Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Reexamination Certificate
2001-03-29
2003-12-02
Turner, Archene (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S408000, C428S469000, C428S472000, C428S698000, C428S701000, C428S044000
Reexamination Certificate
active
06656575
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to multilayer reflective systems with a protective capping layer system according to the claims. The invention also relates to the production method of such multilayer systems.
BACKGROUND OF THE INVENTION
Conventional multilayer systems are produced by depositing materials with different reactive indices or different absorption coefficients on top of each other in several layers on a substrate. They are used in particular as mirrors in the extreme ultraviolet range. The wavelength range between 10 nm and 50 nm is designated as the extreme ultraviolet wavelength range. Other possible applications of multilayer systems are for example, in the visible wavelength range, as antireflective coatings of optical elements.
The reflection of electromagnetic radiation on a multilayer system is based on interference between the radiation which is reflected on the many interfaces of the multilayer system and is approximated by Bragg's law. This reflection is thus of a dispersive nature. The reflectivity of the interface between two such layers for electromagnetic radiation in a wavelength range <50 nm amounts to a few per thousand for angles that are greater than the critical angle. For reflection angles greater than the critical angle reflectivities up to a magnitude of 70% can be obtained. Multilayer systems are therefore used to achieve high reflectivities with maximum angles relative to the layer surface, and can also be used as dispersive elements.
A multilayer system for reflecting short wavelengths consists of successive sets of two or more layers respectively of materials with different refractive indices and thicknesses, for example in the magnitude of the wavelength of the reflected radiation. Partial reflection takes place at each of the interfaces between the different materials, and with a proper choice of the individual layer thicknesses, all partial reflections add up coherently. The overall reflectivity of a multilayer system is determined by the magnitude of the reflection per boundary surface, i.e. the difference of the refractive indices.
Multilayer systems for the extreme ultraviolet wavelength range generally consist of molybdenum-silicon- or molybdenum-beryllium-systems. For special applications multilayer systems are used made up from more than two differing types of layers. The choice of material with all multilayer systems depends heavily on the application's wavelength range.
Multilayer systems are utilized for the extreme ultraviolet to soft x-ray wavelength range amongst other things in lithography for the production of semiconductor components. It is precisely in their being employed in lithography that the multilayer system needs to demonstrate a long life with maximum possible constant reflectivity. On the one hand, the mirrors must show as little radiation damage as possible despite long periods of radiation. Any contamination or radiation damage would result in a shortened lifetime and usage interval, and hence in increased cost of the lithography process. The reflectivity does not fluctuate, but would go down monotonously.
Examinations have shown (J. H. Underwood et al., Applied Optics, Vol.32, 1993, p. 6985-6990), that when kept in air reflectivity decreases with time. Molybdenum silicon multilayer mirrors were examined. In particular, molybdenum used as the outermost layer became completely oxidized to molybdenum trioxide and molybdenum dioxide and contaminated with carbon-containing compounds, so that the absolute reflectivity decreased by 10 to 12%. The oxidization of the silicon layer into silicon dioxide caused a decrease in reflectivity of 4 to 5%.
In order to counter this, in U.S. Pat. No. 5,310,603 it is proposed that the mirrors should be provided with a protective layer of carbon of a thickness of 0.5 to 1 nm. With such mirrors we are dealing with a multilayer system for the soft x-ray range to the extreme ultraviolet wavelength range. For layering materials here use is made of, for example, ruthenium and silicon dioxide or even silicon carbide and hafnium.
In addition, C. Montcalm et al, SPIE Vol.3331, p.42-51 have a critical look at the problem of reduction of reflectivity by oxidization and contamination of the uppermost layer (p.44). Theoretical simulations were carried out for protective layers made from silicon dioxide, boron carbide, boron nitride, carbon, palladium, molybdenum carbide and molybdenum boride.
Moreover, C. Montcalm et al. tested experimentally and specifically for the first time how the reflectivity of multilayer systems changes when used in the context of lithography with extreme ultraviolet wavelengths. Measurements were carried out over a long period in real working conditions. In the course of this it was discovered that reflectivity is greatly decreased by contamination of the multilayer systems through residual substances in a vacuum.
WO 99/24851 describes a two or more layer passivation for multilayer reflectors for the soft x-ray and extreme ultraviolet wavelength range. The passivation consists at least of an under coating and an upper coating. In the case of the under coating it is a matter of the less absorbent material of the multilayer reflector, i.e. silicon in the case of molybdenum silicon mirrors, and beryllium in the case of molybdenum beryllium mirrors. In the case of the upper coating it is a matter of a material that does not oxidize or form an oxidation barrier and protects the layers beneath from oxidization. Quite generally these can be pure elements, carbides, oxides or nitrides. For example, silicon carbide, silicon dioxide or even molybdenum carbide are especially proposed. The thicknesses of the protective layers vary within the range 0.5 to 5 nm and are especially optimized on the mirrors to be protected. The upper coating is applied by precipitation from the gas phase or even controlled oxidization, the process for controlled oxidization not being elaborated in greater detail.
SUMMARY OF THE INVENTION
The present invention has as its task to prepare multilayer systems, especially for the extreme ultraviolet wavelength range, with a longer life span with as constant reflectivity as possible. This task is met by a multilayer system according to the claims. Moreover, the task is met by a process for the production of multilayer systems according to the claimed invention.
DETAILED DESCRIPTION OF THE INVENTION
The multilayer system according to the invention can consist of two or more materials with differing refractive indices or absorption coefficients. By applying a protective layer system from at least one of the substances ruthenium, aluminium oxide, titanium nitride, carbon, molybdenum carbide, silicon carbide or titanium dioxide respectively a situation is achieved where not only are the mirrors passivated against radiation damage, and chemical and mechanical influences, but the reflectivity is also even increased to a small extent. In contrast with conventional multilayer systems without a protective layer, the life span is increased e.g. by a factor of three.
The multilayer systems according to the invention have the advantage that they can be cleaned, without suffering any losses in reflectivity. Here various options for cleaning methods may be employed, whether it be for example ozone cleaning or wet or dry chemical etching.
Moreover, the multilayer system according to the invention shows the positive characteristic, compared with the multilayer systems of the prior art, of increased insensitivity to the partial pressure of water and/or water containing components, which are to be found during use of the multilayer system in a vacuum chamber. This results in the risk from oxidation by water being lessened.
The most important advantage of the multilayer system according to the invention is an improved resistivity against oxidation and contamination.
Ruthenium is an inert material which is resistant to surface deterioration caused, for example, by oxidation. In optical applications it has hitherto been used as a layer with a small re
Bijkerk Frederik
Görts Peter Cornelis
Louis Eric
Oestreich Sebastian
Yakshin Andrey E.
Carl-Zeiss-Stiftung
Hudak, Shunk & Farine Co. LPA
Turner Archene
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