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Extreme Ultraviolet (EUV) Lithography

If wavelengths of light in the range of 11–14 nm are used, it is possible to construct reflecting optics of moderate efficiency (> 60%) using multilayer films. This opens up the possibility of projection optics and reduction imaging. With a numerical aperture of 0.25, a wavelength of ~13.5 nm (this particular choice is explained shortly), and a k₁ value of 0.6, the resolution is

The very short wavelength enables a small numerical aperture to be used, avoiding the problem of self-vignetting in all-reflective optics. Wavelengths in the range 11–14 nm are in the extreme ultraviolet (EUV) or soft x-ray portion of the electromagnetic spectrum, so lithography using such wavelengths is referred to as EUV lithography. Because projection optics are possible at EUV wavelengths, particularly reduction optics, many of the mask problems encountered with 1:1 x-ray lithography are avoided. However, as will be seen, EUV lithography has its own set of challenges.

Reflection occurs at interfaces between materials of different indices of refraction. The larger the difference in refractive index the greater the reflectivity. At wavelengths < 50 nm, all materials have indices of refraction
≈ 1. Thus, it is difficult to create a highly reflective interface. At EUV wavelengths, it has proven possible to make mirrors with moderate reflectivity, in the range of 60–70%, by the use of multilayers. Multilayer reflectors are made by depositing alternating layers of high-Z and low-Z materials, giving a small but effective difference between refractive indices at each interface (Fig. 12.9). By making the periodicity d of the multilayer stack satisfy the Bragg condition,

where θ is defined in Fig. 12.9, the net effect of small reflectivity at each interface is moderately high reflectivity overall when the stack has a sufficient number of layers.

The materials chosen to comprise the multilayer stack must be weak absorbers of EUV light, since the light must be able to penetrate to the lower layers of the film stack. Several stacks have been identified as possible reflectors in the EUV, and these are listed in Table 12.1. Because of beryllium’s toxicity, there is a reluctance to use that metal for EUV lithography, and most effort in EUV lithography today is focused on Mo/Si reflectors. Because reflectance is angle dependent [Eq. (12.5)], graded depositions are needed on optics that have light incident at varying angles over the mirror surfaces.

Reflectance versus wavelength is shown in Fig. 12.10. As can be seen in this figure, reflectance peaks narrowly around a given wavelength. From Eq. (12.8), it is apparent that only a very small wavelength shift causes a small change in peak wavelength (see Problem 12.3.) Even if every mirror in the EUV system has very high peak reflectances, the overall system transmission can be low if these peak reflectances do not occur at nearly the same wavelength. A similar statement is true regarding the need to match the peak reflectance of the masks to that of the projection optics mirrors.⁴ When specifying the wavelength at which peak reflectance occurs, it is important to pay attention to detail. As can be seen in Fig. 12.10, the curve of reflectance versus wavelength is asymmetric.

As discussed in Chapter 5, optical lithography has been practiced where there are light sources that satisfy certain key requirements, particularly narrow bandwidth and high intensity. The optics and masks of each optical lithographic technology have been engineered around this primary requirement of an intense, narrow bandwidth light source. For EUV lithography there are few options for masks and optics. EUV lithography must necessarily be practiced at wavelengths where there are multilayer reflectors with at least moderate reflectance. Unfortunately, intense sources of light may not exist at these wavelengths. Productivity due to low light intensity at the wafer is a concern with EUV lithographic technology, so considerable attention must be paid to multilayer reflectance and sources of EUV light.

References

1. A.A. Krasnoperova, R. Rippstein, A. Flamholz, E. Kratchmer, S. Wind, C. Brooks, and M. Lercel, “Imaging capabilities of proximity x-ray lithography at 70-nm ground rules,” Proc. SPIE 3676, pp. 24–39 (1999).
2. J.A. Folta, S. Bajt, T.W. Barbee, R.F. Grabner, P.B. Mirkarimi, T. Nguyen, M.A. Schmidt, E. Spiller, C.C. Walton, M. Wedowski, and C. Montcalm, “Advances in multilayer reflective coatings for extreme-ultraviolet lithography,” Proc. SPIE 3676, pp. 702–709 (1999).
3. S. Bajt, “Molybdenum-ruthenium/beryllium multilayer coatings,” J. Vac. Sci. Technol. A 18(2), 557–559 (2000).
4. S.D. Hector, E.M. Gullikson, P. Mirkarimi, E. Spiller, P. Kearney and J. Folta, “Multilayer coating requirements for extreme ultraviolet lithography masks,” Proc. SPIE 4562, pp. 863–881 (2002).