Reflective optical components for use in the extreme ultraviolet spectral range (EUV), which encompasses the wavelength range from approximately 10 nm to approximately 50 nm, can be realized with multilayer mirrors containing a generally periodic layer sequence comprising a plurality of layer pairs. A layer pair generally contains two layers composed of different materials, which should have a maximum difference in their optical constants in the wavelength range provided for the use of the component. At least one of the materials, the so-called spacer material, should have a minimum absorption at the wavelength provided. The selection of the materials for the multilayer mirrors is therefore dependent primarily on the wavelength at which the optical component is intended to be used. In the EUV spectral range, therefore, for in each case a specific wavelength range that is usually only a few nanometers wide, there is an optimum material pairing that guarantees a high reflection on account of the optical contrast of the layer materials.
In the wavelength range from approximately 12.5 nm to 14 nm, which is of great importance, in particular, for the development of optical systems for applications in EUV lithography, multilayer mirrors comprising the material pairing molybdenum and silicon are preferably used since there is particularly good optical contrast between these materials in the wavelength range stated. With Mo/Si (molybdenum-silicon) multilayer mirrors it is possible to obtain, for example, a reflection of approximately 70% at a wavelength of 13.5 nm.
For the operation of optical systems for EUV lithography, laser plasma sources that emit at a wavelength of approximately 13.5 nm are provided, in particular, as radiation sources. Since the reflection of the overall optical system in EUV lithography is comparatively low due to the plurality of mirrors, EUV radiation sources of this type have to be operated with high powers in order to compensate for the reflection losses that arise in the optical system. In the vicinity of such a high-power EUV radiation source, EUV multilayer mirrors can be exposed to high temperatures. This is the case, in particular, for an EUV multilayer mirror which is positioned close to an EUV radiation source for beam shaping purposes, for example, as a so-called collector mirror.
At high temperatures the materials molybdenum and silicon tend, however, toward the formation of molybdenum silicide, in particular MoSi2, and toward interdiffusion processes at the interfaces, as is known, for example, from German Patent Publication DE 100 11 547 C2. Therefore, at high application temperatures, there is the risk of degradation of such multilayer mirrors, which significantly reduces the reflection. In addition to reduction of the reflection, the degradation caused by interdiffusion processes and molybdenum silicide formation is also associated with a decrease in the thickness of the layer pairs, which is also referred to as period thickness. This decrease in the period thickness results in a shift in the reflection maximum toward a shorter wavelength. The function of an optical system based on Mo/Si multilayer mirrors can be considerably impaired or even completely destroyed by such degradation processes.
In order to increase the thermal stability of Mo/Si multilayer mirrors, it is known from German Patent Publication DE 100 11 547 C2 to insert a respective barrier layer of Mo2C at the interfaces between the molybdenum layers and the silicon layers. Furthermore, DE 100 11 548 C2 describes the use of barrier layers of MoSi2 for increasing the thermal stability.
Furthermore, it is known from U.S. Pat. No. 6,396,900 to insert barrier layers composed of the material B4C into Mo/Si multilayer mirrors in order to increase the reflection and/or the thermal stability.
The use of such known barrier layers makes it possible to produce layer systems having a high reflection which have an improved thermal stability by comparison with pure Mo/Si layer systems.
In the case of Mo/Si layer systems with barrier layers, however, the technological requirements when producing the barrier layers are comparatively high since the thickness of the barrier layers is generally less than 0.5 nm. In particular, it is difficult to deposit a layer sequence with such thin barrier layers on curved substrates.
This holds true particularly if the angle of incidence of the EUV radiation varies over the surface of the multilayer mirror and, for this reason, the layer sequence has to have a layer thickness gradient in order to meet the Bragg reflection condition at all locations of the mirror surface.