Patent Application
20080073592
Embodiments of the present invention may be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the invention. The drawings, however, should not be taken to be limiting, but are for explanation and understanding only.
Elliptical mirrors may be designed and constructed to collect EUV with a single bounce, a high range of incident light angles and at low cost. The design is compact, potentially easy to cool, and collects a larger proportion of the light from an EUV source than many other designs. As many as fifteen or more elliptical shells may be used to collect light from different types of EUV sources. The shells are reflective and may be used in a vacuum environment to avoid absorption.
A fifteen shell elliptical collector is described below, however, the number of shells and their design may be modified to increase the efficiency of collection and to accommodate different EUV sources. Different materials may also be chosen to increase reflectivity as a function of angle and the lifetime of the collectors.
The described collector is particularly well-suited for discharge produced plasma (DPP) sources. However, it may be adapted to any of a variety of other extreme ultraviolet (EUV) and other light sources.
The cone of light represents a large solid angle. The reflectivity of, for example, a Ru coated elliptical shell may be greater than 60% for angles greater than 25°. For larger angles, multilayer coatings may be used on the elliptical shells. A five bi-layer multilayer mirror of SiC and Ru, each of thicknesses 13.8 nm and 9.9 nm respectively may provide reflectivity of greater than 60% for higher solid angles, such as 32° or more. Other reflective coatings may be used to optimize performance for still higher incident angles.
The elliptical surface of each shell may be based on an ellipse with a different height, but the same foci. This provides a different shape surface on each shell and the surface will be shaped specifically for the range of angles that are anticipated for that shell to reflect. The number of shells and their design may be adapted to suit different light sources and different types of reflectors.
Each shell is able to efficiently reflect light within a certain range of incident angles. The width of the shell will determine the range of incident angles that it will collect. Accordingly, the width of each shell may be selected for the angles at which it works best, or at which it works well. For a Ru coated shell, incident angles of from 0° (tangential) to about 15° show good reflectivity and the reflectivity may still be usable at angles as high as 25° or 32°. Ray tracings such as that of
Another design limitation is the thickness and the width of the shells. Each shell will block light that would otherwise reach a neighboring shell on either side. The amount of light blocked may be reduced by reducing the thickness of each shell (the vertical direction in
Another design parameter is the range of incident angles from the light source that are to be intercepted. As shown in the example of
In one example, the collector uses fifteen shells, as mentioned above. The largest shell has a diameter of about 700 mm, while the smallest shell has a diameter of about 94 mm. Each shell is about 50.8 mm wide and about 1 mm thick. This appears to work well with a particular DPP light source that is about 0.5 mm high and wide and about 2 mm deep. However, for other light sources and other reflectors, other numbers of shells of other sizes may provide better results.
While the shells in the figures are shown as circular, this is related to the choice of light source and other optical elements in the photolithography system. The particular physical shape of the rings may be elliptical, oval, or elongated in more than one dimension in order to suit other light sources and to direct light to other types of optical elements.
The collectors may be fabricated in a variety of different ways. In one example, a glass shell may be used for the substrate or, in other words, as a mandrel. First a release layer is applied to the mandrel surface. Then a seed layer is applied over the release layer. A base layer is applied over the seed layer and a reflective coating is applied over the base coat. In one example, the release layer is a polymer, the seed layer is a metal, the base layer is another metal layer, and the reflective coating is Ru or a multilayer coating.
Depending on the particular choice of materials and intended application, one or more of these layers may not be necessary. The release layer may be avoided by using thermal expansion to release the reflector or by not releasing the reflector from the mandrel. The seed layer may be avoided by combining the seed layer and base layer into single layer or by using a conductive polymer as the release layer.
Considered in more detail, a polymer is sprayed onto the inside of the glass mandrel. This polymer is eventually used as the release layer. A seed layer of Au, Au—Pd. Ni, Cr, Pt, Ag reduction, or any of a variety of other seed materials may then be sputter coated, sprayed, deposited or electroplated over the polymer inside the glass mandrel. The deposited seed layer is then electroplated or electroformed with a base coat of Ni, Ni—Co, Ni—P, Cu or any of a variety of other materials individually or in any combination of two or more materials. The electroplated material may then be machined, or polished, or both to the desired surface roughness value, if necessary. Finally, the electroplated base coat material is sputter coated with Ru or other multilayer materials.
