Reflective optical element and EUV lithography appliance

Abstract

The invention relates to a reflective optical element and an EUV lithography appliance containing one such element, said appliance displaying a low propensity to contamination. According to the invention, the reflective optical element has a protective layer system consisting of at least one layer. The optical characteristics of the protective layer system are between those of a spacer and an absorber or correspond to those of a spacer. The selection of a material with the smallest possible imaginary part and a real part which is as close to 1 as possible in terms of the refractive index leads to a plateau-type reflectivity course according to the thickness of the protective layer system between two thicknesses d1 and d2. The thickness of the protective layer system is selected in such a way that it is less than d2.

Description

CROSS REFERENCE [0001] This application is a continuation-in-part application of International Application No. PCT / EP2004 / 002014, filed Mar. 1, 2004 and published as WO 2004 / 079753 on Sep. 16, 2004, which claims the priority to German Application No. 103 09 084.3, filed Mar. 3, 2003.FIELD OF THE INVENTION [0002] The invention concerns a reflective optical element for the EUV and soft X-ray wavelength region, having a multilayer system and a protective layer system, wherein the side of the multilayer system facing the protective layer system terminates in an absorber layer. Furthermore, the invention concerns an EUV lithography appliance with a reflective optical element of this kind. BACKGROUND OF THE INVENTION [0003] Multilayers are composed of periodic repetitions, and in the most simple case a period consists of two layers. The one layer material should consist of a so-called spacer material, while the other layer material should consist of a so-called absorber material. Spacer m...

AI Technical Summary

Benefits of technology

[0011] The reflective optical elements of the invention have the benefit that their relative insensitivity to thickness variations in the protective layer system also translates into an insensitivity to the build-up of a contamination layer. Without substantial change in reflectivity, much thicker carbon layers can be tolerated than with traditional reflective optical elements. This also has a positive impact on the homogeneity of the imaging, since even thickness fluctuations over the entire area are negligible.

[0013] It has been found that a standing wave field is formed by reflection at the reflective optical element, whose minimum for a protective layer thickness d1 lies in the vacuum at a distance of a fraction of the operating wavelength. Now, if the layer thickness of the protective layer system is increased, the minimum of the standing wave field approaches the surface. Accordingly, the value of the standing wave field at the surface increases until the maximum is also achieved. Thus, the formation of the reflectivity plateau in dependence on the thickness of the protective layer system results because, with increasing layer thickness, the additionally created absorption, i.e., the resulting decrease in reflectivity, is compensated in that reflectivity gains are produced by increasingly constructive interference after a certain layer thickness.

[0014] As an additional effect, fewer photoelectrons are emitted near the minimum of a standing wave field. Since the photoelectrons also break down the hydrocarbons from the residual gas atmosphere into carbon or carbon-containing particles, this has the result of a noticeably slower build-up of the contamination.

[0019] The advantageous properties of the invented reflective optical element have especially positive impact when they are used in an EUV lithography appliance. Especially when several reflective optical elements are connected in succession, the more uniform reflectivity and also more uniform field illumination for lengthy periods of time have especially positive impact. It has been found that even with increasing contamination the wavefront errors in the complex optical systems of EUV lithography appliances can be kept small. A major benefit consists in that fewer cleaning cycles are required for the EUV lithography appliance, thanks to the longer lifetime of the reflective optical elements. This not only reduces the down time, but also the risk of degeneration of the layer homogeneity, greater roughness of the surface, or partial destruction of the uppermost protective layer from too intense cleaning are significantly reduced. In particular, the cleaning processes for the reflective optical elements of the invention can be controlled such that the contamination layer is deliberately not entirely removed, but rather a minimal contamination layer always remains on the uppermost layer. This protects the reflective optical element against being destroyed by too intense cleaning. The thickness of the contamination layer can be measured in traditional manner during its build-up or during the cleaning with a suitable in situ monitoring system.

[0023] In another preferred embodiment, the protective layer system ends toward the vacuum with a layer of a material that is inert to energy injection, that is, to bombardment with EUV protons or to external electric fields. This decreases the probability of spontaneous electron emission, which in turn might split apart the residual gases into reactive cleavage products. Hence, the deposition of contamination on the protective layer system is further reduced. One can influence the inertia to external electromagnetic fields, for example, by giving the surface the lowest possible relief and / or using materials that have a large gap between the valency band and the conduction band. Especially preferred for this are the materials Nb, BN, B4C, Y, amorphous carbon, Si3N4, SiC, as well as silicon dioxide in various stoichiometric relations. The silicon dioxide can be in the amorphous or polycrystalline, or possibly even the crystalline state.

Problems solved by technology

Although EUV lithography appliances have a vacuum or a residual gas atmosphere in their interior, it is not entirely possible to prevent hydrocarbons and / or other carbon compounds from being inside the appliance.

These carbon compounds are split apart by the extreme ultraviolet radiation or by secondary electrons, resulting in the depositing of a carbon-containing contamination film on the optical elements.

This contamination with carbon compounds leads to substantial reflection losses of the functional optical surfaces, which can have a considerable influence on the economic effectiveness of the EUV lithography process.

The contamination leads not only to reflectivity losses, but also to imaging errors, which in the worst case make an imaging impossible.

These significantly increase the operating costs.

But the cleaning cycles not only increase the down time, but also entail the risk of worsening of the homogeneity of the layer thickness of the reflective optical elements and the risk of increasing the surface relief, which leads to further reflectivity losses.

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