1. Field of the Invention
The present invention relates to capping layers for multilayer reflective coatings used in extreme ultraviolet or soft x-ray lithography applications.
2. Description of Related Art
Extreme ultraviolet (EUV) and soft x-ray projection lithography make use of optical elements with highly reflective multilayer coatings. These multilayer coatings typically consist of alternating layers of molybdenum (Mo) and silicon (Si) or molybdenum and beryllium (Be). High EUV reflectivity is essential for lithography applications. A critical limitation to achieving the maximum theoretical peak reflectivity is the oxidation and corrosion of the top layers, which both increases the absorption and degrades the phase coherence of the reflection from these layers.
Although there have been numerous investigations of carbon-based, boron carbide-based, and silicon-based multilayer coatings for EUV mirrors, there has been little work on environmental effects (e.g., oxidation and corrosion) of these structures. Underwood et al. (Applied Optics 32:6985 (1993)) investigated the aging effects of Moxe2x80x94Si multilayers by monitoring the decrease in reflectivity with time. Their experimental results showed a degradation of the Moxe2x80x94Si multilayer reflectance caused by the oxidation of the topmost layer of molybdenum. Underwood et al. identified the oxidation of the molybdenum layer as a potential problem in soft x-ray projection lithography. The proposed solutions were to make silicon the topmost layer, to store the optical elements in an inert atmosphere or vacuum, or to remove the oxidized surface by sputtering or chemical etching. Underwood et al. did not investigate the use of passivating layers.
Mo/Si multilayers with Mo as the top layer have the highest theoretically possible reflectivity; however, Mo is not stable in air and therefore Mo/Si multilayers for EUV optics are usually capped with a Si top layer with a loss in reflectivity of 1.3%. After exposure to air, this layer partly oxidizes and forms SiO2 that absorbs EUV light and reduces the reflectance of the multilayer by about another 1-2% This reflectance of Si capped multilayers will remain unchanged for years if the multilayers are kept at room temperatures. See C. Montcalm, S. Bajt, P. B. Mirkarimi, E. Spiller, F. J. Weber, and J. A. Folta, in xe2x80x9cEmerging Lithographic Technologies IIxe2x80x9d, ed. Y. Vladimirsky, SPIE Vol 3331, 42-51 (1998). However, in a working EUV lithography tool the coatings are exposed to EUV illumination in the presence of low pressure background gases including water, oxygen, and hydrocarbons. L. Klebanoff et al., M. Wedowski et al. references have shown that the reflectance of Si capped Mo/Si multilayers decreased as a function of EUV illumination dose and the amount of water vapor and other background gases in the system.
U.S. Pat. No. 5,958,605, titled xe2x80x9cPassivating Overcoat Bilayer For Multilayer Reflective Coatings For Extreme Ultraviolet Lithographyxe2x80x9d, discloses a passivating overcoat bilayer that is used for multilayer reflective coatings for extreme ultraviolet (EUV) or soft x-ray applications to prevent oxidation and corrosion of the multilayer coating, thereby improving the EUV optical performance. The overcoat bilayer comprises a layer of silicon or beryllium underneath at least one top layer of an elemental or a compound material that resists oxidation and corrosion. Materials for the top layer include carbon, palladium, carbides, borides, nitrides, and oxides. The thicknesses of the two layers that make up the overcoat bilayer are optimized to produce the highest reflectance at the wavelength range of operation. Protective overcoat systems comprising three or more layers are also possible.
It is an object of the present invention to provide a passivating overcoat bilayer for a multilayer reflective coating designed for use in extreme ultraviolet or soft x-ray applications.
It is another object of the invention to provide a bottom overcoat layer that prevents diffusion of a top overcoat layer into the top layer of a multilayer reflective coating.
It is another object of the invention to provide a top overcoat layer made of material that resists oxidation and corrosion and protects a multilayer reflective coating from oxidation.
These and other objects will be apparent based on the disclosure herein.
The present invention is a passivating overcoat bilayer for multilayer reflective coatings for soft x-ray or extreme ultraviolet applications and the method for making such layers. These passivating layers are useful for reflective optical coatings for soft x-ray and extreme ultraviolet wavelengths in applications such as microscopy, astronomy, spectroscopy, laser research, laser cavities and optics, synchrotron optics, and projection lithography.
A passivating overcoat bilayer (also called a xe2x80x9ccappingxe2x80x9d bilayer) is deposited on top of a multilayer coating to prevent oxidation and corrosion of the multilayer coating, thereby improving the EUV optical performance. The multilayer coating can comprise alternating layers of a variety of materials, such as molybdenum-silicon, molybdenum carbide-silicon, molybdenum-beryllium, and molybdenum carbide-beryllium. The passivating bilayer comprises a diffusion resistant layer underneath at least one top layer of an elemental material or compound that resists oxidation and corrosion. Oxidation resistant materials for the top layer (or layers) may include pure elements, such as Ru, Zr, Rh or Pd, and similar materials or compound materials. Diffusion resistant materials for the bottom layer include B4C, Mo and carbon and similar materials or compound materials. The top layer and the bottom layer may each comprise a plurality of layers.
The thickness of each layer that makes up the overcoat bilayer is in the range of about 0.5 to 7 nanometers, and the thicknesses are selected to produce the highest reflectance at the EUV wavelength range of operation. The thickness of the overcoat bilayer will typically be different from the thickness of the pairs of alternating layers in the underlying multilayer coating. The thickness of the two layers in the overcoat are individually optimized so as to provide sufficient chemical protection and to maximize EUV optical performance.