The optic is released from the mandrel by dissolving the polymer. This may be done with heat or with a chemical, such as acetone, or in a variety of other ways such as photodissolution. While the lifetime of the glass shell mandrel may be short in an EUV environment, this does not limit the lifetime of the collector when the final optic does not include the mandrel. Even if the glass mandrel were not released, the optical surface of the optic is opposite the surface of the glass mandrel. As a result, the glass would be at least partially protected. This would extend the lifetime of the glass in the EUV environment.
As an alternative, instead of applying the polymer release layer and the seed layer, the inside diameter of the mandrel may be directly electro-less plated with Ni or another metal. The electroplated material is machined or polished to the desired surface roughness values to minimize flare. The Ru or other multilayer mirror materials may be directly coated over the Ni. Another approach, would be to deposit a seed layer by spraying or physical vapor deposition, followed by electroplating with the material of choice. Finally the optic may be released by cooling. The cooling may be selected to exploit the vastly different coefficients of thermal expansion between the glass and the Ni metal. With either approach, for any specific shape and finish of the optics, the reflecting surfaces can be machined, polished or electro-polished easily without deforming the optic since the mandrel may be used to provide mechanical support until after the polishing is finished.
At block 42, the glass mandrel may be mounted on a holder, such as a self centering holder for spinning. At block 43, a polymer is then spin coated or spray coated inside of the mandrel. A variety of different polymers may be used. Polymers that can be thermally dissociated, or conductive polymers, or polymers that can be dissolved by exposure to light may be used. If a conductive polymer is used then the metal seed layer may not be needed. At block 44, the coated mandrel is baked.
At block 45, a seed coat layer, if needed, is applied, over the polymer. This may be done, for example, by sputtering. At block 46, the inside of the mandrel is coated with a Ni layer directly over the seed layer. This coating may be measured to quantify its roughness. Different techniques may be used including AFM (Atomic Force Microscopy) and interferometry. The surface smoothness is important as this may affect the reflectivity of the eventual reflector.
If at block 47, further smoothing is required, then at block 48, the surface may be polished. Electro-polishing, or in a lower volume fabrication, hand polishing may be applied to the Ni surface. The surface may be measured again, and the roughness quantified again by AFM and/or interferometry.
When the surface is sufficiently smooth, then at block 49, it may be sputter coated with Ru. Alternatively, multilayer mirrors may be fabricated on the inside of the optic and then the thickness and roughness may again be measured. If the surface is good, then at block 50, the optic may be released from the mandrel by dissolving the polymer. Before use, the optic may be tested for quality, including characterizing it at visible and EUV wavelengths.
As an alternative, instead of using a seed layer, the Ni may be applied directly to the polymer. This may result in better adhesion to the mandrel during the rest of the fabrication process. With a Ni seed layer, a Ni—Co or Ni—P layer may be electroplated onto the seed layer, instead of a straight Ni layer. As a further alternative, Cr or Pt may be used as the seed layer instead of Au. As a further alternative, a silver reduction may be sprayed on the interior to act as the seed layer.
A lesser or more complex shell configuration, cross-sectional design, concentric arrangement, and production process may be used than those shown and described herein. Embodiments of the invention may be applied to different reflective materials and constructions. Optical elements may be added to the system for a variety of different reasons. Therefore, the configurations may vary from implementation to implementation depending upon numerous factors, such as price constraints, performance requirements, technological improvements, or other circumstances. Embodiments of the invention may also be applied to other types of photolithography systems that use different materials and devices than those shown and described herein.
In the description above, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. For example, well-known equivalent materials may be substituted in place of those described herein, and similarly, well-known equivalent techniques may be substituted in place of the particular processing techniques disclosed. In other instances, well-known optical elements, structures and techniques have not been shown in detail to avoid obscuring the understanding of this description.
While the embodiments of the invention have been described in terms of several examples, those skilled in the art may recognize that the invention is not limited to the embodiments described, but may be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting.
