity.  

Montcalm, S. Bajt, P.B. Mirkarimi, E. Spiller, F.J. Weber, and J.A. Folta, Multilayer reflective coatings for extreme ultraviolet lithography, Proc. SPIE 3331, 42 51 (1998). 

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S.B. Hill, N.S. Faradzhev, C. Tarrio, T.B. Lucatorto, T.E. Madey, B.V. Yakshinskiy, E. Loginova, and S. Yulin, Accelerated lifetime metrology of EUV multilayer mirrors in hydrocarbon environ ments, Proc. SPIE 6911, 692117 (2008). 

Under EUV photon irradiation in the presence of oxygen or oxidizing agents, the surfaces of EUV optics and reticles are easily oxidized, leading to reflectivity loss of such surfaces. Oxides strongly absorb EUV radiation, and a small increase in oxide film thickness ( 1.5 nm) can decrease EUV multilayer mirror reflectivity by up to 1.6% (absolute). Because oxidation is difficult to remove, even small amounts of oxidation can negatively affect the EUV optics lifetime. The oxidation of silicon-capped EUV multilayer reflectors has been reported to show similar trends as those observed for carbon deposition. In particular, the rate of reflectivity loss due to oxidation of EUV mirrors has been observed to increase with the partial pressure of water in the optics environment, as well as with the EUV radiation intensity (see Eig. 14.16). " ... 

Figure 14.11 EUV spectrum for a Sn droplet obtained with a CO2 drive laser of an LPP system during high-repetition-rate operation in comparison with a high-temperature stable multilayer mirror reflectance curve. (After D.C. Brandt et al. )...

Takase, S. Terashima, Y. Gomei, M. Tanabe, Y. Watanabe, T. Aoki, K. Murakami, S. Matsunari, M. Niihe, and Y. Kakutani, Study of ruthenium capped multilayer mirror for EUV irradiation durability, Proc. SPIE 6151, 615135 (2006). 

Hill, C. Tamo, S.E. Grantham, T.B. Lucatorto, T. Madey, I. Ermanoski, S. Bajt, M. Chandhok, P. Yan, O.R. Wood II, S. Wurm, and N.V. Edwards, EUV testing of multilayer mirrors critical issues, Proc. SPIE 6151, 61510F (2006). 

It must be emphasized that the short attenuation length and low normal-incidence reflectivity of materials in the EUV range present very difficult problems regarding fabricating single-surface mirrors or lenses. The main solution to this problem is the use of multilayer reflectors. Absent these reflectors, EUV lithography as currently practiced would not be possible. 

Figure 14.11 shows the normalized EUV emission spectrum in the region from 8 to 20 nm obtained from a Sn target in an LPP system under development for high-volume manufacturing at Cymer. The peak emission of the C02-laser-produced plasma occurs at 13.5 nm and matches well to the reflectivity curve of multilayer coated mirrors designed for this wavelength. 

Furthermore, Hill et al. reported that mitigation of Ru-capped multilayers by carbon-containing species may result from water-induced desorption and/or displacement of carbon compounds from the exposure chamber and other surfaces therein. Atomic hydrogen has been reported to be elfective in cleaning Sn as well as carbon contaminants from EUV mirrors. ... 

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